Contact us to have your installation tips, photos, and videos posted on this page.  

*NOTE: this project is currently in Beta testing. The half-dozen Spridget owners presently undertaking the conversion will, at their option, share photos, tips, and performance data on this page as their projects are completed.

**HOWEVER: the Eaton blower used here displaces the same 45 cubic inches as that in the Moss Motors' conversion, the beginning drive pulley ratio is similar, and we use the same SU carburetor. Accordingly, the performance-related differences are a 5-rib drive belt (rather than 4-rib), a 1999 Audi A4 intercooler, and 2002 Mini Cooper S bypass valve.

These instructions apply to the A-series engine as longitudinally mounted in the MG Midget, Austin Healey Sprite, and Bugeye, and the Morris Minor—with an alternative intercooler setup to suit its different sheet metal behind the grille.

As for the sequence of installation, you can upgrade to a serpentine belt drive, mount the supercharger, install the intercooler, re-route the heater fresh air intake, prepare the SU HIF44 to run under boost, and install the high-pressure fuel system in any order you wish, while continuing to drive your Spridget or Minor until ready to hook up the blower and begin running on boost.

That said, it is advisable to convert to a serpentine belt drive before fabricating the front supercharger mount, because the final welds—attaching small ears which bolt to the supercharger’s front mounting tabs—require checking the alignment of the pulleys.

Once the components are in place, the final steps are: (a) swap the shorter serpentine belt for a longer one that will reach the blower’s pulley (60 seconds); (b) switch over to the upgraded fuel delivery line (60 minutes); and (c) replace the air cleaner with the snorkels connecting the carburetor to the Audi A4 intercooler and the Mini Cooper S bypass valve circuit (another 60 minutes). If you wish to install a cool air intake or boost gauge, plan on at least a couple of hours—and more likely a morning—for each task.

These online instructions will be expanded as photos become available. As each section is finished, the corresponding hyperlink below will be enabled.

convert to serpentine belt drive

fabricate the supercharger mounts

install the idler pulley

relocate the horns

install the intercooler

re-route the heater fresh air intake

install a high-pressure fuel delivery system

replace the supercharger pulley

install the supercharger & plumb bypass circuit

prepare the carburetor

Establish the correct mixture

install the boost gauge

install a ‘stealth’ cool air intake

select and harvest a used supercharger

rebuild a used supercharger

convert to serpentine belt drive

A good starting point is to convert your naturally aspirated A-series engine to a serpentine belt drive. Once this is accomplished, you can continue to drive your car while installing the other components as your budget and time permit.

The process is covered on a separate page on this website, linked here: Converting to Serpentine Belt Drive.

fabricate the supercharger mounts

 One of the most satisfying, money-saving stages of this conversion is fabricating the mild steel mounts that attach the Eaton M45 supercharger to the ignition side of the engine.

Even if you have never fabricated a part using mild steel, the material’s inherent characteristics make it amenable to a variety of hand and power tools. Also, the design of the mounts is relatively forgiving. In short, fabricating the mounts is within the skill set of an average amateur MG mechanic.

When fabricating the mounts, you should refer to the free downloadable drawings with specifications available on the Purchase Page. However, below are photos of the components you will make (except for the simple support strut, which is cut and drilled once the front mount is bolted onto the engine), and then either weld together or hire someone to do that for you.

Note that the time you are charged to weld up the components should be not much more than an hour, if you deliver the parts clean and with the anti-corrosion coating stripped along the mating surfaces (not yet done in the below photo).

Indeed, the only weld that involves a critical measurement attaches the two components that make up the front mount—and you will provide the welder with the printed diagrams and foolproof index marks on the front mount components, as shown later on.


Here is a finished set of front and rear mounts (and serpentine belt pulleys)s fitted to an A-series engine:

Creative Spridgets DIY supercharger mounts.jpg

The above photo shows the $65.00, late-model Saturn CS-121D alternator recommended in the parts list. However, this conversion may be compatible with other charging options. Contact us for details.

SECTION A: The Rear Mount

The rear mount is composed of three components welded together with simple butt joints.

All of the components are made from inexpensive, 3/16” thick mild steel. They are: (i) an inner bracket that replaces the OEM rear alternator or generator mount, made with 1-1/2” wide angle iron; (ii) a center trapezoid cut from 3” wide flat bar; and (iii) an outer, more complex shape fashioned by cutting and drilling 2” wide angle iron.

 Here are the three components of the rear mount laid out for welding (note that the anti-corrosion coating is sanded off to avoid contaminating the welds):


The easiest shape to cut is the center trapezoid. You should rely on the downloadable CAD drawings for the dimensions, but here are images showing how to mark and cut the part for the 65-degree angle that is a repeating theme in making the rear mount:

*NOTE: While buying steel fom a home improvement store is more convenient and possibly cheaper than local or online jobber suppliers, retail stores apply an anti-corrosion coating that loads up sandpaper and takes more effort to remove for welding, as seen in the below comparison. This NOT a concern with the 1/8" x 1" support strut, as it is not welded but only drilled.


It is not critical that the mating edges be perfectly even, as the welds will fill in any small gouges, file or grinder dressing marks, etc.

