Turbo with 12.5:1 compression
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From: Greenfield, IL
Turbo with 12.5:1 compression Thinking out loud here... I bought a camaro last fall with
799 casting heads ported/polished
Dual valve springs
ls7 lifters
cromemoly 7.4 push rods
42 lb. injectors and racetronics fuel pump kit
Texas Speed 233/239 112 cam
and some other things that are probably not relevant here, previous owner claims the heads were milled 0.1" resulting in 12.5:1 compression. I found some math online that said about every .0075" you take off the heads results in the loss of 1cc of volume putting these heads at about 52 cc. Is this adding up? I'm no expert here. Anyway, I have always wanted a turbo ls1, and know my compression is way too high, so I was thinking about changing heads then started to think instead of swapping heads, could I just find some deep dish forged pistons to regain some cylinder volume and lower compression that way? I have seen up to -28cc pistons for sale. Yes i know the cam is far from optimal, I could change, but I am more concerned if this is a realistic method lowering compression. I have no paperwork documenting the previous mods and I have never heard of people milling that much off a head, but anyway, I would like to hear people's thoughts. Thanks!
799 casting heads ported/polished
Dual valve springs
ls7 lifters
cromemoly 7.4 push rods
42 lb. injectors and racetronics fuel pump kit
Texas Speed 233/239 112 cam
and some other things that are probably not relevant here, previous owner claims the heads were milled 0.1" resulting in 12.5:1 compression. I found some math online that said about every .0075" you take off the heads results in the loss of 1cc of volume putting these heads at about 52 cc. Is this adding up? I'm no expert here. Anyway, I have always wanted a turbo ls1, and know my compression is way too high, so I was thinking about changing heads then started to think instead of swapping heads, could I just find some deep dish forged pistons to regain some cylinder volume and lower compression that way? I have seen up to -28cc pistons for sale. Yes i know the cam is far from optimal, I could change, but I am more concerned if this is a realistic method lowering compression. I have no paperwork documenting the previous mods and I have never heard of people milling that much off a head, but anyway, I would like to hear people's thoughts. Thanks!
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Joined: May 2010
Posts: 558
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Better to get dished pistons and change the rods out if you are going for boost anyway. Changing the heads would still leave you with weak pistons/rods.
That being said, milled heads decrease the already thin the deck thickness of stock heads, so you may have problems keeping them down. Good gaskets and studs would go a long way in helping this though.
I vote pistons/rods to decrease compression.
That being said, milled heads decrease the already thin the deck thickness of stock heads, so you may have problems keeping them down. Good gaskets and studs would go a long way in helping this though.
I vote pistons/rods to decrease compression.
If he took .1 of the heads, you need to get different heads. The decks are way to thin for much boost. 317s would put c.r. in the 10.5 range.
Last edited by quicksilverado; Apr 27, 2012 at 07:10 PM.
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Joined: Jan 2011
Posts: 934
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From: Tulsa
^ Gotta agree with him/her on the good stud/gasket thing. LS9 Gaskets and ARP Studs bare minimum. Also, do you have E85 accessible? If so I say run the CR you have and add some meth. Should spool like a raped ape 
I would get rid of those heads immediately if you decide to run ANY power adder with them. (On a side not a 7.4" pushrod is way too long, you might not be seating the valves all the way providing you are running a non-adjustable rocker setup.)
You need to ask yourself how much power you want to make?
A stock rod/piston can take 600rwhp and live for a long time. You can even push them upwards of over 800rwhp....of course you are on borrowed time and one single mishap in the tune and kiss it all goodbye. Even with a forged piston, if the tune isn't right a forged piston/rod setup might last .5 seconds longer. It's ALL in the tune-up.
The key on a stock rod/piston setup is to have MORE than enough fuel and injector than you need, keep the timing conservative, valve springs, rod bolts, head studs, and LS9 gaskets and it wil last for a long time.
You need to ask yourself how much power you want to make?
A stock rod/piston can take 600rwhp and live for a long time. You can even push them upwards of over 800rwhp....of course you are on borrowed time and one single mishap in the tune and kiss it all goodbye. Even with a forged piston, if the tune isn't right a forged piston/rod setup might last .5 seconds longer. It's ALL in the tune-up.
The key on a stock rod/piston setup is to have MORE than enough fuel and injector than you need, keep the timing conservative, valve springs, rod bolts, head studs, and LS9 gaskets and it wil last for a long time.
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TECH Apprentice
Joined: Apr 2012
Posts: 333
Likes: 4
From: New Zealand
I am at around 10.5:1 on a stock bottom end LS1 and run 6psi boost with ARP fasteners for head and 051 cometic gaskets on a 222/226 cam. It is perfect on pump gas (93) with water/meth (which was worth an instant 45rwhp).
Lose those milled heads and you'll be fine at low boost. If you want more than 8-9lb boost then you need forged pistons more for a slightly lower ring land than anything as stock pistons have a high ring land for emissions and they do not like extreme cylinder pressure.
Lose those milled heads and you'll be fine at low boost. If you want more than 8-9lb boost then you need forged pistons more for a slightly lower ring land than anything as stock pistons have a high ring land for emissions and they do not like extreme cylinder pressure.
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