333pg333

So can we 'safely' assume that up to 500rwhp the stock studs and a cometic h/g are ok?

Tom M'Guinn

I don't doubt the conclusion (after all, the piston is moving up and the spark plug is lighthing the flame -- where's the pressure to go?), but I'm not sure I follow the evidence. The valves are closed on the compression stroke, so if the pressure of a given cylinder's compression stroke is lifting the head, how does that change the intake manifold pressure? Coming back in to the intake through the open valves in the adjacent cylinder? Even so, at any given time, you have one cylinder in each of the 4 strokes, so if you see a rise in cooling jacket pressure, how can you associate it with any particular cylinder (i.e., any given stroke)?

Tom M'Guinn

I'd suggest you need a mighty good state of tune to get 500hp out of stock studs. Before I went to Vitesse stuff, I had to use racewares just to get 350 without the head lifting, using Huntley's ARC2/MAF, etc.

Tom M'Guinn

They both screw in to factory stud holes.

special tool

you are overthinking this.
the pressure goes into the water jacket under compression.
the gasket reseals under combustion.

nothing reverts to the intake - I mentioned the intake because I compared its pressure to the water jaket's pressure to be able to relay this fact.

mark944turbo

The statement about the leakage happening on the compression stroke is wrong.

Way more pressure happens during combustion (unless you have preignition), this is where it would leak. If you are making peak pressure on the compression stroke you have messed up ignition timing and are losing lots of power.

Also, there is a power stroke happening in another cylinder for every compression stroke so there is no way to tell which of the 2 is causing the pressure rise in the coolant jacket using the measurements you described.

How did you come to this conclusion?


Edit- thinking about it more, were you able to see which cylinder was leaking afterwards? This might make the arguement more valid, but it is still unreasonable for combustion to reseal a leak.

special tool

not wrong - common misconception by those who make assumptions.
like I said - water pressure follows boost , NOT combustion.

yes I know what AND when the leakage.

mark944turbo

Maybe the problem you are having is that water pressure follows cylinder pressure, but you cant measure that, but you can measure intake pressure, which is very related, and from there you made a bad assumption.

If you logged crank position and water pressure quick enough, and you knew which cylinder leaked, you would see that the leakage occurs on the combustion stroke.

333pg333

Is this what most that know would agree with. I'm not doubting Tom but from what I understand, the aftermarket studs require a bit of upkeep that I'd like to avoid if possible. What is involved with the re-torquing or how often does it need to be done?
Thanks

mark944turbo

Just as a note, an increase in boost (or power) has less of an effect on max cylinder pressure than most people realize. It produces more area under the crank position/cylinder pressure curve mostly by extending the length, not the peak value. The primary thing that effects peak pressure is compression ratio.

hacker-pschorr

Then keep the stock studs, if your heads lifts and coolant goes all over....pull the head and install different studs.

Porsche switched from studs to bolts with the 928 as power increased. Not sure why, just throwing it out there. Some think it was simply to allow the 32V head to be removed without yanking the motor.

David Floyd

I torqued to 85 ft lbs and ARP told me to let sit overnight and then retorque before assembly ( there was no movement ) and no need to dissassemble and retorque after run in.

No problems at all, for some 30k miles, but I don't run any crazy boost like you ST....

Also using an MLS headgasket

Tom M'Guinn

I've always heat cycled my racewares and then retorqued. Only once did I get a nut that needed more torque. Last time, I set them to 90 and did not retorque and all has been fine up to occassional bursts at 24psi.

Tom M'Guinn

Sorry, wasn't trying to overthink it (I hate that) -- just trying to understand how "the EXACT correspondance of intake manifold pressure to coolant jacket pressure" evidences that the head is lifting on the compression stroke. Thought I might be missing an insight, but I was probably just reading your post too literally.

Laust Pedersen

Porsche’s end-goal is a specific clamping force adequate for the OEM (engine) torque delivery. Their 3-step method is virtually unaffected by thread friction and gets reliably to the desired result if and only if OEM headgasket, studs, nuts and washers are used.

Laust