89 928 turbo

We are in the process of building up a 10psi boosted turbo 89' 928 and are planning on reducing the compression to about 9.0-9.2 with the metal gasket. Only problem is that I have heard about failure of these in several boosted (SC) engines allowing water to go in place where we don't want water to go (like the exhaust pipe).

Anyone can give me some feedback on this and any resolution, I would appreciate it.

JT

1989 S-4 (0- miles- new engine)

slate blue

If you're planning to reduce the comp by having a thicker gasket, then I would reconsider. Do a search because Tony reduced the comp by increasing the bowl size in the piston, this is a smart move as it doesn't interfere with the quench.

Herr-Kuhn

Agree with Greg...reducing CR should be done in the piston and not with the gasket. But it does work, just don't go too far. I have heard issues with leaks on that style of gasket, but some say they work really well. For my twin turbo S4 build I will have stock gaskets, no o-rings.

For what you are doing you should install forgings.

BC

The issue with cometic is the smoothness of the mating surfaces. 50 RA or better. They have fixed the bose-issue that may have lead to coolant leaks OUTSIDE the block. NOT combustion chamber related.

Tom. M

This motor is designed for 9.2 to 1 CR...via forged pistons and changes in bore.....the cometic gaskets will be used due to their percieved strengths over stock gaskets. I think JT is worried about what someone posted a while back regarding using the cometics and not having them seal.
So this sealing issue is related to the head and block surface smoothness Brendan?...

Any reason to not use cometics..or any other viable options wrt head gaskets on turbocharged motors?..

later,
Tom
89GT (mit rear turbo)

BC

Tom- Cometic is vastly superior in "trouble areas" heat and conbustion wise to our stock gaskets. I do believe, however, that the idea is to not go too far with the thickness, as that simply gives more surface for the gasket to get p ushed out, if you catch me on that. Th heat wil sften the steel layer, and if enough pressure is there, then it will move. That is under severe conditions.

Cometic DOES have a new product that SHOULD be very special. They will be useing gass filled rings that sit in the original cometic gaskets, and this should be good for people with sealing issues - they liken it in thier marketing to the fact that some ford heads only have four bolts per cylinder. I think it will be good for use with your open decks. I have not called them on these yet, but I am getting close to needing them.

Tom I have done extensive reading on cometic becuase of those sealing issues (coolant to free air) and it seems that it is a block prep situation, as well as some viton issues they had and fixed. Thats all the info I had from hours of reading. They just work for most people.

Tim Murphy

I am running the standard issue Cometic gaskets in my 8.6:1 engine at about 17psi with no issues at 3000 miles.

We did have a problem on a similar built engine and at about 10k miles it would pressurize the coolant system under boost. After removing the gaskets we found signs of carbon build up between the gasket layers. We worked with Cometic and came up with the idea to emboss the stainless center layer of the gasket and they reworked our gaskets at no additional cost. This is the way we are running today with no issues so far.

Cometic may have made the change to the stock gasket configuration and that is the way you would get any new ones, but I would ask to make sure.


Hey, what happened to my sig line??

Herr-Kuhn

I'm of the opinion if you keep detonation out of the picture, a stock gasket is a very good joint. I've considered upgrading, just not sure if it worth the risk of having leaks. I run 24 psig on an Audi S4 with stock gasket. That car has 250,000 miles on the clock and still runs like a freight train. Goldmember is running 10 psig, stock gaskets. Ex-Callaway is running 12 psig, stock gaskets. These are turbo applications, where the boost is hammered from 3K up to redline.

Detonation will kill a gasket in short order. Boost pressure typically won't dislodge a good stock gasket. Regardless, there is still merit to a stronger gasket....but stronger can mean less sealing capacity.

Imo000

At only 10 psi a regular OEM gasket would be more than sufficient. There is another issue worth considering. If the engine start to severaly detoanate it's a good idea to have the head gasket blow as opposed to breaking the ring lands. My oppinion is that ,unless you are running high boost, keep the OEM style HG as a safetly measure, sort of like a "fuse".

BC

While the stock headgasket is still very usefull, the issue remains that under boost, the towers do move. When you have a "better" headgasket, it will let it stay where it needs to be. When the head lifts (if it does), its not always because of det.

hacker-pschorr

I'm assuming a first gen S4 since the current units are running a Cometic MLS style gasket from the factory, so are the 1.8 turbo engines.

FlyingDog

I think it was Dave Lomas who posted a detailed explanation of what was wrong with the Cometic gaskets. The guys at Motorsport seem to install a fair number and he said the problems had been fixed. You could probably search posts by Ketchmi.

Tim, you are now a "registered user". Your membership probably expired. According to the login system, my membership doesn't exist even though I'm paid through march.

Imo000

I agree, however there shouldn't be any head lifitng at only 10 psi. I'm just saying that for a 10 psi system OEM gaskets are more than enough.

928SS

I'm running cometics RMR .043" w/11.3:1 comp and they leaked around the outside of the block/head joint for about 2 hrs. on initial startup... they quickly swelled/sealed up, and never had any internal leaks... after 1800 fun miles still dry as a bone.

iirc, it was a common issue w/some motors - but they all quickly stopped leaking and are fine. rumored to be related to smoothness/head finishing issue, but never confirmed... did scare the crap out of me though...

CMW

Mike and I plan to install a Cometic head gasket in the GTS... for the untapped extra boost