ENGINE SEIZURES

[bazhart]

We have recently been spending a lot of time and thought on engine cooling for two quite different projects - the 500bhp 3 litre 944/968 based turbo stage 2 project and the problem of 996 3.6, 997 and Cayman S piston seizures and bore scoring. These involved building engines with temperature sensors and on going road tests etc.

The results will be published soon - but they enabled me to resolve a concern I have had for many years - why some 944's (especially turbos) but mainly 944 S2's (and some 968's) suddenly seize up a cylinder or two after running OK for many years and despite there often not being much else apparently wrong with the engine or car.

We are completely changing the coolant flow inside the big turbo engine (for better cooling) because originally it flowed backwards down the block then up through the head gasket and forwards accross the head to the front outlet. Despite there being many coolant passages apparent from the block to the head - the head gasket shields most of those except those at the rear (where the flow passes up to the head).

Usually when we receive an engine or car with a damaged cylinder or two - the head is off and we don't even get to see the head gasket - but I realised that by now (if they are still original) they are all rotting away and this first creates holes between the block and the head where the coolant channels were originally blocked off by the gasket - creating a "short circuit" and preventing the coolant going all the way to the back before coming forward again - creating a hot spot at the rear being hardly cooled at all. Under heavy acceleration the oil may well flow backwards and help cool the No 4 cylinder but No 2 & 3 will probably suffer most (the most common we see seized).

This will be worse in the 2.7, S2 and 968 block because the coolant depth in the cylinders is less and there is only (not two) coolant inlets to the block.

So when the gaskets rot - the engine will run too hot near the cylinder walls - mainly centre and rear (where they usually go). The sealing of the head may still be OK at this point - so anyone with a car that has never had a head gasket by now - I would strongly recomend they get one fiited as a preventative measure.

Baz

 

[DivineE]

Top stuff Barry. I've been caught twice by 944s that seemed to run fine (except the odd tiny puff of oil) and turned out to have scored bores on the 2nd and or 3rd cylinder. I thought it was the ceramic breaking up in the head and debris getting into the bores.

That always felt questionable as a cause, this diagnosis sounds far more reasonable.

Regards,

Ben

 

[barks944]

Great work Barry, thats a great bit of knowledge to share with the community. We had a chat on the phone about 944's this week and my engine rebuild! What do you think is a safe horsepower limit for the 2.5 turbo block given how its cooled (assuming a good head gasket)?

 

[Suffolk944]

Really useful info many thanks - Mines getting a new HG as part of its fettling this winter and not before time it would appear !

 

[Lowtimer]

Thanks, Barry

Even more glad now that I decided to have a new head gasket in March as part of the anticipatory maintenance work, rather than waiting for it to fail.

 

[bazhart]

Cooling is extremely complicated and far more involved the more you look into it.

Top tuners will adjust mixtures, timing and even compression ratios in some engines to compensate for different running temperatures in different cylinders.

Post 1995 Porsche engines have fed all cylinders and heads with coolant individually and this allows flow to equalise the temperatures (although ironically - there is then a potential over-heating problem - as all 6 are near the operating limit so you have 6 times as much chance of them seizing).

Older Porsche's (like my Carerra GT) have the thermostat on top of the engine thus controlling outlet temperatures whereas these (944 and 968 etc) have it on the inlet - which means the temperature inside the engine is always hotter than the thermostat setting anyway and if it gets hot there is a delay in opening the thermostat because the hotter coolant first goes through the rsdiator before getting back to the thermostat - so is slower to react.

Really the cross flow back and the front is bad because the no 1 cylinder is always coolest and the number 1 head always hottest (as all the hot coolant passes it on its way out).

This limits potential for reliable tuning to probably around 400 to 450 bhp (IMHO).

By splitting it in two and using the two outlets on the 968 head (one front and one back for interior heater) I can half the temperature gradient in the engine and hopefully get more out of it reliably.

I think a std 2.5 should never reach enough bhp to trouble the original system but you will need more oil cooling and preferably a cooler thermostat.

