ptuomov

I spent a couple of hourse reading head gasket threads in the 951 etc. forums. I asked them some questions and would like to ask those same questions here.

What head gasket to use for a high-boost 928 engine? Type and brand recommendations?

My plan at this point is not to o-ring the head or cylinder tower. Also, at an extra expense but without a huge inconvenience, I can have the head and the block surfaced to very high degree of straighteness / squareness and to a low average roughness. So I could use MLS gasket if there's a benefit.

Many thanks for all the answers that you've provided on my previous questions!

hacker-pschorr

Define high boost.

Todd is using a Cometic MLS gasket (30psi).
Most Murf cars are using the stock head gasket.
The stock gasket in my 81 is holding up just find to 17psi. I replaced the gaskets before boosting. Good thing I did, they were not in good shape.

Keep the motor from detonating and the head gasket should be fine.

ptuomov

Thanks for your response. It's very valuable to know that the 30-psi-guy who builds these engines for living is using a Cometic MLS gasket. This suggests to me that the best (not cheapest) option is to use Cometic MLS gasket, surface both block and the head to something like 25 RA, and then use undercut head studs.

Instead of high boost, maybe a better way to quote it would be 160 max hp / liter and 125 max lbf-ft / liter? In any case, I think you provided me the info that I needed. This was very valuable info, thanks.

BC

tuomo -

Focus on that block and head surface and you should be fine. Cometics are working very well for Todd.

But no gasket will hold up to detonation.

123quattro

For a cometic gasket you want nearly a mirror finish on the sealing surfaces. Or so says the engine shop I use that builds fancy small block engines for GM.

ptuomov

I can get them better than 25 RA and "damn straight," but it costs a bit more. I'll eat noodles for a week or so, that'll pad the budget.

blown 87

Cometic told me 50 RA was the spec.

I will let you know how it works.


And you guys running 17 and 30 PSI

mark kibort

I had my shop go a little beyond the 50RA spec and its seems to be working well. Only 370rwhp and 220psi static compression, but already 14 race days!

mk

ptuomov

How much beyond?

I am taking my que from the Japanese production cars with MLS gaskets that can pump out high three figures of hp from an engine half the size of 928. The spec for those stock gaskets is much lower roughness than 50RA. That's where my 25 RA guesstimate came.

blown 87

50 RA is pretty slick.

928autobahndreamer

800HP

Looking forward to hear more about that. Please share more details on your build.

toofast928

My head gasket are stock 928. Holds up well to 8 psi. No oil leaks, coolant leaks. I didn't re-torque but did replace the head studs/bolts. I don't think it's practical to take the 928 block over 10 psi (and 10 is high) Too much blow-by gets past the rings and the crankcase begins to pressurize, exaggerating the oil control issue. Also fueling becomes an issue with stock injectors.
Tony

blown 87

I would think so.

ptuomov

What I am interested in may not be practical, I am fine with that as long as it's practicable! ;-)

One of the advantages of MLS gaskets is that they don't collapse or deform much over time, which means that the initial torque doesn't slip away. Or so I've been told.

The blowby is an issue. I am trying to combat it with tight piston to cylinder wall clearances. I've got forged 4032 aluminum pistons coming and their tops will be thermal barrier coated. Skirts will get a moly coating. This combined with nikasil coated aluminum cylinders should allow for very tight piston to cylinder wall clearances, hot or cold.

Another thing that I have planned is a electric crank-case vacuum pump that supposedly can handle some oil mist. Invested about USD 50 in a control harness that will only operate the pump when there's boost above a threshold. Otherwise the vacuum pump that I got from ebay would burn in a couple thousand miles, I've been told. Get the harness here http://www.racetronix.com/product/RX-CEPH/RX-CEPH.html Photo of the pump and the control harness attached.

That's the plan anyway, so far it's only working for me in theory -- I don't even have the pistons from the Australian manufacturer yet. Let me remind you that I don't know what I am talking about and haven't actually done any of this, so so far this is for your comedy value alone.

ptuomov

Me too. :-)

So far, I've actually accomplished close to 318hp, thus so far my posts have only provided some unintentional comedy for the list. If I get something actually built and working at some point, I'll be sure to post!