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Too interfere or not!

 
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dwak  
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PostPosted: Sat May 04, 2002 2:56 pm    Post subject: Reply with quote

It has been suggested elsewhere on this site that '79/'80 931's were non interference and '81/'82's were. Yes or no?
I've noticed earlier pistons have an offset dish and latter day have a centred dish. Was this the cause of the conflict and what,if any, was the improvement??
Is there anyone out there who really knows?
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Paul  
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PostPosted: Sat May 04, 2002 3:23 pm    Post subject: Reply with quote

I have repaired 80 and 81 931 motors that bent valves when the belt failed. I assume all stock 931's are interference motors.
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Geddy T  
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PostPosted: Sat May 04, 2002 4:56 pm    Post subject: Reply with quote

I've heard similar
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Peter_in_AU  



Joined: 29 Jul 2001
Posts: 2740
Location: Sydney, Australia

PostPosted: Sun May 05, 2002 10:40 pm    Post subject: Reply with quote

the interference problem is caused by the diameter of the valves. Basically the combination of the larger non-angled valves means that it is not possible to build cut-outs into the pistons like the NA - the valves are just too close to the cylinder wall.

The only way a 931 would be non-interference would be if it had a 924 head and pistons.
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dwak  
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PostPosted: Sun May 05, 2002 11:19 pm    Post subject: Reply with quote

And the offset dish and latter day centred dish piston, what's that about?
dwak
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Peter_in_AU  



Joined: 29 Jul 2001
Posts: 2740
Location: Sydney, Australia

PostPosted: Sun May 05, 2002 11:34 pm    Post subject: Reply with quote

my uninformed opinion is that the very organic-looking 931 pistons were used to reduce detonation back in the days before intercoolers.
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Paul  
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PostPosted: Mon May 06, 2002 12:40 am    Post subject: Reply with quote

The earlier pistons had more metal on the exhaust side, maybe Porsche wasn't sure that normal looking pistons could stand the heat.

After some experience, they changed the design.
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gohim  
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PostPosted: Mon May 06, 2002 5:25 am    Post subject: Reply with quote

I have checked my collection of Porsche Factory Documentation, and the results of my quick research are mixed. I have read somewhere that engines with the 7.5CR pistons are non-interference, and later engine with the 8.0CR are interference. I can't find that reference now, and looking throught the 924 Turbo Wokshop Manual, with current Supplements shows some reference to being careful when installing the head, or valve damage could occur, but no direct statement that any or all 924T engines are interference, or not-interference.

I guess the safe way to act is to treat all 924T engines as interference type engines. DON'T LET THE TIMING BELT BREAK !
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larso  
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PostPosted: Tue May 07, 2002 1:39 pm    Post subject: Reply with quote

The 7.5 engines are interference design. It doesn't matter what the books say since there are so many mistakes in books.
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924 turbo  
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PostPosted: Tue May 07, 2002 7:53 pm    Post subject: Reply with quote

Yes, Lars knows from personal experience! They're all interference.
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Peter_in_AU  



Joined: 29 Jul 2001
Posts: 2740
Location: Sydney, Australia

PostPosted: Tue May 07, 2002 10:31 pm    Post subject: Reply with quote

next time you have a 931 head off, have a look at how close the edge of the valves are to the cylinder walls - that's why they bang into the pistons. There just isn't enough clearance between the outer edge of the valves and the cylinder wall to allow a cutout in the piston which would leave enough metal to protect the cylinder wall.

Remember that the 924 motor was designed to contain the combustion within the piston not within the head (the 924 head has no combustion chamber).

The only way the low-compression 931s could be non-interference would be if they were fitted with standard 924 heads and valves and had cutouts in the pistons like the 924. It all comes down to the valves
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