Leakdown?

ohmthis · 06-09-2008, 04:04 AM

I'm getting ready to do a leakdown on my 454 mag and I need to know what to look for. I know about the compression, but what about the leakdown? How long do I let it sit until I check, What is a good reading vs. bad, and any other things I should know. Thanks! Anthony

Chris Sunkin · 06-09-2008, 08:17 AM

Leakdown will tell you specifically where any losses might be occuring. It allows you to isolate if you have leaks on intake or exhaust valves or through the rings. Leakdown is measured in percentage of loss over a period of time so keeping track of your readings is helpful in charting cylinder wear. Obviously if you have loss through valves, your problem is immediate. You want to do your testing on a warm motor.

hoozeyurdaddy · 06-09-2008, 10:19 AM

I've never heard of a leakdown that you let sit and wait for a reading, what type of set up is this, the leak down that I've always used, you have a air hose attached and use a double gauge, fist gauge is incoming pressure and second gauge tells you the amount of leakage thru the combustion chamber. a good cyl. will have less than 10% drop from incoming air to leaking air, and a max of 15% diff between cyl. what type do you use?

Chris Sunkin · 06-09-2008, 12:13 PM

I wasn't very clear on that. Over a period of time is meant to refer to doing periodic tests and charting the leak, following cylinder wear and determining when it's time to rebuild the motor.

One other thing about leakdown. You need to measure each cylinder in at least 3 crankshaft positions. I've seen wall damage that didn't show up on a leakdown tests as the test was done with the piston at TDC. I always back off the rockers to test anyway.

hoozeyurdaddy · 06-10-2008, 10:36 PM

Quote:

Originally Posted by Chris Sunkin

I wasn't very clear on that. Over a period of time is meant to refer to doing periodic tests and charting the leak, following cylinder wear and determining when it's time to rebuild the motor.

One other thing about leakdown. You need to measure each cylinder in at least 3 crankshaft positions. I've seen wall damage that didn't show up on a leakdown tests as the test was done with the piston at TDC. I always back off the rockers to test anyway.

OK, I thought I was going to learn something new, but were on the same page, but I have only done them in 2 positions, I think 3 is a good idea, I will start doing that, thanks for sharing.
Larry

ohmthis · 06-13-2008, 03:12 AM

Thanks Guys, I was way off based.

Thunderstruck · 06-13-2008, 01:57 PM

How do you keep the crank from turning at anything other than TDC or BDC? Or close to BDC. Low air pressure? If I am a little bit off of TDC the crank turns. I do use 100 psig air to make the leak down calc easier. Any comments?

Kidnova · 06-13-2008, 03:56 PM

Quote:

Originally Posted by Thunderstruck

How do you keep the crank from turning at anything other than TDC or BDC? Or close to BDC. Low air pressure? If I am a little bit off of TDC the crank turns. I do use 100 psig air to make the leak down calc easier. Any comments?

Long breaker bar, and a 500 pound Gorilla on da crank nut...........Wanna know how I know that ?

Thunderstruck · 06-14-2008, 02:18 PM

So your sayin Bob is a Gorilla?

Budz Motorsports · 06-18-2008, 08:27 AM

A leakdown is a very accurate way to check valves and rings. We have done a few hundred leakdowns here and I have never heard of doing a leakdown in three positions! You must bring the piston up to TDC in the hole you are checking. Three places to look. Ex pipe for bad valve, Throttle body or carb for intake valve, and oil fill for rings. In a new engine just getting sealed after break in should be around 3% for a stock rebuild. Anything over 15% to 20% sounds like its getting tired! My still run fine but not getting all the power it once had.