Thoughts on this built engine for my Turbo FRS
 

https://www.rallysportdirect.com/par...ru-short-block

Looking to build my engine. The shop I would take it to is recommending this engine. Any thoughts?

how much power are you trying to make?
What turbo are you going to be running and at how much boost?

400ish... Gt2860rs...I dunno; what ever is safe for E85 and occasionally 91.

You can make 400whp on E85 without dropping the compression ratio.

That’s jus what they recommend... Why would or wouldn’t you use a different compression ratio? The IAG described it as being safer for boost... I don’t care what hp number I make. I just want my engine to not blow up.

IAG is a great company. Do it!!

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Dropping compression will allow for more power and especially more power on pump gas, but you may find there is less fuel economy and less power down low or off boost with slightly more turbo lag. It is definitely not necessary for modest goals and isn’t too necessary if you have easy access to E85. Unlike a supercharger, the turbo can have its boost modulated better with an electronic boost controller to adjust for 91 or for doing boost by gear or progressive boosting. If you want to go further than 400whp then definitely drop compression.

Exactly why I would not drop compression at 400hp. With good rods, pistons, tune and E85 you will be safe at 400.

I’m not a turbo expert. But isn’t that turbo on the smaller side? Not sure if it can flow enough air to hit 400whp with 10:1 compression. If it can I bet it’ll be maxed out.

350whp on 91 and 425whp on E85 should be perfectly fine and safe on stock compression with forged rods/pistons. It’s largely on the tune and boost controller for safety once you have forged internals.

The trade off is lower compression requires more boost to make the same power So you’ll have more turbo lag/spool up and less low end torque/throttle response, worse MPG, and less off boost power. but assuming your turbo can pump enough air you’ll have more top end potential. But with sub 425whp goals your probably better off with stock compression. Or do 12:1 or 11.5:1 for a happy medium.

Pump gas is really the deciding factor, besides power potential and reliability. I would rather be able to make big power on pump gas than be concerned about low end power.

Are there many people making more than 300whp on 91? 350whp on 91 seems sketchy, even on a built motor. A built motor can help avoid failure from higher power, but not from preignition/detonation.

You may be right. 350whp on 91 may be pushing it. I’ve seen 350whp on 93 though. So maybe 315whp on 91. (Disclaimer dyno #s vary a lot dyno to dyno)
But not really anyone with a built block that kept stock compression and runs 91 that’s posted a dyno that I’ve found. Usually either lowered compression, or uses E85.

Personally if I had E85 and only wanted sub 450whp i would keep stock compression, or only slightly drop it. And just keep the boost low if I ever had to resort to pump.

The other option is lower compression and have more power on pump and e85, but deal with a slight drop in low end, but I’ve seen dynos with lower compression, and there doesn’t seem to be a huge difference. This is also why high compression and FI is just not done. It is better to raise the effective compression ratio than to start with a high static compression ratio.

I would run 10.5:1

Also, this post of this thread had this link, which showed the dyno below for a slightly modded WRX throwing down decent numbers on 93 octane.

http://i.imgur.com/MU08aj3.jpg

That post led to this response, which I'll quote anyways: