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Old 02-08-2006, 12:59 AM
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Differential cylinder pressure/ leakdown tester

I have a differential cylinder pressure tester. Mac tool #CLD200M. I have used it with some success in determining where I have had to direct attention when diagnosing engine malfunctions. It is a simple, common sense tool. Pressurize the cylinder with the piston at TDC and listen for leaks. If air is coming from the carb,you have a leaky intake valve. Hissing in the valve cover, rings are going away. Tailpipe, ex valve. You can do all of that with an old hollowed out sparkplug with a air nipple welded to it. But I know this tool does more It checks differential pressure, Thats why it has 2 gages. I understand that the line pressure will be higher than the cylinder pressure due to the blowing by rings, burnt valves .
My question is: 1) What percent of leakage is normally considered acceptable, what is considered better, what is worse, and when is it time to put the for sale sign on the car? If the gages read 100/99 PSI I would assume that is good. I know I went through the top end on my shovelhead Hardley abelson when The gauges said 100/60. (Do you think I could have run it another year?LOL) What are the ranges inbetween?
Question 2) Is this considered a leakdown tester or is that another tool.. I always thought that a leakdown test was time based. Thanks for reading this, Mikey

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Last edited by powerrodsmike; 02-08-2006 at 12:59 PM. Reason: Make tool name exactly correct.
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Old 02-08-2006, 03:16 AM
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For what it's worth differential pressure can be used to determine flow in the field of instrumentation.For example an "orifice plate" would be sandwiched between two gauges and the flow would be determined by the square root of the pressure difference.I'm really glossing over some details here but I you see the connection.
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Old 02-08-2006, 07:26 PM
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In the aviation industry they use this test to check for valve and cyl wear. Set the engine as close as possible to the TDC of the cyl you are testing. Connect the tester, and set the inlet pressure to 80 psi with the built in regulator. The lower gage should not read below 60 psi. Preferred is around 70 to 72 psi. And there should no more than 10% difference from cyl to cyl.
I hope this clears up some of the confusion.
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Old 02-08-2006, 07:28 PM
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in the automotive world that is a "leakdown tester"...a good engine should not show much over 20% leakage...still more than i'd like to see
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Old 02-08-2006, 08:48 PM
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Thanks guys, people do read my posts. That info is what I was looking for. The actual test was not a mystery to me ,only the resultant pressure loss expressed as a percentage and it's meaning as it related to the engines condition.. My tester came with a chart showing the readings at 3 different input pressures (it has its own regulator) and the 2nd gage reading = % loss. I had one guy tell me if a motor had 3% loss it was time for a rebuild.... I'm working on street rods and semi regular performance cars, not top fuel cars.

So 90% is acceptable and within wear limits. 80% is borderline, and 75% you should get the engine hoist ready. Kind of like grading schoolwork. Thank you, mikey
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Old 02-09-2006, 05:58 AM
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How do you calibrate a leakdown tester? Is there a standard leak?
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Old 02-09-2006, 07:32 AM
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Onovakind,The tester I have has adjusters on the gages. I would think you would put some pressure to the input without hooking it to anything on the output side and set both gages to the same reading. (which would be the actual input pressure) No flow on the outfut side should read as 0% leakdown, Or 100% sealed. Does this sound right to you? Here is a link with a simple pic. (I like simple pics, well, because i'm simple)http://www.engineersedge.com/power_t...sion_check.htm
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Old 02-09-2006, 07:34 AM
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i use a "flowed .080" orifice...a flowed fuel injector nozzle from Enderly fuel injection. it should show 80% leakage. this is just an example of the calibrations i have for mine...other models will vary some..depending on the size and quality of the orifice inteternally, accuracy of the guages etc. They are more of a comparison tool than a "micrometer". They are like tape measures.....4 different tapes...4 slightly different measurements. the only reason i calibrate mine are for checking fuel jets and nozzles for mechanical fuel injection at the track.
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Old 02-09-2006, 07:38 AM
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Powerrodmike, that will work to make sure that both guages read the same psi...but does not check the flow calibration to confirm percentage loss.....there are mathmatical formulas to determine percentages due to variences of orifice size.....but math hurts my head
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Old 02-09-2006, 07:51 AM
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Quote:
Originally Posted by 61 chevy
i use a "flowed .080" orifice...a flowed fuel injector nozzle from Enderly fuel injection. it should show 80% leakage. this is just an example of the calibrations i have for mine...other models will vary some..depending on the size and quality of the orifice inteternally, accuracy of the guages etc. They are more of a comparison tool than a "micrometer". They are like tape measures.....4 different tapes...4 slightly different measurements. the only reason i calibrate mine are for checking fuel jets and nozzles for mechanical fuel injection at the track.

