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valve seating query
I have been working on my friends 2.7 - we stripped the top end down to deal to a bunch of oil leaks. #5 cylinder then also showed a bad leakdown - 85%, so we pulled that and found a damaged face. All the others were around 98%. But since we were in there, and saw a bit of pitting and lots of varnish on # 5 faces, we pulled the others to get them all refaced to same standard.
I have since installed #s 4-6, including cam box, about to do 1-3. Out of curiosity I decided to check the 4-6 leakdowns - expecting good numbers. You can imagine my surprise to find poor numbers and gas leaking from both inlet and exhaust valves. I contacted the machine shop who say they vacuum tested each one before dispatch. They are one of the bigger machine shops in the area, and have a good reputation - altho not P specialists (there are none here). I took the remaining unfitted heads back, and he tested them again. On his vacuum machine, they come up to spec. This is how they test each one, and they measure up. He is only testing at 0.8 bar vac, I am testing at 100 psi pressure. Would the seating improve once the valves have bedded in a bit? Is this just a discrepancy between the measuring systems. These valves were at 98% before we had them cleaned up. Thanks Alan
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83 SC, 82 930 (track) - Stock except for RarlyL8 race headers, RarlyL8 Zork, K27-7006, 22/28 T bars, 007 Fuel head, short 3&4 gears, NGK AFR, Greddy EBC (on the slippery slope), Wevo engine mounts, ERP rear camber adjust and mono balls, Tarret front monoball camber adjust, Elgin cams, 38mm ported heads, 964 IC. 380rwhp @ 0.8bar Apart from above, bone stock:-) |
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07-08-2009, 09:01 PM
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Max Sluiter
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Perhaps it is due to the cold temperature of the metal? When running, the metal heats, expands, and seals better?
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1971 911S, 2.7RS spec MFI engine, suspension mods, lightened Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance |
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07-08-2009, 09:21 PM
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Well the temp is cold for both of us.
But if I took the heads off again and gave them back, there is nothing more he would do - they check out as well as any other valve he has done, and he has done plenty. I can only conclude that the valves would work themselves into a better fit with some engine revolutions. Bit reluctant to box the whole engine up without being confident of this tho. I doubt many do a leakdown before reassembling the engine. Normally before teardown. Maybe this is normal? Alan
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83 SC, 82 930 (track) - Stock except for RarlyL8 race headers, RarlyL8 Zork, K27-7006, 22/28 T bars, 007 Fuel head, short 3&4 gears, NGK AFR, Greddy EBC (on the slippery slope), Wevo engine mounts, ERP rear camber adjust and mono balls, Tarret front monoball camber adjust, Elgin cams, 38mm ported heads, 964 IC. 380rwhp @ 0.8bar Apart from above, bone stock:-) |
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07-08-2009, 10:06 PM
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Join Date: Apr 2009
Location: Cape Vincent, NY
Posts: 841
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The numbers may improve with a little lapping. Other than that, I suppose that a warm engine will perform better. Lap and see if you have a nice wide interface that is concentric.
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1968 911S "Leona" Air goes in and out, blood goes round and round, any variation on this is a bad thing. |
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07-09-2009, 02:43 AM
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1971 Porsche 911S
