Towards the end of 2019 I bought my current daily driver - a 1983 Mk1 Austin Metro 998 HLE. The car was in great condition with 31000 miles on the clock. Not perfect, but in a good, standard, well looked after condition.
After giving it a thorough inspection, a full service and making sure safety critical items like the brakes were 100% I drove it to the French Alps for a snowboarding holiday in early 2020.
The little Metro proved to be a smooth, reliable ride, returning close to 50 mpg over the near 2000 mile journey without a single problem. My only issue was that the 46bhp 998cc engine felt a little underpowered, especially on the long uphill motorway sections.
Later in the year I set out to increase my power with a supercharger. Those who know the Podifold name know this isn’t my first time dabbling with superchargers. But that’s a different story…
I set out to create a suck through supercharger kit that overcome many of the flaws I perceived it the commonly available kits.
The starting point was an Eaton M24 supercharger as fitted to the VW TSI twincharged engine. Because these units are normally operated using a magnetic clutch that disengages at around 3500rpm second hand units are generally in excellent condition having never been worked too hard. Other advantages are: they have the perfect output volume for small bore engines; they are readily available; they are cheap to buy and they have a relatively small physical size.
Once I’d decided on the supercharger I was going to use I started designing the system. The first thing I needed was a belt pulley to attach to the crank and drive the supercharger. The Eaton M24 had a 53mm diameter pulley and is internally geared at 1.93:1 revolutions. Using a boost pressure calculator I’d written ages ago I determined that I’d need a 54mm crank pulley to produce around 7psi whilst remaining in the unit’s thermal efficiency range.
I drew up and had machined from stainless steel a pulley that would be doweled and fitted in place to a standard C-AEG454 crank pulley.
Next thing to address was the position of the s/c. Other systems line up the supercharger and crank pulley with no regard to where this puts the outlet of the s/c relative to the inlet ports. I’ve always thought this was a terrible compromise and knew I wanted the outlet to sit directly between the inlet ports so that the inlet tracts could be equal length and the fueling could be more balanced.
To achieve this I designed a spacer, CNC’d from aluminium, which would move the complete nose of the s/c outwards the required amount to allow the pulleys to be perfectly aligned. This of course meant that I also had to have a new, longer shaft machined for the supercharger nose to re-align all the internal parts.
With that all done and the position of the supercharger locked in I turned my attention to the outlet manifold that connected the super charger to the cylinder head. What I wanted was a manifold that flowed well whilst maintaining a high velocity for the fuel/air mixture by carefully matching the diameter of the inlet runners to the inlet port size of the CAM4810 cylinder head it would be bolted to which happened to be 29.5mm. After much trial and error using 3D printed mock ups I settled on a design that would be CNC’d from aluminium in two halves and welded together.
The final piece to design was an inlet manifold to connect a HIF38 carb to the inlet of the supercharger. Considerations here were to keep the carb as close to the supercharger as possible whilst still allowing clearance for the throttle cable, abutment plate and bonnet closing as well as keeping the flow of air as smooth as possible.
With all the parts designed and manufactured I had some work to do in the engine bay before I could fit everything. The brake and clutch master cylinders were rotated 180 degrees and all the pipework remade and re-routed to provide clearance. Following that I had to remake the lower coolant hose to make room for the s/c belt as well.
With an ABY carb needle fitted and a temporary decompression plate to lower the CR from 10.3:1 down to 8.0:1 I took it to a local rolling road where it made an astonishing 70.5bhp!
That’s a huge 50% increase in power on an otherwise completely standard 38 year old engine with no modifications other than the supercharger and a Lumenition electronic ignition kit. Remember, that’s the factory camshaft, completely unworked CAM4810 head, cast iron exhaust manifold, pea-shooter exhaust and the standard mechanical fuel pump!
What really pleases me though is how it performs. There’s no loss of drivability, it starts instantly (hot and cold), the power comes in smooth with no hesitation, tickover’s still set at around 800rpm and in normal driving you barely know it’s there. Put your foot down though and you hear that distinctive supercharger whine!
