I think that I may not be explaining this well but I will give it another go. Basically you have a constant air supply from a compressor hooked up to a gauge and regulator. With only the air supply connected you adjust the regulator so you have 100psi. Now, without touching the regulator you connect the regulated 100psi air to a cylinder with an air hose that is threaded into the spark plug hole. This is done with valves closed and piston near top dead center. Now when this constant source of 100psi regulated air enters and fills the cylinder some will leak past the rings and possibly past the valves. So, when connecting your 100psi source to the cylinder and the gauge has dropped to 90psi the cylinder is leaking 10% of the air you are feeding it. If the gauge reads 95psi it is leaking 5% of the air and so forth. Each cylinders reading is an independent leakdown percentage and has no relation to the other cylinders. While you want them to be fairly consistent the leakdown number is only based off of the gauge reading and not based off of the other cylinders. (source 100 PSI minus PSI retained by the cylinder= percentage leakdown) Now, on a dual gauge leakdown tester the 1st gauge is pressure and the second reads in percentage leakdown. (same exact method and theory but the second gauge means you don't need to do the simple math of calculating leakdown with the pressure gauge). You will find that on the dual gauge tester when you hook up the air pressure source to the gauges and adjust the regulator to make the leakdown gauge read "set" or "zero percent leakdown" the first gauge will be at 100psi test pressure. This is how everyone I have seen works anyway.
Can a leakdown hurt an engine? There seems to be a few owners/dealers who refuse to have a leakdown performed on cars they have for sale due to the damage it may cause. Is this fear completely unfounded?
I think you hit the nail on the head, the nail being my head. Now I understand. Thanks for the clear explanation, James.
The only way to damage an engine in a leak down test is to leave the wrench (used to rotate the crank) attached to the nose pulley and then if the piston moves under 100 PIS of pressure the wrench hits something in the engine and harms it. The normal operating cylinder pressure of an engine (like the F355) is 1200-1300 PSI (TDC at 3000 RPM 100% throttle). Heck the normal operating pressure at idle is over 100 PSI just to keep the engine turning over. No, the real fear is loosing ones ability to say "no known trouble" in a sentance containing the word "engine".
Exactly. If it isn't 1-2% the engine builder did a crappy job. It is the normal range for a good 355.
When I was young I did exactly that leaning over an engine when I opened the valve to put the 100psi into the cylinder. The breaker bar handle cranked since I was not exactly at TDC and center punched me right in the chest. I fell over as the air was knocked out of me. I learned a lesson that day!!!