What is important are three things: (i) the 5-5/16” measurement of the longest edge is a finished dimension; (ii) the angle that starts at the end of the longest side must be as close to 65 degrees as possible; and (iii) the short side must be precisely perpendicular to the two longer sides—that is, at 90 degrees to them.

The reason for taking care when measuring and cutting this otherwise simple, center component is that it establishes the outboard, third component’s relationship to the vertical plane of the engine block, and thereby determines the supercharger’s position in three-dimensional space. So, take your time when making this part.

Next up is the innermost component. This part is designed to achieve three goals:

First, it reproduces the layout of the slotted 5/16” holes for the bolts that affix the rear OEM generator mount to the block, and their relationship to the single hole that will receive the 8-1/2” long, 5/16-18 bolt which supports both the CS121D internally cooled alternator used with this conversion and the idler pulley: 

*NOTE: the 45-degree angle at the top/rear of the inner component is not essential, but eliminates a sharp corner adjacent to the dipstick and affords more clearance for your fingers when checking the oil.

Second, the outboard side of this part provides a flat mating surface for the first butt weld you will complete:


Third, the part provides more metal than the original mount to allow for longer slots, thus enabling greater fore-aft adjustment of the blower:


To make this inboard component, first cut an approximately 3-3/4” long piece of 1-1/2” wide angle iron (the piece in these photos was a little short, which took away some of the ‘forgiveness’ of the design) and drill the single hole for the alternator pivot bolt.

You should take all longitudinal measurements for the holes in this part from the same end of the stock; lateral measurements are from the flat or outside edge of the angle iron—i.e., not from the middle of the radius formed by the angle iron’s extrusion:

Next, drill and slot the oval holes that provide the gross, initial fore-aft adjustment of the supercharger.

The easiest method is to drill two holes and then connect them with a Dremel or small cutoff wheel mounted in a drill motor. You can use a small file to smooth the borders of the slots, but a quicker method is shown in the third image—an inexpensive pneumatic die grinder ($13.00 on the internet) and a cheap set of carbide burrs.

(We found the Capri forged, thin-body wrenches in the third photo far superior to the soft metal wrenches that came with the grinder, but they cost more than the grinder and bits combined.)

Next is the most complicated component—the outer, third part of the rear mount. However, fashioning it out of 2” wide, 3/16” thick angle iron is not technically difficult; it’s just time-consuming. Be patient, go slowly, and you will be rewarded with an elegant and durable rear mount.

First, cut an 11.5” long piece of angle iron. It is important to end up with at least that much steel; 11.75” is probably a better starting place, as it’s possible to cut the ends a little crooked which will require re-squaring. That process will shave off overall length.

If you look closely at this image, the stock was a hair under 11.5” and this required a higher level of precision in making the part, as you will see later on.


Next, mark one corner to indicate the point from which all of the coming longitudinal measurements are taken. Note that there are only two, diametrically opposite corners of the stock that you can select to make this mark:


Here are the measurements to locate the two, asymmetrical holes that will correspond with the supercharger’s rear mounting tabs:


Now drill the 5/16” holes per your measurements. Although a drill press is shown here (the stock is resting on a piece of wood, with both pieces clamped to the table), a hand drill is fine. Just make sure in any event that you use plenty of oil, adding it often during the process, and let the bit work at a natural pace.

Next, measure and mark the two inner cuts you will make to simultaneously: (i) form the mounting tabs that will bolt onto the blower’s rear mounting ears; (ii) create the relief for the rotor chamber to nestle into the rear mount; and (iii) begin creating the mating edge for welding to the center trapezoid:


Now make the cuts, using either an electric band saw or hack saw (a plasma cutter is not recommended for this operation, as the jagged edges will reduce the width of the tabs), making sure not to slice through the other part of the angle iron. Stop in time to leave roughly the thickness of the stock, or 3/16”:


The next step is to carry the lines formed by the cuts you have just made onto the other side of the angle iron, and then mark the radii and bottom of the relief for the supercharger’s rotor chamber.

It is not critical that the relief have rounded ends. Doing so merely results in a more aesthetically pleasing mount that follows the contours of the supercharger’s ribbed rotor chamber:


Next, cut the end farthest from your initial index mark to form a mating edge which will be welded to the middle trapezoid. The width of the inboard mounting tab thus created is not critical; the only critical dimension resulting from cutting its profile is that you must leave 3.0” at the end of the part to mate with the 90-degree, outboard end of the middle trapezoid you made previously:


Now, cut and shape the relief for the rotor chamber, and also cut the angles beneath it, thereby forming the gross contours of the component. The angles forming the outboard side of the component (the bottom in this picture) may default to 65 degrees to create visual harmony with other components, but this is entirely up to you.


This final image shows the mounting ears rounded off. The quickest method is to cut 45-degree angles with a saw and then grind the steel to form a graceful curve.


The final step before breaking the edges and scuffing off the anti-corrosion finish to prepare for welding is to slot the fore-aft, fine-adjustment holes in the mounting ears.

Refer to the CAD drawings for the location of the holes that form the slots, drill the holes, and then connect them with a Dremel or small cut-off wheel in a drill motor:

rear_mount (center)_slotting(1).jpg
*TIP: keep fiber wheels that get worn down like the one in the below picture (or even smaller), as they are helpful for slotting holes in tight quarters.


