Hi,
I am about to do a leak-down test of my C5. What tool are you guys using to turn the engine at the crank bolt? The steering rack is in the way of any kind of ratchet or bar to hook up to the 24mm socket. Will an offset box end do it?
Thanks for the help,
Steve

 

I thought you could just crank the motor over with the starter to do a leak down test. I always just cranked the starter over when doing an compression test.

 

Hi,
You need the piston to be at TDC for a leak-down test. You are interested in how the rings (and valves) are sealing at the top of the bore because this is where all the action is during combustion. If you pressurize the cylinder with the piston not at TDC it will turn the engine by pushing the piston down. Hey! Maybe I can push one piston down with compressed air and then test the piston that just got pushed up by this action!

Leak down tests pressurize the cylinder and compare the readouts of two pressure gauges that are in series but with a fixed restriction (jet) between them. The first gauge just reads the inlet pressure from the source. The air then goes through a fixed restriction (jet) on its way to the second gauge. This down stream gauge reads lower if any air is being leaked out of the cylinder. The % leak down is read from a calibration chart. For my Travis setup it reads something like: @100psi inlet pressure, if the second gauge reads 98 you have 2% leak down.

A compression test is influenced not only by the rings and valves but also the intake valve closing point. I am trying to get some baseline data before I change my cam and heads soon. One of things I am trying to do is keep the cranking pressure with the new cam very close to stock. This will give some indication that the dynamic compression ratio is the same and the car should be happy with the same octane gas.

Bye for now,
Steve
:seeya

 

If you've got an M6, and are up on jackstands, you can do what I do.. put the car in reverse and put a lug wrench a rear tire.. you can turn the rear tire and rotate the engine this way....

No laughing, it works!!!!!!

 

Hi,
M6 it is! Thanks for the tip. I love inexpensive solutions!
Steve
:seeya

 

SNW, thanks for the info on the leak down test! I appreciate it.

 

nevrenuf,

Along the same line, but I always just put the car into a gear ( in this case, I'd try 3rd....if not fine enough control, the try a lower gear) and simply rock the car.

That's the way we used to center the breaker points on the high point of the distributor cam.

Mark

 

Thats why I love this guy...in a manly sort of way! :smash: :cheers:

 

One of things I am trying to do is keep the cranking pressure with the new cam very close to stock.
--SNW Vette

To accomplish this goal, I would think that you would want a cam with about the same amount of overlap as you have now

 

Hi,
As the piston is coming up on the compression stroke the intake valve is still open for a while. This late closing of the intake bleeds off the intake charge at low rpm’s and lowers the observed compression pressures. I am going to LS6 heads that will raise the static compression ratio to ~10.5 and I am going with a cam that has about the same intake valve closing point as the LS6: ~42 degrees ABDC. I am doing this combination to try and keep about the same octane requirement as my stock engine. Here in CA we only have 91 octane.

At low rpm’s valve overlap dilutes the intake charge at the top of the exhaust stroke giving that wonderful big cam idle sound. My new cam has much more overlap.
Bye for now,
Steve
:seeya