86 928 engine stumper
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Joined: Jun 2001
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From: Ball Ground, GA
Oil consumption in a cylinder does not have the relationship to compression that you might think.
Compression is determined by:
-Quality of sealing by the compression rings on the piston, which is affected by bore quality and the condition of the rings.
-Quality of sealing by the valves and seats.
-Quality of sealing by the block and head, which is not a factor without physical damage, such as a blown gasket.
Oil consumption thru the cylinder is determined by:
-Oil control by the oil ring on the piston, which is affected by bore quality, ring condition, and crank bearing condition. If the bearings are worn, more oil will be thrown on the cylinder walls, and the oil ring may be overwhelmed.
-Oil control at the valve guides, which is determined mainly by valve guide wear and oil seal condition.
As Marc says, oil loss thru the intake guide will end up in the cylinder. There can also be significant oil loss thru the exhaust guide, but that normally goes out the tailpipe and plugs the cat. The extra oil in the cylinder will actually help the compression, not lower it.
Oil going by the rings can make even more improvement in compression. The added oil improves the sealing on the compression rings.
I rebuilt a Corvair engine that 25k later went from no oil consumption to a quart every 75 miles. I checked the compression - new spec was 140 psi, and I had 200 to 225 psi. The oil rings were faulty, and were flooding the compression rings with oil. Bores and compression rings were perfect.
Compression is determined by:
-Quality of sealing by the compression rings on the piston, which is affected by bore quality and the condition of the rings.
-Quality of sealing by the valves and seats.
-Quality of sealing by the block and head, which is not a factor without physical damage, such as a blown gasket.
Oil consumption thru the cylinder is determined by:
-Oil control by the oil ring on the piston, which is affected by bore quality, ring condition, and crank bearing condition. If the bearings are worn, more oil will be thrown on the cylinder walls, and the oil ring may be overwhelmed.
-Oil control at the valve guides, which is determined mainly by valve guide wear and oil seal condition.
As Marc says, oil loss thru the intake guide will end up in the cylinder. There can also be significant oil loss thru the exhaust guide, but that normally goes out the tailpipe and plugs the cat. The extra oil in the cylinder will actually help the compression, not lower it.
Oil going by the rings can make even more improvement in compression. The added oil improves the sealing on the compression rings.
I rebuilt a Corvair engine that 25k later went from no oil consumption to a quart every 75 miles. I checked the compression - new spec was 140 psi, and I had 200 to 225 psi. The oil rings were faulty, and were flooding the compression rings with oil. Bores and compression rings were perfect.
Thread Starter
Instructor
Joined: Sep 2002
Posts: 236
Likes: 0
From: Los Angeles, CA
very educational; thanks!
i had assumed that if the piston couldn't control oil getting past it that it would not produce compression.
so let's say oil is getting past the valve guide. What is involved in repair? pull the head and...?
i had assumed that if the piston couldn't control oil getting past it that it would not produce compression.
so let's say oil is getting past the valve guide. What is involved in repair? pull the head and...?
