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Old 08-21-2007, 06:06 PM   #15
Scooby Specialist
Member#: 100150
Join Date: Nov 2005
Chapter/Region: MAIC
Location: NC
02 V9 STi 6spd
HTA Green, Built EJ205


This is exactly why I was asking. I figured that displacement was going to be my limiting factor. All I've ever heard about as far as getting the motor to rev higher is shims and oil... never anything else so I was wondering why I just can't spin it a bit higher. I obviously didn't think about nor consider most of these other factors. I'm actually running a stock exhaust header right now... stock intake manifold (rotated)... So how do I play this out? Get the car on the dyno and see if it's continuing to make power as it approaches 8k? I had a 7cm Green and new that it would be well choked up before 8k. The SZ55 should have more room... but how much more? Will it be restrictive as well? I am NOT trying to destroy this motor so there is certainly a "for safety's sake" here.
Originally Posted by [email protected] View Post
One question to ask yourself is do you need to go higher. Take away the reliability issue of the build for a second.

You need to look at the engine as a dynamic system. There are many variables that dictate where you make power. Your camshaft, your valve size and your turbo size are some of them. You mentioned those, but that still doesn't touch on intake manifold (runner length and plenum volume), head porting (shape and volume of the runner), compression ratio, exhaust backpressure, or anything of that nature.

Someone touched on piston speed which is an important element when trying to figure out how 'high' to take your engine. Sending it to an arbitrary 9500 is just that...arbitrary. You have to consider intake charge velocity and if it can keep up with the piston as it travels down the bore. Through that you have to decide what is the proper (not saying yours are necessarily incorrect) camshaft and cam timing for what you're looking to do (this has to do with dynamic compression ratio).

What turbo do you have? You have to consider where your turbo will run out of have to consider what turbine and housing you have on the car to determine if it can support that airflow while maintaining a good ratio of boost to backpressure.

There are a ton of elements that dictate where an engine is efficient...if you make peak power early and torque falls off hard, shifting higher is just hurting you, not to mention all of the reliability issues we didn't touch on.

As a general guideline, you want to go higher in revs when your engine displacement is the limiting factor. When that is what's 'holding you back' per se, the focus of the engine build (if you're building it for peak power mind you) is to sustain high rpm torque which obviously translates into horsepower since they are a function of eachother. 300lbs of torque at 10,500 is 600hp. Perfect examples of this would be Formula One cars, they rely on rpm and high rpm volumetric efficiency to yeild high horsepower numbers.
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