Now, sand off the protective coating near the mating edges (on both sides and all adjacent edges), clean the steel to avoid contaminating the welds (brake cleaner works well for this), and jig the parts up on a flat surface for welding:


Weld one side, let the part cool a little before proceeding:

welding rear mount_1st_side.jpg

Note the discoloration along the full length of the initial welds in the above photo. This indicates good penetration and a durable weld.

Now, weld the other sides of the joints:


After breaking all edges, test fit the mount on the engine block. It should extend outward at 90 degrees from the plane formed by the line where the cylinder head joins the block. If it is not perpendicular to the head, readjust the mount by grabbing it in a vise as shown below:

aligning_rear_mount (in vise).jpg

We found it easiest to apply even pressure by using two big crescent (adjustable) wrenches or one paired with a medium-sized monkey (pipe fitting) wrench.

Finally, degrease the mount and finish with your choice of high-temperature engine paint. Once the paint has dried, you may bolt the mount to the block—positioning the original 5/16-24 bolts in the center of the fore-aft adjustment slots and installing your generator or alternator as usual. You may now continue running naturally aspirated while focusing on the front mount.

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SECTION B: The Front Mount
The front mount is easier to make, as it is composed of just two components: a ‘J’-shaped, vertically oriented piece of 3/16” thick, 6” wide mild steel that encircles the supercharger’s snout and will receive its front mounting bolts; and another but more simple shape fashioned from 3/16” thick, 2” wide angle iron. There are then two small mounting tabs welded to the ‘J’-shaped outboard piece with the supercharger in place and aligned, as shown at the end of this section.

Below is a photo of the ‘J’-shaped, outboard component laid out on the mild steel stock. Both of the long angles are 65 degrees; the short 7/8” angle should be roughly perpendicular to the 4-1/2” side to the left of this image, but that is entirely arbitrary; it could just as well parallel the 6-3/4” longest angle above it in this photo (and below it when the part is right-side-up). It could also be a curve rather than a flat ray.

Indeed, the only critical dimensions on this part are: (i) its height—which provides sufficient room to attach the snout’s mounting tabs and the support strut that makes the alternator a stressed member and produces an extremely rigid mounting system; and (ii) its overall length—which allows sufficient inboard/outboard adjustment when jigging the part up for welding, as we will see shortly:


Remember to defer to dimensions on the free, downloadable diagrams available on our purchase page.

This part may be cut with a plasma cutter to save time, in which case a large socket, lid from a jar, or other appropriate shape may be clamped to the steel to guide the torch and produce the circular relief for the blower’s snout.

Alternatively, a bimetal hole saw will produce a very clean radius—taking your time and applying liberal amounts of oil—though the blade may not survive to make more than one part:

New front mount(1).jpg

The remainder of the outer profile can be easily cut with a plasma cutter, a hand-held electric band saw, a ‘saws-all’ with a bimetal blade, a jig saw with an appropriate blade, or more laboriously with a hack saw and plenty of elbow grease.

The other component is fashioned from a length of angle iron, as shown below, and is extremely simple to make. Indeed, the only critical dimensions relate to the 3/8” hole which slides onto the final cylinder head stud (a longer stud is used in place of the shorter one, as discussed later on).

In the below photo, the 1-7/16” dimension locates the hole’s center from the flat, forward-facing side of the 2” wide angle iron (i.e., not from the radius formed by the angle’s extrusion). If this measurement is off slightly, the part will still be viable but its aft side—the top in this photo—may contact the valve cover and require relieving with a file or grinder. As always, defer for dimensions to the free downloadable diagrams available on our purchase page.


The 1/2” dimension locates the hole in an inboard dimension—to the right in this photo—as measured from the cut made 90 degrees or perpendicular to the long axis of the part. In this photo, the dotted line carries the cut across the top of the part, as the author inadvertently made a gouge when cutting the part with a hand-held electric band saw.

This error did not affect the part’s viability. But the longitudinal (that is, perpendicular to the engine’s axis) relationship between the hole’s center and the cut running down the front, vertical portion of the part is important, as that portion rests against the side of the cylinder head, thus creating a cantilever, as shown below:

IMG_2590 (1).jpg

Note that the part being fabricated above features a profile along the edge adjacent to the thermostat housing that matches its contours. This is fine if you have a late model cross-flow radiator, as the housing aims toward the left-hand side of the car.

However, if you have a down-flow, early radiator or a VW Rabbit radiator (like the 1967 MG Midget pictured in these instructions), you will need a Creative Spridgets combination thermostat housing spacer and GM-style temp sensor mount pictured below and available here.


The above photo shows an OEM longer head bolt installed in place of the short bolt, together with a 3/8” I.D. x 1” long spacer. However, it may be possible to retain the short bolt if you use an ARP-style, shorter nut.

The spacer has a square outer profile, so the relief for the OEM thermostat housing must be moved back toward the 3/8” hole for the head bolt and made more square, as shown in the earlier picture enlarged here to emphasize the area in question:


Next, place the two components in a vise and mark the center of the 3/8” cylinder head bolt hole and the center of the large semi-circular relief for the blower’s snout.

