Hay Guys Josh here from New Zealand... I'm Doing a build on my 03 wrx sti... I have built the engine it started life as a EJ255 out of a turbo outback. not much point boring you with the engine specs but has been built well and to make good HP. First dyno run we run a gtx3071R twin scroll and made 315kw 400ft @22psi boost peak power was just over 6500 rpm and would not make any more power with more boost or timing live data showed egp to boost exceeding 1:1. round 2 at the dyno we tried a g30-660 0.84ar basically run into the same issue this time results were different @ 22 psi it made the same power but turbo come in 500 rpm sooner and run out just over 6500 and made another 100ft pound of torque but saw 35-45 psi on the egp. we tried 6 degrees more advance timing and run the lucas octane booster and made 470whp but became knock limited. basically i feel the turbo just isn't big enough on the exhaust side. we are running bp 98 octane fuel and wanting to make around 500-550 whp I would be interested in your thoughts. Cheers
For 98 grade fuel the g30 did quite well. I find that exhaust gas back pressure to be one of the main culprits of why things are limited.
On my car, with just built motor it was the same results for the gen 2 GTX3076r and Blouch Dom 3.0xtr. After building the valve train with oversized ferrea valves and opening up the exhaust side it made about 80whp(not sure what it is in KW) but it was significant.
We then moved to a gen 2 gtx3576R 1.03A/R and easily cleared the 470whp mark on pump fuel with egts in the clear as well as back pressure. With the built valve train and larger cams we shifted our power band in the timeattack car to 4200rpm peak torque and peak power at 7300 rpm, redlining at 8200rpm.
For giggles on the dyno, we set up the Haltech Elite 2500 with flex fuel and a few other goodies and ran pump ethanol and made 600whp at 30psi. Ended up bringing it back down to specifications of the class we race in, but in general i see no point in running twin scrolls or weird sized turbos like the 3076r. Once you get into the 450whp territory and you are still thinking about super duper early spool that cant carry out into the upper revs where a faster car would be, then i wouldve stuck with a smaller stock location fp or blouch turbo and left it at 23psi.
If you are dead set on 500-550 as other have asked before. I would absolutely reccommend larger cams and larger valves along with a 35r sized turbo with the larger hotside. You'll just have to come to terms that it is now a road style competition car and not a torquey street car anymore. If you are limited in octane or at higher elevation(which i doubt you are) then you may possibly be looking at even a larger turbo than the 35r if you dont build your heads. In my area and on track days, there are a few teams that daily 500whp ej257s on a 35r with US 93oct making 490-500whp. However i believe a lot of them are using a water meth injection system.
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