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  #1  
Old 11-02-2007, 12:36 AM
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Compression tester--theoretical question

I have a diesel compression tester with a variety of adapters. There are a number of glow plug hole adapters. Some of these adapters include the pencil tip section on the adapter. The adaptor that fits the 617 pencil plugs does not include the pencil tip length but stops where the pencil tip would normally begin.
How will this effect compression readings?
The combustion chamber is quite small. It seems to me that the pencil tip on the glow plug is quite large in comparison to the size of the combustion chamber. Will this additional area (normally taken up by the pencil tip), now available for the compression chamber, significantly lower the compression reading results?
Or, given the fact that the volume of the pencil tip compared to the full volume of the cylinder with the piston at bdc is relatively small mean that the change in final compression readings will only be negligibly smaller?

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  #2  
Old 11-02-2007, 12:50 AM
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There's also the volume of the pipe leading to the one-way valve in the meter.

The volume of the lost metal compared to the volume with the piston at the top is much more important than comparing to the volume with the piston at the bottom. I'll leave it to you do do the computations, but they're pretty easy to carry out.

In fact, if you know the volume of the combustion chamber with the piston at the top, the swept volume, the volume of metal removed and the indicated pressure, you can easily compute the actual pressure. The question is, is the difference in pressure greater than the accuracy of your gauge. That I do not know.
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  #3  
Old 11-02-2007, 12:52 AM
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There's a schrader type valve in the glow plug adapter so the volume of the hose leading to the gauge won't matter. I have no idea on the exact volume of the combustion chamber and I don't know how to perform the calculations anyway.
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1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
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  #4  
Old 11-02-2007, 12:58 AM
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The volume below the valve is what I am talking about.

And I know that when you say that you don't know how to perform the calculations, it just means that you haven't thought about them. But that's fine, since without knowing the exact volume of the chamber with the piston at the top, you can't do them.

And in any case, it really doesn't matter. You know that you're reaching pressures that are not lower than the gauge sees (although what it registers may not be true, of course). So if they are high enough and fairly close across the cylinders, you're good to go.
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  #5  
Old 11-02-2007, 01:01 AM
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That valve is right at the end of the adaptor, where the pencil would normally begin, inside the head.
I raise the question because it puzzles me as to why the compression kit has adaptors that include the length of the pencil tip and some without. Why would they make adapters with fake pencil tips if that volume did not make a difference?
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  #6  
Old 11-02-2007, 02:28 AM
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Of course it makes a difference. Any volume change in the chamber will affect the compression ratio and thus the compression. With everything else the same, an increase in volume in the chamber will lower the compression ratio. How much? To know that, you must know the exact volume of that chamber.
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  #7  
Old 11-02-2007, 05:13 AM
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Let's try the calculation

The 5 cylinder engine has a displacement of 3000 cc's. Each cylinder is 600 cc's. Compression ratio from what I have read is 22.5:1.

The compression ratio being the difference in volume from the cylinder at bottom dead center to the volume at top dead center, the displacement of the cylinder must be 21.5 times the volume of the combustion chamber.

Divide 600cc by 21.5, the combustion chamber is 27.9cc. I just happened to have a glow plug in the trunk of my car at work tonight. Filled a 25 cc graduated cylinder up to 20 cc's, stuck the glow plug in the water until it reached the threading. It displaced 1.5 cc's.

Using the compression meter plug instead of the glow plug gives us a combustion chamber volume of 29.4. The resulting combustion ratio is 21.4 (600+29.4/29.4). A 5% difference. Theoretically at sea level with an air pressure of 14.7 psi, the engine should have a compression gage reading of 315 (14.7x21.4) psi. Slightly less at higher altitudes.

In practice, carbon buildup reduces the size of the combustion chamber and higher readings can result.

A better way to figure the problem out is to read a previous thread which quoted the shop manual. 350-400 is excellent. 275 to 350 is good. 225 to 275 is acceptable. The key being that there should not be more than 30 psi difference between cylinders. If you are below 240, factor in the 5%.
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  #8  
Old 11-02-2007, 08:35 AM
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Excellent analysis.

Bottom line, it is not significant unless you happen to be at the bottom of the allowable compression readings.

