Cylinder Compression - nominal and variation questions
#1
Melting Slicks
Thread Starter
Cylinder Compression - nominal and variation questions
Got the '65 supplement yesterday and saw that for the 327/300 the nominal cylinder compression is supposed to be 160.
A compression test on my car showed the following:
1: 175
2: 175
3: 175
4: 175
5: 175
6: 175
7: 180
8: 150 (160 after addition of oil)
Two questions:
1. I'm pretty sure I have corvette cylinder heads for a c2 corvette since the clean air tube hole is there - how can my compression be ~25 lbs above the value indicated in teh service manual. (haven't taken the valve covers off yet)
2. Should I worry about that cylinder with lower compression. The engine runs very well - no smoking, no oil consumption, smoooooooooooth. I have no history on this engine.
Thanks!
A compression test on my car showed the following:
1: 175
2: 175
3: 175
4: 175
5: 175
6: 175
7: 180
8: 150 (160 after addition of oil)
Two questions:
1. I'm pretty sure I have corvette cylinder heads for a c2 corvette since the clean air tube hole is there - how can my compression be ~25 lbs above the value indicated in teh service manual. (haven't taken the valve covers off yet)
2. Should I worry about that cylinder with lower compression. The engine runs very well - no smoking, no oil consumption, smoooooooooooth. I have no history on this engine.
Thanks!
#2
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Re: Cylinder Compression - nominal and variation questions (Allcoupedup)
Allcoupedup,
Compression readings can be affected by excess carbon and deposits in the combustion chamber (deposits equal less area equals higher compression ratio equals higher readings).
Also cam variations can alter readings as well. A high overlap cam will read less on a compression test. Note that if a cam lobe has worn, the reading for that cylinder can be higher than the others (worn cam lobe equals less overlap equals higher reading).
Your addition of oil in the cylinder that was reading low (which then climbed 10 points) indicates worn rings. However, some question arise.
1. Did you block open the throttle when you ran the test?
2. Was the engine "warm" when you ran the test?
3. Did you turn over the engine the same amount for each cylinder?
4. Was the low reading cylinder the last one tested (indicating that maybe the battery was weaker and therefore cranking speed was lower which equals a lower reading)?
5. Did you remove all of the plugs before testing?
These items will affect the readings (but normally not that much if the battery is in good shape).
Before condeming the engine, I would recommend a "Cylinder Leakdown test". This is a far better way of checking the "seal" of the engine, and will point to the area that is "leaking". If you need further info on this test, look up past posts, or ask me. You do need a Leakdown tester (about $60) and an air compressor capable of supplying a well regulated 100 psi.
Good luck,
[Modified by Plasticman, 11:50 AM 12/13/2002]
Compression readings can be affected by excess carbon and deposits in the combustion chamber (deposits equal less area equals higher compression ratio equals higher readings).
Also cam variations can alter readings as well. A high overlap cam will read less on a compression test. Note that if a cam lobe has worn, the reading for that cylinder can be higher than the others (worn cam lobe equals less overlap equals higher reading).
Your addition of oil in the cylinder that was reading low (which then climbed 10 points) indicates worn rings. However, some question arise.
1. Did you block open the throttle when you ran the test?
2. Was the engine "warm" when you ran the test?
3. Did you turn over the engine the same amount for each cylinder?
4. Was the low reading cylinder the last one tested (indicating that maybe the battery was weaker and therefore cranking speed was lower which equals a lower reading)?
5. Did you remove all of the plugs before testing?
These items will affect the readings (but normally not that much if the battery is in good shape).
Before condeming the engine, I would recommend a "Cylinder Leakdown test". This is a far better way of checking the "seal" of the engine, and will point to the area that is "leaking". If you need further info on this test, look up past posts, or ask me. You do need a Leakdown tester (about $60) and an air compressor capable of supplying a well regulated 100 psi.