Defer to the downloadable diagram for the appropriate measurement, then measure the distance between the centers using a carpenter’s square and small adjustable or fixed square or triangle:


Double check the diagram for the correct measurement, adjust the components in the vise as necessary, clamp them up tight and make three index marks:


Finally, prepare the steel, align the parts using your index marks, and weld as shown in this photo series. The discoloration shows where the mount was welded on the reverse side—along the bottom and outboard end of the angle iron:

It is now time to fabricate and weld the two front mounting tabs onto the outboard-aft side of the mount. This must be done with the mounts bolted to the engine and the supercharger temporarily suspended from the rear mount to align the pulley.

First, install the rear mount and position it in the center of the fore-aft adjustment:


Now, hang the front mount on the cylinder head. These photos show the alternator with idler pulley and support strut installed:

Next, bolt the supercharger to the rear mount, again positioning the bolts in the center of their fore-aft range:


As stated earlier, in order to align the pulleys you must have converted to a serpentine belt drive as described here.

Using a framing square or long straight-edge, check pulley alignment both above the supercharger’s drive shaft and against the water pump pulley, and below the shaft and against the alternator and crankshaft pulleys, as shown here:

Make adjustments as necessary, using the twin 5/16” bolts affixing the rear mount to the engine block for gross fore-aft ‘range’ adjustments, and then moving to the bolts holding the supercharger to the rear mount for finer adjustments.

Once the alignment is reasonably close (you will finalize it later), fabricate and weld on the tabs connecting the supercharger’s mounting ears to the front mount as shown here. The dimensions should be confirmed by frequent test-fitting to make sure the tabs afford sufficient fore-aft adjustment—but this process is greatly simplified by bolting the supercharger to the rear mount and aligning the pulleys.

The lower/inboard tab:

Begin by cutting the basic shape from mild steel angle iron:

Next, mark and drill the longer leg of the angle iron, make two parallel cuts, and finish with a round file to form a U-shaped channel:

Round the edge that will face forward and test fit the mounting tab:


Break the edges and strip off the protective coating to produce the finished part, ready for welding


Now jig up the lower mounting tab and weld in place. Be sure to remove the protective coating on the front mount itself for an inch or so around the tab:

The upper/outboard tab:

The upper mounting tab is much simpler. Begin by cutting a 1-1/4” section of 2” mild steel angle iron and rounding the longer leg. Then, drill two holes in the longer leg:


Connect the holes with a Dremel or small cutter wheel in a drill motor, and test fit the mounting tab, ensuring that the slot provides sufficient fore-aft adjustment. At the same time, mark the front mount to indicate where you will remove the protective coating before welding:


*Note: if you are using rosin core wire instead of shielding gas, you may wish to put a rag over the blower to avoid the splatter in the above photo.

Here are the tabs welded onto the aft side of the front mount:


Finally, as with the front mount, degrease the part, break the edges, and finish with your preferred color of high-temperature engine paint:

Creative Spridgets DIY supercharger mounts.jpg

The above mounts are painted OEM olive green, which contrasts nicely with the silver and black components, but this is a custom project. Do whatever strikes your fancy!

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Installing the idler pulley

This conversion uses an inexpensive, late model Saturn CS121D internally cooled alternator (though it is compatible with the earlier Saturn and OEM Lucas alternators, and with additional fidgeting, even early Spridget generators).

A single 8-1/2” long 5/16-18 bolt acts as the alternator pivot and mount for the 54mm idler pulley—Dayco part no. 8535, which turns on a combination bushing and spacer. This photo series shows the simple setup:

The highest quality combination bushing and spacer we could find is actually a lawnmower part. Its internal diameter is 3/8” but this does not affect is usefulness in this application.

The bushing’s external diameter where it inserts into the idler pulley is a few thousandths over. Turning it for a light press fit takes just seconds using a drill motor secured in a bench vise, a length of 3/8” all thread or bolt, a couple of nuts and some washers. This photo series shows how to turn the bushing:

Use very light, even pressure on the file and check the fit often, as the operation only takes a few seconds. Make sure your are rotating the bushing opposite to the cutting direction of the file’s teeth, or things will go much slower.

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relocate the horns

If your horns are located at the bottom of the well behind the grille and just in front of the radiator, they will need to be relocated to make room for the intercooler. While reusing the OEM horns is possible, we found that modern horns are both louder and more compact.

Here is one of the original Lucas horns fitted to the 1967 MG Midget test mule featured in these instructions compared with a horn from a 2009 Volvo C30 (which is similar to the S40):


The Volvo horns have a similar tone to OEM Spridget horns but are slightly brighter and noticeably louder. They nestle underneath the sheet metal on either side of the bonnet latch mechanism, requiring only a single hole to receive the mounting bolt:


The leads from the horn’s pigtail hanging down in the first above photo will easily reach the OEM loom, which used to run down alongside the vertical support. The bonnet release cable is unaffected. (see the second above photo).

If you wish to mount these horns, they are Volvo part nos. 30796711 and 30796712 (low and high tones). The pigtails are not stocked by most Volvo dealers but are used for various auto electronic components. Common part numbers include NAPA 2-18457 and ACDelco PT113.

The horns do not draw much current, but you can install a relay if you like.

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installing the intercooler

For the standard body MG Midget and Austin Healey Sprite, we recommend using the OEM intercooler from a turbocharged, 1.8 liter 1999 Audi A4 below:


Installation requires removing the grille (a handful of bolts above, two Phillips screws below), and relocating the horns as described in the preceding section if they are mounted down in the well in front of the radiator.

