I'm in the middle of a forged 2.0 build (over on Projects as 'Budget DIY Rebuild'). But at the moment it's only going to be running a TD05-16G and supporting mods for 350bhp max.

I'm thinking about what heads to use at the moment and least have narrowed it down to wanting v3/4 heads to suit the age of the car, manifold etc. I've got a pair of good WRX4 heads there but the plan I'm toying with is to fit an STi3/4 complete valvetrain in there so converting them to STi3/4 heads.

I'm wondering, in the interests of keeping the budget down, whether I will see that much benefit from the STi head conversion over the standard WRX, which already has overbucket shims anyway. My lift measurements of these WRX cams vs. some STi3/4 that I measured up before show that the WRX run at 8.38 inlet/8.44 exhaust and the STi 8.49 inlet/8.63 exhaust, no idea of differences in duration though. Of course the STi springs would allow higher rpm and the exhaust valves are sodium filled.

So the question is, for 350bhp on a forged 2.0, are STi3/4 heads gonna make that much difference over WRX3/4? At what sort of tune would the STi heads start to make themselves useful? I've heard they rev to 8000-8250, but are STi heads making more power over 7000 rpm or is it just extra headroom before you hit the limiter? All advice appreciated.

 

my standard v3 heads are running out of puff at low to mid 400bhp. The next step is v3/4 sti heads which flow much better and have wilder cams than uk ones. You get better mid range with sti heads but at 350 target i wouldnt worry for the moment.

 

So Andy, they should do the job for the time being then without converting to STi. What's your rev limit on standard v3 heads? Any idea where peak power is?

 

Rev limiter set at 7050. Peak power is around 6-6.5k (dont know for definite, not RR).

With the right setup and mapping, it will fly, even on Uk heads

Andy

 

Do you know if they will take any more revs, say to 7500? Not that I particularly want to use over 7000, but it gives more headroom before hitting the pretty harsh rev limiter.

 

I was advised that you can dislodge a shim if you run high revs on uk heads. Besides with my 2.5ltr there is no need to go beyond 6.5 rpm.

 

Yes I suppose that's a possibility. It's not particularly that I want to wring it out at 7000rpm all day, but when I take it on track, it's annoying if you need to stay in gear and you're already at 6500 and you know if you leave it you're going to run into the limiter, violently!