MP or Cometic Head Gasket Size?

[Miss89baron]

I was curious looking at the FWD head gasket sizes. The MP gaskets say stock thickness is .040-.042. Then cometics have three sizes .051, .065, .074. Why does the .051 say the thickness is slightly less than stock, and .065 is equal to stock if the stock MP gaskets are .042. May be a dumb questions.

[speeduphoria]

I believe the stock gasket is .065" then .040" compressed, so thats your answer

[Miss89baron]

Quote:

Originally Posted by speeduphoria I believe the stock gasket is .065" then .040" compressed, so thats your answer

So the MP gaskets are listed as compressed thickness, .040, and the cometic are listed as non-compressed thickness? What determines which cometic thickness you want?

[Screamin4banger]

Quote:

Originally Posted by Miss89baron What determines which cometic thickness you want?

Your overall engine blueprint. IE: if you shave your head and use a stock HG, the CR will go up, Use a thicker HG to compensate.
N/A builds will respond to thinner HG to raise CR.
Remember, geometrically, the hole in the headgasket is a cylinder (no matter how shallow) and its volume will need to be factored.

Cam degreeing should always be on your mind, especially when it comes to decked blocks, shaved heads, or non-standard headgasket volumes.

[Miss89baron]

Quote:

Originally Posted by Screamin4banger Your overall engine blueprint. IE: if you shave your head and use a stock HG, the CR will go up, Use a thicker HG to compensate. N/A builds will respond to thinner HG to raise CR. Remember, geometrically, the hole in the headgasket is a cylinder (no matter how shallow) and its volume will need to be factored. Cam degreeing should always be on your mind, especially when it comes to decked blocks, shaved heads, or non-standard headgasket volumes.

I was going to run the .051 cometic listed slightly less than stick. I'm installing the Venolia forged pistons. Figured with the 3 cm dish, I'd be OK with a tiny bit more CR. With a hybrid 2 stage turbo and no deck or shaved block, do you think the .051 will work fine?

[Screamin4banger]

Quote:

Originally Posted by Miss89baron I was going to run the .051 cometic listed slightly less than stick. I'm installing the Venolia forged pistons. Figured with the 3 cm dish, I'd be OK with a tiny bit more CR. With a hybrid 2 stage turbo and no deck or shaved block, do you think the .051 will work fine?

I can't answer that and here's why:
Unless you've had that engine since day one, and can attest that its never had a head gasket failure or replacement, you can't know if the head or block has been shaved.
Is the head off right now? have you checked the head and deck for straightness with a straight-edge and feeler-gauge? Are they truly flat? even if they are, keep in mind that Cometics aren't stock-type Headgaskets. As I understand it, the block and head need to be finished to a different RA than stock. -So there's some machine work there.

And, again, as the distance between the cam and crank change, cam timing will be affected.

BTW, I'm not at all familiar with your build-up. if this is a blueprinted build-up, you will be CC'ing your chambers, factoring the cylinder volume at TDC, and adding that to the volume you come up for the headgasket. At that point, you will have answered your own question.
Knowledge is power.

[speeduphoria]

for pump gas HP your best with lower CR and more boost. Then you'll have less off boost performance so its a trade off.

So that being said you should make a plan of how much boost and what kind of gas your going to run and then decide

Oh and in some other publications the stock head gasket is listed as .065"

[Miss89baron]

Quote:

Originally Posted by Screamin4banger I can't answer that and here's why: Unless you've had that engine since day one, and can attest that its never had a head gasket failure or replacement, you can't know if the head or block has been shaved. Is the head off right now? have you checked the head and deck for straightness with a straight-edge and feeler-gauge? Are they truly flat? even if they are, keep in mind that Cometics aren't stock-type Headgaskets. As I understand it, the block and head need to be finished to a different RA than stock. -So there's some machine work there. And, again, as the distance between the cam and crank change, cam timing will be affected. BTW, I'm not at all familiar with your build-up. if this is a blueprinted build-up, you will be CC'ing your chambers, factoring the cylinder volume at TDC, and adding that to the volume you come up for the headgasket. At that point, you will have answered your own question. Knowledge is power.