Below is a mock-up showing how the intercooler and tight-radius U-shaped snorkels are assembled, absent the sheet metal for clarity. The snorkel on the left-hand side must be trimmed to fit under the bonnet’s latch support sheet metal, due to the intercooler’s asymmetrical barbs:

intercooler w_hoses.jpg

Once you have removed the grille and horns, you must cut two holes in the front sheet metal to allow the snorkels to pass through, as shown in the following photos. You will use an arbored, bimetal hole saw available at any home improvement store as shown in this slideshow:

Each hole will be cut from below, but these pictures show the pilot hole being drilled from above. If you cannot find or borrow a right-angle drilling attachment for the pilot hole, simply dimple the metal with a punch from above, then drill the pilot hole from below, too.

*TIP: a seam intrudes into the circumference of the large hole you will cut. Use gentle pressure and hold the drill tightly while pressing upward, or the hole saw may grab the seam and cause the drill's handle to bust you in the chops.

You may wish to repaint the exposed metal edges to retard rust (not a concern in northern-central New Mexico). Then, slice some fuel/vapor hose and slip it onto the vertical support to protect the intercooler.

Finally, slip the joiners into the holes. These photos show imported, lathe-turned aluminum pieces, but extruded aluminum nipples are much less expensive and work just as well:

Once the holes are cut, installing the intercooler is straightforward, as seen in this photo series. You may choose to place a piece of thin rubber beneath the intercooler, but the core is tough and will not be subject to much vibration:

The intercooler will not foul the grille, though you may have to modify or forgo using the lower two mounting tabs, as the holes for their mounting screws were eliminated by the snorkel holes. If you look carefully in the right-hand lower corner of the second photo, you will see the lower mounting tabs have been bent upward:

If you will be driving your car before completing the project, you may wish to cap off the intercooler. While tape will work perfectly well, these flexible PVC caps are also available:

cap for intercooler joiner.jpg

Finally, to maximize intercooler efficiency and increase power, you may wish to cut a slot in the lower valance beneath the grille.

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reroute the heater fresh air intake

Retaining the heater and fresh air ventilation system is not difficult with this conversion. However, on early cars like the 1967 MG Midget in these photos, the OEM fresh air intake for the heater/ventilation system must be rerouted, as the supercharger mounts directly in its path.

The method is straightforward: simply remove the OEM duct, air control vane, and snail-shaped fan, and either relocate or replace each component.

These instructions utilize large diameter silicone hose and metal connectors, but the cost can be reduced (together with the weight) by using aluminized HVAC duct between the fan and air control vane. That said, one upside to the more expensive silicone hose is that it will not conduct heat if you wish to use the system for fresh air ventilation instead of merely heating the cockpit.

The inline fan shown here, an aluminum racing model from Revotec (available in either 3” or 4”), can also be replaced with an inexpensive plastic marine bilge fan to save still more money. The 4” Revotec auto racing fan shown here moves a lot of air, looks nice, and is very light but is also expensive (though often cheaper if purchased directly from the UK):


After removing the OEM duct, air control vane, and fan, assemble the fan and silicone reducing connectors and elbows and metal nipples (these are steel, though aluminum would be much lighter) to produce the following configuration:


The above shows a 4” fan and duct toward the heater box with a 4” to 3” inline reducer aiming forward toward the front sheet metal, because that is the largest duct that will fit between the inner and outer fenders:

*NOTE: One or both of the 45-degree 4” silicone elbow’s legs, and the forward-facing leg of the 90-degree 4” silicone elbow may have to trimmed slightly to position the tract inline with the heater box’s inlet..

Affix the fan’s legs to the wheelhouse sheet metal, keeping in mind that nuts and lock washers should go inside the engine compartment to avoid frozen fasteners down the road.

Next, the fan assembly is connected to the OEM air control vane—moved by the shortened and dramatically smoother-operating OEM cable (we positioned the lever atop the affair as shown below). The air control vane is connected to the heater box by a 4” to 3-1/4” inline silicone reducer:


The insect mesh that originally mounted directly to the OEM fan on the 1967 MG Midget shown here is moved up to the front sheet metal, to allow room for the intercooler snorkel. An aluminum or steel trim ring would make for a less spartan installation than shown here, which simply uses washers to grip the mesh:


Now, trim at least 1” or more from the larger leg of a 4” to 3” silicone reducing elbow—and be extra careful when using a razor knife:


This will allow the elbow to clear the right-hand tank of the dual-pass 1979 VW Rabbit radiator shown here, or the angled sheet metal piece that connects the radiator supports to the right-hand inner fender (or wheelhouse) if you are running an OEM-sized radiator:

*TIP: Work the trimmed end of the elbow over the sheet metal flange in the bodywork by using your fingers inside the elbow. You may have to insert your hand first, like a glove, work the elbow onto the flange, then hold it in place with your left hand while extracting the right.

Next, trim the flared portion off one end of a connector nipple, as the aluminized HVAC duct is too inflexible to attach otherwise. We used a Dremel with a cutter wheel and held the nipple by clamping it into the silicone elbow, then securing the hose clamp in a bench vise (just loosen the clamp to rotate the nipple as you go):


Be sure to protect your eyes and lungs when performing this operation. Ideally, a full face shield is better than just goggles when using cutoff wheels of any kind. Don’t forget to break the edges on the nipple after using the cutoff wheel.

