In answer to the original question the VAG 2L TFSI with the ko4 turbo has the most tuning potential on a stock turbo and engine. Up from 240-260bhp depending what car it is in to 380bhp with a few bolt on mods. I can't think of another engine that can get 50% power increase with a stock engine and turbo. With uprated rods and a bigger turbo they are starting to hit 500bhp in track cars.
I'll agree with you that keeping the turbo stock aswell there is very little out there to compete, but there is certainly one other engine i can think of that has extremely good tuning potential. The old Saab B2x2/B2x4 series engines were so over engineered that they can be tuned to over 500hp on standard internals. This kind of tuning only requires suitable bolt ons. With the standard turbo you can take it from 225hp to about 310hp, which is the absolute limit for the TD04 15T, with a hybrid 19T compressor wheel this is around 360hp. Best i've seen so far is a guy who has fitted a holset VGT to a B204. (2.0) The turbo is the same sort of size as a GT35 but with the spool up capabilities of a T28. The guy has built it in a 9000 for the time being as a test bed, apparently he is planning on putting something like it into his reliant kitten.
Another one put into an E30
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They are most commonly put into vauxhall's as the bellhousing bolts match, making it easier to fit. The best bit about these engines is that the engine management system is home tunable. You can even live map it on the dyno yourself, have live data via car pc and perform data logging of all input parameters.
Only problem is the cars aren't what could ever be termed hot hatch as they were never small, and don't handle well enough. The modern tat that you get out of them nowadays is crap, you need forged internals to reliably get past 300hp on the B2x5 series. Don't know much about the ecotec series (which is what the 4 pot VXR's use aswell as the 93 aero).
My own car should be remapped again soon up from 185hp to around 270hp with only a bigger downpipe. (which cost the grand total of £35 for a SS decat) I'll be at the expensive stage after this, I'll need bigger injectors, a better clutch and a bigger turbo. (already got the turbo, which i'm keeping till the original T25 surrenders as its already pushing 1.4 bar)
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