First off, I want to thank everyone who contributes to this board... lots of information. Formalities aside, I realize that in order to run more than approx. 10psi of boost, compression must be lowered. However, what is the exact reason for this? What I am trying to determine here is what is least stressful to the engine in a "big picture" sort of way:

10psi from a KW s/c kit with 11:1 compression (user MrHonda), or

18psi from a gt30 and 3mm gasket ~ 9:1 comp. (my current setup that blew a gasket due to head lift, or so is the theory at the moment)

In other words, is it better to run low boost with high CR, or low CR and higher boost?

My goal is to be able to maintain boost for longer periods of time in the higher gears (4-6th) while minimizing the risk of the head lifting off the block or the head bolts giving away, and overall stress. An example would be a really long straight at a track, or a long pull up to redline 5th. InlinePro suggested pulling timing to reduce stress, at the expense of absolute power. ARP head studs are another option.

As for power goals, I am looking for 340 to no more than 400 WHP. I am not obsessed with peak HP. Things like response, drivability, power delivery and RELATIVE reliability are more important. Your advice is welcome, thanks

 

I think the lower CR reduce the chance of detonation on pump gas. To compare those two you would have to look at the effective compression ratio for both setups.

ECR = ((boost psi/14.7) + 1) X engine compression ratio

10psi from a KW s/c kit with 11:1 compression

ECR=18.85



18psi from a gt30 and 3mm gasket ~ 9:1 comp

ECR=20.02

 

It's better to run high boost and low CR. It just costs a lot more to do it right on a car like the S2000.

 

from what i've seen on the board, most engine issues in terms of failures/reliability issues are tied closely to:
1/ stupidity or error
2/ poor quality tune, often accompanied by #1 (typ running lean)
and you can nearly eliminate problem source #2 by either using a pretuned kit or a competent tuner.

a few people have had hardware failures from poor installs or just worn out parts after enough mileage accumulates, but that's not common yet. to get a real answer, we'll have to wait a few more years. even then, i think the difference you're talking about is just noise compared to the install quality, tune, hardware quality, and other variables.

here's a personal theory: over time, if under-the-hood temperatures are very high (like 100F+ over stock), that could cause stress (due to thermal cycling) in places honda engineers weren't focusing their analyses on, so it has the potential to break parts unexpectedly. because of that concept, to me, if there is a large difference in UTH temps for the two applications you're comparing, i'd prefer the lower temp solution, as that would be a bigger long-term-reliability concern to me than the combination of boost pressure/CR (not to mention better power per psi boost). that's 100% speculation based on the reasoning i gave, so take it, leave it, or throw it up for discussion until it's beat to death.

 

Squeezing air and gas causes it to heat up. The higher the CR, the more the mixture is squeezed. If the temperature reaches the flash point of the mixture, or it is squeezed tight enogh, it will detonate. Turbocharging adds more fuel and air resulting in the mixture getting squeezed even harder. It's sort of like a deisel. That's why you see a formula for effective compression for turbo cars.

 

because most people don't want to run c16/detonate their motor. lowering compression allows you to run more boost and stay on pump gas longer. the higher compression makes pump gas ignite sooner or "pre-ignite/detonate" because of its lower octane rating. Thus the reason people either lower compression (more reliable and cheaper in the long run) or run race gas i.e. higher octane rating fuels (which is pricey already and unless you are racing, it is a waste for your dayly commute.)

 

To summarize what SgtB and 9KRDLIN said so well, lowering compression ratio allows you to run higher boost and still keep the pistons inside the cylinders.