bagger

I've read the other threads on this subject but they have not addressed the details of these tests. I’m hoping for a more detailed explanation than “hook up the leak down tester and read the % leak down”. My best guess is that the test procedures are something like the following.

Leak Down Test:
1. run car until it is warmed up based on ??? then turn engine off
2. set piston to TDC, and lock crank shaft in place
3. pressurize that cylinder through the spark plug hole to an initial pressure of ??? psi
4. wait ??? seconds, record the final pressure
5. leakdown percentage = ((initial pressure / final pressure) -1.00 ) x 100

Compression Test:
1. run car until it is warmed up based on ??? then turn engine off
2. replace spark plug with pressure tap to a pressure gage
3. hand crank the piston to TDC
4. record pressure

Can anybody confirm, correct, and/or fill in the blanks? Can anybody recommend other threads or sites that explain the tests?

m42racer

First you need to understand the differences between the 2 tests. Both are very different and give slightly different information, one based upon known info.

The Leakdown, shows the amount of leakage, based upon 100% of cylinder filling. From your earlier questions, both tests should be done at normal running temps. This way any expansion etc can be factored in. I know that sometimes these tests are done under other conditions, but they are done under these same conditions always. Comparisions are then apples for apples.

To answer your questions,

1. run engine until it shows normal operating temps.
2. remove all Plugs.
3. turn engine to TDC compression. The use of a whistle here is handy. It will tell you when you are at TDC compression. If you start with cyl # 1, the TDC marks will be up at the fan ring. Remember, they come up 2 times. If the engine is at TDC overlap, the whistle won't whistle. If you can remove the Valve covers, the cam lobes will be facing away from the rockers. Valves closed. If you know the firing order, mark the pulley every 120 degrees. Then as you count off the firing order, one of these marks should always be up at the fan ring.
4. Install your leakdown hose. Connect the tester and read the amount of leakage. Some leakdown testers are better than others. The type that show 100% on 1 gauge and the leakage on another are the best. Most types require around 80 psi line pressure. Adjust until the gauge showing 100% reads this amount. They normally tell you.
Now, the leakdown will show where the problem is. Listen thro the OPEN throttle for air coming out. This shows leakage past the Intake valve. Same goes for the exhaust. Put your ear up to the tail pipe if you can. Now take off the engine breather and listen or put your finger over the hole. Don't be alarmed by what you hear/ feel from the breather. You can expect leakage from 2 places typically. The exhaust and the breather. The amount of leakage should be the concern. Numbers as low as 2% and as high as 20% are typical. High numbers must always be considered against loss of performance, oil consumption etc. If you feel no loss, your ok. Other factors will tell you what going on too. The plugs will be oilly, if the rings are going away. All things need to be considered. I have seen leakdown tests done by experts, and they always hit(tap) the valve before they accept a number. This way any dirt etc on the seat is eliminated. Be careful here. On my race car, we see leakdown numbers as high as 20%, but the power is not affected. Every type of engine is different.

Now for the Compression test.

Same thing applies here for temp. get to normal operating temp.

1. remove all plugs.

2. Fit hose and crank engine with starter. Make sure throttle is wide open. Do this for each cylinder. Firing order makes no differece here.
3. You will read in psi, or some other number. Look for a big difference between cylinders. The amount of pressure that you should see has to be known from spec's or manufacturers spec's. I know in my case, my engine builder tests my engine on the dyno after the breakin etc. They hand me a print out of every cylinder. I then can do comp tests throughout my engines life cycle, and compare the numbers. When I see some changes, I then do a leakdown to see where the problem is, and to compare leakage to that which was done also on the dyno.

For your tests, if you know what the correct cranking PSI should be, then this test will be easier. If you see something odd, then do a leakdown.

Again, both show different numbers, but each show the condition of the sealing of the engine.

Hope this helps.

bagger

m42racer,

Thanks for all the detail, this is great! You answered all my questions about the compression test. The leak down test has more involved than I thought though so I have more questions.

Regarding “3.” for the leak down test:
What is a whistle and how do you use it? Does it make a sound to tell you when the piston as at TDC?

Regarding “4.” for the leak down test:
I still don’t have a clear idea of how you determine the % leak down. You said “Connect the tester and read the amount of leakage.” as if there will be a single pressure value on the second gauge that allows you to determine % leak down. But I think of leakage as a flow rate with measurement units of pressure (or mass) lost per unit time. Am I incorrect or does the tester have a time measurement built into it?

Your notes on trouble shooting to find the source of the problem are useful. That part of the test seems like it would be the most revealing assuming that the leak down rate was not acceptable.

m42racer

bagger,

My engine builder made me a small whistle that I put over the hose I screw into my cylinder. It just pushes on. As soon as I hear it whistle, I know I'm on the right stroke. It will not tell you when you are on TDC, only the marks will do that.

As for the percent of leakage, the gauge shows this. There is a measured orifice in the gauge assy. One gauge is set to 100% by adjusting typically a ****, the other gauge shows the leakage, based upon how much air passes thro this orifice. My engine builder tests my gauge against his calibrated orifice, several times a season.

Alan Herod

Another easy is to have a small manifold with two adjacent gauges, one which measures inlet pressure -- and the other which measures cylinder pressure when connected. Set inlet pressure to 100 psi -- other gauge will read cylinder pressure, which in this case, pressure in psi will equal percent. As discussed previously engine operating temperature, valves properly adjusted, TDC for the cylinder being tested.

Actually the higher the inlet pressure the better, but then you have to do math. (percent = inlet/cylinder pressure). Cylinder holds 97%, you have 3% leak-down.

bagger

Okay, I think that I understand how it works. In the direction of the air flow there is a compressor air supply, a pressure guage (I'll call it the up stream pressure guage), an orifice, a second pressure guage (I'll call it the down stream pressure guage), then the engine cylinder. A leak in the engine means that air will flow through the orifice and that the up stream guage will see a higher pressure than the down stream guage. You determine leak-down based on the readings from the guages. Do I have the right?

If you want to use two different leak-down testers and get the same result, that means you have to have the same orifice size and shape on both testers right? Does that mean that there is a standard orifice for all leak down testers?

m42racer

bagger,

That is right. all leakdown testers are made differently. Many are similar in design and concept, but I'm sure all will give different readings. I know I used to use a Snap On one, and my engine builder told me to change to the type that has 2 gauges. It all comes down to the make, and how good they are. You should ask for a calibration test and orifice. It should also be pointed out, that any test is better than none, and if the very worst tester is used, any change of leakage will be shown, just maybe not the correct amount. As for using 2 testers, again, I'm sure the same results will be hard to repeat. It all comes down to change, if measured on the same unit.

bagger

m42racer,

Okay that makes sense too. The goal of the leak down test is only to compare all the cylinders on a given engine. I think that I've read that all cylinders should have leak down values within 10% of eachother. So probably the main thing to make sure of is that the orifice is always clean. The compression test is probably the one where you want a calibrated gage so that you can get an accurate pressure measurement.

m42racer and Alan Herod,

Thanks for your responses, I appreciate the time that you took.