Tom W
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  #9  
Old 11-02-2007, 10:19 AM
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Quote:
Originally Posted by alamostation View Post
The 5 cylinder engine has a displacement of 3000 cc's. Each cylinder is 600 cc's. Compression ratio from what I have read is 22.5:1.

The compression ratio being the difference in volume from the cylinder at bottom dead center to the volume at top dead center, the displacement of the cylinder must be 21.5 times the volume of the combustion chamber.

Divide 600cc by 21.5, the combustion chamber is 27.9cc. I just happened to have a glow plug in the trunk of my car at work tonight. Filled a 25 cc graduated cylinder up to 20 cc's, stuck the glow plug in the water until it reached the threading. It displaced 1.5 cc's.

Using the compression meter plug instead of the glow plug gives us a combustion chamber volume of 29.4. The resulting combustion ratio is 21.4 (600+29.4/29.4). A 5% difference. Theoretically at sea level with an air pressure of 14.7 psi, the engine should have a compression gage reading of 315 (14.7x21.4) psi. Slightly less at higher altitudes.

In practice, carbon buildup reduces the size of the combustion chamber and higher readings can result.

A better way to figure the problem out is to read a previous thread which quoted the shop manual. 350-400 is excellent. 275 to 350 is good. 225 to 275 is acceptable. The key being that there should not be more than 30 psi difference between cylinders. If you are below 240, factor in the 5%.
Excellent.
I'm thinking that they must include the pencil tip on the other adapters for smaller engines where the volume of the tip is even more significant given the smaller combustion chambers. (1.5L having a 10% lower reading given your calculations)
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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
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  #10  
Old 11-02-2007, 10:40 AM
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it does not matter for compression test. you could have a 30 gallon tank connected to the injector hole. you are not measuring the Compression ratio, you are measuring how much air is being pressurized accross several piston strokes and how much is leaking out to get a total pressure reading.
it would matter for a ratio of bdc displacement to TDC displacement, but that is not what a compression test is for.
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  #11  
Old 12-27-2007, 12:24 PM
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does a lack of response indicate that I am correct, or nobody wants to put their $.02?...
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"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
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1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Last edited by vstech; 12-27-2007 at 01:53 PM.
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  #12  
Old 12-27-2007, 02:36 PM
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>>does a lack of response indicate that I am correct

I don't think so. Over a few compression cycles, the reading on the gauge goes up from zero - thets just because you take some air from the cylinder to pressurise the pipe and gauge - the one way valve in the gauge tip prevents it all just leaking away between compressions.

As the pressure in the pipe and gauge nears the peak pressure in the cylinder, a smaller and smaller volume of air passes from the cylinder, via the one-way valve to the pipe and gauge.

Eventually, you reach the equilibrium situation where the gauge doesn't move much as each compression passes. However, this pressure isn't a sum over a number of compressions, it's just an approximation to the peak pressure reached in the cylinder, and as such, is representative of the peak pressure in one cycle, and this isn't in any way a sum, or an average pressure.
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  #13  
Old 12-27-2007, 07:39 PM
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How about like this:
Take the Max pressure 400 psi/ 600cc = 0.667 psi/ 1cc
If the tip of the glow plug took up a volume of 2cc; removing that volume would be a loss of 1.334 psi from the Max pressure. Max pressure without the glow plug tip would be; 398.666.

He can take the actual pressure reading he gets devide it by 600cc and use that amount of psi per 1cc ounce he knows the volume of the glow plug tip and subtract it from the total reading that he gets.
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  #14  
Old 12-27-2007, 08:17 PM
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Your analysis ignores the unswept volume, Diesel911. You will lose a lot more than 1.334 psi.

Say you have 600 swept volume and 21:1 compression. This means that (600+u)/u=21/1 or 600=20u giving u=30. This is with the plug in place.

Say that the plug displaces 2cc. Then we compare (600+30)/30=21 with (600+32)/32=19.75. You have lost 1.25 bar, or about 17.5psi.

Last edited by Matt L; 12-27-2007 at 08:29 PM.
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  #15  
Old 12-27-2007, 11:06 PM
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so, if I put a cylinder capable of handling 400psi in the circuit with the compression gauge, you are saying I will get a substantially lower compression pressure on the gauge? say a 30cc chamber plumbed into the tester hose. what would my 350PSI compression cylinder read?

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