Cranking Compression Question
Thread Starter
Burning Brakes
Joined: Apr 2001
Posts: 752
Likes: 1
From: Springfield Oregon
Cranking Compression Question
I did a compression test on my motor, all plugs removed, throttle open, motor warmed up, and got 140psi (+/-1psi) on each cylinder. I have a 350(+.040) stock replacement short block with 76cc heads milled .006. I'm not sure what the static compression is, but the cranking readings seem low compared to what I've seen most people post. The motor only has about 15K on it, and I have a Summit cam 204/214, 420/442 lift installed straight up. The intake lobe centerline is 107 and the LDA is 112. Question is, are these readings abnormally low? I thought about advancing the cam 4deg., but I'm not sure if it allready has advance ground into it and another 4deg. would be excessive. The thing just feels soft until 2-2500RPM. I thought with this small of a cam I should have a little more bottom end. I know part of the problem is a stock converter and 3.08's. Would like to hear any thoughts on this. Thanks.
Melting Slicks
Joined: Dec 2000
Posts: 2,041
Likes: 0
From: Wichita, KS
Re: Cranking Compression Question (ORVette)
Guess that I'll watch how this goes too. I just checked mine yesterday and got 145 to 150 on all cylinders. Until reading this I had thought that it was normal.
Cam specs are: Comp XE262, duration IN 262 EX 270, Lobe center 106 degrees, Lobe seperation 110 degrees.
Engine: L48 with ? combustion chambers. Were 76c.c. now milled .030 with .015 head gasket.
Cam specs are: Comp XE262, duration IN 262 EX 270, Lobe center 106 degrees, Lobe seperation 110 degrees.
Engine: L48 with ? combustion chambers. Were 76c.c. now milled .030 with .015 head gasket.
Racer
Joined: May 2000
Posts: 386
Likes: 0
From: Niedernhausen, Germany
Re: Cranking Compression Question (ORVette)
When you cranked the engine for compression testing, did you open the throttle FULL? Further more, all other spark plugs ought to be removed, too.
You are right, you reading seems a little low (on a stock SB, it should read around 160psi) BUT: The deviation from cylinder to cylinder that you have stated indicate for a sound engine. The low reading results either from not opening the throttle or/and the cam.
BTW, if you want to check the engine, a blow-by-test is much more accurate and can tell you where the problem comes from - piston rings, head gasket, inktake gasket, valves (in case there is one). You can probably borrow the tools for that in a shop...
Good luck,
Best r
You are right, you reading seems a little low (on a stock SB, it should read around 160psi) BUT: The deviation from cylinder to cylinder that you have stated indicate for a sound engine. The low reading results either from not opening the throttle or/and the cam.
BTW, if you want to check the engine, a blow-by-test is much more accurate and can tell you where the problem comes from - piston rings, head gasket, inktake gasket, valves (in case there is one). You can probably borrow the tools for that in a shop...
Good luck,
Best r
Race Director
Joined: Oct 2000
Posts: 14,956
Likes: 79
From: central FL, near the beach.
Re: Cranking Compression Question (ORVette)
when it was rebuilt, was it honed with a torque plate?
Are the pistons dished?
my guess is near normal for dished, low by 20-30 psi if flat tops.
but it will run on Terrible Herbets gas!
Are the pistons dished?
my guess is near normal for dished, low by 20-30 psi if flat tops.
but it will run on Terrible Herbets gas!
Thread Starter
Burning Brakes
Joined: Apr 2001
Posts: 752
Likes: 1
From: Springfield Oregon
Re: Cranking Compression Question (drives61)
When I tested it I had all the plugs out and the throttle linkage full open, allthough I didn't block open the secondaries. I do not know about the rebuilding technique, but it probably wasn't done with a torque plate. It is a mass produced rebuilt block distributed by PowerMaster(Cummins Northwest). It was in the car when I bought it. I don't remember if the pistons were dished from when I replaced the heads, but I do remember they had 4 valve relief cuts and also the outer 3/16 or so of them was cut down at like a 45deg. angle. It does run great, plugs look perfect, steady 18 in. vac.@600RPM, just kind of a dog out of the hole.
Drifting
Joined: Nov 2000
Posts: 1,361
Likes: 0
From: Honolulu Hawaii
Re: Cranking Compression Question (ORVette)
Sounds like dished pistons. Another possible cause for lower compression is the use of thicker composition head gasket than the stock steel shim.
A good book to read about cranking compression is David Vizard's "How to Build Max Horsepower on a Budget".
A good book to read about cranking compression is David Vizard's "How to Build Max Horsepower on a Budget".
