Fabrik8

That's cool, but what engine, what modifications (or lack thereof), what turbo, what charge air temp, what altitude, where in the power band, etc????? That one setup may only have gained 4-5 HP, but a different setup may have gained 30HP (or a lot more) with the same C/R increase.
It's kind of like saying that some guy lost 1MPH on the top speed of his car by putting his pet cat in the back seat. That's great, but what kind of car? A 600HP Murcielago isn't going to lose 1MPH with a pet cat (and doesn't have a back seat anyway) although a worn out Geo Metro might. Better yet, I might have put a turbo on my engine and only gained 10HP. That might make complete sense if my engine is from a 250 or 400cc ATV or dirt bike. Get my point? Hint: Randomly throwing around numbers with no info is useless bullshit.

Without knowing everything about the engine in question, you can't draw any conclusions from that. The only thing you can say for sure is that for a certain amount of boost, you're going to make more power with a higher C/R than a lower C/R, and that assumes you're tuned so that both setups are safe. How much power is what you don't know. It's all about control of combustion; lower C/R lowers efficiency (and available mechanical energy), and is only worthwhile for safety against detonation. Power is all about cylinder pressure, and whether you get that pressure from high C/R or boost doesn't matter. But, it's easier to safely control combustion with boost than with a high C/R, which is where detonation (actually pre-ignition) comes into play. So you can safely create higher cylinder pressure by running higher boost because you have more control to keep pre-ignition from happening.

SO basically, if you are running mild levels of boost, and are within the safety margin of a high compression setup, go for it. Change your game plan if you want to go over 10-15 PSI though. That's a general rule of thumb, and will be different for different engines under different conditions. Regardless, you have better low end power, better power off boost, and faster turbo spool with a higher C/R than a lower C/R, and that's fact that can't be argued.

99EJ6T

all fjt stocks are the 75 mm ones right now. soon he will have the 75.5 ones in, approx 4 more weeks for the first batch. his are the ones that are making the 400+ power plants. other ones you can buy off ebay have not made as much power.

BuiltforSin

I thought he already had the 75.5mm in?

99EJ6T

nope. he just placed the order on the second. and the wait should be four to five weeks after he ordered them (or so he was told). i should know, i pre-ordered from him on d-series.org. second set of the first batch, should be comming to me!

Jarrod

Thank you Fabrik8. I just had this discusion with someone the other day.