Here is the affair attached to the OEM sheet metal flange. The 3” aluminized duct hanging down at the right-hand edge of the photo is threaded up inside the fender and attached to the Revotec’s inlet:


Loosen the hose clamp on the larger leg slightly and rotate the elbow upward toward the natural path of the aluminized duct, then work the duct onto the nipple and tighten all of the clamps.

You can see in the next image that we found a standard clamp best for the narrow flange pressed into the OEM sheet metal, whereas T-bolt style clamps worked better for both ends of the modified nipple:


installing a high-pressure fuel delivery system

A blow-through system running under boost may be operated by merely capping the SU HIF44’s fuel float chamber vent and engine breather pipes, and adding a high-pressure pump capable of at least 20 psi. paired with a rising-rate fuel pressure regulator, as discussed here.

However, another method is to add a new, higher-volume feed line, utilize the OEM delivery line as a return line to a swirl tank in the boot, which feeds a true, EFI-spec high-pressure fuel pump such as the Walbro universal 50 psi. model shown in the following photos (though non-immersed, inline pumps from a 2009 Volvo or ‘80s GM vehicle work well, too). Overflow from the swirl tank goes back into the OEM Spridget tank via the filler neck. This combination provides ample fuel delivery regardless of cornering, acceleration, or braking force, angle of climb or descent.

Another benefit of this setup is that the fuel stays relatively cool, since it is constantly circulating through several feet of steel and rubber line.  Boiling fuel under the bonnet is effectively eliminated.

Regardless of which option you choose, as with the other stages, you can install components in whatever order you want--so long as you save converting the OEM 1/4" delivery line into a return line for last, as you'll need it to feed your the SU carb (or carbs) in the interim.  Accordingly, the following steps are in no particular order.

STEP 1 - Install the fuel filter

Although you may use whatever filter you like (many people like the ‘80s GM EFI-spec inline filters), the ‘67 Midget test mule uses a '90s Subaru canister-style, EFI-spec fuel filter and quick-release bracket ($5.00 at a salvage yard). This setup mounts under the bonnet for easy access, between the carburetor or throttle body (shown in these pictures) and pressure regulator. Here are the filter components and mounting location.  The high pressure feed line from the boot is not yet attached in the last photo; the filter’s inlet nipple still has its plastic dust cap installed:

Really, the only trick is determining the ideal location by positioning the assembled filter and bracket, then removing the left-hand front wheel and working from the wheel well to mark the bolt hole locations with a punch and drilling the holes. Here is their approximate location (the ‘67 Midget test mule has a tubular front shock conversion):

STEP 2 - Install hi-pressure pump and swirl tank

The swirl tank bolts to the right-hand frame member in the boot, and to a simple dog-leg bracket made from aluminum stock available at your local home improvement store:

The high-pressure pump mounts adjacent to the swirl tank, as close as possible to the right-hand frame member to afford room for the spare tire, while allowing for the 3/8" feed line to curve without kinking:

STEP 3 - Drill holes, install grommets

You must drill three holes: one for the new 5/16" stainless steel delivery line that heads forward to the engine bay, and two more just to the right of the stiffening rib for the short pieces harvested from the coil to connect the swirl tank to the OEM (or aftermarket) low-pressure fuel pump, and to connect to the OEM 1/4" line that will now serve as a return line.

These pictures show their approximate location relative to the pump, and again from under the car, relative to the pump mounting bolts.  The smaller grommet is evidently for wiring and was on the car when purchased (unused):

STEP 4 - Form new 5/16" stainless steel delivery line

Forming and installing this line is perhaps the most physically demanding task in this conversion.  The car must be on a hoist (if you are lucky enough to have access to one), or raised high enough to allow for a creeper and elbow room to bend stainless steel line--which is much stiffer than regular mild steel line.

The picture to the right below is NOT the way to go; our method is at left--jack stands up front, rear suspension on ramps at rear (on wooden boosters for more clearance). In the alternative, you could put jack stands under the axle tube, but that leaves less room for scooting around on a creeper:

*NOTE: the car's weight must be on the springs, as you want to have some sense of how much clearance to leave for suspension travel above the axle when forming the lines.

Now comes the tricky part, as shown in the following slide show.  

Note that putting a pressure union in the center of the new delivery line, so that it is comprised of two sections, will vastly simplify bending up the line.

Working from beneath the boot, uncoil the stainless line slightly at one end by hand, so that you have perhaps 10~12" with a little larger radius than the natural coil. Then, relax the coil on the last round and work the spiral around the axle over the differential, so that you can insert the end through the grommet into the boot:

Next, bend the line away from the pump's outlet roughly 90 degrees or slightly less:

Now, back under the car and pull the bend you've made a hair through the grommet. Then, relax the coil a bit and push it up into the axle arch, and when reasonably close, zip tie it to the axle or exhaust pipe to hold the bend roughly parallel to the arch:

Now, back to the boot, where you will mark the line in two ways: first, to show where it comes through the grommet; second, with a longitudinal line showing what will become the inside of the next curve:

Next, pull the line back into the boot a little ways to make room for the tool, and using the index mark you've made, bend toward the pump.  Then push the line back through to bring the run close to the pump outlet to judge the angle.  The line must have a straight section just the right length to insert into the pump fitting and slip the female AN flange freely onto the line:

It is now time to mark and cut the line.  When marking, make sure you know how far the line inserts into the pump outlet fitting (not much).  A mini-tubing cutter helps here:

Finally, attach the fittings to the line and tighten everything down.  It is much easier to do this if you unbolt the pump first, then reattach the clamps.  However, do not forget that you must have a straight section for the AN sleeve and brass bead to slip on freely:

It's now time to go back under the car and finish forming the curve to match the axle arch, and then make the bend to send the line forward as shown in this series.  You will notice that in the fifth picture, the unused portion of the coil has been cut off.  

