Compression Test

[Dusty1]

After a year of getting the bugs out of my 340, I got brave enough to do a compression test. I guess the results explain a few of the issues I had. I was told a '68 340 would run about 120 lbs. , is this correct? So is 155 lbs. too much?

[Chryco Psycho]

No , that is what I would expect to see

[Dusty1]

155 is normal pressure? I really didn't know what it would have. Thanks

[67vertman]

My 120,000 mile 318 had 135 to 140 psi, so 155 is good.

[Dusty1]

Thanks Ron

[cudabob496]

My 120,000 mile 318 had 135 to 140 psi, so 155 is good.

where are you in so Cal?

[AussieMark]

Did you do the test with a warm engine, all spark plugs removed and gas pedal fully pressed down? I would expect between 160 and 180 this engine was a high compression, premium fuel engine and should actually have the same test results as a street Hemi. The main thing to look for is equal pressures from each cylinder with only small differences.

[blown motor]

5000 miles after the rebuild my 340 tested one cylinder at 148 and the rest at 155.

[Dusty1]

Cold motor, all other plugs in and didn't touch the gas, just cranked it. It didn't very 5 lbs. on every cylinder. Sounds like I needed a little more instruction!

[DocMel]

I really dont focus on what one or two readings might give me.  I take the old school approach, take all 8 readings and ensure the highest and lowest reading does not differ more than 10%  But single reading on a suspect cylinder to diagnose a prob is not a bad idea

[AussieMark]

I really dont focus on what one or two readings might give me.  I take the old school approach, take all 8 readings and ensure the highest and lowest reading does not differ more than 10%  But single reading on a suspect cylinder to diagnose a prob is not a bad idea   Exactly the more equal the pressures are the better

[brasil]

mine showed 175 to 180 psi on all cylinders  when I checked the compression last year....

Greetings Jurgen

[734406pk]

After a year of getting the bugs out of my 340, I got brave enough to do a compression test. I guess the results explain a few of the issues I had. I was told a '68 340 would run about 120 lbs. , is this correct? So is 155 lbs. too much?

What kind of issues are you having?

[Dusty1]

After the rebuild, a couple of years ago, I would only get a couple of hundred miles on it before it would push the felpro head gasket out from under the left head. Put a cometic gasket on it and it started blowing water out under the header toward the front of the left head. After a lot of consultation on this site, I put new ARP bolts in and used copper spray sealant on the new cometic gasket. 1000 miles and no problems yet. Just now able to get to things like this instead of tearing things back apart.

[734406pk]

After the rebuild, a couple of years ago, I would only get a couple of hundred miles on it before it would push the felpro head gasket out from under the left head. Put a cometic gasket on it and it started blowing water out under the header toward the front of the left head. After a lot of consultation on this site, I put new ARP bolts in and used copper spray sealant on the new cometic gasket. 1000 miles and no problems yet. Just now able to get to things like this instead of tearing things back apart.

 That's strange, only the left cylinder head. Are these aluminum heads? Hopefully it will now stay sealed with the ARP bolts and Cometic gaskets. Best of luck!