DIY Compression Testing
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Can someone explain why the throttle needs to be wide open? I did the test w/ throttle closed and got 250, 245, 245, and 235psi. Note I cranked until the needle wouldn't go up anymore. Is this ok or should I have timed the cranking?
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Joined: Apr 2001
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From: Fort Worth
My notes on a few items...
Honda's numbers
Honda's nominal number of 228 psi I think is a little low (see below) but I think is a good target. Above that and you should (mechanically) be OK. I would use this standard for a new motor, which will not seal as well as one that has some miles on it, and not show as high of numbers. Their minimum number, though (135 psi) is complete and utter BS. 135 psi would be typical of a garden tractor, not an F2xC. If your engine blows 135 psi, there is something horribly, horribly wrong with it. See below for more info on typical numbers.
How long to test?
I find that this is probably the greatest contributor to inaccurate or inconsistent numbers. I've had some engines take as long as 10 seconds of holding the starter button to peak out the gauge. Ideally it should only take about 5 seconds, that's enough to get the cylinder of a healthy motor w/strong battery to spin the crank enough to get one good compression stroke on the cylinder. Sometimes it takes more than one, and sometimes the battery comes into play, not spinning the engine as fast. I always test for 10 seconds, or until the gauge peaks out. See below for more pointers or how to do the test to insure the results are accurate.
Wide-open-throttle testing and pulling the spark plugs
Doing this reduces the load on the starter and effectively increases the speed at which the test is conducted. Given the smaller battery in the S2, this is not a bad practice and I recommend it. Not doing so will not have a huge impact on the numbers but may make it harder to determine if the engine has issues or not if the battery is a little weak.
Warm testing
The engine doesn't have to be warm. Testing warm will generally provide better numbers as the engine usually seals better when warm. I prefer doing a test cold as I'd rather get worst case numbers than best case.
Pulling the fuel pump fuse
This isn't necessary. You'll smell fuel and the cylinder walls will get a little wet but it's not going to impact test results enough to detect. I suppose it could create a fire hazard if you smoke around your motor but that would be a stupid thing to do regardless.
Wet testing
Some people are fans of putting a little oil in the cylinder to also aid with ring seal. I think this is unnecessary on an engine that doesn't have problems (again, I would rather have the worst case).
Altitude
Altitude has an effect on results. Keep this in mind if you are much higher (or I suppose lower) than sea level:
Typical numbers
I have been challenged on this, but I know what test results I've gotten. My gauge isn't the most expensive but it seems to test well against others I've seen. Keep in mind that your gauge might not be great but it is more than likely repeatable. That is probably the most important thing to remember, and cylinder-to-cylinder the numbers should be consistent. However, don't lose sight of the fact that the engine may have a problem affecting all cylinders (such as all the intake valves being bent, or all the exhaust valves being burnt).
In general, I see healthy motors blow 235-255 psi. I wouldn't necessarily freak if I got 220 psi, but I would want to know more (see below). Also, don't get excited if you get 180 psi across the board thinking that you are fine since they are all the same (see above). I would recommend in that instance that you check your gauge (or procedure) against another car/person.
What causes low numbers?
Lots of things. The primary driver behind the pressure developed is the compression ratio (assuming proper ring and valve seal). A higher compression motor will generate higher numbers than a lower compression motor. A 9:1 CR Civic might make 160 psi in good health, but an S2000 at 11:1 CR should be much higher than this (well over 200psi).
After static compression ratio, ring seal and valve seal are the next two most important items. The rings seal up the combustion chamber by preventing, or at least reducing, the blowby out of the combustion chamber down into the crankcase. If the rings or cylinder wall or worn, this seal will be compromised and the cylinder can leak out of the chamber into the crankcase. This occurs usually as a result of cylinder wall wear, which may be because of old age (glazing) or poor oiling/low oil (creating scoring/scuffing). With forced induction or poor fuel management, detonation can cause scoring, or in extreme cases, fracture ring lands and then damage the cylinder walls.