That is because you will only need another 18" at most from the front left corner of the opening by the front of the oil pan to end up just forward of the motor mount near the OEM line:


STEP 5 - Attach AN fittings and connect filter & pressure regulator

This step is simple, though turning wrenches on the fittings is much easier with the radiator removed.  Note that the 5/16" fuel hose must be EFI spec.

The 5/16” delivery hose connects to the filter’s inlet pipe (marked on the body of the Subaru filter), and thence to the carburetor or throttle body. The 1/4” return hose (which can be regular fuel/vent hose, not EFI-spec hose) runs from the pressure regulator’s outlet (bottom nipple) to the OEM delivery line—which will become the return line.

STEP 6 - Form swirl tank lines and attach hoses

Cut two pieces from the leftover coil about 18" long, leaving the natural curve intact, then bend as shown in this series and insert through grommets and connect to swirl tank:

All that remains when you are ready to operate under boost is to disconnect the stock low-pressure pump from the 1/4" OEM feed line headed forward to the engine bay and connect it to the lower steel line pictured above, and which feeds into the swirl tank. Then, connect the 1/4" OEM feed line (now the return line) to the upper of the swirl tank lines.

Here is a detail showing how the overflow line is routed to an NPT nipple screwed into a hole drilled in the OEM fill pipe:

*WARNING: you must NOT attempt to drill the hole in the steel filler pipe while it is still attached to the gas tank, as the drill bit may create a spark and ignite the fuel vapor.

The fuel system is now complete.

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replace the supercharger pulley

The M45 mounted to the Mercedes donor car is fitted with a pulley made of pressed steel welded to a hub, and must be removed and replaced with the Smoothflow M45 Modular Pulley System.

At least one Spridget owner has successfully used a standard two-jaw pulley remover, as shown here:


If you attempt this method, you will need quite long wrenches to achieve sufficient torque.

In any case, be sure to apply liberal amount of penetrating oil to the shaft and let it soak in before beginning removal.

Another method is using a dedicated supercharger pulley removal tool. Various companies offer these pullers. We prefer the Metco Motorsports puller pictured below, and offer to rent the tool to purchasers of our M45 supercharger manifold for the cost of shipping both ways (with a refundable damage deposit), by clicking here.

*WARNING: when using this tool, you MUST keep the horseshoe-shaped steel plate positioned so that the long bolts remain aligned with AND parallel to the larger, central bolt throughout the process. Otherwise, the thin metal of the pulley’s bell will distort and greatly complicate removal.

It is also advisable to use a high-quality lubricant on the threads and tip where the tool contacts the supercharger shaft (we include a quantity of ARP fastener lubricant with the puller rental) to maximize the tool’s efficiency and reduce wear.

Use a 1/2” drive breaker bar and socket on the central bolt, and a piece of heavy pipe or other suitable cheater bar to hold the tool’s handle.

*NOTE: it is advisable to replace the snout oil seal before installing the new pulley hub.

To install the Smoothflow M45 Modular Pulley System’s steel hub, you may use the special tool or—preferable in our opinion—an arbor press. The only benefit of using the special tool is that you can complete the procedure without opening the supercharger’s gear case. (This is not a concern, as it is straightforward to reseal it with Permatex anaerobic sealant. Rotor timing is not involved in opening the gear case.)

The following photo series shows how to use a standard arbor press to install the pulley. Be sure to lubricate the shaft before pressing on the hub.

The third image emphasizes that you must fully support the supercharger’s aluminum snout assembly during the pressing operation to avoid shattering it.

Be sure to press the hub just until it is flush with the end of the shaft. As shown above, the longer end of the hub goes forward, or away from the supercharger snout.

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prepare the carburetor

The SU HIF44 is easily converted to run under boost as described here. In addition, however, some Spridgets will encounter slight contact between the sloping bonnet and the dashpot cap on top of the vacuum chamber.

There are two solutions.

First is to replace the standard cap and damper rod with a low profile version, SU part no. CUD2901 (around $20), shown side by side with a standard cap and rod for comparison:


This should create sufficient clearance for the bonnet to close.

Note that the metal clip on the damper rod has no function in this application and should be removed. We grabbed it in a vise and cut it with a dremel, then twisted a flat blade screwdriver to break the clip off without damaging the rod:


Using the shorter cap will limit upward travel of the vacuum piston and needle by approximately 1mm, thus limiting wide open throttle (WOT) operation, although the carburetor’s function is not otherwise affected. The piston is easily shortened to eliminate this effect, and we will add a step to this section showing how to perform this modification soon.

