The axial force in the studs/bolts is driven by the torque applied, and the coefficient of friction between the rotated parts (bolt or nut head and threads).
The strength of the stud or bolt material will not increase the clamping force directly, but will allow a greater preload to be applied to the stud without yielding it.
The 17-4PH studs are stiffer than the standard ones (which they should be is they are stronger), the clamping force on the joint may be greater when everything warms up. Since they will stretch less for the same applied force as a weaker stud would.

If your running high boost, I don't think it is a bandade to use a higher preload/torque than standard. I expect the purpose of the stronger studs is to tolerate a higher preload and provide a greater clamping force to counter the increase boot pressure trying to open the head gasket joint.
Generally in bolted joint design, you apply the highest preload all the parts can take without failing, this provided the best sealing and fatigue performance.

Have you got this engine up and running again with these studs? If so I would be interested to know how much torque you have applied (and how much this is above standard).
Its possible to calculate the stress in the stud from the applied torque and stud and nut size. I can calculate this for you if you want to know how close to yield you are with a certain torque. I would need to know the stud size/thread and nuts used, and what you are lubricating the thread with?

There is an web calculator here that will determine the axil force due to toque too. https://www.engineersedge.com/calculators/torque_calc.htm

You may find the thread in the block is limiting over the stud.

Pete

Hi Pete,

Yes had them in for about a year with mixed results....

Have steadily increased torque from first try at 70lb ft to 110lb ft at the last attempt.

Have managed to blow the HG with all 3 attempts so far - it may well be that significantly more clamping torque is required - the two rear most nuts seem to resist holding torque as well as the others though and I'm just about to dump the studs for stock bolts. Given that the map I'm using is no longer injection timing based (and thus producing less boost), and that I'm managing the wastegate via the map now, the bolts should (fingers crossed) have much less to deal with and the torque should be more controlled due to the stretch...

@gordongraham the best person to ask is sabredylan on the d2bc, he's the one doing the tweaking.

Incidentally, performance is better with less boost since more exhaust can flow via the wastegate.

That's interesting sounds like they need more torque get get some stretch in them. If the studs are too stiff you can getbussues with understanding after warming up and cooling down.
I hope you have more luck with the normal studs this time round.

Pete

I've bought new main & big end shells, bottom end gasket kit & rings for the pistons too. Will be giving it a glaze bust this time round as it was looking a bit smooth at the last change.

Fun and games.

Bennehboy said:

Hi Pete,

Yes had them in for about a year with mixed results....

Have steadily increased torque from first try at 70lb ft to 110lb ft at the last attempt.

Have managed to blow the HG with all 3 attempts so far - it may well be that significantly more clamping torque is required - the two rear most nuts seem to resist holding torque as well as the others though and I'm just about to dump the studs for stock bolts. Given that the map I'm using is no longer injection timing based (and thus producing less boost), and that I'm managing the wastegate via the map now, the bolts should (fingers crossed) have much less to deal with and the torque should be more controlled due to the stretch...

@gordongraham the best person to ask is sabredylan on the d2bc, he's the one doing the tweaking.

Incidentally, performance is better with less boost since more exhaust can flow via the wastegate.

Click to expand...

you have basically answered your own question by accidently it performs better lower boost

accidentally you mean less HOT air pumping into the engine .... more boost means more heat less dense .. the intercooler can only do so much an some what limited and the turbo also has a AR ratio also

are you dead set sure you dont have any warpage issues ? just becouse there are no cracks does not mean its not warped either

ps when was the last time your injection system was tested ? ..a injector compensating can also play a role when it comes to disputing heat evenly across the engine but not enough to melt a piston either ......specially in your case most issiues are at the rear of the engine over a time period

No warpage. Nothing melted. The Turbo is good for the boost level that it was running. I think the performance change is down to the fact that the exhaust exit route is larger with the wastegate being open sooner.

Injectors are very much one of the quantities that are unknown, probably well worth having them checked.