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Old 12-09-2018, 07:26 PM
86911TLCAB 86911TLCAB is offline
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Default Top dead center for leak down test

Want to do leak downs on my 6 cyl e36. Does it matter if the cylinder is at the tdc during compression stroke or exhaust stroke?

Thanks.
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Old 12-09-2018, 08:39 PM
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Originally Posted by 86911TLCAB View Post
Want to do leak downs on my 6 cyl e36. Does it matter if the cylinder is at the tdc during compression stroke or exhaust stroke?

Thanks.
my guess is compression as tat is what i have always done. Want to make sure both valves are closed.
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Old 12-09-2018, 10:27 PM
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I guess that is my question. Aren't the valves for a particular cylinder also closed at tdc for the exhaust stroke? Hence the leak down value would be the same? I guess I can try it and find out..thanks Bill.
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Old 12-09-2018, 10:42 PM
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Google "valve overlap"
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Old 12-10-2018, 10:14 AM
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thanks Bill...i will do that...
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Old 12-10-2018, 10:19 AM
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Valves closed, piston at the top. Find wherever that occurs and you are good If you are going through the effort, measure the piston travel with a dowel/whatever and watch the valves.

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Old 12-10-2018, 10:33 AM
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in the 911 i can easily tell that i am at TDC during the compression stroke because i can see the rotor as it goes around...i haven't done leakdowns on the BMW, but other than putting a straw or a dowel into the spark plug hole and waiting till it is out as far as it will go, i don't know how to tell if it is in the compression or exhaust stroke without removing valve cover gasket...i don't think the notch on the case is visible...but it would make sense to me as these are interference engines that the valves must be closed at the top position on both strokes...will give it a whack and see what i find...thanks guys...
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Old 12-10-2018, 10:51 AM
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Unless Porsche BMW is doing something really weird, both valves will be open (slightly) at TDC during the transition from exhaust to intake stroke. This is the valve overlap referred to earlier.

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Old 12-10-2018, 03:14 PM
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ok...so looks like i need to do this during the compression stroke...
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Old 12-10-2018, 03:39 PM
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Quote:
Originally Posted by 86911TLCAB View Post
ok...so looks like i need to do this during the compression stroke...
Absolutely. Must be on the compression stroke.
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