blown 944

It's funny to me that the cometic is being recommended when there may be a tuning problem going on.

evil 944t

+1

rberry951

The gentleman he purchased the car from has had my chip in it since 2006. I believe if there were a tuning issue there would have been a failure before now.

jmj951: What boost pressure are you running at the track?

Regards,
Russell

evil 944t

Russell, I'm not blaming your chips rather than there could be something throwing the a/f's off or the boost was raised or something..

My point is that these HG's don't blow because they are faulty or weak.

rberry951

I agree, I emailed him this morning asking what boost pressure he is running, I'm wondering if the old stock wastegate is causing boost spikes and/or such erratic boost pressure changes that it is throwing the AFR way off. Also, these chips are not optimized for 100 octane fuel. The lack of timing advance could be causing the EGTs to go through the roof, hence contributing to the head gasket failure.

Regards,
Russell

TurboTommy

"Also, these chips are not optimized for 100 octane fuel. The lack of timing advance could be causing the EGTs to go through the roof, hence contributing to the head gasket failure."

Russell
What do you mean by that; I don't quite understand why higher fuel octane has anything to do with it.

jmj951

I don't have a real boost gauge, just the factory gauge. It never sees 2 Bar. The only modification that could affect boost is the Lindsey Boost Enhancer. As I understand it, if it is left at its default setting, it should not increase total boost, just let it come on earlier and improve acceleration. I was also a little concerned that I've been running too lean all this time because my NB AFR gauge doesn't even come up to lean until I'm on the gas a little, and doesn't hit the center/stoich until about 2k RPM's. I have resolved all of the vacuum problems and I have excellent vacuum, but I haven't checked for any blink codes (still need to make my tester) or considered what else may need to be sealed up.

See this thread:
https://rennlist.com/forums/944-turbo-and-turbo-s-forum/455265-new-non-leaky-cometic-gaskets.html

I had read this a while back so I knew to not even try the Cometic yet. Maybe the problems are resolved now, but I prefer not to be in the trial group and probably shouldn't be anyway since I'm not getting the block or head machined (at least not *this time*).

I have ordered a stock head gasket set, new water pump, belts and rollers, the OAS o-rings, a throttle body reseal kit, and I am getting the Lindsey Steam Vent kit. I've seen recommendations about using new head studs (mostly as added protection) and have also gotten feedback saying I can use the original hardware with no problems - I'm still running stock boost, so I'm going to pass on the $300 head stud kit for now. I also really wanted to upgrade the valve springs and install new seals, as that seems like a very smart thing to do right now, but it adds a lot of cost to this project and I just can't absorb that much all at once. I'm also going to delay the new wiring harnesses and WB O2 sensor. I am really interested in the detail on the WB but it's not in the cards for now. Coolant hoses and vacuum lines were all done earlier this year, so they should be in great shape.

Thanks to all that have replied, I will post pics after the teardown this weekend, and I'll probably have a few questions as I go through this.

blown 944

If you do not want to get a WB on there right away you may want to take it to a dyno and get an accurate reading.

Make sure they know that the AFR is the main concern so the O2 sensor being used is calibrated correctly.

Bri Bro

+1 . The WB is needed to insure all the money spent works. So many things can cause an incorrect AFR that is good to keep a constant eye on it. These engines will run a long time if the AFR and boost isn't abused.

BTW: EGT monitoring does the same thing but is a little harder to install and monitor. Planes use EGT all the time.

jmj951

Updated front page with a pic.

APKhaos

That looks like a combustion pressure failure. Detonation, overboost, or both.

blown 944

my thoughts

if you look at the next cylinder to the left you will see that the ring is already being pushed outward (bow in composite too), also you will se that the composite material is already degraded just to the right of bad cyl.

My thoughts are (this was the same with my recent failure) that the composite material breaks down over time and just the pressure alone forces the ring past the top of the cylinder with a weakened material this is easier to do.

In my experience detonation will leave a peppered look to the area that was overheating. Mine was like brand new. I can't tell with this one but it doesn't appear to be burned. However t is hard to tell from the photo.

jmj951

Thanks for the feedback, I was personally hoping that the corrosion of the gasket was probably the main culprit. In the area where the fire ring failed, the gasket is corroded very badly in a wide band, almost completely across the bottom section. I just don't see how it can hold together very well with big chunks of it corroding away. I don't see any burn marks or any indication of overheating in one particular area of the head over another. The pistons have a bit of carbon buildup on all of them, and especially the tops of the valves, but the carbon buildup is coming off very easily. The intake valve stems are very clean as they should be. The block and head are cleaning up pretty easily, no stubborn pieces of gasket or carbon.

Willard Bridgham 3

When you replace the head gasket, get new high-strength head bolts.

Stock head bolts are stretch bolts and have failed by definition the first time they are used. A b**ch to get old ones out, but well worth the effort.

jmj951

I thought about this a lot. I had some people saying it was okay to reuse the existing hardware, some recommending the Raceware studs to beef it up. Due to cost and budget limitations and the fact that I need this car ready for my next track day this Thursday (last of the year, probably my last one for quite a while) I decided to skip this $300+ upgrade. I don't have stud extractors either (or have access to them), so that would add even more to the cost. If I ever do any serious upgrades on the engine, or if this head gasket doesn't last and I have to do another one, I suppose I'll go with the Raceware studs and give the new 951 Cometic head gasket a try. I don't think that weak or excessively stretching studs is the real issue here. The gasket is literally falling apart where it faces the open cooling cavities around the cylinders, and it appears to have weakened the gasket beyond the point of being able to put up with track use. I don't know if stronger studs really could have helped there. My new head gasket will get one day of track use - if I can get it back together in time for Thursday - and then just some daily driver use until I get through my upcoming move and things settle down again. It may be about nine months before I hit the track again, but then I'll have access to a shop, a lift, and a lot of specialty tools at my next location, so it will be more practical to start doing some serious work on it then. If this head gasket fails early, I will be bumping this thread with details.