Engine compression
12-05-2007 | 08:05 PM
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From: lindenhurst, NY
Engine compression
Does anyone know what the compression should be for a big block nitro engine???? I have an XTM 457, a FORCE 26 and an AXIAL 28RR. I own a compression tester and have been searching far and wide for any specs or at least a range of what it should be.
12-05-2007 | 08:18 PM
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From: Somerset, UNITED KINGDOM
RE: Engine compression
Blimey that's some large type face your using there!
Now onto the topic,no i have no clue as to what the actual reading (in either psi or bar) would be but i'm sure that if you measure what you have and post your results we'll be able to make a reasoned judgement.
Now onto the topic,no i have no clue as to what the actual reading (in either psi or bar) would be but i'm sure that if you measure what you have and post your results we'll be able to make a reasoned judgement.
12-05-2007 | 08:37 PM
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From: , PA
RE: Engine compression
I dont think you will fine actual compression numbers on nitro engines. You will have to go by the signs of a weak engine like hard starting while hot, loss of power adn real easy to pull when cold. Usually if you can pull ont eh pull start and the truck lifts with it then you are good. If you have no resistance while pulling most likely its worn.
12-05-2007 | 09:52 PM
#5
RE: Engine compression
Compression pressures using a gauge are only a guide to follow any wear in a particular engine. Even this only applies if you use the same gauge all the time or if you want to compare figures with someone else then they have to be using the exact same gauge as well. All gauges have a dead volume inside them which includes whatever tubing leads from the engine to the part inside the gauge that takes the measurement. All of this dead volume of air has to be compressed which gives the affect of having a much larger combustion chamber volume. Basically this means that the smaller the engine you test the lower the pressure is going to read.
In theory, if the engine had 10:1 compression then the gauge should read about 150psi (disregarding any heating of the air caused by the compression). The dead volume reduces this figure considerably. The other thing is that compression only starts when the exhaust port closes so instead of compressing the full swept volume of the engine you're only compressing maybe 3/4 of the swept volume. Combine the dead volume loss with the swept volume loss and instead of reading 150psi you read about 60psi, in other words it's just a guide.
The best compression tester is your fingers. Turn the engine up against compression until it just starts to get into the pinch area and hold it there. If the engine is in good condition you should feel the compression trying to push the piston back down for several seconds. Basically that's a leak down test. The rule of thumb for old iron piston engines was that if you could still feel at least some compression after 5 seconds then it was fine. An ABC should hold for longer than you have patience if you put the piston up into the pinch. I did this with a Rossi and after 45 minutes I figured that was long enough
. Putting the piston into the pinch isn't a fair test though because there's no pinch once warm.
In theory, if the engine had 10:1 compression then the gauge should read about 150psi (disregarding any heating of the air caused by the compression). The dead volume reduces this figure considerably. The other thing is that compression only starts when the exhaust port closes so instead of compressing the full swept volume of the engine you're only compressing maybe 3/4 of the swept volume. Combine the dead volume loss with the swept volume loss and instead of reading 150psi you read about 60psi, in other words it's just a guide.
The best compression tester is your fingers. Turn the engine up against compression until it just starts to get into the pinch area and hold it there. If the engine is in good condition you should feel the compression trying to push the piston back down for several seconds. Basically that's a leak down test. The rule of thumb for old iron piston engines was that if you could still feel at least some compression after 5 seconds then it was fine. An ABC should hold for longer than you have patience if you put the piston up into the pinch. I did this with a Rossi and after 45 minutes I figured that was long enough
. Putting the piston into the pinch isn't a fair test though because there's no pinch once warm.
12-07-2007 | 06:36 AM
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From: , WV
RE: Engine compression
check those numbers against a weedeater, chainsaw, blower etc and see how that type of 2 stroke stacks up. I'd guess the numbers you got were probably good for all the reasons downunder gave
12-07-2007 | 11:26 PM
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From: lindenhurst, NY
RE: Engine compression
ORIGINAL: downunder
Compression pressures using a gauge are only a guide to follow any wear in a particular engine. Even this only applies if you use the same gauge all the time or if you want to compare figures with someone else then they have to be using the exact same gauge as well. All gauges have a dead volume inside them which includes whatever tubing leads from the engine to the part inside the gauge that takes the measurement. All of this dead volume of air has to be compressed which gives the affect of having a much larger combustion chamber volume. Basically this means that the smaller the engine you test the lower the pressure is going to read.
In theory, if the engine had 10:1 compression then the gauge should read about 150psi (disregarding any heating of the air caused by the compression). The dead volume reduces this figure considerably. The other thing is that compression only starts when the exhaust port closes so instead of compressing the full swept volume of the engine you're only compressing maybe 3/4 of the swept volume. Combine the dead volume loss with the swept volume loss and instead of reading 150psi you read about 60psi, in other words it's just a guide.
The best compression tester is your fingers. Turn the engine up against compression until it just starts to get into the pinch area and hold it there. If the engine is in good condition you should feel the compression trying to push the piston back down for several seconds. Basically that's a leak down test. The rule of thumb for old iron piston engines was that if you could still feel at least some compression after 5 seconds then it was fine. An ABC should hold for longer than you have patience if you put the piston up into the pinch. I did this with a Rossi and after 45 minutes I figured that was long enough. Putting the piston into the pinch isn't a fair test though because there's no pinch once warm.
Compression pressures using a gauge are only a guide to follow any wear in a particular engine. Even this only applies if you use the same gauge all the time or if you want to compare figures with someone else then they have to be using the exact same gauge as well. All gauges have a dead volume inside them which includes whatever tubing leads from the engine to the part inside the gauge that takes the measurement. All of this dead volume of air has to be compressed which gives the affect of having a much larger combustion chamber volume. Basically this means that the smaller the engine you test the lower the pressure is going to read.
In theory, if the engine had 10:1 compression then the gauge should read about 150psi (disregarding any heating of the air caused by the compression). The dead volume reduces this figure considerably. The other thing is that compression only starts when the exhaust port closes so instead of compressing the full swept volume of the engine you're only compressing maybe 3/4 of the swept volume. Combine the dead volume loss with the swept volume loss and instead of reading 150psi you read about 60psi, in other words it's just a guide.
The best compression tester is your fingers. Turn the engine up against compression until it just starts to get into the pinch area and hold it there. If the engine is in good condition you should feel the compression trying to push the piston back down for several seconds. Basically that's a leak down test. The rule of thumb for old iron piston engines was that if you could still feel at least some compression after 5 seconds then it was fine. An ABC should hold for longer than you have patience if you put the piston up into the pinch. I did this with a Rossi and after 45 minutes I figured that was long enough
. Putting the piston into the pinch isn't a fair test though because there's no pinch once warm.
I e-mailed axial on the reading I got and they said that it was up to spec and not to bother with a compression tester.