Valve seal is equally important. This does not refer to the valve stem seals, but rather the edge of the valve where it mates with the valve seat in the combustion chamber. As an engine ages, hot exhaust gases take their toll on these surfaces. Exhaust valves pit or "burn" and the sealing is reduced. This is exaggerated/accelerated with forced induction, or engines that run lean.
Bent valves can cause this as well. With the F2xC's valve timing, the intake valves can be bent as a result of an over-rev.
Finally, the head gasket can contribute to poor compression as a result of overheating (head or block warpage) or very old age (gasket failure). Compression leaks past the gasket into another cylinder, the water jacket (cooling system), or oil supply or return port (crankcase).
Note that on an older motor, as carbon is deposited on the top of the piston, the height of the piston, and the static compression ratio of the motor is effectively increased. This may result in higher numbers if you compare a car with some miles on it vs a new car.
This is anecdotal evidence from F20C's I've worked on recently and what I found wrong with them. These were done pretty close to sea level (a few hundred feet above):
245 to 255 across the board: healthy
225 psi: badly pitted exhaust valves (resulting in about 12% cylinder leakage)
210 psi in cyl 4: badly pitted exhaust valves and some scoring of the cyl walls resulting in about 20% cylinder leakage
150 psi across the board: badly pitted exhaust valves and bent intake valves resulting in about 40% leakage
This doesn't mean that with these numbers you will have these problems. Again, there are a lot of possible contributing factors.
So what do I do next?
If you think you've done the test right, and don't like your numbers or want to know more, you should have a leakdown (cylinder leakage) test done. This can determine if there truly is cylinder leakage, and where the leakage is going.
Hopefully this helps answer some questions for some of you... and explain things better.
Honda's numbers
Honda's nominal number of 228 psi I think is a little low (see below) but I think is a good target. Above that and you should (mechanically) be OK. I would use this standard for a new motor, which will not seal as well as one that has some miles on it, and not show as high of numbers. Their minimum number, though (135 psi) is complete and utter BS. 135 psi would be typical of a garden tractor, not an F2xC. If your engine blows 135 psi, there is something horribly, horribly wrong with it. See below for more info on typical numbers.
How long to test?
I find that this is probably the greatest contributor to inaccurate or inconsistent numbers. I've had some engines take as long as 10 seconds of holding the starter button to peak out the gauge. Ideally it should only take about 5 seconds, that's enough to get the cylinder of a healthy motor w/strong battery to spin the crank enough to get one good compression stroke on the cylinder. Sometimes it takes more than one, and sometimes the battery comes into play, not spinning the engine as fast. I always test for 10 seconds, or until the gauge peaks out. See below for more pointers or how to do the test to insure the results are accurate.
Wide-open-throttle testing and pulling the spark plugs
Doing this reduces the load on the starter and effectively increases the speed at which the test is conducted. Given the smaller battery in the S2, this is not a bad practice and I recommend it. Not doing so will not have a huge impact on the numbers but may make it harder to determine if the engine has issues or not if the battery is a little weak.
Warm testing
The engine doesn't have to be warm. Testing warm will generally provide better numbers as the engine usually seals better when warm. I prefer doing a test cold as I'd rather get worst case numbers than best case.
Pulling the fuel pump fuse
This isn't necessary. You'll smell fuel and the cylinder walls will get a little wet but it's not going to impact test results enough to detect. I suppose it could create a fire hazard if you smoke around your motor but that would be a stupid thing to do regardless.
Wet testing
Some people are fans of putting a little oil in the cylinder to also aid with ring seal. I think this is unnecessary on an engine that doesn't have problems (again, I would rather have the worst case).
Altitude
Altitude has an effect on results. Keep this in mind if you are much higher (or I suppose lower) than sea level:
Originally Posted by gernby
I'm not sure how reliable these numbers are, but I found a site that gave these conversion factors for compression numbers at various altitudes.