Good luck,
[Modified by Plasticman, 11:50 AM 12/13/2002]
#3
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Re: Cylinder Compression - nominal and variation questions (Allcoupedup)
#8 looks a bit on the soft side, but not by much; did you do the compression tests with the engine warm and the throttle blocked wide open, with 5 or 6 compression strokes on each cylinder? If not, that would explain the difference between your readings and the "nominal" spec. I'd do a leakdown test on #8 to see if the loss is rings or valves.
#4
Melting Slicks
Thread Starter
Re: Cylinder Compression - nominal and variation questions (Plasticman)
1. Throttle was wide open
2. Engine was warm
3. Turned it over for 4 or 5 compression strokes
4. Actually the high reading was the last one tested
5. All plugs were removed
I'm nowhere close to condeming this engine as mife wife would KILL me if I asked for 4k in the budget. I just want to make sure that I'm not causing excessive damage to the engine.
The test one done a few days after a 300 mile trip. I've driven it roughly 600 miles since then and ran a quart of risolene through. Any suggestions (aside from the leakdown test) as to what to do next?
Thanks again
Allcoupedup
2. Engine was warm
3. Turned it over for 4 or 5 compression strokes
4. Actually the high reading was the last one tested
5. All plugs were removed
I'm nowhere close to condeming this engine as mife wife would KILL me if I asked for 4k in the budget. I just want to make sure that I'm not causing excessive damage to the engine.
The test one done a few days after a 300 mile trip. I've driven it roughly 600 miles since then and ran a quart of risolene through. Any suggestions (aside from the leakdown test) as to what to do next?
Thanks again
Allcoupedup
#5
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Re: Cylinder Compression - nominal and variation questions (Allcoupedup)
Allcoupedup,
Sounds like you have done everything I would do. If you don't have a leakdown tester (and since your in the general area), drive on over some day and we can test it here (SW suburbs near Lemont).
Plasticman
Sounds like you have done everything I would do. If you don't have a leakdown tester (and since your in the general area), drive on over some day and we can test it here (SW suburbs near Lemont).
Plasticman
#6
Melting Slicks
Thread Starter
Re: Cylinder Compression - nominal and variation questions (Plasticman)
Plasticman,
I really appreciate the knowledge/advice. I just may take you up on your offer once Corvette season returns.
One more question - assuming the higher compression is due to carbon deposits is there any safe way to reduce these deposits? Should I be concerned about having high compression?
Thanks
I really appreciate the knowledge/advice. I just may take you up on your offer once Corvette season returns.
One more question - assuming the higher compression is due to carbon deposits is there any safe way to reduce these deposits? Should I be concerned about having high compression?
Thanks
#7
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Re: Cylinder Compression - nominal and variation questions (Allcoupedup)
Allcoupedup,
I doubt that your compression is "too high" due to deposits with todays "cleaner" burning fuel (I would bet on a different cam or milled heads being the cause, but I am always being "surprised" by what I learn each day). Do you have any problems with "knock"? If you do, there can be any number of reasons for that (including "high compression" due to deposits or milled heads), such as ignition timing being off, wrong plugs, poor or wrong fuel (regular vs. premuim), engine running hotter than normal, lean fuel/air mixture (carb. not adjusted correctly, or jets/rods incorrect or metering rod spring too weak, or intake manifold or carb. vacuum leak), on & on. If you don't have any problem with audible knock, then your OK. As I stated earlier, higher than normal reading can be caused by several items (of which carbon deposits are just one). Unless you are seeing other symptoms, I would not be concerned. Typically, a compression test is just one way used to verify if the engine is in good/bad shape. I would be more concerned with the one cylinder that reads low, but a leak down test still should be the next step (far better way of determing if there is a leak and where it is "going"). I have also seen several times where a valve may not seat correctly for one test (and read low), and then seal properly the next time it is tested (and then read "normal"). Most people would take the later test results and say the engine is OK, but when this happens, I suspect valve guides as being worn (very normal in a high milage Chevy engine). But without excess oil burning on start up, is this a reason to rebuild the motor? Depends on how the engine is used. If just a nice reliable street engine, I would say not, unless you need something to do with your extra time and cash (or there is another "problem" as well). If a race engine, then absolutely it needs to be rebuilt, since peak performance will not be obtained when valves are not sealing 100% of the time.
