70 LS5 Static Compression Test
Thread Starter
Melting Slicks
Joined: Apr 2007
Posts: 3,189
Likes: 111
From: Tolland CT
70 LS5 Static Compression Test
I've been working on getting the idle to settle out. Vacuum levels seemed low, around 14...
Checking the basics, I did a compression test.
Reads between 200 - 210 on all cylinders.
Seems a bit high though I've never heard of this being an issue.
I've read about cam profile, timing, having an effect on static compression PSI. Generally went the other way, lower compression readings due to timing and overlap.
Are these pressures typical for a stock 70 LS5?
I don't know what cam is in there, it was installed a very long time ago.
Not sure I have the paperwork.
Would there be any reason to check cam timing? Degree the cam?
Looks like a pain in the butt to do it in the car. If it were a radical cam I'd expect lower compression due to overlap?
I had a reputable shop assemble the motor for me. Installed a double roller chain. Wondered at times if it could be off, giving me a low vacuum. I would think it would show up with lower compression if the valve timing is that far off.
I have not tried to adjust the idle mixture screws yet. The carb was recently rebuilt, mixture screws set to 2 turns as received.
Checking the basics, I did a compression test.
Reads between 200 - 210 on all cylinders.
Seems a bit high though I've never heard of this being an issue.
I've read about cam profile, timing, having an effect on static compression PSI. Generally went the other way, lower compression readings due to timing and overlap.
Are these pressures typical for a stock 70 LS5?
I don't know what cam is in there, it was installed a very long time ago.
Not sure I have the paperwork.
Would there be any reason to check cam timing? Degree the cam?
Looks like a pain in the butt to do it in the car. If it were a radical cam I'd expect lower compression due to overlap?
I had a reputable shop assemble the motor for me. Installed a double roller chain. Wondered at times if it could be off, giving me a low vacuum. I would think it would show up with lower compression if the valve timing is that far off.
I have not tried to adjust the idle mixture screws yet. The carb was recently rebuilt, mixture screws set to 2 turns as received.
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Compression is fine. In addition to the total number you get as a maximum, you also need to look at how it gets there. 1st stroke, 3rd and total.
You are correct- all the cam timing and duration will make a difference.
If your cam is "radical", it won't idle very smooth, torque will probably be pretty low off a stop. Ignition timing will also affect the vacuum.
Go back to the basic stuff and look for a vacuum leak. These cars have lots of vacuum driven accessories and a leak in a couple of spots and idle and vacuum readings get a little funny.
You are correct- all the cam timing and duration will make a difference.
If your cam is "radical", it won't idle very smooth, torque will probably be pretty low off a stop. Ignition timing will also affect the vacuum.
Go back to the basic stuff and look for a vacuum leak. These cars have lots of vacuum driven accessories and a leak in a couple of spots and idle and vacuum readings get a little funny.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
With engine idling, remove the main vacuum line coming off the intake manifold and connect the vacuum gauge in its place. With no other vacuum items connected [and unable to leak], the reading you get will be the maximum possible with that engine set-up. If that reading is significantly higher than the 14 in. Hg you previously obtained, you have some leaks in the vacuum systems somewhere. If you get the same vacuum level, it is what it is....
Thread Starter
Melting Slicks
Joined: Apr 2007
Posts: 3,189
Likes: 111
From: Tolland CT
I'm getting about 15 now when connected directly to the manifold, no accessories.
Shouldn't be a radical cam, I recall asking for cam with similar characteristics as stock. I just want to drive the car without having to tinker a lot with it. Cam as installed many years ago as I mentioned earlier. I'd have to dig a bit to see if I have any info on what's in there.
Still more to do with checking timing and adjusting idle mixture. I set the mixture screws to 4 turns out as spec'd in the Service manual. Didn't get a chance to try adjusting in our out to see if vacuum improved.
PS: Haven't ruled out a leak at the base of the carb. Don't think there's a leak but haven't checked.
Shouldn't be a radical cam, I recall asking for cam with similar characteristics as stock. I just want to drive the car without having to tinker a lot with it. Cam as installed many years ago as I mentioned earlier. I'd have to dig a bit to see if I have any info on what's in there.
Still more to do with checking timing and adjusting idle mixture. I set the mixture screws to 4 turns out as spec'd in the Service manual. Didn't get a chance to try adjusting in our out to see if vacuum improved.
PS: Haven't ruled out a leak at the base of the carb. Don't think there's a leak but haven't checked.
Last edited by BBCorv70; Sep 5, 2010 at 10:35 PM.
