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Negative overlap? Cometic and ARP ?'s

VB242

VB242

I.M. Weasel
Joined
Sep 21, 2011
Location
Driving the No Malarkey Bus
So I was talking to my machinist about why my exhaust manifold gaskets burnt up. He was claiming the overlap in the cam with a blown head gasket is what caused them to burn out. I think I have the ipd turbo cam, it was free.... I thought negative overlap numbers meant the intake and exhaust valves were never open at the same time, he contends the engine wouldn't run without some overlap. I plan on spinning the cam around once I get it back to see. I don't want to argue with him since he takes care of my tuning mistakes. Also he's claiming I can reuse my cometic 2.6 mm even though it was leaking enough to pressurize the cooling system. I'd hate to bolt everything back together and have it fail.l, opinions or experience? Some of the black coating is off where it blew through near the fire rings. Also can I use moly assembly lube instead of the ARP lube when I slap this thing back together?
 
VB242 said:
So I was talking to my machinist about why my exhaust manifold gaskets burnt up. He was claiming the overlap in the cam with a blown head gasket is what caused them to burn out.
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Huh? I can't imagine how a head gasket leak would take out the exhaust gaskets. What brand exhaust gaskets are you using? Volvo and Elring look nice. Felpro is worse than re-using the old ones.

The turbo cams have some overlap when measuring just as the valves start to move. If measured at the common US 0.050" lift point, the overlap might be negative depending on the cam.
 
I'd agree, blown HG shouldn't kill exhaust gaskets. I'd guess loose nuts on the manifold to be a bigger issue there.
Moly lube on the ARP should be fine. Seems like ARP had 3 different torque specs, dry, engine oil, and moly lube.
I've heard of people reusing the Cometic's, though when I spoke with them, they said no, they were not intended to be reused. If you did reuse, I'd put some hylomar on them to make up for the coating that's peeled off.
 
Cheap cardboard exhaust manifold gaskets have been known to burn out. Spend the time to locate the better quality gaskets.
 
They are the metal jacketed gaskets that come in the elring head set from FCP so I assume they are elring gaskets. I've done the hylomar before, so just where the black stuff is missing or the whole gasket? I ordered ARP ultra lube from Jegs. Interesting on the cam specs, I'm going to watch as I spin the camshaft.
 
To understand a bit more about your cam overlap. The IPD cam has 110 degrees overlap or lobe angle separation. The V15 n/a cam has 109 degrees lobe separation. The V15 turbo cam uses 112 degrees. The IPD cam is leaning more to the n/a side of things but is still a low over lap cam so it is good to use with a turbo.

We used to use the copper spray gasket maker on head gaskets. But I would go by the advice above and probably just use the Elring head gasket you got.

Always use new locking nuts on the exhaust manifold studs. The flanged copper plated ones.
Also are all the studs in good condition? They can pull out of the head since the threads are in aluminum.
 
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tryingbe said:
Is your engine block perfectly flat?
Is your cylinder head perfectly flat?

If the answer is "don't know", use a stock head gasket.

A surface finish of 50 RA is what you need for the MLS head gasket to work.
https://www.cometic.com/ft-2800-technical-support.html
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Yes the block has been decked and the head just had a couple thousandths off, not a concern. I see you're missing the part where my cometic is 2.6mm thick to make up for previous head cuttings. So no the elring won't work for me. All of the exhaust manifold nuts were tight, I don't have the fat turbo manifold washers, I'm not sure they would make a difference. I was running fairly high boost 22-24 psi perhaps the back pressure blew out the gaskets. This isn't my first head gasket rodeo, I think I have some hylomar somewhere in storage I'll try to dig that expensive ish up.
 
gsellstr said:
Napa has it in spray form I think.
strange on the exhaust gaskets, unless your manifold isn't flat.
.100" head gasket? Damn son!
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I had the manifold flattened a couple head gaskets ago, I'm going to cut the supports between the runners per machinist recommendation, even though it's cast he said they might give enough to stay sealed
 
I would get the thick manfold wathers. They are supposed to be there to spread and distribute the torque on the manifold. They even have a litttle 'ut' stamped on them so you put them on the correct way.
 
VB242 said:
my cometic is 2.6mm thick to make up for previous head cuttings.
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What's your piston protrusion? Can you use a thinner head gasket for "tight squish" (look it up here if unsure), and get an adjustable cam gear to adjust for shaved head?

VB242 said:
I don't have the fat turbo manifold washers
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Oh, I'd assume those are important to allow a bit of give between the different head and manifold metals. If you look at the manifold carefully, you'll notice that only one of the holes is "tight" and the rest have some slop for side-to-side stud movement. Maybe the engine builders here can comment?
 
The cam card usually specifies valve events at .050" lift and at something close to .006" lift. Armed with that and a little subtraction, you'll know what's going on overlap-wise. As bobxyz mentioned above, not uncommon on cams set up for boost to see no overlap at .050" -- but I've yet to see a cam that didn't have some overlap at .006".
 
bobxyz said:
What's your piston protrusion? Can you use a thinner head gasket for "tight squish" (look it up here if unsure), and get an adjustable cam gear to adjust for shaved head?


Oh, I'd assume those are important to allow a bit of give between the different head and manifold metals. If you look at the manifold carefully, you'll notice that only one of the holes is "tight" and the rest have some slop for side-to-side stud movement. Maybe the engine builders here can comment?
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I've been down the tight squish road and like 8.5-1 cr better. They are even with the block according to machinist I measured 2 thou out but I'm not a professional. Maybe hiperfauto can hook me up with the correct washers, I owe him some business.
 
lummert said:
I was going to suggest cooper spray.
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No.

Why does Cometic recommend MLS gaskets to be installed dry?

Cometic Multi-Layer Steel (MLS) head gaskets go on dry because they are coated with a sealant. Each MLS head gasket is coated with a .001" thick Viton rubber that is bonded to the outer stainless steel layers. Adding an additional sealer can hinder the performance of an MLS head gasket.

https://www.cometic.com/ft-2800-technical-support.html#q31025
 
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