JET951

so what if aftermarket head gaskets are used, like the MLS one

tedesco

I am running M14x1.5 head studs. They were specially made for the engine and cost me as much as the raceware studs. M14 is the maximum you can use without getting into problems with the nuts because the sokets don´t fit anymore into the head. Needs only minimum work.
I never understood why you have to use high grad steel in an alu block to increase clamping force. The weakest link wound anyway always be the alu.
What you achive with the M14 studs is that the circumference of the load carrying aluminium in the block increases with increases strength. The second point is that the bigger studd diameter makes up for a stiffer stud which means less tension under load and less torque necessary to keep the head on the block. This also helps to avoid distorsions of the head and block due to excessive torque.

Chris White

I haven’t heard anybody addressing the timing issue –
If you are advancing the timing or running higher pressure (or both) then you are moving the peak pressure curve earlier in the combustion cycle. If you move enough of that curve to BTDC then you can see some very high pressures during compression.

special tool

toddk911

But then ST won't have full boost at 2,800 anymore.

toddk911

ST: Without going into major detail, would you say your spool up even with that #10 is due to the turbo design (VR) or your tuning skills???

I guess your answer will be "a combination of both"

evil 944t

[QUOTE=tedesco] The weakest link wound anyway always be the alu.
What you achive with the M14 studs is that the circumference of the load carrying aluminium in the block increases with increases strength. The second point is that the bigger studd diameter makes up for a stiffer stud which means less tension under load and less torque necessary to keep the head on the block. QUOTE]

I agree, Even if the stud is huge. The threaded portion remains weak.

mark944turbo

"If you move enough of that curve to BTDC then you can see some very high pressures during compression."

Yea, and never will they be higher than the peak pressure during combustion unless you have a horrible, power killing tune.

Chris White

True – if there is no preignition.
If the rise in pressure is enough to promote spontaneous combustion before TDC the resulting pressure spikes will be pretty nasty. Irregular flame fronts after TDC aren’t great but they are not anywhere near as nasty as pre TDC ‘pings’.

Tom M'Guinn

Now admittedly, the sum total of my formal education in mechanical engineering came from Mr. Whoopee's 3DBB, so consider the source, but it seems logical that the head lifts on the compression stroke. On the combustion stroke, if things are working well, the pistons moves down to make more space for the pressure, but on the compression stroke, the piston is moving up and there is no place for the increasing pressure to go, especially if the flame starts too soon or if it detonates on its own. Overly simple?

Rogue_Ant

Hmm, considering you want Peak Cylinder Pressure at ~15ATDC, that is where I would expect the head to lift.... under normal combustion (I.E. no knock/pre-ignition).




Rogue

mark944turbo

If more pressure was made during compression then the engine would make negative horsepower because the crankshaft is getting pushed backwards by pressure before top dead center.

Rogue is correct, as a general rule a peak pressure location of 10-15 deg ATDC produces the most power. What you really want to maximize is the integral of cylinder pressure * change in cylinder volume. This is the info you really need to find your best timing (or a good dyno).

special tool

Wow kid, you really need to hit the books.
The combustion of the other pitstons carry through the high pressure of the compression of the subject.

Also - I am not merely looking for the highest cylinder pressure - there are additional mechanical factors that are contributing.

mark944turbo

"If more pressure was made during compression then the engine would make negative horsepower because the crankshaft is getting pushed backwards by pressure before top dead center."

Keyword up there is MORE. I am well aware of the pressure that exists before TDC and why it is acceptable. I was trying to explain in a simple way why what Tom said was wrong.

"The combustion of the other pitstons carry through the high pressure of the compression of the subject."

This wouldnt work too well if all cylinders had more pressure before top dead center. The engine would be going nowhere.


What additional mechanical factors are you talking about? Thrust force of the piston against the cylinder wall? Thats relevant but it is also greater during combustion. I dont see where you are going with this, at all.

Perhaps if you posted where and with what sensor you were measuring intake pressure and the data itself it would help explain your mistake.

Chris White

Correct.

Most of the cylinder head lifting I have seen is in conjunction with the initial onset of detonation – not really detectable and not sever enough to do physical damage – but the pressure spikes are way up there and help cause the head lifting. I have typically seen this with over advanced spark running at only mildly elevated boost (15psi).

So – the first thing I would do is back off the timing a tad (a couple of degrees is good) and run the same boost. If it doesn’t lift then you found the problem.