These cylinder compression test results seem low

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Don R
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Re: These cylinder compression test results seem low

#31

Post by Don R »

True, for what it's worth mine has a Schrader valve in the spark plug end.
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Re: These cylinder compression test results seem low

#32

Post by robin1731 »

pjlogue wrote:The dead space in the hose (If there is no schreder valve at the end of the hose) can't collect/store pumped air. The piston pushes air into the hose on compression stroke and it comes right back out when the piston comes down. It is the same as increasing the combustion chamber volume when the piston is TDC. If you double your combustion chamber volume when the piston is TDC you half your compression ratio. The hose volume on a pressure gage with no schreder valve at the end acts the same as increasing the combustion chamber volume.

-P.

So you are talking about a compression gauge with no schrader valve at all? In the hose anyplace?

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Re: These cylinder compression test results seem low

#33

Post by pjlogue »

The amount of dead space depends on where the valve is. If the valve is right at the spark plug adapter there is very little dead space and the pressure is almost equal to what it would be when the piston is at TDC on the compression stroke. When the valve is at the gauge side of the hose/tube all the volume in the tube is dead space because the air pushed into the tube comes back out when the piston retracts. This doesn't happen if the valve is right at the spark plug adapter. If the valve is right at the adapter any air pushed into the tune stays there as the valve prevents it from coming back out.

If you had a compression tester without any valve in it all you would see, when cranking the engine, is the needle flopping back and forth with a final reading of zero PSI.

-P.
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Re: These cylinder compression test results seem low

#34

Post by robin1731 »

OK, guess I was misreading. Earlier (post #15) you said there was a difference in a gauge used for cars and one for bikes. Why would there be any difference? An air pump is an air pump. I don't see why it would be any different.

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Re: These cylinder compression test results seem low

#35

Post by pjlogue »

A compression tester for a bike or any small displacement engine will have a minimal dead space with the schreder valve at the plug adapter. With a car engine (ones from the days of big block engines) the dead space in the hose going to the gauge was a very small percentage of the combustion chamber volume. The combustion chamber volume of the GL1000 is roughly 28-30cc. The hose at 18" in length can have 8-10cc. This extra dead space will bring a compression of 170 PSI down to 119 PSI.

I would hate to see folks here tearing down an engine thinking they have a compression problem when the don't.

-P.
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Re: These cylinder compression test results seem low

#36

Post by robin1731 »

Interesting. Where did you find this information? Have a link to it?

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Re: These cylinder compression test results seem low

#37

Post by pjlogue »

The GL1000 has a compression ratio of 9.2:1. https://www.motorcyclespecs.co.za/model ... 0%2078.htm

The total displacement of the bike is 999cc. 999/4= 249.75 cc displacement per cylinder. 249.75/9.2=27.15 cc. This is the combustion chamber volume which is the dead space. (my estimate of 28-30 cc was a little high, but close)

If you double the dead space (without changing the displacement) you reduce the pressure of the dead space (at TDC) by 1/2. Any tube/hose attached to the combustion chamber in essence becomes part of the dead space if the air can go back into the combustion chamber. Once air is past a shrader valve it is no longer part of the dead space. If the volume in the tube/hose past the shrader valve is large, it will take longer (more revolutions of the engine) to reach the maximum pressure but it doesn't change the final reading. It boils down to basic physics.

The one thing I do not understand is how an engine (or compressor) can attain a compression pressure higher than the maximum pressure in the dead space at TDC. If the GL1000 has a compression ratio of 9.2:1 then the maximum pressure in the dead space (combustion chamber) should be: 14.7 PSI @( standard atmospheric pressure) X 9.2 (compression ratio) = 135.24 PSI. This pressure, since the gauge reads zero at normal atmospheric pressure of 14.7 psi, should actually be 120.54 PSI. Honda states the compression on a good engine should be ~171 PSI. The only thing I can think of is the way the carb intake may act as a velocity stack and use the mass of the moving air to slightly compress the incoming charge. a ~30% increase in compression seems to be quite high. Anybody have any thoughts/explanation for this?

