Hello every one. I installed a Holley on my 79 150 460 and since I did this I'm getting some blue smoke puffing out of my pvc port, could this be old gunk being burned off or bad rings well to be sure I'm going to do a compression test on it, but I've never done one before and I was wondering what would be the best kit to buy? And also would some one be so kind as to explain a wet compression test please??
and also what is the psi range i'm looking for? any thing over a 100?
Thanks a ton

 

I think any compression checker would work. I mainly look for somethink that is out of wack with the other cylinders. There are a lot of variables like how long its been sitting and have you turned it over a bunch off times to wash down the cylinders with gas. It should be over a 100 psi. I just look for the ones that are way off the others.

 

My 92 460 just tested at 145-150 on all cyls. Factory Ford Reman'd motor with about 7000 miles. Biggest thing I've heard is that all cyls should be fairly close.

 

A wet test is like a dry test...

A wet test includes a dry test first to get the base line compression of each cylinder while it is "dry".

The wet part is doing the dry again but this time after squirting about a table spoon or so of engine oil into the cylinder. The engine oil wets the piston rings and may increase compression. If compression increases it means the rings are bad. If the compression does not rise, then probably there are bad valves, guides, cracks or holes.

Roughly you should be seeing about 150psi of compression(depends on your engine model). You want to see all cylinders within about 10 percent of the others, say between 135psi and 165 psi. Tighter grouping is better. You don't want to see two cylinders next to each other with similar low compression which could indicate a head gasket leak between the cylinders.
So for example a decent compression test might be 145 150 155 140 160 157 148 143.
A bad one might be 110 110 150 155 145 150 145 145, the two 110 readings might be a bad gasket. Or 120 155 110 160 145 0 120 130 might indicate worn cylinders/piston rings etc and possibly one holed piston.


I prefer the screwin guages since you don't have to hold them in place while testing. The rubber nipple ones are sometimes easier to use but require a helper to crank the engine, unless you have a remote start.


Good Luck,

Jim Henderson

 

If you want to simplify, take lowest pressure reading, divide by highest reading, and multiply by 100 for the percentage. Lowest number should be within 75% of highest, according to a few mechanics I know.

Example: low reading 140 psi high reading 160 psi: 140 / 160 X 100 = 87.5 %, which is an acceptable difference.

I agree that the screw-in compression testers are better. Most times you will get one that has adapters that screw on for different plug sizes, and you can unscrew one of these and attach it to an air line from your compressor to hold a valve closed while you change a valve spring or valve stem seal, or other valve train part. Two tools in one! lol

 

Thanks guys so very much for the info. Just one thing

(squirting about a table spoon or so of engine oil into the cylinder. The engine oil wets the piston rings and may increase compression. If compression increases it means the rings are bad. If the compression does not rise, then probably there are bad valves, guides, cracks or holes) So if I get a higher wet psi over the dry test psi then bad rings and if the psi does not rise above the dry test number then bad valves? how does that work? I most be missing some thing.. But thank you very much again.

 

The wet test is mostly used if you get a low reading on some cyls. Say your at 140s on 6 cyls, 120s on 1 cyl and one cyl is in the 90s. You'd do a wet test on the low cyls to find out if your problem is in the shortblock or in the heads. If the oil brings the #s up then you know the problem is the piston/rings, because the oil helps worn rings seal better, but doesn't help worn valves.

You can always do a wet test on a good motor just to check the status of the rings/heads as well...

 

Oh so if the psi stays the same! then its a valve I gotcha now.. thanks guys a ton

 

Right: A valve that doesn't close all the way because of carbon around the seat, or other reasons such as a crack near the valve, broken guide, etc means the cylinder won't seal properly, and the pressure just vents out that area instead of building normally.
Sounds like you got it.

 

Any way you do it low compression and high miles means you should rebuild the whole engine. If you determine that it needs valve work and rebuild the heads it won't be long before your tearing it back apart to do the bottom end. So the advice is do it right, do it once. Good luck, Rich.

 

What cobraguy said........ been there done that, do it right the first time it's cheaper in the long run!

 

thanks guys =)