Compression Check
Racer
Joined: Mar 2008
Posts: 428
Likes: 0
Seeing that the piston is at an 45 degree angle how do you get oil on the top side of the piston.
And any time you add that much oil to a cylinder the compression is always going to be higher,
Jackfits compression test was 180 to 200 but the leak down was up to 40 percent leak down no wonder the oil was getting black in a 100 miles.
And any time you add that much oil to a cylinder the compression is always going to be higher,
Jackfits compression test was 180 to 200 but the leak down was up to 40 percent leak down no wonder the oil was getting black in a 100 miles.
Instructor
Joined: Jun 2009
Posts: 157
Likes: 1
From: Sebring florida
The accepted method when doing a leak down test on airplanes is to find TDC for that cyl, put 80# of air into the gague with the regulator on the unit and read the second gague. If you get 76 to 78 on gague #2, thats as close as it will ever get to perfection. You mwill never see 80# in and 80# on the cyl. Often we rock the prop back and forth to seat the valves better and rings in the bottom of the grove.
This will only tell you the condition of the cylinders/ rings and not berrings, nor camshaft.
This will only tell you the condition of the cylinders/ rings and not berrings, nor camshaft.
Drifting
Joined: Dec 2005
Posts: 1,360
Likes: 3
From: NORTHEAST
Most mechanics don't really use a spoon for a spoonful of oil, they squirt it in and spin the motor before rechecking compression. By your reasoning, every time fuel enters the chamber, the compression is raised. The wet test is to look for a significant increase in compression, not just a minor increase in psi. Large amounts of carbon deposits will also raise the compression ratio falsely. These are basic engine troubleshooting concepts that every ASE mechanic knows.
Leak down is the only way to go.
Burning Brakes
Joined: Jun 2008
Posts: 1,086
Likes: 1
From: Michigan
After I thought about it I could do a leak down test.
I will be pulling the engine anyway because we want to spice up the engine compartment and it will make it a lot easier to do the leak test.
I will order the one 08vycpe has linked to.
I never gave the leak down test any thought because I really did not care about the valves in the old heads.
The leak down test will be a sure way to check the rings then? Even if a valve leaks? Should any air escape the rings?
I will be pulling the engine anyway because we want to spice up the engine compartment and it will make it a lot easier to do the leak test.
I will order the one 08vycpe has linked to.
I never gave the leak down test any thought because I really did not care about the valves in the old heads.
The leak down test will be a sure way to check the rings then? Even if a valve leaks? Should any air escape the rings?
Since you are taking the engine out anyway, drop the oil pan, take the caps off the crank bearings and do a visual inspection of the journals. Then, check the crank bearing clearances. It's easy to do. Be sure to install the caps in the same place they came off. When reinstalling the oil pan put on new front and rear main seals and a new pan gasket.
If the leak down check is o.k. and the crank bearings and journals check out o.k. and your engine seems to be running well I wouldn't bother with the cost of rebuilding it.
BTW...the reason you have to crank the engine over at least 4 times to do a compression check is to assure the cylinder you are checking passes through the compression stroke (intake, compression, power, exhaust). Sometime between 1 and 4 cranks it will pass through that compression stroke and you will get a reading. It has nothing to do with building up more pressure.
Drifting
Joined: Dec 2005
Posts: 1,360
Likes: 3
From: NORTHEAST
NO shyt the comment was directed at this statement
Most mechanics don't really use a spoon for a spoonful of oil, they squirt it in and spin the motor before rechecking compression. By your reasoning, every time fuel enters the chamber, the compression is raised.
Advanced
Joined: Jan 2008
Posts: 99
Likes: 0
From: Chesapeake Virginia
I agree about a leak down check. A compression check doesn't tell you much. Compression is nothing more than a function of cylinder volumn differences from BDC to TDC.
I found it easier to turn the engine with a wrench on the harmonic balancer and all the spark plugs removed. I started at #1 cylinder at TDC and checked each cylinder in the firing order as I rotated the engine. You have to hold it at TDC for a few seconds to check the reading because the pressure wants to push the piston down.