-P.
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Re: These cylinder compression test results seem low

#38

Post by robin1731 »

Again, please, where did you find this information? Do you have a link to this?
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Re: These cylinder compression test results seem low

#39

Post by CYBORG »

I was taught the important number was the number difference between cyl. on the same engine. And I would think the location above sea level would have an effect on the number
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Re: These cylinder compression test results seem low

#40

Post by robin1731 »

CYBORG wrote:I was taught the important number was the number difference between cyl. on the same engine. And I would think the location above sea level would have an effect on the number

Absolutely true. And if someone takes an engine apart just going by compression numbers they shouldn't be doing that type of work. IMO. More diagnosis to be done before that. I've said that many times. People talk about compression numbers but don't mention leakdown numbers.

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ericheath
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Re: These cylinder compression test results seem low

#41

Post by ericheath »

I never heard of the psi being a ratio of standard air pressure but a static ratio of maximum volume in the combustion chamber at bottom dead center to the maximum volume of the combustion chamber at top dead center.

So mathematically you have to know the volume of the combustion chamber in the head, the volume of the cylinder, the volume of the piston’s dome positive or negative, and whether the piston comes fully to the top of the cylinder or not. A few years ago I measured 1000 head chambers and as best as I recall it was more like 36 cc’s. Others have posted similar findings. The 1000 piston stops just shy of the top of the cylinder. 249/9.2 does give a combustion chamber of 27ccs. So the dome would need to displace 9ccs at top dead center. As best as I recall, it was nearly ten ccs with my crude measurements. The 1000 piston does not come fully to the top of the cylinder so my measurements of ten ccs would need to subtract the amount from the piston not coming fully to the top. I measured the gap at 0.040”

These are static, non-moving compression numbers. Once the crank is in motion all kinds of things happen because of valve timing.

So where did Honda get the posted numbers? No clue.

Now with all that said, the original post comment about extra space in the hose of our compression testers does have merit (in my mind) that it will lower the overal reading unless the schrader valve is at the spark plug hole. I can see it dropping substantially. Mine appears to have a pretty large volume. I might go check it.
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ericheath
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Re: These cylinder compression test results seem low

#42

Post by ericheath »

BCF1C1EF-3216-4275-AADD-4AAB46C3D161.jpeg
BCF1C1EF-3216-4275-AADD-4AAB46C3D161.jpeg (308.9 KiB) Viewed 260 times
Mine holds ten ccs of water!! That’s quite a bit, enough to cause significantly lower readings.

I’m feeling a LOT better about my readings. If it’s supposed to be 171psi with a combustion chamber of 27ccs, my readings are about 25% off.

140psi Times 1.25 = 175psi Bingo
Whatever I suggest here should be given ample time for a moderator to delicately correct. I apologize in advance.
77 WING, 1200 engine with 77 heads, cams, gl1100 foot pegs, Magna V65 front end, 764A carbs, [-gone Suzuki M109 monoshock--, replaced with gl1100 shocks] gl 1200 swing arm, gl1500 final drive, wheel and rear brakes Valkyrie seat, Meanstreak tank, Sportster pipes, Power Arc ignition off crank.
77 Wing. black
83 Wing, in pieces
"Continuing education is important even if the subject matter is fairly useless (as in this case)."---Greg Foresi
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Re: These cylinder compression test results seem low

#43

Post by pjlogue »

robin1731 wrote:Again, please, where did you find this information? Do you have a link to this?
I posted the link about the GL1000 stats. The rest is physics. P1 X V1 = P2 X V2. I don't have a link for this as it's stuff I have learned over a long time.

-P.
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Re: These cylinder compression test results seem low

#44

Post by pjlogue »

CYBORG wrote:I was taught the important number was the number difference between cyl. on the same engine. And I would think the location above sea level would have an effect on the number
The altitude does have an impact on compression. At 9,000 feet your compression would be roughly 128 PSI for an engine that had 170 psi at sea level.

-P.
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Re: These cylinder compression test results seem low

#45

Post by CYBORG »

Just saying. So book numbers are more or less meaningless. Except to be used as a tool to trouble shoot, along with other things, to help find the cause of a certain problem
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