DIY Supercharger Conversion with Intercooler for 948, 1098 and 1275 A-series MG Midget, Austin Healey Sprite, and Morris Minor - Overview
The venerable A-Series engine is the beating heart of some of Britain's most beloved automobiles, but achieving a meaningful increase in horsepower means dismantling the engine and spending serious money. Also, modifying an A-Series engine to achieve 90 to 100 h.p. impacts its street manners and fuel economy.
A viable alternative is forced induction, via a turbocharger or supercharger.
supercharging
According to Eaton’s website, “[u]nlike turbocharging, Eaton superchargers provide instant throttle response and torque across the engine's entire powerband.” Also, superchargers do not require custom exhaust work or a dedicated oil feed line, and they do not raise temperatures under the bonnet like a turbocharger.
But until now, only three supercharger systems fit under the Spridget's sloping bonnet: the inefficient and increasingly rare Judson setup; the Moss Motors Eaton-based kit, which costs $4,600, and the High Performance Products twin-screw offering, which begins at $6,000. Mama mia!
Importantly, none of the above superchargers can be fitted with an intercooler. As a result, the amount of boost these ‘draw-through’ systems can handle without harmful detonation and back-firing is quite limited.
But now, there's another option: the Creative Spridgets DIY intercooled and dynamically bypassed supercharger conversion for the 948, 1098 and 1275 A-series MG Midget, Austin Healey Sprite, and Morris Minor.
Here is a video clip of the engine bay at idle:
Here is a video showing acceleration from 0-75 mph up a gentle grade with a 2.9” supercharger pulley fitted to an internally stock 1275 A-Series engine, which generated 5.0 psi boost at 6,000 ft. / 1,829 m. elevation. The temperature was 85 F. / 29.4 C:
Compare the above to this 0-96 mph ride-along in Doug McConnell’s car, running a supercharger-specific cam, 3.1” blower pulley and 3.55 differential, which developed 3 psi of boost at 89’ above sea level…
Here the car easily paces traffic and overtakes slower vehicles on the freeway, effortlessly surging from 72 mph to 80 mph (Datsun 5-speed transmission with 3.9:1 differential). You can also hear the vacuum-actuated bypass valve operating each time the driver backs off the throttle:
Best of all, adding a supercharger and intercooler to your A-Series powered vehicle can be done for well under $3,000—a savings of $1,600 to $3,000! If you already run a single SU HIF44 carburetor and find a serviceable used supercharger at a ‘you pull it’ auto recycler, your cost may be closer to $2,500.
But that’s for an intercooled system featuring a vacuum-modulated bypass valve from the 2002 Mini Cooper S for improved economy, reduced wear on the supercharger and serpentine belt, and smooth throttle transitions.
Click here to review the installation instructions, click here for FAQs, or continue reading for benefits and an overview of the project.
benefits and OVERVIEW
Installing a supercharger and intercooler on a stock A-Series engine can increase horsepower 40% to 50%, without detonation or impacting your Spridget’s smooth idle and street manners.
Standing-start acceleration is dramatically improved, as are passing and hill-climbing in top gear.
Increased torque means you shift gears less often and can push a taller differential ratio. Commute on the freeway in your Spridget or Moggy!
Our intercooled supercharger conversion is compatible with a single carburetor or electronic fuel injection.**
Bolt-on modifications like a long-center-branch header, free-flowing 1.75" exhaust system, up to seven smaller ‘high boost’ supercharger pulleys, and high-lift rocker arms will further enhance your performance.
Although you do not need any internal modifications to supercharge your A-Series engine (the red ‘67 MG Midget shown above and in the installation instructions has an internally stock engine) you may consult an experienced engine-builder about a supercharger-specific camshaft and larger valves for neck-snapping acceleration.
**Note: we designed a bespoke GM temperature sensor mount to enable EFI for 1275cc A-Series engines, or to run an electric temperature gauge, pictured below and available by clicking here.
However, the sensor mount raises the thermostat housing .40”, creating clearance necessary to supercharge 1275-powered cars with early, down-flow radiators or those fitted with a VW Rabbit dual-pass aluminum crossflow radiator, as described here. The 948 and 1098 engines do not need this adapter, and it will not fit their cylinder heads.
*Note: Moss’s alternator conversion kit is $270. If you run a generator and wish to upgrade to an alternator for our conversion, you will save yet another $200. See the serpentine belt conversion page for details.
But there are still more performance and cost advantages to this conversion!
Moss offers a $190 ‘high boost’ pulley kit, consisting of one smaller pulley, an adjuster bracket, and hardware. With our DIY conversion, you may order three progressively smaller, ‘high boost’ pulleys from Smoothflow Pulleys, a second idler and two bushings in our online parts list, and a fastener and washers from your hardware store, for the same price as the Moss Motors high boost kit:
the supercharger
Our intercooled supercharger conversion utilizes Eaton's M45 (45 C.I.D.) blower as configured for at least two Mercedes Benz cars, which a European colleague has informed us may bear different designations. The only apparent difference is that the drain plug for the gear box may be closer to the inlet/outlet flange—meaningless for this conversion.