So the sized orifice you use to calibrate the tool is used like a "standard" to check the internal fixed orifice in the tool.It is a mac tools brand so I'd imagine it is close enough for the work I do if the gages are right. I was looking for info relating to my original question a few days ago and found a page:
http://www.globalair.com/discussions...le.asp?msgID=3
It has reference to calibrations using a built in "Master orifice" specifically for checking the tool and getting a "calibrated reading". I didn't understand what I was reading there at the time, But with what you just explained to me ,it all makes sense to me now. Although with my newfound knowledge it has raised another question. For a tester to accurately show a percentage directly correlating to cylinder efficiency, Wouldn't the tool's internal orifice have to be sized for a given cylinder volume? Or the leak gage reading be somehow adjusted or compensated for. Being as how a large cylinder with properly seated rings etc would show a greater loss than a smaller cylinder would even though both were working with the same effieciency. So in order to use the same tool on various displacement motors , You would have to use different "standard" orifices to get your 80% baseline reading to start from. Most of the things I work on are within a small range of cylinder displacement sizes, but I do like to understand how stuff works., Thanks, mikey
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Old 02-09-2006, 09:35 AM
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How about if you just measured leakdown directly in scfh at a given pressure with a variable area flowmeter like these?
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Old 02-09-2006, 10:36 AM
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Onovakind- I have a few of those left over from a test lab demo a buddy of mine did. Would there be a way to find the standard to gage it against though. I would think you would need to know how much volume of air (or whatever test gas you chose to use) was acceptable to bypass the rings and valves and still be operating at an acceptable efficiency in order to test it using that method. Personally, any math beyond the third grade level befuddles me. I have no problem leaving the calculations up to someone else who can perform them. Like 61 says he uses an .080" jet for his calibration. Would that orifice give a 80% benchmark for a 44 Cubic inch displacement cylinder or 63 cid cylinder. If .080" was the 80% benchmark for a 44 cid then i would think that a larger orifice "was in order for a 63. I know there is an easy way for you to figure out what the acceptable bypass would be if you knew the orifice sizes that matched the displacements. Also I would think that alot of calculations could be done away with if the test was time based. Like say you pressurized a cylinder with 800 psi and timed the amount of time it took to "Leakdown" to a given pressure. The amount of time would indicate seal quality. The longer time the better. Like the more holes in the bucket, the less time it takes the water to dribble out. That would automatically compensate (to a point),for different cylinder volumes, Different amounts surface area for the rings to seal on, etc. Many conditions are time based in a motor anyway. The tool could be simple, maybe a High pressure tank charged with air regulated to 500 t0 800 psi output, a cylinder adapter set with a valve hooked to a timer of some sort. I would make sure that the breaker bar wasn't hooked to the crank when I did that test though. I know that the 100 psi test pressure used with my tester swings that puppy mighty fast if I'm not right on TDC. I could have sworn that I heard of this type of test somewhere before. Thanks for reading, mikey
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Old 02-09-2006, 12:23 PM
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I would guess something in the range of 5-50 scfh would work as a leak tester and compare favorably to one that uses a .040" orifice. I would expect that less than 5 scfh at 100# would be an excellent seal, less than 1% leakdown. 29 scfh would equate to about 15% leakdown. This could all go out the window if my calculations are wrong, but you can check them at:

http://www.efunda.com/formulae/fluid...meter.cfm#calc
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Old 02-09-2006, 01:22 PM
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Onovakind- I looked at that page. That is like 27 POUNDS of math. I will just take your word for it that your calculations are right. Did you use the .040" orifice for any particular reason. Also, what is the ring circumference you expect those cfm ratings to corelate to? I have a dwyer 5-50 scfm flowmeter identical to the on in your pic. Because I am such a big fan of empirical testing (by necessity, I assure you) I will do a test with both apparatus the next time I do a leakdown test. My Old yeller pickup would be a good candidate. I initially thought this thread would be a flop, but it is turning out kind of fun. Later, mikey
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Old 02-09-2006, 01:43 PM
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I chose the .040" orifice because it appeared on a couple of tester sites:

http://www.xs11.com/tips/misc/misc3.shtml

http://www.engineersedge.com/power_t...sion_check.htm

Precious little info on any of the commercially manufactured ones....

For the calculator I used .25" for the pipe size, .04" as the orifice size, 10 psi as the difference in pressure, 1.29 kg.m for the density, .7 as the coefficient and got my answer in ft/min to get the midrange of the gauge, about .4 scfh or 24 scfh. If you put in 1 as the pressure difference you get .124 scfm, or about 7.5 scfh. A 5-50 scfh flow gauge would show leakdowns of less than 1% to about 45% using an .040" orifice as the standard.

You must be careful when selecting the operating pressures, as you can reach critical flow situations if your absolute downstream pressure gets too low.

Last edited by onovakind67; 02-09-2006 at 01:58 PM.
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