As for removing excess carbon/combustion chamber deposits, there are a couple of ways to do this (short of removing the heads and scrapping them out).
1. Full acceleration through the gears has always been my favorite method (as long as knock isn't trying to tear the engine apart!).
2. Older mechanics used to pour a pint or quart of water down the carb. throat of the fast idling engine (at about 1500 to 2000 RPM). Not enough to "drown" the engine & kill it, but enough to cause copious amounts of steam to emit from the exhaust pipes. Along with this steam will come the majority of deposits (this steam "steam cleans" the combustion chambers). This does work, but need to be careful about how fast the water is poured. Have heard of some who poured ATF (auto trans fluid), but have never seen the logic in this (seems like it would add more deposits than it would remove).
You are wise to check out the engine (with these tests) when it is running properly. When/if a problem does appear, you can use the readings that you obtained earlier to guage where the real problem lies. Just don't "read" too much into them that you cause yourself grief when there really is no actual problem. Glad to have you over when Spring comes. Any other questions, keep on asking.
Plasticman
I doubt that your compression is "too high" due to deposits with todays "cleaner" burning fuel (I would bet on a different cam or milled heads being the cause, but I am always being "surprised" by what I learn each day). Do you have any problems with "knock"? If you do, there can be any number of reasons for that (including "high compression" due to deposits or milled heads), such as ignition timing being off, wrong plugs, poor or wrong fuel (regular vs. premuim), engine running hotter than normal, lean fuel/air mixture (carb. not adjusted correctly, or jets/rods incorrect or metering rod spring too weak, or intake manifold or carb. vacuum leak), on & on. If you don't have any problem with audible knock, then your OK. As I stated earlier, higher than normal reading can be caused by several items (of which carbon deposits are just one). Unless you are seeing other symptoms, I would not be concerned. Typically, a compression test is just one way used to verify if the engine is in good/bad shape. I would be more concerned with the one cylinder that reads low, but a leak down test still should be the next step (far better way of determing if there is a leak and where it is "going"). I have also seen several times where a valve may not seat correctly for one test (and read low), and then seal properly the next time it is tested (and then read "normal"). Most people would take the later test results and say the engine is OK, but when this happens, I suspect valve guides as being worn (very normal in a high milage Chevy engine). But without excess oil burning on start up, is this a reason to rebuild the motor? Depends on how the engine is used. If just a nice reliable street engine, I would say not, unless you need something to do with your extra time and cash (or there is another "problem" as well). If a race engine, then absolutely it needs to be rebuilt, since peak performance will not be obtained when valves are not sealing 100% of the time.
As for removing excess carbon/combustion chamber deposits, there are a couple of ways to do this (short of removing the heads and scrapping them out).
1. Full acceleration through the gears has always been my favorite method (as long as knock isn't trying to tear the engine apart!).
2. Older mechanics used to pour a pint or quart of water down the carb. throat of the fast idling engine (at about 1500 to 2000 RPM). Not enough to "drown" the engine & kill it, but enough to cause copious amounts of steam to emit from the exhaust pipes. Along with this steam will come the majority of deposits (this steam "steam cleans" the combustion chambers). This does work, but need to be careful about how fast the water is poured. Have heard of some who poured ATF (auto trans fluid), but have never seen the logic in this (seems like it would add more deposits than it would remove).
You are wise to check out the engine (with these tests) when it is running properly. When/if a problem does appear, you can use the readings that you obtained earlier to guage where the real problem lies. Just don't "read" too much into them that you cause yourself grief when there really is no actual problem. Glad to have you over when Spring comes. Any other questions, keep on asking.
Plasticman
#9
Melting Slicks
Thread Starter
Re: Cylinder Compression - nominal and variation questions (Plasticman)
Now that you mention pouring stuff in the intake I remember seeing my wife's dad pour some stuff called sea foam into a vacuum tube in her integra. After seeing what happened I swore I would never subject an engine to such abuse! The dash lit up like a christmas tree and it let out some funky colored smoke all over the place..... violent shaking.....sputtering....
