irun4cops

running 20psi with a f1r and a 3 inch pulley on my forged lunati 383 ls6.

Had 10:1 compression on my old setup which included stock ls1 heads...

Now im going with etp 245cc heads that have a 72cc chamber, about 5 i think? cc bigger than my stock heads. I forget, its been a while since i looked at the numbers... but my question is this...

Which is the best way to go... a thick or a thin head gasket from cometicx. I think they make them from 15 all the way up to 120 thickness.

I was thinking about going with a thicker gasket to reduce my compression, but am i shooting myself in the foot... i would assume that a thicker gasket has a much higher chance of blowing out under large amounts of power and boost. Also, if I do go with a thicker gasket, do i tighten things down a bit more than usuall?

let me know, im leaning towards a thicker gasket, but would like to hear from people who have tried both ways. If i go with the thicker, i can lower my compression to about 9:1

elias_799

iTrader: (8)

for 20 psi i would personally want something like 8.5:1 compression ratio and nothing over 9.0:1. but that's just me, i like to be on a safe side when i spend money.

good luck

irun4cops

so you think a very thick gasket, like say 80+ would be okay with that much pressure, and not bust or leak?

Ryne @ CMS

iTrader: (10)

with a centrifigal blower with that much boost i would run around 9.5:1 compression and i would try reach that compression using a standard thickness gasket (like between .040"-.060" thick) and utilizing the proper piston and chamber dimension.

Avengeance

iTrader: (4)

IMO I would use as thin as possible.... and use APR studs in the process.

That kind of boost I would definitely stud it no matter what gasket you use.

ls1290

iTrader: (20)

Hammer applied to nail head correctly

irun4cops

yeah, i have arp head studs. Any special advice on how much to tighten them down? What about any special advice on using any kind of sealant or anything to prep the block for the head gasket? What can you use to clean the block face, just a razor blade? Ive heard not to use a wire brush like one on a wizzy wheel because it can distort the true flatness of the block? All advice welcome...

If i do go with a 30-60 thickness, i will be in the 9.5:1 range

irun4cops

Your build is similar to mine... what pulley size are you running on your blower? How much boost are you making? What size are your heads? Ls2 motor i take it? I still just have an ls6 Ill stay with this motor until my next car. Still undecided if im going to pick up another vette... or get a ford GT and try to make something around the 1200hp range in that car...

Avengeance

iTrader: (4)

Just follow the torque sequence from GM.. but use the torque specs given by ARP.

I work at a Ford service dept, I know I know.. has good job security though, and we regularly stud the Powerstrokes after they blow head gaskets. Customers ante up the difference for the ARPs and they get something like an additional 120lbft put on the studs vs stock bolts.

For cleaning the heads... brake clean/carb clean and when I did my heads I used a plastic razor blade. Less chance of cutting in, takes more time but Id rather do it right and not F up the surface.

DeltaT

iTrader: (5)

And the block needs a fine finish as required by Cometic to seal correctly. Check their website for a description. If you are using their MLS gaskets the thinnest they will go is .027" - I just bought a set and asked them.

Which pistons are you using? Do they have a quench pad (more of a D-shaped dish) or are they a round dish or flattop?

Jim

irun4cops

they are not a D, they are a circle dish all the way around the piston with about a 1/4 inch that starts out flat and flush with the deck, and then it goes down in.

DeltaT

iTrader: (5)

OK, so you want to get the raised ring portion (the quench ring) piston-to-head clearance set around .040". I don't know your deck height or installed piston height but you can use a piece of modeling clay on the ring section of the piston (do one on both sides - 1 on the lifter valley side and 1 on the exhaust side of the piston), install the head with no gasket and turn the motor over slowly by hand. Remove the head and measure the now flattened clay ***** for thickness. If they are different, take the average height of them both. Now select a gasket thickness that, when added to the average clay thickness will hit around .040" and you should be good to go.

I have a d-cup piston that matches my heads, but here's a picture (shown with the FelPro gakset I use for mockup, not the expensive Cometic) that shows the clay after slicing it in half with an Xacto knife:



BTW, putting WD40 on the clay before using makes it more accurate and cleans up easily. Put some WD40 on the knife blade before you cut, as it will distort the clay without it.

Jim

irun4cops

i understand what you are saying about the .40 thickness. Im not sure on this current setup that I will be able to get to that however.

Before I was running the .51 mls gasket, and had the 68cc ls1 heads. Ive now gone up to the 72cc etp heads, and have decided to stay with the .51 gaskets. So.. thats going to lower my compression from 10:1 down to 9.6:1 or somethign in that area. Anyway, needless to say, if im running a .51 gasket, im AT LEAST .51 away from the cylinder head when the piston gets to the top. So that being the case, i dont think that this motor setup that i have right now can be optimised to the extent that you are describing. Even if i did go with a thinner gasket, id still be shy of the .40 clearance I would assume. Also, i think the pistons that I have are probably not the greatest either, for they are not flush like yours are on half the face... mine are dished the entire circle of the piston, and have a 1/4 inch ring around the outside of them that is "flush" or close to level with my deck height. The rest of the circle is dished down in, so .40 clearance is more like 1/4th inch clearance

I do enjoy talking about all this stuff tho, next time i build a motor, ill be an expert

DeltaT

iTrader: (5)

.040" is a target for quench, not an absolute. Common thinking is that once you get above about .060", you have no quench benefits anymore. On mine I got it to .048" which was fine without going to a thin copper gasket and needing to o-ring the block.

Depending on your heads, even with those pistons you can still get some amount of quench benefits, so don't give up on it. It can really help keep detonation down.

Jim