Could you guys please tell me the best way to do a compression and vacuum test on my 66. I have an idea, but I want to make sure I do it right. Any info would be great. Thanks

 

well, you need a compression tester. then you disable ignition and prop throttle wide open. pull a plug and install tool, crank engine 4-5 revolutions, record measurement, reinstall plug and move to the next cylinder and do the same. all your readings should be in the neighborhood of 120psi and have no more that 20% between cylinders. for vacuum test istall the guage to manifold vacuum and idle the engine you should have a steady 17 -21 inhg reading. this will be lower and a little jumpy if you have a radical cam

 

just wanted to add.

you are best off removing all spark plugs before the test and line them up on a sheet from left to right (cylinder #1 to #8). This way you can crossreference your compression test with the according spark plug face

as towards 120psi, this fully depends on what enmgine you are using (compression ratio). my stock 289 was around 150psi and a stock 72 302 was around 120psi on each.
if one cylinder is down in the test (less than the others) then squirt a bit of oil into cylinder through spark plug hole (max 1 teaspoon for this recipe). If readings improves after doing that then the problem is piston rings (the added oil will create a seal on the ring)

as jwog says crank with full throttle and make sure to write down what the pressure was at the 3rd compression stroke and after the last. It shouldn't go higher after 3rd or 4th latest.

It's the easiest if you have a helper. I did it once on my own, but could only do it because I have a remote starter and 3 hands ;-)

When you buy/rent a comression tester don't get one of the push-on testers. get one that you can screw in to the spark plug threads (way more accurate). You don't want to measure and then wonder if you pressed hard enough ...

 

+1 ^^^sorry forgot about the wet compression test.

 

Kalli is right on. Remove all the plugs, get a screw in type tester, disconnect your igintion coil, and cycle it through several strokes.

I will add one thing. You are looking for consistency. The amount of pressure a cylinder creates will depend on the compression ratio, cam event timing, heads, and intake. My 351C tested at over 230psi. A janer 289 will be closer to 130. It is all relative. What you are looking for is a deviation of less than 10-15% between all cylinders. If you have more than that, then you should think about a rebuild (valves, rings, or both).

 

230? holy ....

 

hahaha...yeh 230psi for 6 of the cylinders, 190 for 1, and 95 for 1. That was a stock 351 4V closed chamber Cleveland. It had obvious problems with at least two cylinder's valves, but you get my point.

Not sure what the 383 would test out at. I may throw a gauge on it when I change the plugs next.

 

As another reference, my 331 with 9.3:1, a Comp Cams Magnum 280H, and rhoads lifters (which bleed down and create more dynamic compression below 3k) reads ~160psi.

 

Sounds good guys for the compression test. I already bought a nice compression tester. As far as vacuum, can I test anywhere, is one place better than another? I had someone help me last year, and we tested off of the base of the carb. We got low readings, (motor is all new) so I either have a leak somewhere, or I am testing in wrong spot. Thanks

 

You have to find the right port on the carb. There's a ported vacuum port (which is usually hooked to the vacuum advance) and a manifold vacuum port, which has full manifold vacuum all the time. The latter is the one you need to use.