Baz

 

[Frenchy]

Cheers Barry,
I have a cometic HG on my 250+ turbo seems bomb proof ?
I will have a look at receipts for the 220 to ascertain when and if it has been changed the car has been to Hartech a number of times in the past.

 

[Neil Haughey]

Great stuff as usual Baz, very interesting reading and glad I had a new HG and head clean up done on my S2 race car.

 

[sulzeruk]

I took the head of a 1986 turbo that ran well with no smoke or oil loss recently, 1 and 4 cylinders were perfect, 2 and 3 had seen the cheese grater! So if anyone needs a good bottom end for a liner job or repair, let me know,
Alasdair

 

[barks944]

I need the lines that go from the oil cooler to the engine block if you have those!

 

[zcacogp]

>WorriesAboutTheHeadGasketOnHisS2At166kMiles<

[]


Oli.

 

[barks944]

Whats the consensus on Cometic gaskets?

 

[Lowtimer]

I think providing it's a decent make, and you are not going insane with the boost levels, it's the age of the gasket that matters, and whether it was properly fitted, rather than one make or another. I had Barry's folk put a standard one on my car earlier this year, though if I were going for anything over about 350 bhp (which I'm not) I might have made more examination of alternatives.

 

[DivineE]

ORIGINAL: barks944

Whats the consensus on Cometic gaskets?

Touch wood it seems to be pretty good on WUF. Rick had no problems and I have had no problems either running 24psi ish.

I'd always heard horror stories about them leaking water into the cylinders but some guys in the states think that might have had more to do with poor machining on the face of the head or block. They seem to be less forgiving of defects than a squidgable gasket

Regards,

Ben

 

[barks944]

Seems like they rely on a very thin layer of vitron rubber to seal properly. This would sugges that surface finish is very important with the cometic gasket. With a very flat mirror finish surface it will probably seal to very high cylinder pressure and last almost forever!

 

[bertelli_1]

Cometic are very good quality. In fact vauxhall use a silmilar gasket on their recent diesels & say you can reuse it a dozen times!

 

[barks944]

For somone who might be lifting the head relatively often this could offer quite a lot of saving. Standard gaskets are one use right?

 

[MarkK]

My red tuned Turbo has run a Cometic HG for a few years no problems so far..

 

[sawood12]

Cometic gaskets have a reputation for leaking. Alot of people have put this down to warped heads but some people who have machined heads have also had problems. About 18 months ago there was a good thread from the guys at Lindsay Racing who had looked into this with Cometic and learned that the Cometic gaskets for the 944 were actually gaskets for 928's - the assumption being that they are the same. In fact they are not quite identical and there are some very small differences in the positioning of the cooling passages where the coolant passes through the HG. Depending upon the tolerance stack-ups of cylinder head and block cooling passages there could be some misalignment of the hole in the HG lining up with the passages which leads to leaking - which explains why some cars have a problem and some don't even if the owner has skimmed the head. They then worked with Cometic to develop a gasket specifically for 944's. I have not followed this one up so don't know where it got to.

Clearly a multi-layered SS gasket will always be more robust and have more longevity than a normal gasket and I personally think they are a good piece of kit. No sense in putting anything in there that is not the best. In fact reading the HG replacement section of my Ford Focus Haynes Manual I was interested to read that they are fitted with a multi-layered SS gasket - so if it is good enough for a Ford Focus it is good enough for a 944! I am not a subscriber to the view that a HG is some form of 'safety valve' that will hopefully blow before the engine if something goes wrong. This is a bit of a false hope approach - there is no guarantee that the HG will blow if things go pear shaped - better to prevent the engine from blowing in the first place.

Good observation on the cooling issue linking with old HG's. I've always been of the view that it is better to pro-actively replace consumable items like HG's but many others are of the view that you leave them till they need replacement.

 

[Fat Albert]

Good thread, I will have to go back through the receipts for mine as I have a feeling that it is still on the original at 206k and 22 years...