I’ve never leaked down a v8 car engine but have done many motorcycle engines. With bikes it was easy because you leave it in gear, manually rotate to tdc on cylinder, step on rear brake to keep engine from rotating and apply air.

With v8 can one remove all the spark plugs, manually rotate engine from crank nut to tdc and apply air to cylinder without engine rotating?

I don’t think I could bump the engine with starter to tdc of the desired cylinder.

Hard to keep them from rotating. I never worried about it. After all you are leaking to find where it's not sealing.

My opinion.

Dave

If you have help someone can hold them with a break over.

AS LONG AS ITS ON TOP IT WILL SET STILL,most of the time i can put the hose in and use starter

I don't worry about getting them exactly to TDC. Just reasonably close so the valves are closed, and then follow the firing order rotating 90* each time.

I only remove one plug at a time so the engine is more resistant to rotating from the air pressure.

I always leave all the plugs in except the cylinder I am testing. Put the hose in it and bump it until its at TDC. Have you regulator already set at 100 and plug in the hose.
Like Rusty said if its at TDC it should not move.
If it does its not perfectly at TDC, if you cant get it to stop moving then hold the crank with a long breaker bar. Having a helper wont hurt either

I rotate to tdc on the balancer and it will almost always hold, but I've verified that the balancer is right. I've done lots where it wouldn't hold and it pushes to bdc and I'd all 8 there. I don't know that there's much of a difference between top and bottom, but I was told that checking at the top was the only way to correctly test. I watched Jenkins leak an engine at the track and they held the crank with a bar. BTW, it leaked 4% and they changed engines. 4 was as good as I could get back in the late 70's, but after seeing that 4 was junk, I got it down to 2.

quote:

Originally posted by Ron Gusack:
I rotate to tdc on the balancer and it will almost always hold, but I've verified that the balancer is right. I've done lots where it wouldn't hold and it pushes to bdc and I'd all 8 there. I don't know that there's much of a difference between top and bottom, but I was told that checking at the top was the only way to correctly test. I watched Jenkins leak an engine at the track and they held the crank with a bar. BTW, it leaked 4% and they changed engines. 4 was as good as I could get back in the late 70's, but after seeing that 4 was junk, I got it down to 2.

Ron, it's funny. I have leaked a new engine and had 2-3% across the board. Leaked it later and had 20% on a couple and less but not great on the others. Running Methanol is probably why but the engine ran the same at 20 as it did at 2. I have always gas ported my pistons though so that probably has a lot to do with it. It wasn't leaking through the valves though.

Like Steve said leave the other 7 plugs in it while checking one.

Lots of good advice here. If its leaking down you will find it. You should also able to determine where its leaking down. Had a valve seat erode away on my BBC few season ago. Could hear it really well in the intake. And also when cranking it over - which is why I started looking.

Had about 40percent leak down vs other cylinders. Thought wow I really didn't see much impact on performance. I had it fixed and car ran exactly the same.

Point I am trying to make is even it is leaking down a bunch it likely may not effect performance much if at all. You are testing in a static state with constant pressure. When the piston is moving its split second compression and pressure.

If its running good and repeating well - I would not put much stock in the leak down results.

^^^^^^ I’m agreeing with this. Looking at 7 yrs of log books says it runs as fast now as it did in 2021 when I switched it from MFI to EFI. Thanks for everyones advice.

I tagged an exhaust valve in the burnout at the bracket finals, heard it hit, (ting) and could hear it at idle, staged up expecting to be at least a tenth off, and went dead on. Raced it 3 more rounds that way.

Note: replaced the bent valve and ran those heads another couple of seasons, when that valve seat shattered, causing a nasty failure.

simplet way. Pull pushrods throw the air to it
It will spin to bdc?That will tell you if valves are good and halfass cherck on rings. Now if doing leak test to check ring seal leakage TDC is important and since no blower snout for ratchet. A brealer bar and buddy required. yes if perfect TDC it will sit still but least bi off and it is going to spin.

Have done few hundred leak downs with a=top alcohol cars. Some times with easy method and between rounds the hard way but with blower it is lot easier. About only thing easier.