poltergeist 02

iTrader: (65)

I have looked and looked and can't seem to find the answer i'm looking for. Here is what i have:

stock bottom LS1
799 heads 64.43cc (stock)
GM MLS .052 compressed thickness gaskets

How much will i need to mill these heads to achieve a 10.8:1 - 11.0:1 compresson ratio?

01ssreda4

iTrader: (96)

there's a formula, someone around here know's it. But I'd guess it would be in the neighborhood of .030-.040.

baxsom

iTrader: (6)

what will that do to your PTV clearance?

01ssreda4

iTrader: (96)

With a 228/232 612/612 cam and .010 off some 799s I had .100 and .120 for PTV clearance. Those heads got another .030 taken off to bring me to 11 to 1 with 60cc chambers. My PTV is now .070 and .090 respectively. These figures are with some combustion chamber work.

427LS7HCI

Recommended minimum is .080 on the intake side and .100 on the exhaust. Don't miss any gears and overrev it

427LS7HCI

.006 off = 63cc for 10.6:1 compression
.0012 off = 62cc for 10.7
.0018 off = 61cc for 10.8
.0024 off = 60cc for 10.9
.0030 off = 59cc for 11.0
.0036 off = 58cc for 11.1

01ssreda4

iTrader: (96)

Understood. A lot less likely of missing a gear with an auto, a lot less likely to have problems with good valve control, and a lot less likely with a conservative limiter in-place. Plus the PTV was spec'd by the heads guys so I trust their judgement.

bww3588

iTrader: (49)

your numbers are a little off according to my calculations. .0065 removed/cc is a good rule of thumb.

stock = 64.5 = 10.7
-.0065 = 63.5 = 10.8
-.013 = 62.5 = 10.9
etc...

i would remove .0065 from the head and go with a .040 gasket which will get you right at 11.1:1 with .033 quench area. the increased quench area will increase power and throttle response more than the same compression with a thicker gasket.

so with the stock gasket and .027 removed to get 11.1:1 wouldnt be as good a setup as less removed and a thinner gasket. plus it will leave more on the deck for further milling if needed because of some sort of failure.

bww3588

iTrader: (49)

.070 and .090 is not a bad PTV. obviously, you have done your homework and know what is safe. if it was in front of a 6 speed car, I would say you would probably be pushing your luck, but I agree being in front of an auto with a good limiter, your safe. you should only have to worry about a converter flare to spike the revs.

01ssreda4

iTrader: (96)

Actually, those numbers aren't mine. The initial PTV measurement was done by me during a previous install and the milling was left to A.I. They made the suggestion and I said ok based on their judgment. They also milled the intake flange to accommodate.

427LS7HCI

Stock 799/243 is 10.5:1 Comp ratio

I just had stock 799/243 milled to 63cc which puts my compression at 10.6 with the stock GM MLS head gasket.


I agree, quench is more important than mill to reduce hot spots and prevent detonation.

poltergeist 02

iTrader: (65)

alot of mixed answers here? would like to be certain before going though with this.

c5z16z

good thread,

same heads 243s on my ls6

a guy from work told me to mill maybe .0065 and use a .040 cometic gasket for a tight quench,,

but i dnt know if i will have P to V issues cause of my cam

a comp XE-R 236 238 .601 .605 112+4 LSA ,,

i also want to make sure before i do this with my heads i just dont want to flycut pistons



what cam is ( poltergeist 02 ) using with the milled heads?

bww3588

iTrader: (49)

427...10.6 is right with stock gaskets. If he were to take 1 cc off the chamber (.0065) milled and run a .012 thinner gasket it will get him right at 11:1 with better quench area.

c5z16z

do you think my cam will clear my pistons clearances doing the same .0065 and .040 cometics

you seem like you know what you are talking about thanks sir..

junior,

bww3588

iTrader: (49)

It looks like it would be very very close. I would mock it up and clay it before any milling to get a starting point. Mock it up without gaskets (carefully), set your valves to zero lash and roll the engine over by hand 2 full revolutions. Then pull everything apart and measure the thinnest part of the clay. You need at least .080 on the intake and .100 exhaust. Take your measured numbers and add your crushed gasket thickness (.040) and then subtract .080 and .100 respectively. That will give you how much you can safely mill off the heads.

I personally hate giving a definate yes or no to piston to valve clearance questions as a lot more than just lift and duration play into the final product.

poltergeist 02

iTrader: (65)

so i would need to cut .016-.017 off the heads to get me to 10.8:1 or 11.0:1?

bww3588

iTrader: (49)

.020 should get you 11:1

bww3588

iTrader: (49)

But again, I would cut .007 off and run a thinner gasket. You will see better results from the same compression increase.

427LS7HCI

My old 243s (64.4cc), stock valves with a 234/238 .598/.605 111+2lsa (109 icl) and a .040 cometic gasket didn't clear with a flycut being required, so I ran stock GM MLS gaskets. Your cam is a tad larger (236/238) on the intake side with a 108 icl (112+4), with a slight mill (.0065) and .040 gaskets, It's not going to clear without flycutting.

With a 234/238 .598/.605 111LSA (109 ICL) cam with 243 heads (stock 2.00/1.55 valves) and stock GM MLS head gaskets, I would have .064-.069" P to V on the intake side at 62cc.