ptuomov

Hi All --

I am a noob from the 928 forum. I am planning a 928 turbo project. I have a question about head gaskets, and since the hp/liter on your side of the RL is decisively higher than at our side, you have more experience with head gaskets.

So the question is, what's the best head gasket to use if one plans to produce high hp/liter using high boost and high peak firing pressures? I am interested both in style and in brand that you would recommend.

Here are some background info on the project:
- open deck and stock aluminium cylinder towers
- about 8.5:1 static compression
- pent roof 4v head
- piston and head combine to a compact combustion chamber
- 20+ psi boost
- air-to-air intercooler
- 7300 redline
- hp/liter in the ranges of those 400hp+ 951s that I see here
- max torque electronically limited to 600 lbf-ft using a boost controller

I figured that with the experience and expertise here, why spend time trying to reinvent the wheel.

Any recommendations for
- gasket type
- gasket brand
- web links or other reading on the topic
would be greatly appreciated.

Thanks in advance!

Lorax

Well, I have had good experience with both the cometic MLS and the widefire.

MLS can have the tendency to leak if your surfaces aren't perfect or if installation isn't done properly.

I have run 21+psi on mine with the widefire currently, but I think good tuning has more to do with the life of my headgasket than anything.

xsboost90

i run a stock turbo gasket - also good to use nice new stock studs - 21psi so far. True- tuning has more to do w/ the gaskets than the pressure in most cases.

ptuomov

Lorax, xsboost90 --

I have the opportunity to finish the heads to high smoothness (as low as 25 RA), but it'll cost more. If the MLS gaskets make a difference, I am willing to pay to get the surfaces smooth enough to accept MLS gaskets. If not, then I'll probably go with the cheaper standard surface smoothness / roughness of about 50 RA.

More noob questions: At what point do people switch from stock gaskets to MLS gaskets? Do they fail less frequently than the stock gaskets, given the same boost levels? Do you have an idea why the MLS gaskets have failed when they have?

Many thanks for your responses!

Tom M'Guinn

20+ psi on a 7300rpm 4-valve V8 seems like overkill for 600hp! I make 500+hp at the flywheel at 22psi with my 2-valve 3 liter, with a 6500 rev limit.

At any rate, I use an o-ringed block, widefire gasket and Raceware studs torqued to 85 ft. lbs. (along with machined flat surfaces). Kind of an old-school approach in light of the new MLS gaskets, but it should be more than fine for a 600hp V8.

Rock

I've heard from a very reputable 951 guy that your safest bet is a Widefire headgasket. Its like the stock gasket except it has a little more surface on the rings around the cylinder wall.

He said with proper tuning there is no need for the cometic gasket. Also, as an added safety, he said that he would rather have a widefire gasket blow than have a cometic gasket on. The reason is that in case of detonation or the tuning being off, the widefire will blow as the cometic will hold. That means if there is a problem and the headgasket doesnt give, youre going to be looking at cooking a piston or cylinder walls.

Lorax

Cometic gasket will blow too if you try. Just ask chris white.

I have heard from several people, and I believe our own special tool has actually suggested this as well, that the cometic gasket can actually add rigidity to the block when properly installed. I don't know if that's true or not, just putting it out there. It's worth a shot at least with our open deck blocks.

In any case if you are going to be running high boost there needs to be some kind of safety features - something in your EMS that will pull timing if you AFR goes above a certain level at higher rpm's etc. That is the best way to prevent any problems.

I agree in part with the philosophy that rock is talking about, except I chose the widefire because I am depending on my EMS to prevent a blown head gasket, and I don't want to worry about the cometic leaking.

What are you planning for EMS?

All together HG failure does not seem to be a huge issue with the high boost 951's. Maybe I'm wrong about that, it has just been my observation. Once in my old car I ran out of gas. I had to walk ot a station and I accidentally got 87 instead of 93. I got in the car and boosted to 21psi on the way home... twice. I could audibly hear the knock loud and clear (it sounded like dozens of marbles bouncing in my exhaust) and it still didn't pop the stock head gasket.

Van

Flatness is more important than than surface finish. You'll need half-a-thousandth or better (0.0005"), or a strip of cigarette pack cellophane under your parallel bar, if you plan on running a metal head gasket.

I'm not part of the crazy boost league - but I have a copper head gasket with an o-ringed head in case I ever want more boost.

Good luck with your project - make sure you show us pics!

Chris White

Oh noooo….not the head gasket question again (along with the typical answers!)
Here is what I have learned over the years –
Headgaskets are ‘safety fuses’ is just a dumb idea. I have cracked a cylinder top to bottom on a stock head gasket and I have melted a Cometic without any damage to the block or head. Both were due to some really extreme engine management issues – but the general idea is that head gaskets are not designed as a ‘safety fuse’ and certainly don’t but into one design over another based on that concept.
One poster pointed out that tuning is important, that is right on. I think I could get wet tissue paper to work as a head gasket if the tuning is correct! If the tuning is wrong then nothing will last.
Selection comes down to having the right feature for the right use.
Stock – works well as long as the head and block are flat
Wide Fire – essentially the same as stock
Oring (with wide fire) – makes up for a small amount of warping / out of levelness.
Cometic – head and block MUST be flat and smooth – gasket is reusable as long as it not too old (they do get deposits on them with age that makes it difficult to reuse)

Chris White

BTW - you will hear a lot about Air Fuel ratios….its important, but nowhere near as important as ignition timing. The smallest about of detonation can cause serious damage when on boost and running too much advance will cause detonation every time.
Make sure you have some pretty good control of ignition timing, it’s a lot different under boost that normally aspirated!

ptuomov

Lorax, Chris White, xsboost90: Agreed on AFR and spark advance. The 928 computers have been thoroughly hacked and we have a full control over those parameters in our neck of woods. I plan to tune this relatively conservatively, conservative meaning low knock counts and good cooling. In addition, the '87+ models have knock sensors and automatic cylinder-specific spark retard, so if something like bad fuel happens, one is not automatically phucked.

Tom M'Guinn: I am limiting the torque to 600 lbf-ft, which translates to hp using the formula (rpm/5250)*600. At 5250 rpm, the engine will deliver 600 hp. At if the torque keeps up at 600 lbf-ft, at 7000 rpm the engine delivers 800 hp. This is nothing extreme from 5 liter turbo by your standard, but it's more than 600 hp.

Another consideration. I am not planning to o-ring the head or the block for many reasons, most importantly because I don't want any cracks in the cylinder tower. It is my understanding that MLS gaskets should not be used with o-rings, and that the gasket itself performs a similar function. For this reason, I am leaning toward MLS if I can get the block and head straight and smooth.

V2Rocket

an 800HP 928...



dear god...

Lorax

It's about time for a few of those.

ptuomov

It's just a goal, I am not counting my chicken...

Lorax

Definitely post some results over here whatever you achieve, I have always wondered why their aren't more 928's in the 700-1000hp league.

That would be a monster machine.