The 782 head is off getting ported. The head was off once on this engine for a new turbo and it was totally stock with 47,000 miles. No shaving just cleaning with MP gasket back on. Everything checked out straight and flat. What is RA and machine work required? I have a cam sprocket adjuster so I can adjust timing as needed.

[Screamin4banger]

RA = Roughness Average

Here is some info I stole from some site on the interwebz

Quote:

Surface finish is also very important too. For many years, most aftermarket gasket manufacturers said a surface finish of 55 to 110 microinches RA (roughness average), or 60 to 125 RMS (root mean square) is acceptable for conventional gaskets. The preferred range has traditionally been 80 to 100 RA. More recently, though, some gasket manufacturers have changed their recommendations because today's engines are lighter, and castings are thinner and less rigid. The latest recommendations for nonasbestos and graphite gaskets is a surface finish of 30 to 110 RA for cast iron head and block combinations, with a preferred range of 60 to 100 RA and 30 to 60 RA for aluminum heads on cast iron blocks with a preferred range of 50 to 60 RA. For MLS gaskets, a surface finish of 30 RA or smoother is usually required.

[Miss89baron]

Quote:

Originally Posted by speeduphoria for pump gas HP your best with lower CR and more boost. Then you'll have less off boost performance so its a trade off. So that being said you should make a plan of how much boost and what kind of gas your going to run and then decide Oh and in some other publications the stock head gasket is listed as .065"

I will be running around 22 psi boost. I typically pump 1/3 100 octane Sunoco race and 2/3 91 octane pump gas. Theoretically I should be around 94 octane is what I have been running. Wouldn't mind going straight pump 91 octane. Methanol injection is a future possibility to help with cooling the intake.

If stock is .065 and I run .051, that should retard my timing almost 2 degrees. Should I advance my cam time 2 degrees to compensate, leave it alone, or return the .051 and get a .065? The CR would be a little higher, not sure if that's a good thing on a turbo all ported out or not.

[moparfwdsleeper]

Cometic .74 FTW

[Screamin4banger]

Quote:

Originally Posted by Miss89baron The 782 head is off getting ported.

Are you having any chamber work done?
Un-shrouding the valves will add volume and decrease CR (all other things being equal).

[The Pope]

the stock head gasket has been a called a 40 for years and the MP gasket is said to be 20 thicker at 60. MP can make a stronger gasket at 60 than at 40 and compress it more. Most all of us with ported heads or people running G casting can run a stock thickness gasket and the static compression is lower than with an MP.

Myself I have ported combustion chambers and I'm running a .051 Cometic and it'll still be lower compression than stock. On the turbo cars it doesn't matter than much. MP gasket and a G head? Better get some more timing on the cal.

[Miss89baron]

Quote:

Originally Posted by The Pope the stock head gasket has been a called a 40 for years and the MP gasket is said to be 20 thicker at 60. MP can make a stronger gasket at 60 than at 40 and compress it more. Most all of us with ported heads or people running G casting can run a stock thickness gasket and the static compression is lower than with an MP. Myself I have ported combustion chambers and I'm running a .051 Cometic and it'll still be lower compression than stock. On the turbo cars it doesn't matter than much. MP gasket and a G head? Better get some more timing on the cal.

So your saying a 782 2.5 CB non-milled surface would still be lower CR than stock or MP with a .051 cometic. So proly not that important or performance difference going to .065 with turbo. My base timing is now 15 and cam is center.

[The Pope]

Quote:

Originally Posted by Miss89baron So your saying a 782 2.5 CB non-milled surface would still be lower CR than stock or MP with a .051 cometic. So proly not that important or performance difference going to .065 with turbo. My base timing is now 15 and cam is center.

The .051 is half way between stock and MP gasket. I would reduce base timing to 12-14 and advance the cam 2-4 degrees.