Boost/comp. ratio question, Question for the boost tech heads Options

[celica3sgte]

I've been throwing this question back and forth for awhile. In the future I plan on building a 3s. Most of the crazy 3s builds I've seen are running around 22-24 psi with a lowered compression ratio. If you had the choice, would you rather a)lower compression and add more boost or b)keep the compression ratio at 8.5:1 and limit the boost. Or would you say that you'd get the same result either way. Personally I'd go with option A. But I really don't know whether one or the other is more or less hazardous. What do you think is safer,,if either?

[Supersprynt]

I think option B would be more hazardous because the more compressed the air gets the hotter it will be and thats more prone to detonation.

[lagos]

i would keep the stock compression, just so that the car dosnt feel like a dog when driving off boost. how much power are you looking for at the wheels?

[jgreening]

stock compression isn't dangerous for a FI application. The danger comes from running pump gas at those levels. Is this going to be a street car? drag? what applications? and what are your hp goals?

[jgreening]

I am responding to a PM from Steve.

Assuming you are going for over 400 whp, it will take a killer big turbo to get there on a 3sgte and pump gas. I have seen exactly ONE dyno plot of a 3sgte making over 400 whp on pump gas and it had everything (stroker, huge A/W intercooler, standalone, totally reworked head, GT35R, etc. etc.) On this application, the car barely TOUCHED 400whp at about 8000 RPMs. You have to remember that lowering the compression ratio will result in lower power - even in forced induction applications. Its done routinely for engines that were not boosted from the factory for safety reasons. However, I have not seen any evidence that lowering the compression ratio beyond 8.5/1 is necessary so long as the rest of the car is set up properly. In addition, you will have to accept the fact that you probably won't be making 200 ft/lbs of torque until 4000 RPM's. I don't know about you guys, but I routinely drive in the 2000-5000 range which means I would be either dropping out of boost with that application or reving the snot out of the car. By the time, you hit 3rd gear, you would be breaking the highest speed limit. These goals are somewhat incompatible with street use - ESPECIALLY on a FWD application due to traction issues.

If I had to do a build again with a 3s, I would definitely do it in a MR2 so the car would be RWD. My mods would be a 3s block stroker, GT3076R, AEM or Nemesis, Intake manifold, headwork, forged internals, and full-race exhaust manifold. The bad part is the thing could blow up at anytime no matter how safe the tune. If you read alot on mr2oc.com you will see that all of the big boys blow up sooner or later. They don't routinely post about it so you have to read between the lines. Does anyone know of a car making over 350whp that has done so continually (running the boost required to do so) for over a year or so? You really don't see it all that often.

In my personal opinion, when you venture beyond 300whp in a 4 cylinder car, you have to be ready to work on your car on a moments notice and be prepared financially for a catastrophic failure at anytime.

I guess what I am trying to say is that stock compression isn't really the problem. Its running a motor designed for 170whp at over twice that amount. No matter how good the engineering from Toyota, we have to recognize that failure at these levels is inevitable. Its not a matter of if, but when.