Lifting Head
 

My head seems to be lifting under high (20+ psi) boost. (Coolant pressurizes and pukes up, no mixing, no wet plugs.) New WFHG. Raceware studs torqued and retorqued to spec. Fairly fresh head.

Can I hear from other Raceware stud users -- are you able to run 20+ (on high octane, w/ good a/f) without the head lifting?

Has anyone confirmed a cylinder sinking or shifting? If so, how?

I don’t think there would be lift - the Raceware’s won’t stretch. Did you check the flatness of the block and head? Raceware studs are less forgiving of warps and such.

Chris White

Is the head O-ringed?

"I don’t think there would be lift - the Raceware’s won’t stretch. Did you check the flatness of the block and head? Raceware studs are less forgiving of warps and such."

Could you elaborate Chris? I dont see how the "forgivingness" would vary.

And, another thing to check, a blockage in the cooling system causing that high pressure. Check and make sure it is cylinder pressure dependant and not on rpm.

The stock head studs are a stretch type fastener – they have the ability to maintain clamping force over a variety of conditions – such as dimensional changes due to expansion and head ‘movement’. They will continue to provide claming force even if the head lifts a little or the head/block changes shape.
The aftermarket studs (reaceware / ARP) stud do not stretch and can only exert the proper clamping force at exactly the height they were installed. If there is any movement the clamping force can change dramatically.
Imagine a warped head that is torqued to spec – as the head heats and cools the warp may change thus changing the clamping force, a stretch fastener will compensate somewhat bit a non-stretch fastener may loose all clamping force.

All that begin said there is one other possibility – did you check all the hose clamps on the cooling system? If one is just slightly loose it may leak coolant only under high coolant pressure (like the added head of high boost). Get a pressure tester to see if the coolant system will hold over 15psi.

Chris White

Tom,

Take it to a shop that can check the coolant for exhaust gasses. If the head is lifting or the head gasket is failing, it should have exhaust gasses present in the coolant. If none are present, I would look to cooling system issues.

Remember the 944 cooling system is designed to operate at 7 lbs of pressure. This reduces the boiling point (or puking as you say) of the coolant. If your cooling system will not hold pressure, it will "puke" sooner.

And this not from pressure, but the lack thereof.

And Tom....when you get this car sorted out.....I wanna ride when you're running 20+psi. Yahoooooooooooooooooooooooooooooooooo

Tom,

If the water pump bearing seal fails (known 944 maintence item) your system will not pressurize. Same if the radiator tank seals fail. I have a Stant pressure tester. If you are ever in my neighborhood, we can check your system.

Mike,

I am running 20psi..........

7 lbs radiator cap? Not what I use…

Chris, what exactly do you use?

Stock calls for IIRC, a 16PSI cap. 1.1 Bar I believe.

I had this same puking after WOT runs if I didn't let it cool WAY down before shuting down. The combination of some pinhole leaks in the overflow tank and a bad tank cap that didn't quite hold pressure is what caused mine. I haven't had it happen since I replaced those two items.
Cheers,
Jason

The stock radiator cap is 1.5 bar which is21.75 psi. This is way higher than most engines use.

Thanks for all your thoughts. To respond:

The head was machined recently so I assumed it was flat, but I did not check it or the deck. I may be doing so shortly I guess.

No o-ringing on my head or deck.

It is always possible to be fooled, but I am fairly sure combustion pressure is getting into the water jackets -- I routed the over flow tube up to the windshield, and could see it blow coolant like a nitrous purge line when I got on the boost or shortly thereafter. When I turn the boost down, less comes out.

I am using a 1.5 bar cap (stock), and tried a back-up cap, just in case, without success.

I have a pressure tester. It holds 15psi. That, and the fact that I don't loose coolant if I stay off the boost, makes me think it takes the higher combustion pressure to breach the HG. (Or, I have a crack somewhere, or a cylinder is moving.)

I may try retorquing the head without pulling it off, to a higher torque. If that does not work, then I guess I'll pull the head and check for flatness. If something is slightly warped, would stock head studs help?