0' = 1.0
1000' = .9711
2000' = .9428
3000' = .9151
4000' = .8881
5000' = .8617
6000' = .8359
7000' = .8106
8000' = .7860
So it seems that an S2000 that generated compression values of 250 PSI near sea level would probably do about 215 PSI at 5000'.
0' = 1.0
1000' = .9711
2000' = .9428
3000' = .9151
4000' = .8881
5000' = .8617
6000' = .8359
7000' = .8106
8000' = .7860
So it seems that an S2000 that generated compression values of 250 PSI near sea level would probably do about 215 PSI at 5000'.
I have been challenged on this, but I know what test results I've gotten. My gauge isn't the most expensive but it seems to test well against others I've seen. Keep in mind that your gauge might not be great but it is more than likely repeatable. That is probably the most important thing to remember, and cylinder-to-cylinder the numbers should be consistent. However, don't lose sight of the fact that the engine may have a problem affecting all cylinders (such as all the intake valves being bent, or all the exhaust valves being burnt).
In general, I see healthy motors blow 235-255 psi. I wouldn't necessarily freak if I got 220 psi, but I would want to know more (see below). Also, don't get excited if you get 180 psi across the board thinking that you are fine since they are all the same (see above). I would recommend in that instance that you check your gauge (or procedure) against another car/person.
What causes low numbers?
Lots of things. The primary driver behind the pressure developed is the compression ratio (assuming proper ring and valve seal). A higher compression motor will generate higher numbers than a lower compression motor. A 9:1 CR Civic might make 160 psi in good health, but an S2000 at 11:1 CR should be much higher than this (well over 200psi).
After static compression ratio, ring seal and valve seal are the next two most important items. The rings seal up the combustion chamber by preventing, or at least reducing, the blowby out of the combustion chamber down into the crankcase. If the rings or cylinder wall or worn, this seal will be compromised and the cylinder can leak out of the chamber into the crankcase. This occurs usually as a result of cylinder wall wear, which may be because of old age (glazing) or poor oiling/low oil (creating scoring/scuffing). With forced induction or poor fuel management, detonation can cause scoring, or in extreme cases, fracture ring lands and then damage the cylinder walls.
Valve seal is equally important. This does not refer to the valve stem seals, but rather the edge of the valve where it mates with the valve seat in the combustion chamber. As an engine ages, hot exhaust gases take their toll on these surfaces. Exhaust valves pit or "burn" and the sealing is reduced. This is exaggerated/accelerated with forced induction, or engines that run lean.
Bent valves can cause this as well. With the F2xC's valve timing, the intake valves can be bent as a result of an over-rev.
Finally, the head gasket can contribute to poor compression as a result of overheating (head or block warpage) or very old age (gasket failure). Compression leaks past the gasket into another cylinder, the water jacket (cooling system), or oil supply or return port (crankcase).
Note that on an older motor, as carbon is deposited on the top of the piston, the height of the piston, and the static compression ratio of the motor is effectively increased. This may result in higher numbers if you compare a car with some miles on it vs a new car.
This is anecdotal evidence from F20C's I've worked on recently and what I found wrong with them. These were done pretty close to sea level (a few hundred feet above):
245 to 255 across the board: healthy
225 psi: badly pitted exhaust valves (resulting in about 12% cylinder leakage)
210 psi in cyl 4: badly pitted exhaust valves and some scoring of the cyl walls resulting in about 20% cylinder leakage
150 psi across the board: badly pitted exhaust valves and bent intake valves resulting in about 40% leakage
This doesn't mean that with these numbers you will have these problems. Again, there are a lot of possible contributing factors.
So what do I do next?
If you think you've done the test right, and don't like your numbers or want to know more, you should have a leakdown (cylinder leakage) test done. This can determine if there truly is cylinder leakage, and where the leakage is going.