Look for model no. 207018 or no. 206127, which is OEM equipment on the 1999 to 2003 Mercedes Benz SLK230 Kompressor coupe and some 1999-2000 Mercedes Benz C230 models—but in any case without an electronic clutch. It is at least possible that certain 1998 and 2004 models may feature this blower as well.
If you locate other models fitted with this blower, please let us know so that we may update this page.
This supercharger has the same, 45 cubic inch displacement as the unit fitted to the 2002 Mini Cooper S. However, it has a simpler design, is more compact at 11" long, 6.5" wide and 5" high, and weighs only 16 lbs. (7.26 kg). Here are some photos comparing the above-listed designations:
Eaton Model No. 207018 (above)
Eaton Model No. 206127 (above)
This supercharger features inlet and outlet ports on the same side of the housing. This configuration allowed us to engineer a one-piece inlet/outlet manifold that uses the stock Mercedes gasket:
A single inlet/outlet manifold not only reduces production cost, a savings we pass on to you; it also enables a supercharger conversion that fits neatly beneath the Spridget's sloping bonnet, with the intercooler hidden behind the grille:
Here is a diagram showing the flow of air through the system and identifying related components under the bonnet:
This compact supercharger also affords ready access to ignition components and maintenance items. You can remove the distributor without disconnecting the high tension wires from the cap, replace the spark plugs with a standard ratchet and spark plug socket, access the oil dipstick and valve train as usual, and remove the serpentine belt in seconds to service the water pump or alternator:
While fitting an intercooler is optional, doing so provides a cooler, denser mixture and eliminates the need to remove the engine to install low-compression pistons or retard the ignition timing.
Also eliminated are detonation from a heated intake charge—common where the supercharger bolts to the head via a shallow plenum manifold—and back-firing from gasoline condensing in the blower’s rotor chamber when the engine is cold.
As discussed here, this ‘stealth’ Audi A4 intercooler hidden behind the grille reduced intake charge temperature by as much as 127.8 F. The horns, and the heater control vane (in 1967 Spridgets only), are relocated as described here:
The system is designed to generate 5.0 psi. (.345 Bars) of boost at sea level on a stock 1275cc A-Series engine, running a 3.1" supercharger pulley—what we call the mild-mannered ‘Stage I’ setup. We needed a 2.9” pulley to generate 5.0 psi at 6,000 ft. / 1,829 meters, with an ambient temperature of 94 F. / 34.44 C.
We use the modular supercharger pulley kit offered by Smoothflow Pulleys pictured below, which offers pulleys in 11 different sizes to fit their hub—three larger and seven smaller than 3.1”—so you can reduce or increase the amount of boost and power from mild to wild to suit your needs:
The Smoothflow modular pulley kit also positions the pulley properly in a fore-aft plane, per the free downloadable mount diagrams.
The same pulley ratio may produce different boost at different elevations, and of course internal engine modifications and the condition of your engine will yield varying results. Please use our contact form and we will post the figures you achieve in a table available in the online instructions webpage.
To get started, review the FAQs and step-by-step online instructions. Then visit our purchasing page to download free plans to fabricate the supercharger mounts, and to order the core components in the parts list, including the 5-rib crankshaft and water pump pulleys required for the serpentine belt conversion.
You can then run your car naturally aspirated while completing the balance of the conversion in stages to suit your budget and time constraints.
turbocharging
Turbochargers have been fitted to many powerplants including the A-Series engine, but no kits, fabrication templates, or instructions are available for the A-Series. Also, turbochargers require custom exhaust work, a dedicated oil feed line, and exhibit a lag in generating boost.
However, if you prefer a turbo, you no longer need to find a costly MG Metro intake manifold and clumsy cast snorkel adapter, which requires hacking a hole in your Spridget's bonnet for clearance.
Creative Spridgets now offers a ‘tunable’ CNC-machined ram pipe & snorkel or performance air cleaner adapter for use when pressurizing the SU HIF44 carburetor on any vehicle, available here and pictured below. the same ram pipe & adapter will also work on draw-through forced-induction applications and on naturally aspirated cars, due to its patented ‘convertible’ design.
*Note: the last two images are of a Mazda RX7, demonstrating the broad array of potential applications of these adapters to enable your projects.
The ram pipe is optimized to accelerate the intake charge and it comes with three interchangeable restrictor rings to achieve the correct pressure differential between the Venturi (jet) and the float chamber as discussed by David Vizzard in “Tuning the A-Series Engine” at p. 346.
The SU HIF44 mounts directly to a Minispares C-AHT770 cast aluminum manifold shown above, which costs just $67 including shipping from the UK.
You can convert a standard HIF44 for pressurized operation as described here.
You can fit an Audi A4 intercooler behind the grille in the nose of your Spridget for denser mixture and more power as described here.