I came across a chance to get some cp 9.0:1 pistons for a pos car i dont want in my yard anymore so its hard to pass them up. I have always liked the idea of adding more low end torque into the sr. Im not one to post without researching but theres not really any long pro con threads with dynos to back it up. Heres the situation. I want something with fast spool and between 325-375 hp. I want the car to shine mostly with handling and coming strong out of turns. Im not really looking for a drift maching. My previous plans were to stay stock compression and go with a gt2871r .64 and some jwt s3 cams. How much do spool and low end actually benefit? What are some of the best turbo/cam setups for 9.0:1? Is it worth it? Any experiences/opinions are appreciated.

Well, just some quick technical info to get you going with your info. running 9:1 compression raises your effective compression ratio with boost, e.g. @ 10lbs you'd be making about the same pressure as 12lbs on 8.5:1.
Which translates to your effective compression ratio @ 10lbs on 9:1 is 15.1:1, compared to 15.4:1 @ 12lbs on 8.5:1.
So, you're more efficient @ a lower boost setting, but basically seeing the same pressures, it's a trade off.
Yeah, imagine having 20:1 compression pistons in an N/A engine. That's the pressure you get in there @ 18lbs with 9:1 pistons.

I was just going through the camshaft mega thread the-camshaft-mega-thread-t210484.html
by DrifterProdigy85 » 04 Dec 2006 21:30
Quick reference of Turbo and Cams to choose.

T25...Stock, 256/256
T28...Stock, 256/256, 256/264
GT28RS....256/256, 256/264, 264/264
GT2871R .64.....256/264, 264/264
GT2871R .86.....264/264, 264/272, 272/272
GT3071R .63.....264/264, 264/272
GT30R.....264/272
GT35R.....272/272


I just finished a week long intense study course on solar energy...my head isn't fully clear to properly process the car info I know yet lol. Hopefully someone else can come and sort out some more for you. I tried

i appreciate the effort man. I doubt i would run 18 lbs with 9.0:1. Im fairly certain that without a very proper tune it wouldnt last too long. This car will be driven maybe 250 days out of the year so its basically gonna be a daily driver minus winter. I wish i could get a hold of drifterprodigy ive read the entire camshaft megathread and it seems like hes done a lot of toying around with different setups. If I remember correct hes owned a gt2871r in both .64 and .86. and I believe hes ran a 9.0:1 setup. I want quick spool so i was originally thinking .64 but if spool is greatly affected i would go .86 Although im also wondering if thats even necessary considering i wouldnt even get to where the .86 shines which is in high boost. Theres a lot to consider. Thanks for the reply.

Im also curious as to how much torquier 9.0:1 will be before spool, say from a dead stop. I know there will be a difference, just not sure how much.

anybody have any experience with this?

For torque i really cant notice it at all but in top end for sure, i can feel it a bit quicker than before.

Raising your base comp ratio is a great way to gain better off boost response and increase overall power. There are only cons if you don't have the supplemental upgrades to take advantage of it. I wouldn't hesitate to run 9:1 on the street with a proper tune and supporting mods. Tuning is key here as it is with every other modification. If you don't take the time to do things right you'll be doing them over.

You're not going to gain anything. Once you get into 2nd and 3rd gear turbos like the 2871R are going to produce boost around 3000 RPM, and anything above 8.5:1 is just going to force you to cut back on ignition timing, negating any small gain you may get.

Spend good money on manifold design, reduce piping lengths, and invest in good management. You'd be amazed at how much more drivable a turbo engine is with a properly tuned Motec M-series.

i have messed around with both. although its been almost a year since i drove my car. but i had a gt2871 .64 o na stock motor, then a .86 on a 2.2 with 9 to 1 actually a little higher then 9 to 1 and it spooled the same as the .64 on a stock motor. i was having some leaking fuel injector problems so i wasnt getting the full potential of higher compression and big bore.

but to answer your question. 9 to 1 will add more power of boost and as stated above give you more power per pound of boost

only con is if you have a bad tune. i run it and love it, but i'm on stand alone. my spool with a .86 gt2871r is much quicker than i wouldn't have anticipated.

PantherRacer wrote:Well, just some quick technical info to get you going with your info. running 9:1 compression raises your effective compression ratio with boost, e.g. @ 10lbs you'd be making about the same pressure as 12lbs on 8.5:1.
Which translates to your effective compression ratio @ 10lbs on 9:1 is 15.1:1, compared to 15.4:1 @ 12lbs on 8.5:1.
So, you're more efficient @ a lower boost setting, but basically seeing the same pressures, it's a trade off.
Yeah, imagine having 20:1 compression pistons in an N/A engine. That's the pressure you get in there @ 18lbs with 9:1 pistons.

Sorry to pick on you but this is so incorrect my head hurts. It looks to me as though you just multiplied the compression ration by the atmospheres experienced at different pressure levels. That's not how it works. Just because you are throwing two times atmospheric pressure(1.0 bar or 14.7psi because 0 on a boost gauge is 1 atmosphere) at an engine does not mean your cylinders are filling up to 1 bar and therefore have double their displacement worth of air in them. The actual math is pretty complicated and not really needed here but suffice to say the difference between 8.5 and 9:1 compression is negligible at sane boost levels and especially on a reasonably sized turbo as with all mods so long as you tune it right and take care to always have the good gas in it you'll be fine. I'd be willing to bet with a 2871R .64 you'll be pushing the turbo out of it's efficiency range before you start getting detonation provided a good tune.