Hopefully this helps answer some questions for some of you... and explain things better.
Joined: Apr 2006
Posts: 95
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From: Appleton, WI
Originally Posted by overst33r,Oct 25 2008, 04:00 AM
WHEW! we got it out with a file end and a hammer. Jammed the file end into the adapter and unscrewed it.
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Well I did this test today and I'm happy with the overall results. I'm at 110,000 miles at I'm blowing 240 on all four. 
I do have one question though maybe someone can answer. I had all four plugs out and fuel fuse pulled. When I tested the cylinder closest to the front of the car everything was relatively quiet. When I tested the other 3 front to back I could hear a very noticeable puffing or popping sound. Why did I not hear that on the front one? I had the gas pedal pressed on all four and I checked it twice.
I do have one question though maybe someone can answer. I had all four plugs out and fuel fuse pulled. When I tested the cylinder closest to the front of the car everything was relatively quiet. When I tested the other 3 front to back I could hear a very noticeable puffing or popping sound. Why did I not hear that on the front one? I had the gas pedal pressed on all four and I checked it twice.
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Joined: Oct 2007
Posts: 28,698
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From: Mooresville, NC
How much does the reading change when you go WOT?
I did mine recently but forgot to hold the gas pedal down. Got 225 +/- 1 psi in each cylinder. 130k on the engine.
I know consistency is key but just wondering how much it would throw the readings off.
I did mine recently but forgot to hold the gas pedal down. Got 225 +/- 1 psi in each cylinder. 130k on the engine.
I know consistency is key but just wondering how much it would throw the readings off.
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Joined: May 2006
Posts: 9,409
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From: Calgary
Originally Posted by Moddiction,Jul 6 2010, 09:13 AM
How much does the reading change when you go WOT?
I did mine recently but forgot to hold the gas pedal down. Got 225 +/- 1 psi in each cylinder. 130k on the engine.
I know consistency is key but just wondering how much it would throw the readings off.
I did mine recently but forgot to hold the gas pedal down. Got 225 +/- 1 psi in each cylinder. 130k on the engine.
I know consistency is key but just wondering how much it would throw the readings off.
I didn't hold the gas pedal down, nor did I warm up the engine first. I will be doing another test at some point for more accurate numbers.
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Originally Posted by zeroedout,Jul 6 2010, 06:43 AM
Well I did this test today and I'm happy with the overall results. I'm at 110,000 miles at I'm blowing 240 on all four. I do have one question though maybe someone can answer. I had all four plugs out and fuel fuse pulled. When I tested the cylinder closest to the front of the car everything was relatively quiet. When I tested the other 3 front to back I could hear a very noticeable puffing or popping sound. Why did I not hear that on the front one? I had the gas pedal pressed on all four and I checked it twice.
I do have one question though maybe someone can answer. I had all four plugs out and fuel fuse pulled. When I tested the cylinder closest to the front of the car everything was relatively quiet. When I tested the other 3 front to back I could hear a very noticeable puffing or popping sound. Why did I not hear that on the front one? I had the gas pedal pressed on all four and I checked it twice.
since I had put the spark plug in and tightened it down it wouldn't come out by screwing the hose back in. I tried the thread lock idea and the thread lock worked, but the adapter was in too tight to get out with just twisting the hose. So now I'm stuck with the adapter in the hole as well as the hose. 
I ended up cutting the hose off at the top of the valve cover. Then I took my long skinny flat head screw driver and tapped it down the hose until it wouldn't go any more. I gave it a good twist and it all came out together.
I'm so glad it came out. I thought I was in big trouble. I'm going to have to go buy a new compression checker, but I don't even care about that. I don't even know why that tester has an adapter to it. I think on my new one I'm just going to thread lock the adapter to the hose attachment. I think we need to add to the DIY about the adapter possibility of coming off.I'm glad for the diy though thank you.
