I worked on my new ride toady afterwork.
I found a fouled plug, and changed it....now it runs on 8...funny eh?

I also took thistime to check compression.....and I have a question...What is good compression for a stock 289?

 

Ok....heres what I got...."Cold"

Cyl 1.= 115psi
Cyl 2.= 120
Cyl 3.= 107
Cyl 4.= 120
Cyl 5.= 115
Cyl 6.= 120
Cyl 7.= 110
Cyl 8.= 103

I dont think the variation is that great...but then again...its 40 years old.

 

Bu^^^^ Because I wanna know too

 

I can not recall what the cranking compression was when mine was stock. But I would guess that is pretty respectable for stock...how many miles are on the engine? For comparison purposes my 2k mile 11.5-1 solid roller 302 has 165psi give or take a couple all the way around.

 

this engine has 148000 on it. 1967 289

 

The difference from low to high is more important than the actual number. I run around 90psi with my high compression 302. The reasons are the cam and altitude.

You really want to be within 10-15% for the cylinders. Another thing you should do is add a little bit of oil (a squirt or two from a oil gun is enough) and check the pressure with the oil in the cylinder. It will increase the pressure just due to the increased sealing power of the liquid, but the amount of increase will tell you what kind of shape your rings/walls are in.

 

+1

its more about the variation in the numbers.....if one of the cylinders is off then you'll notice one of them being more than 20% off than all the others

 

well to wark out you compression divide by atomspheric pressure which is circa 14.4 psi if the number are guage pressures (the guage reads zero when not connected) If they are absolute pressures minus 14.4 off them before dividing by 14.4. But as the other have said variation is the key.

 

So if I use this formula, if I have 8.7:1 compression ratio, I should read 125.28 psi.with a pressure gauge,
or if it a 9.2:1 compression I should be see a 132.48 psi. reading

This would mean the cylinder that is running 103 psi. has a 7.15 compression ratio, and the 120 psi. cylinder is running a 8.3 compression ratio.
Sound right?

Although I am just 14.17% different from highest to lowest on my readings, my worst cylinder is 20.77% below the 130 psi reading of a new cylinder. and my best cylinder is 7.7% below the new cylinder reading.

I am only drilling this topic because I am wondering if it would be worth getting the heads rebuilt now, and put them back on the blockand running it, for a year or so, then pull the engine and get the bottom end done later. or if the engine is so far gone, I need to pull it and rebuild the whole thing and just look at the car from the kitchen windowtill I can afford to pay for the whole shabang.

 

It seem u have some issue from the compression test do the wet compression as mike suggested to get a better idea of the issues. If you compressions are then more homogenous you have found out there is problem with rings/walls.