Ok all you pro engine builders out there; help me out.  What is an acceptable number for a leak down test on a race engine???  What are the correct procedure to follow to do the test??  Any help appreciated.  

The leakdown test is used for evaluation of engine conditions, hot or cold.  At TDC of the power stroke you inject 100 psi of air and measure how much leaks out.  The important part of this test is where the leaking air is leaking to.  The are 4 areas and you can identify them by listening for air leak noises:  1) the oil pan.  Listen on the valve cover.  This will indicate cylinder/ring/piston/head gasket problems.  2) the carburetor which indicates an intake valve seating problem.  3) the exhaust system which indicates an exhaust valve seating problem.  4) the radiator with the cap off, which will indicate a gasket or cracked cylinder.

Now let me caution you about leakdown numbers.  It’s similar to racing dyno numbers or flowbench numbers.  You have to understand your test equipment and keep with that tester.  Sunnen makes a great tester, but uses a lower injected test pressure, around 50 psi.  Their leakdown numbers will be ½ that of a Snap-On tester which uses 100 psi for their test pressure.  The orifice size in the regulator also influences the leakdown number.  The Snap-On tester will show 8% as pretty good.  Anything 10% and lower is good on that tester.  Any leakage through the intake valve is bad.  A couple of % through the exhaust can be caused by carbon/rust in the seat.  Always write down the leakdown % numbers to the corresponding cylinders and look at consistency.  1% deviation across the board is good.  At 3% or more you need to start looking at why.  Remember, this is an evaluation tool.  

A damper marked every 90 degrees, or at the TDC of each cylinder is very helpful in the leakdown process.  T find the power TDC for #1 you can put you thumb over the hose as you turn the engine over either by wrench or a remote starter button.  You’ll feel the air begin to escape the hose as the piston approaches TDC.  Both rockers will be at rest with the valves closed.  Set the damper to the 0 degree timing mark.  Plug the tester into the cylinder and take the reading.  Using a wrench, turn the engine over to 90 degrees (V8’s) and plug the tester into the next cylinder in the firing order.  Continue to the next and so on.

To measure the true performance of the piston rings/cylinder wall/sealing a blowby meter is needed, such as B&B’s.  This device measures cfm of blowby out of the crankcase.  This leakage is comprised of gases that escaped the combustion area past the rings during actual full power running.  A good engine will have less than 2 cfm all of the time.  You can put one in the car and monitor it visually, via a camera or the data acquisition system.  We always use it on the dyno.

A quick tip:  round and straight cylinder walls are essential basics.  Sacrifice this and no other performance enhancements make sense.