leak down test

[oldschool]

what would be a good-bad % reading?
should you do do it with a warm motor?
does it really matter if the piston is right at tdc,or just both valves closed?

[John_Kunkel]

The preferred piston position is TDC and both valves must be closed or you'll get 100% leakdown (think about it).

5-10% leakdown is considered good, 20% is OK, anything over 30% is supposed to mean there are problems. I'm sure opinions vary.

[firefighter3931]

Like John said the piston must be at the top of the bore and the rocker arms backed off (both valves closed).

I would be happy with 10% or thereabouts. 

If mine had 20% it would be coming apart for inspection. 



Ron

[John_Kunkel]

I'm very skeptical of the usefulness of leakdown testing, I am from the aircraft maintenance industry where cylinder leakdown testing is often called "the A&P mechanic's full employment insurance test"; 25% leakdown is often cited as the absolute limit and a lot of cylinders are removed from good running engines because of it.

If you look at the procedure, air is passed through a .040" orifice (that's 0.00125 square inches) so any leak or leaks with a combined area equal to that orifice will show 100% leakdown.

Using a 440 as an example, the piston surface area is 14.66 square inches (over 11,000 times larger than the size of the test orifice) so 100% leakdown in a static test could go entirely unnoticed by the motor in the dynamic atmosphere of the running combustion chamber. Blowby is another issue entirely.

I see leakdown testing as a useful tool for finding large leaks like burned valves, broken rings, etc. but I wouldn't condemn an otherwise healthy engine because of excessive leakdown if visual (borescope) testing shows no anomalies.

Stepping down from soapbox.

[firefighter3931]

John, i hear what you're saying and i use the leakdown to determine where the leak is coming from. If i hear air coming from the intake manifold or out the exhaust then it's not to difficult to figure out there's a valve sealing problem...burnt valve or burnt seat.

If there is no audible leakage in those areas and it's venting into the crankcase then obviously it's a ring issue and that warrants attention in my books....if it's losing close to 20%

The original poster is asking because his motor is making considerably less power than a couple of others that are identical builds. It was suggested that a leakdown test might shed some light on the situation.


http://www.dodgecharger.com/forum/index.php/topic,43279.0.html



Ron

[oldschool]

I'm very skeptical of the usefulness of leakdown testing, I am from the aircraft maintenance industry where cylinder leakdown testing is often called "the A&P mechanic's full employment insurance test"; 25% leakdown is often cited as the absolute limit and a lot of cylinders are removed from good running engines because of it.

If you look at the procedure, air is passed through a .040" orifice (that's 0.00125 square inches) so any leak or leaks with a combined area equal to that orifice will show 100% leakdown.

Using a 440 as an example, the piston surface area is 14.66 square inches (over 11,000 times larger than the size of the test orifice) so 100% leakdown in a static test could go entirely unnoticed by the motor in the dynamic atmosphere of the running combustion chamber. Blowby is another issue entirely.

I see leakdown testing as a useful tool for finding large leaks like burned valves, broken rings, etc. but I wouldn't condemn an otherwise healthy engine because of excessive leakdown if visual (borescope) testing shows no anomalies.

Stepping down from soapbox.

i know exactly what you are saying,as i do a leak down test on my bonanza at every annual.

[oldschool]

John, i hear what you're saying and i use the leakdown to determine where the leak is coming from. If i hear air coming from the intake manifold or out the exhaust then it's not to difficult to figure out there's a valve sealing problem...burnt valve or burnt seat.

If there is no audible leakage in those areas and it's venting into the crankcase then obviously it's a ring issue and that warrants attention in my books....if it's losing close to 20%

The original poster is asking because his motor is making considerably less power than a couple of others that are identical builds. It was suggested that a leakdown test might shed some light on the situation.


http://www.dodgecharger.com/forum/index.php/topic,43279.0.html



Ron

ron,i have not done a hot engine test yet.been very busy.when i do i'll post the results.
also going to change the carb.

thanks again for your insight