What is best way to lower compression ratio for FI? - Pontiac GTO Forum
Pontiac GTO Pontiac GTO Forum

» Auto Insurance
Wheel & Tire Center

Go Back   Pontiac GTO Forum > The 2004-2006 GTO > Engine Discussions
Register Home Forum Active Topics Photo Gallery Garage Search Today's Posts Mark Forums Read Auto EscrowInsurance

GtoForum.com is the premier Pontiac GTO Forum on the internet. Registered Users do not see the above ads.
Reply
 
LinkBack Thread Tools Display Modes
Old 01-23-2007, 08:58 PM   #1 (permalink)
 
Join Date: Feb 2006
Location: Tyler, Texas
Posts: 28
My Photos: (0)
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
txcharlie is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 01-24-2007, 03:01 PM   #2 (permalink)
 
Dwill's Avatar
 
Join Date: Nov 2006
Location: Jersey/Philly
Posts: 18
My Photos: (0)
I'm no engine guru, but I'm pretty sure a thicker head gasket would lower your compression. Someone else chime in here.
Dwill is offline   Reply With Quote
Old 01-24-2007, 03:32 PM   #3 (permalink)
 
b_a_betterperson's Avatar
 
Join Date: Feb 2005
Posts: 2,100
My Photos: (9)
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.
b_a_betterperson is offline   Reply With Quote
Old 01-25-2007, 08:10 AM   #4 (permalink)
440 RWHP/400 RWTQ
 
marathonman's Avatar
 
Join Date: Jun 2005
Location: Clearwater, Fl
Posts: 202
My Photos: (0)
LQ9 heads.
__________________
2005 IBM M6
Weld Prostars
Flowmaster Super 40's
Chris White Guage Changes
HPE Cam Package(S-Cam, AR LT's,Lingenfelter CAI,UD Pulley, X-Pipe, Ported Factory Intake)
BMR Drag Bags
440/396
marathonman is offline   Reply With Quote
Old 04-21-2007, 03:14 PM   #5 (permalink)
 
jed.only's Avatar
 
Join Date: Apr 2007
Posts: 3
My Photos: (0)
Quote:
Originally Posted by marathonman View Post
LQ9 heads.
heads are the easiest way, and they flow really well.
jed.only is offline   Reply With Quote
Old 04-22-2007, 12:01 AM   #6 (permalink)
 
koman's Avatar
 
Join Date: Sep 2006
Location: north chucktown sc
Posts: 730
My Photos: (0)
Send a message via AIM to koman Send a message via MSN to koman Send a message via Yahoo to koman
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.
__________________
06 impulse blue/black/ m12 1 of 252
spoilerless with dba 4000 fronts. gone

78 chevy nova 2dr sbc 400/th350/8.5 (4 jackstand mod)

92 ford rustang lx notchback 5.0/t-5/8.8 LS still stock
FMS shorties, bbk x ,fms catback, air pump overrode.
koman is offline   Reply With Quote
Old 04-22-2007, 09:39 AM   #7 (permalink)
 
bg2m8o's Avatar
 
Join Date: Jul 2006
Location: Bakersfield, CA
Posts: 353
My Photos: (0)
Quote:
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.
bg2m8o is offline   Reply With Quote
Old 05-17-2007, 08:28 PM   #8 (permalink)
 
black04a4's Avatar
 
Join Date: Jan 2007
Location: Ocean Springs, MS
Posts: 4
My Photos: (0)
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.
__________________
2004 Phantom Black A4 GTO - Flowmaster catback, Dynatech LT's,Cat delete, Comp 220*/224* 530"/534" 112 lsa, Comp 1.75 Rockers,TCI Streetfighter 3K stall
Procharger P1SC
Tuned at home with HPT
black04a4 is offline   Reply With Quote
Old 05-17-2007, 10:36 PM   #9 (permalink)
 
GM4life's Avatar
 
Join Date: Mar 2007
Location: RAF Lakenheath, UK
Posts: 4,162
My Photos: (15)
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.
GM4life is offline   Reply With Quote
Old 05-18-2007, 01:00 AM   #10 (permalink)
 
LYNMUP's Avatar
 
Join Date: Nov 2005
Location: Ft Walton Beach, FL
Posts: 689
My Photos: (0)
Check out GM high Tech's buyer's guide to heads. They list flow and compression ratios to help you decide.
__________________
'05 QSM M6
LYNMUP is offline   Reply With Quote
Sponsored Links
Advertisement
 
Reply

  Pontiac GTO Forum > The 2004-2006 GTO > Engine Discussions


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Powered by vBadvanced CMPS v3.2.2

All times are GMT -5. The time now is 04:51 AM.



Powered by vBulletin® Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
SEO by vBSEO 3.3.2
Garage Plus vBulletin Plugins by Drive Thru Online, Inc.