The second solution is to use the Maniflow L159 manifold, which positions the carburetor lower and is available direct from the UK for $198 plus shipping, or $261 to $330 domestically (IM48-20, C-AHT669, or C-AHT770M).

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establish the correct mixture

It is important to arrive at the correct mixture for your elevation, though a supercharged engine is arguably less sensitive to moderate elevation changes than its naturally aspirated counterpart.

The Moss Motors M45-based system uses the same SU HIF44 carburetor as the present DIY conversion. Moss recommends a BCA needle, 4.5oz spring, and Marvel Mystery brand air tool oil in the damper. However, Moss also claims to install a modified jet in the HIF44 it provides. At any rate, the distinction between the Moss draw-through and this blow-through setup should make little difference in mixture.

We are about to begin Beta testing of needle/spring/oil combinations, as well as the necessity of the jets Moss installs in the underside of the vacuum piston (they may not be necessary in a ‘blow-through’ setup, where pressure builds upstream of the piston).

We will provide Air/Fuel Ratio (AFR) and other data gathered from the 1967 1275cc test mule in the table below. “TBD” in the below table indicates the data in that column is “to be determined.”

We ask that you email us your data so that we can assist A-series owners determine a good starting point for their own projects. You can remain anonymous (i.e., ‘Test Vehicle # __’) or include your name, and you can add videos of track time, etc., or a slide show on the ‘customer cars’ page linked here.

*To calculate your supercharger drive ratio, divide the radius or diameter of the supercharger pulley by the radius or diameter of the crankshaft pulley.

test vehicle # 1 - 1967 MG Midget

Elevation/Air Temp Engine & mods Blower Carburetor, needle, spring Ignition Pulley Ratio
6,000'/TBD 1275, 1.5:1 rockers Eaton M45 SU HIF44, TBD, 4.5 oz. Distributor, Pertronix .84:1
Intercooler Bypass valve Boost @ 5,000 rpm Boost @ 6,000 rpm AFR @ idle AFR @ WOT
'99 Audi A4 1.8L Turbo '02 Mini Cooper S TBD TBD TBD TBD
Diff/top gear 0-60 MPH 60-100 MPH MPG Avg MPG Highway
3.9:1/.846:1 TBD TBD TBD TBD

The standing start 0-60 mph test should be conducted without dumping the clutch to avoid any significant traction variable.

The 60-100 rolling acceleration test should be conducted in your car’s top gear and on level ground.

The average fuel economy test should ideally include a mix of city and highway driving, with the overall distance logged provided.

For example, Test Veh. # 1 will typically traverse 1 mile at 10~15 mph (unpaved road), 8 miles at 50~60 mph (two-lane blacktop, descending 1,000 ft.), and 18 miles at 75-80 mph (interstate) to reach the city, followed by various distances of stop-and-go driving at 35~50 mph, before reversing course.

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Select and harvest a used supercharger

The M45 (45 C.I.D) Eaton roots-style blower, model no. 207018, sourced from either a 1999 to 2003 Mercedes Benz SLK230 Kompressor coupe or from certain 1999-2000 Mercedes Benz C230 models—but in either case, without an electronic clutch.

This supercharger can be had from self-service chains such as U-Pull-&-Pay and similar business for as little as $55.54 (including core and environmental charges). However, the cast manifold and plastic air box to which it is attached are typically extra, and extracting the supercharger from the bowels of the Mercedes donor car may take over two hours.

Click here for an article on how to remove a similar Mercedes supercharger.

Another money-saving alternative is to buy a supercharger from a recycling yard that has already removed it from the vehicle. Prices vary, but expect to pay five to eight times as much—and often you will get the manifold and air box, with mass air sensor attached, as depicted here (this one was priced at $450):


Either way, try to confirm the mileage of the vehicle. The blower pictured above was the ideal situation: a 2000 Mercedes SLK 230 that had been totaled with just 10,000 miles on the clock.

If you are fortunate enough to find a low-mileage blower, all you need do is unbolt it from the manifold and air box (which can be sold to recover some of your investment), remove the OEM steel pulley, install the SmoothFlow modular pulley system linked from our 5-rib crankshaft pulley’s purchase page, and change the synthetic oil in the gear box.

In any case, be sure to cut or remove the drive belt on the donor vehicle and rotate the pulley, listening carefully for grinding or metal-to-metal contact. This is not necessarily a deal-breaker, as the snout’s bearings and seals are easily replaced as discussed elsewhere in these instructions.

If you are unsure whether the rotor chamber has been gouged due to extended service of the bearings or abuse by the donor car’s owner, and you are looking at a potentially viable $56 blower, it may be worth your while to remove the unit to peer into the rotor chamber and inspect for damage.

Light scuffing on the tips of the rotor lobes and around the circumference of the chamber are not a concern, as the boost level needed to multiply the performance of the A-series engine is very modest; deep scratches into the casting and on the tips of the lobe disqualify the unit.

If the wear or scuffing is light, the unit is viable—but if the ‘pocket’-style needle bearings pressed into the rear of the rotor housing are going (likely if the car has over 100,000 miles), they can be difficult to extract. We are developing a special tool to extract the bearings for a nominal charge, but until that service is available, try to locate a blower that, if anything, just needs a snout rebuild—which is well within the skills of the typical ‘shade tree’ MG mechanic.

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