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Bigger heads lower compression

This is a discussion on Bigger heads lower compression within the GTO forums, part of the Vehicle Specific category; Hi guys, I got a quicky, i have LS1 heads that have a CC of 65, but my new LS6 ...

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    Need Sand ??? Naf's Avatar
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    Bigger heads lower compression

    Hi guys,

    I got a quicky, i have LS1 heads that have a CC of 65, but my new LS6 heads have a CC of 68...Higher flow though...

    What are my possible out comes?

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    Junior Member Gunny's Goat's Avatar
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    HI Naf,

    You say your LS1 heads have 65cc, CC, so I guess you had them cut down from the stock 67(66.67). Then you say you have some LS6 heads that are 68cc. LS6 heads come stock at 64.45cc (65). However, your outcome is not going to be that much, if the only thing you do is change the heads. The LS6 is a further refined version of the LS1 head, meaning differences between it and the LS1 head are relatively few. It has an enlarged intake port volume of 210cc, the LS1 is 200cc, and at almost 65cc the combustion chamber is slightly smaller and more efficient. This design raises compression to 10.5:1, from 10.1:1, shortens burn times, and ultimately means less ignition timing advance is required to produce the same power. Less advance allows more efficient combustion, the LS6 heads allow the engine to produce more torque. The exhaust port volume was increased to 75cc and a D-shape that improves flow. However, to take advantage of this you need an exhaust system that can flow better than stock, and a Cam that can utilize the improvements made to the LS6 head. There are other differences between the heads, such as the Valves, and springs. But I'm sure you already know this. I guess what I'm trying to say is, if in fact your heads are as stated, you will probley loose HP/TQ when you switch. But if they are stock, you will have a minimal increase with the LS6 heads. But if you change Cams, put on Headers, and improve exhaust, you can pick up over 100HP and 80 to 100lbs of TQ. However, if you would have had your LS1 heads ported/polished, and cut down to the same size combustion chamber as the LS6 heads the result would have been the same, and you could saveed alot of money. But now that you have them, you could make 400 + HP easy. Just changing Heads, stock to stock, I'd say you could get 30 to 40 HP at the most, with 25-30lbs of TQ. Good Luck with your project. Keep us informed. Gunny
    2004 GTO, A4, Torrid Red/Blk, 1 of 1031, Yank SS3600, Dart-225s 62cc, Cometic .040, CompCams 230/236-.613/.608-112LSA, Hi Tech 7.350 pushrods, Kook 1 7/8, CAT/Reson. delete, 3" H-pipe, super 40s, Ported TB, TB bypass, 160TS, SLP MAF, JBA wires, NGK TR-55, New ERA CAI, tuned = 392hp/344tq

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    Gunny,

    Thanx for your advice, i do have headers with 1 3/4" pipes and a true dual 3" exhaust to compliment it. My cam is to be amazing.... 0.601 lift intake and exhaust, with a 117 LSA....I forget the durations, but my friend went from 291hp to 333hp just with this cam....Completely Stock setup...
    Myself i have a stock block with FAST 90 intake, 80mm throttle body, CAI, short headers, midpipes and corsa exhaust 346ho and 354ftlb...The heads i got are ported and polished along with this wicked cam and my new exhaust system should yield me a comfy 420hp and 390ftlb...No shakes, no loss of drivability...
    American Muscle, Australian Design, Japanese Methods

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    Junior Member Gunny's Goat's Avatar
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    Naf, sounds good. Wow, thats a big LSA. With a good tune it will be hard to tell you have a big cam, should idle almost like stock, and have a huge TQ curve. What valve train did you go with? Did you get the thick wall push rods? When they ported/polished your heads, did they open the CC to 68cc? If they did, you could get head gaskets with a thickness of .040, stock is .057. This would raise your compression ration back up. What size valves do you have? I'd be interested in what your duration is, if you can remember. Is there anyone over there that can tune an LS, or do you have to do it yourself?
    How are those brakes coming? I've been in your area, and know there are alot of roads you can play on

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    Veteran 0rion's Avatar
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    maybe patriot ls6 "style" heads with welded chambers? I'm not smart enough to figure all of this out on my own so I would have to read a little but my gut would tell me those heads would be better suited for FI or spray. I just went with 5.3L heads with 61cc chambers that'll put my compression at 10.75:1. I'm not sure where your compression would be with those heads but I would guess a tad below 10:1 with stock gaskets.

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    Talking

    Quote Originally Posted by Gunny's Goat View Post
    How are those brakes coming? I've been in your area, and know there are alot of roads you can play on
    The brakes were awesome, however, the car is no longer...Car meets Palm tree...I am fine, only a few scartchs

    There are tune shops one in praticualar i always go to...I will take your suggestion for the head gasket into consideration, on the next car, but as of now i have to pause all upgrades...

    take care

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    Junior Member Gunny's Goat's Avatar
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    Naf, I'm glad to hear you are OK. Very sorry about your car tho. Are you going to be able to get the brakes off, and maybe use them later. What are you looking for next, or can it be fixed? Or are you taking a break for a little while, and being thankful your still alive?

    Good luck, and take care of yourself.

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    Oh i am sellin the whole lot for 8000$

    The lucky buyer will get over 15,000$ worth of usable upgrades and brand new componets as well...

    The only thing i am taking from this is the LSX 90 mm intake manifold and my brand new heads...

    I am seriously considering a 2005 Cadi CTS-v...Its brand new and US Spec so parts wouldnt be an issue....only 775miles and for a steal...And its a manual

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