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So if I want to purchase the equipment necessary to do a leak down test, is there a way, using this equipment, without major disassembly, that I can determine where the weakness is? Valves or Rings?
 

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Yes, by using a leakdown tester, the results shown will give an indication of how well the combustion chamber is sealed without any disassembly.

Normally, you get the engine to operating temperature.
Remove spark plugs.
Rotate crankahsft to get CUT (cylinder under test) at compression TDC.
Install gauge into plug hole, usually thread a hose and atach the manifold.
Whether you have a single-gauge unit or a double-gauge unit, you must supply it with over 100 PSI shop air as read by the first gauge or gauge on the shop air supply.
If there is a regulator, then adjust the reg to read 100 PSI on the tester, otherwise, turn down shop air supply to read 100 PSI being supplied.
Read second gauge, or the only gauge, on the manifold. This indication is the amount of sealing, ex. 100-20 = 80 PSI. or 20% leakage.

Leakage is presented as a percentage. I believe any leakage over 15% is warrantied an emgine rebuild.

Listen for air leaking from the valves at either the carb throat, leaky intake, exhaust pipe, leaky exhaust valve, or the oil fill cap, leaky rings.

When done, clean & recoil air hose, clean tester and store back in tool box, replace spark plugs and reconnect plug wires, and enjoy a cold drink of your choice.
 

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Is there a standard for this percentage? If I tell you my fuel pressure is 7.5 psig, you can go the NIST and find out exactly what one psig is. How much air leaks out for 1% leakage?
 

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1 percent of 100 pounds of supplied shop air is 1 pound...15 percent leakage would be 85 lbs of shop air that stays in the cylinder, and 15 lbs that escapes through rings, valves, etc...
 

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1 percent of 100 pounds of supplied shop air is 1 pound...15 percent leakage would be 85 lbs of shop air that stays in the cylinder, and 15 lbs that escapes through rings, valves, etc...
How is this calibrated? Is there a standard of leakage? Is my MAC tester going to be the same as your Longacre unit, a Jegs or a Snap-On setup? If I get different readings from each unit, which one is right, if any? Could I use one that reads a lower percentage for my own engines and one that reads higher to demonstrate how poorly my competition builds engines?
 

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as far as i know, a leak down tester is just a pressure guage... you put 100 lbs of compressed air in, and whatever reads on the guage is contained pressure...(shop air - leaking air=contained air pressure)if you supply 100 lbs, and have a reading of 85 on the guage, you are leaking 15 lbs or 15 percent...now understand that i am probably not as versed with these guages as everett, and maybe he`ll chime in to confirm or deny, but thats what i get out of it...
 

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It seems the others here catch the arithmetic. Maybe nova can enlighten us.

The tester must know the supplied air pressure. Using 100 PSIG makes the process easy.
 

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Testers I have used, have a psi reading in the reverse, normal reads is CCW with has the digits reading from 100 to 0 psi. Also attached to gauge is a simple regulator. You plug in to get your stable psi of supply of over 100# and regulate to that , then plug in hose coming off regulator and gauge and gauge will read what escapes on the low reading side of gauge. That reading is your loss. Listen for air at crankcase vents, intake manifolds and exhaust pipes. That will tell you where air is going. 15% is not bad for a street motor. I like to keep race motors under 10%. Tester I use is a Moroso unit, cira 1976.
 

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It seems the others here catch the arithmetic. Maybe nova can enlighten us.

The tester must know the supplied air pressure. Using 100 PSIG makes the process easy.
The arithmetic is easy, it's the units that are in question. What does 10% leakdown mean? How much air goes through the cylinder to get 10% leakdown?

There is no standard regarding the size of the restriction orifice and that is what leads to differences in readings between leak-down testers from different manufacturers (Some poorly designed units do not include a restriction orifice at all). In addition, large engines and small engines will be measured in exactly the same way (compared to the same orifice) but a small leak in a large engine would be a large leak in a small engine. The non standard size of the restriction orifice determines the reading which therefore differs for each design.

http://en.wikipedia.org/wiki/Leak-down_tester

A better method would be to use some standard flow measurement, such as SCFH and measure it directly using a rotameter. Now Joe's shop can do a leakdown test and get the same results as Pete's place with a standard measuring instrument.
 

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A rhetorical question is one that implies its own answer, such as "What business is it of yours?" Asking a question to which you may know the answer isn't technically a rhetorical question since the answer isn't implied in the question but is more correctly defined as hypophora and anthypophora, a classic Greek rhetorical technique.
 

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sounds like using a guage showing "best" for your engines and a different guage showing "worse" for your competitors engines is a piss poor business practice imo...one would think that an educated sounding man such as yourself wouldnt have something like this in mind...not that this is one of your practices...(i AM NOT saying that)where is the restrictor oriface on your guage???why would such a guage need a restrictor oriface???if air is leaking out of lets say the valves, why would it need to leak in addition out of the guage...the perpose of the guage is not to blead off air, but tell you that air is bleeding off...again, i`m not an expert on these guages, but seems simple...you put air in...it leaks out of THE ENGINE...the guage tells you how much constant pressure the cylinder will contain in psi...lost pressure is converted into a percentage to determine the condition of an engine...
 

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Agreed, nova, measuring air flow with a flowmeter is the correct method to measure an air leak, but using gauges makes the process easier and cheaper.
 

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sounds like using a guage showing "best" for your engines and a different guage showing "worse" for your competitors engines is a piss poor business practice imo...one would think that an educated sounding man such as yourself wouldnt have something like this in mind...not that this is one of your practices...(i AM NOT saying that)where is the restrictor oriface on your guage???why would such a guage need a restrictor oriface???if air is leaking out of lets say the valves, why would it need to leak in addition out of the guage...the perpose of the guage is not to blead off air, but tell you that air is bleeding off...again, i`m not an expert on these guages, but seems simple...you put air in...it leaks out of THE ENGINE...the guage tells you how much constant pressure the cylinder will contain in psi...lost pressure is converted into a percentage to determine the condition of an engine...
The gauge is just a gauge, it doesn't have anything to do with the pressure it reads other than it may or may not be accurate. On a 2-gauge system the orifice creates a drop in pressure relative to the orifice and the cylinder leaks, and the gauges are meant to read this pressure drop. The orifice is an intrusive measuring device, as it will affect the flow reading based on the size of the leak. Discounting effects such as critical flow and gauge accuracy, a more reliable method of calculating the cylinder leakage would be to vary the pressure upstream of the nozzle to maintain a fixed cylinder pressure and use the pressure differential to calculate the flow. The pressure drop across an orifice is equal to the square root of the flow, so low pressure drops will be inherently more inaccurate than higher pressure drops. Most commercial orifice-based flowmeters have a programmable low cut-off point, otherwise the gain becomes very high.
 
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