RE: woof's Porsche 930 Turbo

[demolitiondan]

We have done our homework so my postings were 100% worhtwhile. I knew I'd heard about the head studs somehwere but couldn't remember where - just goes to show that I don't listen (properly). Question is did Chris N hear it from JZ, a self-fulfilling prophecy? Was the stud cracked before JZ tried to re-torque it? Actually I was very impressed with JZ and particularly Dave who dealt with me and I believe they made a very fair and professional assessment of my car.

I am not on this forum to spout off, I'm here because i want to learn and by the look of things I'm going to have to do that fast. I don't want to seem arrogant either, if there's surprises due with my new chariot I want to know about 'em. I've been picking brains all over the place which is why I couldn't remember where I'd heard about the head stud issue - I should probably have done my research better. (with regards to witty retorts)

I'm still determined to do the track day, I may only own this car for six months and as I intend to sell it in better condition than I bought it this may be my only opportunity to use it properly. There are people looking forward to a few laps this weekend and it is not my intention to disappoint even if it costs me a few hundred quid more in repairs. Incidentally if you were to turn up at Cadwell and find me you would be more than welcome to jump in for a few laps.

>> Edited by demolitiondan on Thursday 16th October 23:52

[chris_n]

demolitiondan said:

Question is did Chris N hear it from JZ,

No, though I do use them and share your high regard for them. It came from my addictive reading of any bulletin board, book or car mag covering Porsches! Plus a bit of personal experience. I had a strange noise on my 930 on full boost that sounded like a cross between a whistle and a wet fart! Broken studs were suspected but turned out I just had a couple that needed torquing down so I got off lightly. IIRC it's all to do with the different rates of expansion between the steel studs and the ally barrels combined with the high cylinder pressures on the turbo.

One of the reasons I trust JZ is that I took the car in to them having prepared myself for the worst and told them I thought I might have a broken stud or studs and they rang me and told me they only needed torquing down. A less reputable bunch could have taken advantage.

Anyway, good luck with the car and keep us posted on progress.

Chris

[DemolitionDan]

Iguana,

I've had a pop at the calculation and of course you were closer to the mark than I was!

Very roughly the calculation is:

Torque=304 lb/ft Stroke=74.4mm(2.93") Bore=97mm (3.82")

Force at piston = (12/(2.93/2))*304=2490lb
Area of piston = PI*(3.82/2)sq=11.46sq.in.
Cylinder pressure = 2490/11.46=218psi

This of course ignores the fact that there is a second piston contributing to the torque only 60 degrees after the first one but also ignores the fact that the piston is driving the con-rod at an angle and the fact that some of the work being done by the firing pistons is being used to induct air, exhaust gases and compress the mixture in the other cylinders.

So of course my calculation doesn't give you an exact figure of what the cylinder pressure is, it only tells you what order of magnitude the pressure is likely to be. To work out exact pressure it would be better to base your calculation on chemistry rather than physics.

[Clubsport]

Dan,this is a good place to pick tips up on owning 911.
I have been fortunate enough to own various examples for coming up to 10 years.The thing is we get the tips from the specialists and technicians we meet & use.

Everytime i visit my technician he has at least 5 interesting non standard Porker to be fettled..One of the best things you can do is get on good terms with a specialist who will help you out at a reasonable cost.

I visited my techie guru on monday after something was not quite right with my car on Sunday,,,all much better,loads of advice couldn't get him off the phone all for the cost of a round of drinks.