jung4g

OWNED!!

Whitey, you rule.

I'd agree with the 9+:1 opinion. That way you can run less boost (= less heat) for the same power, with better low end. It's just easier on the system as a whole. Honestly though, tuning is WAY more important than comp ratios. You can blow any engine improperly tuned, but make the theorists turn over in their graves with a nicely tuned monster.

Reddy

The only reason to run low compression is becuase there are limitations when running pump gas. 9:1 is the best choice

BoosTedZSix

i wouldnt run lower that 9:1. me personally id do 9.5:1 8.5 is too low. u get the run more boost but its probably not gonna make any difference since u would have to run more psi just to try and make as much power as someone with 9 or 9.5:1.

baldur

Yes it's all a compromise.
8.0:1 compression allows just about optimal timing on pump gas at 20psi boost generally.
9:1 is great for 10psi at optimal spark timing on pump gas.
Old school F1 cars were limited to 102 octane fuel, they ran as high as 70psi boost. The compression was so low that coupled with the bad evaporative characteristics of their fuel they had trouble starting the cars at higher altitudes.
With fuel good enough the compression ratio becomes pretty much irrelevant (Although I don't think anyone is going to build a 14:1 force fed engine).

There are 2 ways to make lots of power and do it safely:
Low compression and lots of boost, sure it will hurt off-boost performance a little bit but the difference isn't day and night, and usually greatly exaggerated. Spark timing for example has a much bigger effect than compression ratio on bottom-end torque.
High octane and lots of boost, costs more in the long run usually but it's still good.

Then there's the method that has a record of not making lots of power reliably for extended periods:
High compression, lots of boost and pump fuel. The efficiency of the engine is being blown out the exhaust if the spark timing has to be very retarded at full throttle, and generates lots of heat in the exhaust valves,ports and manifold. You can reduce this heat by going overly rich with the mixture, losing more power. The end is usually detonation.

And the final bit: Tune it right, no matter how it's built. You really need to listen to the engine, preferrably with a stethoscope to check for detonation.

StanB

I really appreciate the feedbacks.

How much does a skimmed cylinder head affect the compression ratio? In a NA car, one would skim the head to increase compression. So, if the cylinder head was skimmed and I use a set of 8.5:1 pistons, wouldn't the compression be bumped up to maybe 8.7:1 or 8.8:1? If my assessment is correct, I would be within the 9.1:1 range which most people recommend. Does this make any sense or is it bullshit?

OVERALL I KNOW TUNING IS THE KEY.

baldur

And another thing to add, it depends a lot on the application. An engine that can maybe survive in quarter mile drags may perharps melt on a top speed run or in circuit racing.

StanB

Any opinion on my assessment?

projekteg

8:1 = ---- on a honda, but is more forgiving with tuning
9:1 = good for a boosted honda running pump gas
9.5:1 = ideal for a honda with good tuning on pump gas
10:1 or higher = great for power if you have great tuning and don't run too much boost on pump gas or only plan or running race gas

HMT-Admin

10.1

just dont have leed do your tuning!!!

tranceminister

Hah, why? I thought Leed was the "Emril" of tuning. Just whip something up and "Bam!". You've got a hell of a Honda