Right way to do Leak Down Test?

I have a 505 street/strip engine it has less than 50 miles on it and 4 passes. I did a compression check on it and all 8 holes were pretty close (165-170psi). I bought a leak down tester to check it further. What is the best engine warm or cold or doesn't it matter? Thanks Mike

Some people might say warm, I do everything cold. Cold is easy to be consistent. When you start a test warm it is cooling off every minute you work on it.

True

You are not in a hurry so I would do it warm a couple cylinders at a time.

I do everything cold as well.

Put the piston at TDC, true TDC. If you get it at true TDC the crank will not turn. But you may need to set up each cylinder 2-3 times to get it there. If its not there it will turn the crank. DO NOT attempt to put a breaker bar on the crank and hold it, you will get hurt. You would be amazed at the amount of force 100 PSI on a 440 piston generates (it is enough to drive through a well adjusted E brake about 1 foot every few minutes)

Also, don't expect 2-3% leak down numbers. I was never a huge believer in leak down tests, and as time goes on the results I've seen have pushed me more away from them. The best results I ever got was 2% "Hero" results on a late model engine that made 125 PSI cranking compression (150-160 would be ideal) and did not run that great.

Thanks for the pointer. I'm not expecting any huge differences since compression test was pretty consistent.

Cold is easy to be consistent.

If it's in the same heated garage.
Ambient isn't consistent.

I do them both ways, dead cold after sitting overnight in the shop and then warmed up to at least 180F. I do it at TDC usually but you can test with the pistons down in the cylinders like piston engine aircraft mechanics due
They remove all the plugs and use the prop to bring the piston up far enough so both valves are closed and due the test there Some will do it at TDC also but not all of them do that
I use a aircraft gauge now, two gauges and reset the first gauge pressure so it has 80 lbs. pressure with air going into the cylinder being tested at TDC and read the pressure lost with the second gauge going to the cylinders with 80 lbs. on the first gauge

As an aircraft mechanic I've done lots of these, the publication AC 43.13-1b outlines the correct procedure. The test is done on a warm engine and, to start the test, air is introduced into the cylinder while the piston approaches TDC and is finalized with the piston at TDC. The airplane has an advantage because of the prop, you can grab a prop blade and manipulate the piston position instead of just parking it at TDC. This assures that the ring is fully seated against to bottom of the ring land.

Thanks everyone

Agreed, but the temp you start the test at will likely be the same temp you finish with, unless you are super slow lol.

That is interesting, and makes total sense. If you have a balancer that is marked every 90 agrees and you do your leak down test following the firing order you should be good on this aspect as long as you don't pass the mark and back it up.

I have rock the crank backwards and forwards to see if it make any difference, it can , but don't move it much more than 2 to 4 degrees from TDC , it will blow the piston down if you do

From my experience, rocking back and forth from TDC a little does improve the test results.

One thing I've noticed is tapping open a valve when the cylinder is under pressure (soft mallet) can lower the leakdown percentage as the pressure shock squares the pistons/rings in the bore.

I've seen 3 to 6%
The piston and ring design has a lot to do with leak down results.
All the stock aircraft pistons I've seen have tapered rings sides, (not square or parallel like car motors have) like some H.D. diesel motors do, correct John ?

So, which one is a better determinant of whether or not it's time for a refresh?

I look for 15% or worst on my race motors and similar on hot street motors.
That would be with 65 lbs. to the cylinder being tested with 80 lbs. out put pressure on my test gauge pressure

I just did a comp test on my 360. Nominal 12.5 compression. 180+ on 1, 4, 5, & 7. 190+ on 6, 200 on 2, 210 on 3 & 8. This was dead cold. Just for curiosity's sake, I sprayed some WD40 in #1 and retested. It gained a few psi. Thoughts?

I haven't done a compression tests in more years than I can remember, 20 yrs+
I do remember doing them both cold and hot and adding oil into the weak cylinders to see if that helped the rings seal better, if it didn't increase more than a little bit I was taught it was not the rings leaking, it was usually the valve train causing those issues.
Using a leak down meter you can feel the air leaking around the rings by blocking off one breather and holding your hand over the other one to see if it builds pressure up in the crankcase or not and you can hear it leak by the valves on the carb side and the exhaust sides if it is bad valve sealing
I do use a brass hammer to pop open the valves when doing a leak down test that I hear leakage from the valves, sometimes it will help and on others not very much
I use them as tools to help me under stand what condition the heads, blocks and rings are in

Lots of varied replies but my 2cents as a dyno guy for years... just do it cold and bump it to help rings for a 'best' reading...it's really more a troubleshooting aid than anything. I've seen some motors leak REALLY bad and still make about the same power as they did at the start/fresh.

The cylinder leak down test is humorously referred to as "the A&P mechanics guaranteed employment test" because leak down of 25% or more in aircraft is considered "fail" even though the engine runs fine and makes plenty of power.

Consider the makeup of the tester, air flows through an orifice that is .040" in diameter...that is an opening of 0.00125 square inches in a cylinder that can be anywhere from 30 to 70 cubic inches in volume.

To whom are you referring John.

Are testers NOT allowed to wear make-up???

I've been an engineer mechanic for over forty years and NEVER done a leakdown test...
What does it prove?
Compression is being lost past the rings and valves and/or the head gasket.
A good mechanic would know if there is a problem by the signs?

We always know, that when an engine is making good power, then its worn out and ready to BLOW!
The trick is, knowing when to make the LAST run before this happens...

A leak down tester is a great tool to diagnose something before it gets worse. I'll use my small block as an example. Car was running fine, ran a new best of 10.54 just a few weeks before I did a leak down and compression test just as a health test. I found 2 cylinders with leaky exhaust valves that amounted to about a 35% leak down number, the good cylinders averaged 3%. So off with the heads, new hardened exhaust seats and now the whole motor averages 3% leak down. I haven't beat that new best ET yet but I have added about 80lbs of interior back and my MPH has gone up so its making more power. I guess I could have just waited until it was backfiring from trashed exhaust seats but the leak down test seemed like a good idea.

With a marked balancer its about a 10 minute test and can give you some great info to keep you from chasing your tail.

MAKEUP

noun

Definition of makeup:

the way in which the parts or ingredients of something are put together

Apologies John, looks like my attempt at limey humour has failed...


Running without unleaded seats in 2020 Wow, how did you last this long lol...
American gas must be a lot better than UK fuel, (gas).

Built a 340 for a guy who ran it down the track for one day, seats destroyed...
Heads had lasted 40 years till then...

UK John without makeup...

and to add to what John said, there are different orifice sizes based on bore size. Put in 80 PSI and you should read better than 60 for some engines, 42-43 psi for others as long as it is not through the exhaust.