by mart924 » Fri Dec 10, 2010 9:53 pm
HI there,
Dont think I did the post you are talking about, but here goes! The head on the 924 is extremely restrictive and that is where the problem lies. The turbo head has a combustion chamber whereas the normally aspirated head doesnt. Both pistons have combustion chambers, the turbo head has the combustion chamber to lower the compression ratio. The theory goes that you can use Golf pistons and rods which utilise a flat (ish) topped piston with the turbo head. There is nothing to stop you using carburettors, or for example a megasquirt with this - but the head is still the weak point, especially the hump in the intake port where the water jacket runs.
Achillies heel two is the valve seats, which are shrunk into the head. Larger ones would be required for bigger valves to be fitted. The valves aren't bad but are very thick and carry lots of weight as well as having a thick stem. Added to this, the valve guides are of good material, but impinge on the inlet port. New bullet nose guides would probably provide a useful increase in flow. If you add a decent bottom end, with everything lightened, and dry sump it, you would end up with a half decent 924 engine - but - again - in comparison to other 2 litre engines, you may struggle for outright horsepower.
Be interested to see how you get on
Martin