100 hp per litre out of ANY Alfa Romeo engine is definitely possible if you want to. Why we talk about the Alfa V6 as though it is a piece of sh!t and cannot make horsepower I do not understand
My Sud engine produced 125hp per litre. If I could not make a reliable 300 hp out of a 2 valve 3 litre v6 Alfa engine I would be embarrassed. Surely a serious compression ratio, porting + fueling, cams (thus larger valves and new springs), tuned extractors and balancing is all that is required. To make it reliable, lighter and stronger pistons and rods, etc. might be required but we ran standard Sud rods (after shot peening and balancing them) up to 9000 rpm with no problems.
An engine is an engine (and we do not have the long stroke and old fashioned combustion chamber issue like the Nord engine here, but even Nord engines can make over 100 hp per litre). There is a guy on the FerrariLife site who apparently made a Harley Davidson engine produce 100 hp per litre and they are complete rubbish.
In fact with a 12:1 compression ratio proper cams I would have thought above 350hp was possible.
Would a turbo be cheaper ... don't know?. Motorracing is all about how much money you can spend. If I was you I would buy a spare engine (this is a must and I will always have one if I ever race again) and I would slowly build that up as a race engine. At least as this is a spare you can keep on racing
, and once it is the actual race engine if it fails you have old reliable to fall back on.
Remember though the more racy the engine is the more you will look at your gearbox wishing it had decent ratios ... so think carefully when you design the engine.