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Joined: 20 May 2009
Posts: 1000
Location: Bolton Lancashire

PostPosted: Mon Jun 17, 2019 6:20 pm    Post subject: Reply with quote

Yes Luddite, broken studs were common due to corrosion between the head and cylinders.

Of course power in an engine is all down to the compression pressure at ignition, which is a function of cylinder filling and static or geometric compression ratio.

That's why turbos and superchargers work and why high compression pistons do as well (within detonation limits).

It is also why the Porsche engines with variable cam timing (and later ones with lift as well) produce good power in the mid range because they can increase the air flow at high revs (bigger inlets and wilder cam timing) without losing the cylinder filling at lower revs (when the cam timing is softer and/or the lift reduced).

It is also why oversized engines produce better power at both mid-range and top end. Better mid-range because of increased flow while there is more time and better top end because although the breathing limit of the engine has been reached the combustion pressure is pushing down on a bigger piston.

As the revs increase there is less time for the compressed air to leak past the rings - and this is why at low revs the Carerra's with worn rings would be gutless and why - as soon as they came on the cam and the piston speed increased - they took off like a 2 stroke.

With new rings they got back a lot of bottom end but the jerk wasn't so noticeable.

Click here for the Hartech

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Joined: 18 Dec 2018
Posts: 539
Location: Scotland

PostPosted: Tue Jun 18, 2019 8:23 am    Post subject: Reply with quote

Baz many thanks for the further education, VERY much appreciated.

Apologies for drifting somewhat off topic which was related to bore score issues and wandering into air cooled issues, though it does seem that there is potential for general enlightenment that might be thought worthwhile..? Happy to leave it to the mods to determine..

When you posted the pics of the broken studs alongside the piston rings, it took me back a bit and caused the grey matter to contemplate the cause of the stud failures in the past and wondered if some 30 or so years later perhaps the passage of that amount of time and irregular usage might showed up to cause further issues for the 3 and 3.2 litre engines studs..?

I note you site corrosion between the head and cylinders as a cause of the studs braking... interesting... would you care to expand on that...? Pun not intended (-:

Ref Bruce Anderson`s Porsche 911 Performance Handbook (1987) it seems he suggests that the original issue was down to the differential in the rate of expansion betwixt Dilivar stud and the components it held together, further exacerbated by corrosion of the stud in time..? To offset the corrosion issue it seems Porsche initially plated the studs (1980 on) and in time (circa 1984) may have utilised epoxy coating ...?

From that which you typed Baz... In the grand scheme of things it seems replacing worn piston rings created an interesting shift in the drivers perceptions of engine performance in that the effect of increasing engine compression efficiency, actually reduced the sensation of boost that was previously experienced with worn rings when the rise in piston speed overcame the lower piston speed compression losses due to the worn rings... Entirely logical, but not something I had ever contemplated...

Yup, have a basic understanding relative to the effects of volumetric efficiency increases via turbo`s, the advantages of inter-coolers and superchargers and the benefits and otherwise of fitting high lift cams and up to 11:1 pistons, the latter difference being very much felt when trying to kick start old Triumphs and the like after uprating the compression ratio...A lot of mistakes made but some increase in understanding gained in time relative to altering the balance involved in the original design engineers concept... (-:

I have ZERO hands on experience of modern engines and not much in the way of understanding relative to the mechanical operation to achieve VVT though remember well many years back, filing my Norton crankcases to provide enough clearance to allow my then new Polydyne profile cams to rotate without hitting the front of the case..

Every day has potential to be a school day.... Question
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Joined: 29 Jun 2010
Posts: 16211
Location: Kent

PostPosted: Thu Jun 20, 2019 11:57 am    Post subject: Reply with quote

The discussion regarding Norton motorcycles etc has been moved to a new thread.

See --> Barton Motors and Norton motorcycles
2007 Guards Red 997 Turbo Tiptronic
ex 2004 Polar Silver 996T Tiptronic
ex 2002 Seal Grey 996.2 C4 Tiptronic
ex 1978 Silver 924 Manual
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