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PORSCHEV
Joined: 02 Nov 2002 Posts: 1901 Location: Cedar Lake Nova Scotia, Canada
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Posted: Tue Jul 22, 2008 9:04 am Post subject: Building a Non-interfence engine |
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I think Chrenan would be best to answer since he has built the bigger valved engine. I know once he went with bigger valves it became an interference engine. I am wondering if this is only because the valve releifs are too small on the pistons to allow clearence in case of a timing belt failure. Or does this happen with more lift in the cam.....In that case I am running the same risk.
If it is the valve reliefs in the pistons..could one have them machined larger to accept the larger valves?
Thanks in advance _________________ 1976 924
5 lug conversion, 17'C2 wheels,custom body work,327 vette engine.
1978-#53 "D" track racer. |
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ideola
Joined: 01 Oct 2004 Posts: 15548 Location: Spring Lake MI
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Posted: Tue Jul 22, 2008 9:26 am Post subject: Re: Building a Non-interfence engine |
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PORSCHEV wrote: | could one have them machined larger to accept the larger valves? |
Why not? Fairly standard practice here in motown...just need to be able to identify the valve centers. This is typically done by disassembling the head and using a punch thru the valve stem to mark the precise center...should be doable on our pistons like any other. My 928 hybrid motor was prepped in this way as well. _________________ erstwhile owner of just about every 924 variant ever made |
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Chrenan
Joined: 15 Jan 2003 Posts: 3903 Location: Canada
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Posted: Tue Jul 22, 2008 12:22 pm Post subject: |
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From my rough measurements, I think it is only the valve reliefs that were too small. I think the lift on the cam would be OK with larger reliefs, but I only have a stage 1 cam, not very aggresive in order to be able to keep the CIS. _________________ 1987 951 - M193 Version for Japan |
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endwrench
Joined: 07 Dec 2002 Posts: 1631 Location: Victor, Montana
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Posted: Tue Jul 22, 2008 1:59 pm Post subject: |
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This is just a guess but I believe if you go with the monster valves like Chrenens and then try to open up the reliefs on the piston tops you may end up with a VERY thin and sharp edge near the outside circumference of the piston. This could possibly be removed completly but you may expose the top ringland to more abuse than wanted.
Todd _________________ '79 924NA. Rebuilt 9.5:1, MSDS header, Mega Squirt Injection, MJLJ-EDIS Ignition, 1.6L Whipple Charger and Intercooler, 10lbs Boost, 944 Trans, Custom HD Clutch.
"simsport" said....superchargers are better than turbos its official!.... |
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PORSCHEV
Joined: 02 Nov 2002 Posts: 1901 Location: Cedar Lake Nova Scotia, Canada
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Posted: Tue Jul 22, 2008 10:15 pm Post subject: |
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I looked at the pistons and there was a fair amount of meat before the edge of the piston IIRC. You are only talking about 2mm at the most? The only disadvantage is it will lower the CR. I don't have a clue how to figure by how much. _________________ 1976 924
5 lug conversion, 17'C2 wheels,custom body work,327 vette engine.
1978-#53 "D" track racer. |
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Chrenan
Joined: 15 Jan 2003 Posts: 3903 Location: Canada
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Posted: Tue Jul 22, 2008 11:57 pm Post subject: |
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Todd makes a good point. The big valves are very close to the edge of the cylinder. If I had to do it over (which I might as I have a spare engine in the garage calling my name) I would go custom pistons. The compression might be slighlty lower, but it would be non-interference. _________________ 1987 951 - M193 Version for Japan |
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ideola
Joined: 01 Oct 2004 Posts: 15548 Location: Spring Lake MI
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Posted: Wed Jul 23, 2008 12:31 am Post subject: |
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If you go custom pistons, you can slightly change the volume of the dish to achieve the CR you want... _________________ erstwhile owner of just about every 924 variant ever made |
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