Originally Posted by gboutch
What is normal cylinder leakage, I just tested all of mine using BDC and valves closed? Sounded like air was going past rings.
A competition engine should be 1-2 percent
A street engine in good condition should be 10-15%, less is always better.
A street engine in worn but servicable condition 15-20%
A street engine showing 20% or more is reaching the overhaul point.
The difference range between the low and high leak cylinders shouldn't exceed 4-5%
Leaking past the rings is expected and normal if the engine has gapped rings as threre will always be some leakage through the ring gap even if nowhere else.
Testing at BDC gives the best reading you'll ever see as at TDC there is usually more bore wear from where the operating pressures are the greatest and the lubrication the least.
Leakage past the exhaust valves needs to be addressed as when operating that leakage is not compressed air but flames hot enough to erode the seat material which means this situation only gets worse.
Older model engines with low silicon cast pistons will leak more than newer engines with hypereutectic amounts of silicon because the hypers have a tighter clearance which keeps the rings better aligned to the cylinder wall. This can also be said for forged pistons where in general forgings have more clearance than cast so the rings don't address the wall quite as well and within forgings the 2618 alloy needs wider clearances than the 4032 so one would expect the 2618 to show more leakage again because of ring alignment to the cylinder wall. All the for-going assumes a lot of other things have been done correctly in terms of optimizing skirt to wall clearances for the piston materail type and having the correcct wall finish for the type of ring material i.e. straight cast, chrome plated, moly filled or plasma sprayed. So this is a gross generality of what to expect from a pretty new engine.