Maybe I'll have to rethink....
Don't have any knock or any bad symptoms. In fact I am surprised at how well it runs... the only thing that concerns me is the lack of power - I haven't really gotten it over 4000 RPMS though - I just don't feel comfortable doing that to an engine I know nothing about.
Thanks
Maybe I'll have to rethink....
Don't have any knock or any bad symptoms. In fact I am surprised at how well it runs... the only thing that concerns me is the lack of power - I haven't really gotten it over 4000 RPMS though - I just don't feel comfortable doing that to an engine I know nothing about.
Thanks
#10
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Re: Cylinder Compression - nominal and variation questions (Allcoupedup)
Allcoupedup,
I am not too sure about adding water (or any comercial equivelent) to a vehicle with a catalytic converter and computer. Certainly could "light up" the dash as you say (as well as clog up the converter)!
Also, the speed that the water is poured in is critical to prevent the violent shaking you saw, but some engine reaction (shaking) will still be necessary to get the deposits removed.
The absense of knock is a "good" thing. The lack of power is not, but everything is relative. Understand that you don't know the history of the engine. Also, a tall rear gear can make it seem like the engine is struggling, when the gear is the cause. Can do a few things to determine the engine basics:
Plug condition is first. If all the plugs are looking a lite to meduim tan, then good. Some engines (especially newer computer controlled versions) run with stark white plugs as normal, but I would not expect or want that on a carbureted engine.
Your compression check also says that nothing there should be the cause for a noticable lack of power.
Removal of a valve cover and then using a dial indicator can tell you some cam info (valve lift specifically) which will indicate if the cam is a "performance" cam, stock, etc. Could be a "wild" cam that does not even come in until 4K, but your high compression check readings make me doubt that.
Bet the dist. is tired, and could use some dialling in, as well as a little TLC (like a frozen cent. advance, worn bushings, etc.). If it has a vacuum advance can, it could also be shot (leaking). By the way, did I mention I have my own Sun Machine? Seen plenty of dist.'s needing some extra care.
Hope that the oil pressure is good, and if so, no problem in winding it on up. 5K is bottom redline on any SBC, so feel free to go at least that high to get the feel for the engine's characteristics.
Lots of potential here. Just part of the fun of owning an older Vette.
Plasticman
[Modified by Plasticman, 12:16 AM 12/17/2002]
I am not too sure about adding water (or any comercial equivelent) to a vehicle with a catalytic converter and computer. Certainly could "light up" the dash as you say (as well as clog up the converter)!
Also, the speed that the water is poured in is critical to prevent the violent shaking you saw, but some engine reaction (shaking) will still be necessary to get the deposits removed.
The absense of knock is a "good" thing. The lack of power is not, but everything is relative. Understand that you don't know the history of the engine. Also, a tall rear gear can make it seem like the engine is struggling, when the gear is the cause. Can do a few things to determine the engine basics:
Plug condition is first. If all the plugs are looking a lite to meduim tan, then good. Some engines (especially newer computer controlled versions) run with stark white plugs as normal, but I would not expect or want that on a carbureted engine.
Your compression check also says that nothing there should be the cause for a noticable lack of power.
Removal of a valve cover and then using a dial indicator can tell you some cam info (valve lift specifically) which will indicate if the cam is a "performance" cam, stock, etc. Could be a "wild" cam that does not even come in until 4K, but your high compression check readings make me doubt that.
Bet the dist. is tired, and could use some dialling in, as well as a little TLC (like a frozen cent. advance, worn bushings, etc.). If it has a vacuum advance can, it could also be shot (leaking). By the way, did I mention I have my own Sun Machine? Seen plenty of dist.'s needing some extra care.
Hope that the oil pressure is good, and if so, no problem in winding it on up. 5K is bottom redline on any SBC, so feel free to go at least that high to get the feel for the engine's characteristics.
Lots of potential here. Just part of the fun of owning an older Vette.
Plasticman
[Modified by Plasticman, 12:16 AM 12/17/2002]