Check each cylinder at 90 psi. Even a decay of 20% is considered o.k. My '73 SB ranged from 9% to 12% loss.
I found it easier to turn the engine with a wrench on the harmonic balancer and all the spark plugs removed. I started at #1 cylinder at TDC and checked each cylinder in the firing order as I rotated the engine. You have to hold it at TDC for a few seconds to check the reading because the pressure wants to push the piston down.
Check each cylinder at 90 psi. Even a decay of 20% is considered o.k. My '73 SB ranged from 9% to 12% loss.
Instructor
Joined: Jun 2009
Posts: 157
Likes: 1
From: Sebring florida
The leak down is instant. Thats why it's called "compression differential ". The cyl's wont hold air like a balloon.
I'm not sure why we use 80# of air in the cyl, except that much more than that it becomes dangrous. I've got a 520 ci 6 cylinder eng on my airplane and when performing a differential compression test you have hang on to the prop. If not on TDC when air pressure is applied it will rotate with grate force and could split your head open. I guess I'm warning you to be carefull when applying the air pressure.
I'm not sure why we use 80# of air in the cyl, except that much more than that it becomes dangrous. I've got a 520 ci 6 cylinder eng on my airplane and when performing a differential compression test you have hang on to the prop. If not on TDC when air pressure is applied it will rotate with grate force and could split your head open. I guess I'm warning you to be carefull when applying the air pressure.
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Burning Brakes
Joined: Jun 2008
Posts: 1,086
Likes: 1
From: Michigan
You really don't need to hold it at TDC for any length of time. With a socket and a breaker bar on the balancer even though the air at 90 psi wants to push the piston down you can hold it quite easily and even control the piston travel.
Be sure to index the engine through the firing order in the clockwise direction when in front of the engine looking at it.
Be sure to index the engine through the firing order in the clockwise direction when in front of the engine looking at it.
Doing a rebuild on a 58,000-mile engine is rediculous unless you have a specific glaring problem.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
90 psi on a 4 inch diameter piston will generate about 1000 pounds of force pushing on the crank. 
If the piston is at TDC, there is no 'moment arm' on the crank, so it won't turn. If it is off by just 5-10 degrees...look out! It is best to fix the crank in position (long breaker bar or ratchet with socket on the damper bolt and restrained by frame rail, etc.) so that the crank can't spin.
If the piston is at TDC, there is no 'moment arm' on the crank, so it won't turn. If it is off by just 5-10 degrees...look out! It is best to fix the crank in position (long breaker bar or ratchet with socket on the damper bolt and restrained by frame rail, etc.) so that the crank can't spin.
Le Mans Master
Joined: Nov 2008
Posts: 5,691
Likes: 12
From: columbus oh
Burning Brakes
Joined: Jun 2008
Posts: 1,086
Likes: 1
From: Michigan
90 psi on a 4 inch diameter piston will generate about 1000 pounds of force pushing on the crank. If the piston is at TDC, there is no 'moment arm' on the crank, so it won't turn. If it is off by just 5-10 degrees...look out! It is best to fix the crank in position (long breaker bar or ratchet with socket on the damper bolt and restrained by frame rail, etc.) so that the crank can't spin.
If the piston is at TDC, there is no 'moment arm' on the crank, so it won't turn. If it is off by just 5-10 degrees...look out! It is best to fix the crank in position (long breaker bar or ratchet with socket on the damper bolt and restrained by frame rail, etc.) so that the crank can't spin.I used a 15" bar over a socket wrench.
Last edited by 08vycpe; Nov 18, 2009 at 05:07 PM.
Burning Brakes
Joined: Jun 2008
Posts: 1,086
Likes: 1
From: Michigan
To the OP: 7T1vette brings up a point to remember. There is a push/pull of the valve **** to grab the 90 psi and stop the flow of air from your compressor. At TDC the volume is very small and as the volume increases as the piston starts to move, that very small volume of 90 psi increases significantly. I didn't use the shut off but you probably should since it is a feature of the kit.
