Compression 402 shortblock, milled heads?
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Joined: Aug 2002
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From: Hartsdale, NY
Compression 402 shortblock, milled heads? Ok so a few years back I converted my LS1 into an LS6 using SDPC ported heads that were shaved (milled) 60 thousands which increased the static compression Vs. stock.
If I go with a new 402 inch shortblock and use the old milled 60 LS6 heads, will that result in too much compression for a pump gas street car?
If yes, what is the remedy; using dish pistons, thicker gaskets, different heads?
If I go with a new 402 inch shortblock and use the old milled 60 LS6 heads, will that result in too much compression for a pump gas street car?
If yes, what is the remedy; using dish pistons, thicker gaskets, different heads?
Are you sure it was .060" milled, or did they mill the 64cc LS6 heads down to 60cc (roughly .024" removed)? At approximately 1cc for every .006 removed, your combustion chambers may be as small as 54cc, and you might also have the potential for intake alignment issues.
The 220/220 cam in your sig, installed at a 112* ICL, with a .053 head gasket (stock thickness) and a 60cc combustion chamber would yield a static compression of 11.28:1 and a DCR of 8.84:1 - about the max you could have run with such a thick gasket. Any advance, a thinner gasket, or smaller chamber would probably have resulted in detonation.
What will also help to determine what kind of compression you can get away with on pump fuel is the cam you are going to run. Any idea yet how big you'll be running?
PS - Any dyno #'s or track times from your current setup? I'm interested in a similar duration cam and AFR heads when the time comes.
The 220/220 cam in your sig, installed at a 112* ICL, with a .053 head gasket (stock thickness) and a 60cc combustion chamber would yield a static compression of 11.28:1 and a DCR of 8.84:1 - about the max you could have run with such a thick gasket. Any advance, a thinner gasket, or smaller chamber would probably have resulted in detonation.
What will also help to determine what kind of compression you can get away with on pump fuel is the cam you are going to run. Any idea yet how big you'll be running?
PS - Any dyno #'s or track times from your current setup? I'm interested in a similar duration cam and AFR heads when the time comes.
When I had the car in my sig...Those heads were milled about .060", or maybe even .070", and My intake was tight to go on. I had to use the proper torque sequence, and be really careful. It made a few popping sounds, not real loud or anything, but it went on. Be really careful with your intake if your heads are milled that much!!! My compression was around 11.3 - 11.5:1 and the car ran fine on pump gas. It did diesel off a little bit, you know that slight running after you shut it off like a diesel, but not for more than a second.
Last edited by moeZ28; Jan 14, 2008 at 08:35 PM. Reason: more info to add
Thread Starter
TECH Veteran
Joined: Aug 2002
Posts: 4,055
Likes: 1
From: Hartsdale, NY
Yeh, the heads were really milled 60 thou... this was the first ported head from SDPC many years ago. I needed special lenght hardened pushrods because the heads were so much different from the factory spec.
I've had the LS6 intake off many times and it is a tad tricky but it does go on and off OK.
My car never deisels - even on regular gas and stock PCM/tune. No dino done or track times.
So if I go with a 402 I would likely need more than -2 dished pistons?
I've had the LS6 intake off many times and it is a tad tricky but it does go on and off OK.
My car never deisels - even on regular gas and stock PCM/tune. No dino done or track times.
So if I go with a 402 I would likely need more than -2 dished pistons?
The problem is, you can build around those heads now, but if you decide you want a different head later, most heads for the larger bores have larger chambers. Basically, if you go with a large enough dish now, different heads will later leave you with relatively low compression.
For example, TSP L92 Heads are 67-72cc
Thought - What is the casting number on the heads? I'm assuming it is a 243 head, but it's possible that it is the similar 035 or 317 head that has the same port design. If that was milled & ported, some vendors sell tham as an "LS6 style" head.
For example, TSP L92 Heads are 67-72cc
Thought - What is the casting number on the heads? I'm assuming it is a 243 head, but it's possible that it is the similar 035 or 317 head that has the same port design. If that was milled & ported, some vendors sell tham as an "LS6 style" head.
