LS1IMPULSE

iTrader: (42)

I know, I know. Overthinking a little but I cant seem to decide with all the info out their. It WILL be a low comp. engine though.

Basic setup will be:
Forged LQ9
billet caps
GM #317 heads

Intercooled ST80R
Meth Inj.

Running up to 15lbs max, untill new pcm.

SuperZ

iTrader: (2)

Your increments are a little close together.Youll just have to see what piston dishes you can get.Id say anywhere between 8.5 and 9.0

LS1IMPULSE

iTrader: (42)

yeah i put them close 8.5-9 is what im thinking, it will have -15cc pistons but i can tailor the CR regardless, I want to get the quench as close to .050 as possible and be able to mill just a little off the heads to be sure they are tru.

Rickys S/C Z28

iTrader: (13)

with the bigger motor id got with 9:1 i just had my motor built(347) and went higer then any of your choise i went 9:8:1 but im not going to run pass 10 psi

52172

iTrader: (2)

9.8:1

cablebandit

iTrader: (37)

with meth/93 for street...10.0 and race gas at the track

CALL911

iTrader: (17)

Pump gas (93 octane), or race gas? If 15#'s of boost is your max, and you'll be intercooled and meth injected on pump gas, you should be fine in the 9.0-9.5:1 range. If on race gas only, I would say 10:1

FWIW, I am running a setup that would take 22-24#'s of boost on pump gas, and I am at 8.8:1.

LS1IMPULSE

iTrader: (42)

pump 93, I will run meth, and may go over 15#'s but for now 15 will be the max.

CALL911

iTrader: (17)

If thats the case, then you'll be fine with up to about 9.5:1 on the safe side, but if you plan up to say 18-20#'s of boost, you might want to lean towards the 9.0:1 if you are going to stay on the pump gas.

ABeasst

iTrader: (5)

I wished I would have gotten 20cc dishes and done 9:1 instead I'll be at 8:1 with 32cc dishes.

Zombie

iTrader: (2)

there isn't an 8.0:1 choice

red beast

9:1 is what I chose....

Josh@KY-Turbo

iTrader: (120)

9:1-9.5:1. That my Vote.

Websy21

iTrader: (34)

How come you told me 8.5 on my 370 than Josh? LOL

LS1IMPULSE

iTrader: (42)

any more votes?

Photochop

iTrader: (74)

9.5:1 is what I ended up going to. Only because I had flat tops with -2cc valve reliefs already in the motor and just put a set of stock 317s on with some good springs to bring it down from 10.3 to 9.5:1. Had I known I'd be rebuilding the entire motor I probably would've gone back with some 8.7:1 pistons.

JL ws-6

iTrader: (50)

Intresting thread. I'd be intrested to know what all the 7 second lsx cars are running for compression. I have a funny feeling that they are running more then you think, to help with spooling the big turbo's, and they can make the same power with less boost pressure, probably less boost fluctuation between shifts this way as well.

Put this another way, if fuel isn't an issue, what would make more power, 8-1 compression with x amount of boost, or 12 -1 compression with less boost, knowing that no matter which way you go about it, you can only get away with so much cylinder pressure before the head gaskets, studs and pistons will fail?

Websy21

iTrader: (34)

not really the discussion here as this has been talked about frequently but for the oposition the happy medium seems to be around 9:1 however I chose 8.5 for max boost on pump gas. Eaqch to his own and every setup is different. If you plan to push it hard and run race gas, go to the track allot, etc than go witht he higher CR. If you are happy with lower boost and max power, again higher Cr but if you want to go all out on the street/track whever and have a better piece of mind on your expensive build than its a no brainer. Gio with a lower CR hands down. Up to about 9:1 I believe you will be ok, meth for the added security too but some believe to think its a bandaid. I have a kit, if I need it and will set it up to mist a bit in for the added effect of cooling and octane/boost, etc.

JL ws-6

iTrader: (50)

I'm not really talking about anything that would be on the street, more a max effort race build, track only car. Intresting though, I just wonder what the ratio of boost for what compression is the allowable amount for pump gas, compared to what the ratio is for an all out race setup.

Better yet, if there is a definite # for cylinder pressure for a given fuel, does it really matter how you get to that, wether it be at 5 psi and 12 to 1 compression or 12 psi and 8 to 1

Websy21

iTrader: (34)

I see your point as I often pondered this as well but would more or less have to do some extensive testing to gather the info. If you are willing to push the envelope than the higher CR is the better choice, I want my engine to last myslef and mostly on the street. I do agree though that even on race gas there would be a certain CR that would be considered to high to be efficient at a low boost leve. There has to be that magic, or close to number for CR/boost ammount to achieve the best performance. I wasn't going for this so thats as far as I got. I wanted close to max power on pump, so I picked 8.5 myself and wouldn't go any higher than 9:1 for the street and 9.5 for more of a race car. Max effort would probably be up in the 10:1+ range but couldn't imagine the risk involved and number of passes before dealing with issues and/or rebuild.