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Leakdown Testing

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Leakdown Testing

Post by ScottAndrews » Wed Dec 08, 2004 3:11 pm

I was asked on Roafly to provide some info on doing leakdown testing. I figured this might be useful for y'all here too.

All engines leak. The compression test records the highest pressures reached during the piston cycle. The limitation is that even with a leaky cylinder it is possible to obtain high pressures for a brief moment. The idea of the leak test is to really identify where thigs are leaking and how much they are leaking

Here is a link to an article on building and using your own leakdownm tester. You need a good air compressor to do this test.

The article is pretty informative, but here are some tips.

The ?damper valve is very important. I made mine from an NPT nipple (this is the pipe hardware they use for compressed air; look around the compressed air stuff at the hardware store). I filled it with JB weld (use the quick stuff) and let it harden. I then drilled a .060 ole through it.

The engine side fitting needs to screw into the plug hole. I used the hose and fitting from my screw-in compression tester.

Set the compressor to about 100 PSI at the end of the hose.
***IMPORTANT*** If you cannot sustain reasonable pressure on the compressor side, then your compressor is too small. I use a 8.5 cfm 30 gallon unit (usually for painting). I have seen people with leaky heads that could not use the little portable units. A good test for this is to open the tester valve all the way. Check the maximum pressure you can sustain on the compressor side. This is your baseline. Even if you can?t reach 100 PSI, you simply record this number and divide all your leakdown numbers by it (to get percentages). If you can?t sustain at least about 50 PSI, then get a bigger compressor

Remove all the plugs, and take off the valve cover so you can see the valves.

Shut off the air valve, screw in the fitting snug in to cyl #1

Rotate the crank until the valves are both closed and the rockers appear to be on the lowest part of the cam lobes.

Crack the air valve (a little at a time. DO NOT DO IT ALL AT ONCE OR THE CRANK WILL SPIN, AND THE WRENCH YOU ARE USING TO TURN IT WILL HIT SOMETHING-like maybe your arm...BAAAAD JU JU).. The crank will try to turn. Close the valve and rotate the crank in the opposite direction a little. Repeat this until the crank does not turn. You are basically balancing in piston right exactly on TDC where it has no mechanical advantage.

Now when the crank stays put with the valve fully open, measure the air pressure on the engine side of the damper valve. Unlike the diagram in the link, my tester has TWO gauges, one on either side of the damper. This is preferred since you can get rid of the hose losses. In this case you adjust the compressor output pressure until the compressor side tester valve reads 100 PSI. Otherwise you have the pressure drop in the hose adding an error.

With 100 PSI on the compressor side of the damper valve record the pressure on the engine side valve. Listen for where the air is coming from. It may be blowing up from the timing cover (around the timing chain, or it may be whistling in the exhaust manifold, and it may even be blowing out the adjacent plug holes. Since you engine is in the car, you should use a small plastic hose to examine this. Stick one end in your ear, and then listen to the various places. Record what you hear.

Do this for each cylinder.

You are looking for about 95% pressure retention. Anything below 90% implies a problem. Leaks in adjacent plug holes imply cylinder to cylinder leaks (mine had this on two cylinders-bad head gasket, but no coolant leaks). Leaks into the exhaust imply carboned or burned exhaust valves. Leaks into the timing cover imply rings.

Typically ring losses will be accompanied by low compression in that cylinder. The compression reading will rise when you squirt a tablespoon of oil into the cylinder and spin the engine a few times and then repeat the compression test.

So to summarize:
You are seeking info on three things:
1) Ring leaks-found by combination of dry and wet compression tests and confirmed by leakdown blow through the timing cover
2) Head gasket leaks between cylinders (quite common since the cylinders are close); confirmed by leadkown blow through adjacent plug holes
3) Valve leaks (typically exhaust); confirmed by leakdown blow into the exhaust (or intake) manifold
4) Gross head gasket leaks into the cooling system; confirmed by air bubbles in the coolant (although anything this gross would probably make the engine put out clouds of white steam...)

Chris Wright

Post by Chris Wright » Fri Oct 14, 2011 7:04 pm

I just noticed that the procedure above uses an orifice of 0.060" in its differential compression tester, which is correct for a bore of over 5", but our engines are smaller than that, so the correct orifice is .040" x 1/4" long.

Here is the source I used:
The specifications for the orifice size in the compression tester can be found in the FAA's publication Advisory Circular 43.13-1B paragraph 8-14b. For an engine cylinder having less than a 5.00-inch bore; 0.040-inch orifice diameter; .250 inch long; and a 60-degree approach angle. For an engine cylinder with 5.00 inch bore and over: 0.060 inch orifice diameter, .250 inch long, 60 degree approach angle. The publication specifies a reading of 60/80 with this internal orifice.


Post by ScottAndrews » Fri Oct 14, 2011 9:02 pm

OK, so if you use a 0.06 orifice then the numbers will be better than they really are.. so if you have crappy leakdown numbers with a .060 orifice then you will have really crappy numbers with an 0.04 orifice.

Good info though.

I wonder what bore has to do with it. THe pressure drop seems to me to be related more to leak rate. For example, if I had an engine with no leaks, then the values for pressure drop would be the same regardless for orifice or bore size.

Similarly, if I had an engine with no sealing at all, I would see a massive pressure drop, but while the observed drop for a 0.04 orifice might be higher than for a 0.06 orifice, this would only be a result of the feed hose. Suppose I put an infinite pressure tank on one side of the orifice, and had an open pipe on the other side. Let's suppose I made that pipe >5"in dia. The pressure on the open side would be atmospheric. The pressure on the tank side would be 100 psi (or whatever I set it to). Now, let's do that same thought experiment with a 4" dia pipe. As long as that pipe isn't 1000 feet long, so I am measuring the back pressure and the pipe fills with air, the pressure in the pipe will stil be atmospheric. So, the drop wil be the same.

I am sure the FAA approach yields good results, but it seem to be a pretty arbitrary distinction. Probably has a lot to do with the fact that a 5" bore has more perimeter, and so it has a higher typical leak rate.

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