Leak down test questions

[bull]

I'm trying to do a leak down test on my 383 today but I'm running into a couple questions. First off, how can you tell if a given piston is at TDC on the compression stroke without taking off the valve covers? I assume it's when the rotor is pointed at the plug wire that corresponds with the cylinder you're trying to test? Or should I just take the valve covers off? Also, will the test be negatively affected if the engine has not been brought up to operating temp? It's been out of the car and on the floor for a while now so I have no way of starting it so it can get oil up into the cylinders.

Even though I'm running this test I've almost talked myself into just rebuilding the engine regardless. I bought a good block a while back and then today I noticed that my original block has a broken motor mount ear on the drivers side. Aside from that I really don't know what went in to this engine the last time it was rebuilt. Of all the previous owners the only one I've ever been in contact with was the guy I bought it from and he did nothing with the car in the six years he had it so I don't know the compression ratio or anything.

[aifilaw]

you can get a ballpark by hooking the starter up to it, and putting a compression tester on that cylinder, ensure the rest of the plugs are out of the motor, get a fresh battery and spin her until it shows max psi, calculator your dynamic CR from that... only way to really tell is to take off a cylinder head and do some measuring.
It's a safe bet that the engine is in good shape if all cylinders are within 5% of each other, but no way to tell what the cam is, or how much its bleeding off.
a piston will be "near" TDC when the rotor points at its respective plug wire....but if you really want to know where TDC is you need a degree wheel

[bull]

I already tried the starter thing a few weeks ago but it wouldn't engage. The starter probably went out while it was sitting aroud.

I'm starting to think there's no way for me to get any real accurate results with this test on a motor that's out of the car. It won't be properly lubed, it won't be at operating temperature so the pistons can expand and it's very difficult to say for sure when the pistons are @ TDC, at least visually. I ran the test on one cylinder that I thought was @ TDC and I had blowby coming out the exhaust pipe so either my testing procedure is already flawed or the engine needs rebuilt.

[Todd Wilson]

Put your thumb over the spark plug hole and turn the motor. When it pushs air up against your thumb then you know you are on the compression stroke. Put a straw in the hole and keep turning motor until the straw stops moving up!


Todd

[bull]

Put your thumb over the spark plug hole and turn the motor. When it pushs air up against your thumb then you know you are on the compression stroke. Put a straw in the hole and keep turning motor until the straw stops moving up!


Todd

Ok but once it's there what about potential blow by from the test because the cylinder is not lubed and the rings and piston cannot expand because it's not warm? I suppose I can spray some lube into the spark plug hole but I can't run it to warm it up unless I build a stand.

[firefighter3931]

It's best to warm up the motor when doing these types of tests, inmo. If the engine has been sitting awhile chances are that there will be some minor flash rust on the cylinder walls....which will usually clean up with a little run time. A motor in that condition is difficult to evaluate, unfortunately. 



Ron

[bull]

That's what I figured. I have to assume I'm going to get some erroneous test results if I can't run the engine. I emailed a guy about possibly having the engine put on a dyno and checked that way because I'd kind of like to know the hp numbers and torque too. We'll see what he says.

[Todd Wilson]

Sounds to me like you either need to install it and go  or rebuild it since its out.


Todd

[bull]

Sounds to me like you either need to install it and go  or rebuild it since its out.


Todd

Yea, I'd rebuild it before I ever install it just to see if it runs good. Putting an engine in just to face the possibility of having to pull it right back out again does not sound like the definition of efficiency.