m73m95

With larger diameter studs, there is also more inches of thread around the stud (Larger circumference). I don't know the exact difference, of course, but it would be more difficult to pull them out of the block than the smaller diameter studs (If there is adequate block aluminum around the new larger hole)

Forgot about the new head.... Even at lower boost, you're probably getting quite a bit more air in the cylinder.

rlm328

Race ware and cosmetic gaskets. No problems so far. With the ARP studs and their lubricant you get very consistent torque numbers which means even loading on the gasket. Which in my opinion is a good thing.

DDP

I think it has more to do with the head design and the actual head deforming rather than lack of head stud strength. Meaning it will take more head studs than stronger head studs. It's probably a combination of the large bore size of the 944 (essentially the spacing of the head studs) and the weakness of head.

You have to compare what's going on in the Porsche world with what's going on with DSM's. They're simply pushing much harder than we are here and we can learn from a lot of what they experience. Yeah, a different engine but the physics haven't changed.

DDP

PD/CEP have a larger head stud setup that might help the issue, although it won't solve it. Just talk to Dave, Sid.

Tom M'Guinn

Agreed. I've been down this road with my 3 liter, which is almost 9:1 static compression. After trying just about everything else (steam vents, multiple studs types and torques strategies, skimming heads, o-rings, etc.) the real issue was the ignition timing. Not sure if you have any way of monitoring for knock, but I bet you a beer that you are getting detonation which is causing the HG failures. If you can dyno tune with knock detection, that's best. Otherwise, try reducing your ignition timing on boost fairly significantly (e.g., 6 to 8 degrees, but watch your EGTs) compared to your old motor and I bet the head gasket holds. Once you've eliminated the knock, you can then dial in the timing to the point of max power per boost level without detonation. You have to be careful that you don't dial out the timing so much that the EGTs become too high. If so, you have to back off on the boost until you can run enough timing to bring the EGTs in line. In that case, a higher overlap cam might an help bring down the effective compression and let you cheat it back up a bit, at the cost of idle and emissions quality.

While chasing my problem, I had Heritage make me up a new block with the big Millege/Heritage studs. They are mighty impressive and the torque spec is unreal -- I have no doubt, all things equal, they will hold more pressure at the limit. But I do not view them as essential for anything but the most extreme (or extremely used) motor.

DDP

Thanks for that, Tom. What boost pressures did you run into issues at 9:1 compression?

Tom M'Guinn

Well, I'd say the concept is more important than any numbers I might throw out, since every custom motor will have it's own tuning sweet spots. (18psi from a K26 is not the same as 18psi from a big turbo, cams differ, flow differs, combustion chamber shapes vary, etc.) I can tell you at least on my motor, 6 to 8 degrees is the difference between blowing gaskets at 14psi or holding strong at 20psi (both on 100 octane -- E85 may be different). The higher the boost, the more timing needs to come out to keep knock counts down, but at some point it's not very productive, since the power from the extra boost is offset with the lower ignition timing (and the EGTs climb toward unfriendly territory). That's where E85 and/or water injection may come in handy...

Adonay

Have you tried "new" stock head studs i have had no head lifts or head-gasket problems on my car. I wil never go for anything other than stock and a cup gasket "widefire".

blown 944

Tom, thank you greatly for sharing. I certainly appreciate it.

The compression is definately a limiter. It feels great around town, but for making high hp I am not sold on it. I have never had these issues with the stock compression pistons. I was experimenting and obviously my past experience doesn't apply here (high comp blown alcohol)

We have pulled the timing down to less than 20 degrees at peak tq, which is far less than we ever did before.

So at this point, using your experience (that I trust) I may just call it a wash. I think I'm going to just follow through with the o-rings,new studs, and composite and limit my boost level. It is making good power based on maf readings and my mph via gps even at 19 psi. It seems to hold pressure fine at that boost.

Time to get the stroker done ......

blown 944

That is what I'm going to do first. Along with o-rings. I see where you're coming from bc I have felt the same. However, the compression is a game changer.

I'll let everyone know how it works out

333pg333

So you've had no headgasket issues whatsoever while running 26psi?? How many miles have you done like this and what sort of miles? Fast road or track miles with sustained boost pressures?

Tom M'Guinn

Water injection would probably help.

As for high compression turbo motors, the new modern cars have systems to handle it (my DD 335is has 10.2:1), but back when our cars were designed, there was a reason Porsche started off with 6.5:1 on their first street turbo car...

reno808

why not use a deck plate?

Duke

I have the PD large diameter head studs in my engine @ 175 Nm.
A side note, Milledge warned me about the Heritage studs not having the correct stretch design. But maybe they've updated their design.

Tom M'Guinn

When was that? I called Jon on the phone about a year ago and he said the only place to get his studs now is Heritage. He recommended I call heritage for them. They both said they developed the studs together...