What is best way to lower compression ratio for FI? - Pontiac GTO Forum
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post #1 of 14 (permalink) Old 01-23-2007, 08:58 PM Thread Starter
 
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What is best way to lower compression ratio for FI?

I read somewhere that you can change your compression ratio with a cam or even changing the rocker ratio. Is this true? I haven't decided what FI I want to use, but regardless of what I decide, I am a little worried about using FI on my LS2 with stock compression. If I change the heads with a bigger combustion chamber, will that lower my compression ratio? I know just enough on the subject to be dangerous....any input would be helpfull.
I have heard good things about the L92 heads and L76 intake. It should improve flow, but will these heads lower my compression?

Charlie
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post #2 of 14 (permalink) Old 01-24-2007, 03:01 PM
 
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I'm no engine guru, but I'm pretty sure a thicker head gasket would lower your compression. Someone else chime in here.
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post #3 of 14 (permalink) Old 01-24-2007, 03:32 PM
 
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Yeah, a thicker head gasket will do the trick. A lot of guys who run old high-compression muscle cars do it so they run on today's pump gas.
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post #4 of 14 (permalink) Old 01-25-2007, 08:10 AM
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LQ9 heads.

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post #5 of 14 (permalink) Old 04-21-2007, 03:14 PM
 
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Originally Posted by marathonman View Post
LQ9 heads.
heads are the easiest way, and they flow really well.
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post #6 of 14 (permalink) Old 04-22-2007, 12:01 AM
 
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i've heard about alot of good results using the stock ls2 long block and FI. i'm not sure how true it is but i've read that the stock cam is excellent for a turbo. head gaskets will only net you about 0.1-0.2 points of decrease on the compression side of things. the l92/l76 h/i combo have had great results but i have heard some people complain about it being a mix and match parts headache/ nightmare. you'd use the l92 head gaskets for the heads seeing how the bore is smaller on the ls2 than the l92. other than a larger chamber head which i think the l92 is a 72 cc and the stock 243s are 66 cc it would be changing the pistons to inverted domes. if you go that route go with the great 8.0 to 9.5 :1 and boost it loose. i'd love to see that one run. or just keep your motor intact, throw about 7 psi of boost from a procharger with intercooler and call it a day. i drove a procharged ls1 gto which power after 5300 rpm was useless because it then became a tire roast.

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post #7 of 14 (permalink) Old 04-22-2007, 09:39 AM
 
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Originally Posted by txcharlie View Post
I read somewhere that you can change your compression ratio with a cam or even changing the rocker ratio. Is this true? I haven't decided what FI I want to use, but regardless of what I decide, I am a little worried about using FI on my LS2 with stock compression. If I change the heads with a bigger combustion chamber, will that lower my compression ratio? I know just enough on the subject to be dangerous....any input would be helpfull.
I have heard good things about the L92 heads and L76 intake. It should improve flow, but will these heads lower my compression?

Charlie
Compression RATIO cannot be altered without changing the volume of the combustion chambers. Compression pressure can be altered by using a camshaft with more overlap than your current cam, but once FI is in the mix, this small change is academic. If you want lower baseline CR, the only thing to do is make the chambers larger, by relieving them, using multiple gaskets (bad idea) or using heads with larger chambers. Rocker ratio can have the same effect, but ratio mainly affects lift, and to a small degree, duration as well. If you want to run mad boost, an engine with an 11:1 CR is going to have to be severely built to handle it. As for the L92 heads, I dont know what the cc's are on these, but it is easy enough to look up. If they have larger chambers, your CR will drop.
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post #8 of 14 (permalink) Old 05-17-2007, 08:28 PM
 
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You won't have anything to worry about going FI with the factory long block. With 7-8 psi you'll make more power than you can put to the ground, and the engine will live a long happy life. If you want to get crazy and go 10-12 psi you'll probably still be ok with a very good tune and meth injection.

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post #9 of 14 (permalink) Old 05-17-2007, 10:36 PM
 
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Just like said above the LQ4/LQ9(Truck head) is the way to go, the chambers on them are 71cc and have the same port volume as the LS6/LS2 head. It's been awile for me but changing the cam duration is only changing your dynamic compression ratio, changing the volume of the combustion chamber effects you static compression ratio.
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post #10 of 14 (permalink) Old 05-18-2007, 01:00 AM
 
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Check out GM high Tech's buyer's guide to heads. They list flow and compression ratios to help you decide.

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