Compression Test Values - SBC
Thread Starter
Safety Car
Joined: Apr 2001
Posts: 3,525
Likes: 2
From: Houston TX
Compression Test Values - SBC
Ran this subject on C4 forum, thought you guys may be able to add something. My 408 has been giving me some problems as of late, that I had dismissed for tuning issues.
The 408 has about 3000 miles since being built. I have been taking it easy on the motor due to the fact that I am waiting to install the FAST system that sits on the workbench.
Ran a cranking compression test. Motor was cold, throttle was 1/2 open, all plugs removed. Checked the valves (solid roller cam) while I had the covers off; all checked OK.
The values were:
1: 152 2: 152
3: 165 4: 160
5: 162 6: 142
7: 160 8: 148
Wondering if the fact that I am running approx: 9.2:1 compression (SC application), and a very large cam (240°+ @0.050" and 114 CL) may be causing some of the issues. I had never run a comp test on this motor prior.
Thoughts?
Aaron
The 408 has about 3000 miles since being built. I have been taking it easy on the motor due to the fact that I am waiting to install the FAST system that sits on the workbench.
Ran a cranking compression test. Motor was cold, throttle was 1/2 open, all plugs removed. Checked the valves (solid roller cam) while I had the covers off; all checked OK.
The values were:
1: 152 2: 152
3: 165 4: 160
5: 162 6: 142
7: 160 8: 148
Wondering if the fact that I am running approx: 9.2:1 compression (SC application), and a very large cam (240°+ @0.050" and 114 CL) may be causing some of the issues. I had never run a comp test on this motor prior.
Thoughts?
Aaron
Pro
Joined: Jun 2001
Posts: 683
Likes: 0
From: Calgary
Re: Compression Test Values - SBC (AKS Racing)
shouldn't the throttle be wide open?
Those cranking values seem about right considering the compression and the big cam. What tuning issues do you have?
If your application wasn't supercharged, your cranking compression would be too low. Basically not enough compression for the cam. If compression and cam are matched, your dynamic compression should be around 200 for a high performance motor, 220 for street/strip, and 240 for a race motor.
[Modified by 74vetteman, 8:48 PM 10/20/2002]
Those cranking values seem about right considering the compression and the big cam. What tuning issues do you have?
If your application wasn't supercharged, your cranking compression would be too low. Basically not enough compression for the cam. If compression and cam are matched, your dynamic compression should be around 200 for a high performance motor, 220 for street/strip, and 240 for a race motor.
[Modified by 74vetteman, 8:48 PM 10/20/2002]
Race Director
Joined: Nov 2000
Posts: 11,872
Likes: 12
From: Waterloo ontario Canada
Re: Compression Test Values - SBC (AKS Racing)
I don't do compression tests anymore. The cam totally destroys any descent readings. Even with 11.25 compression with a healthy cam the readings are not over 165-170. Instead I do a yearly leak down test. Even if you don't have a proper leak down tester just make an adaptor and put in a 100-125 air line. You quickly hear any bad cylinder. I have a high precision leak down tester but still just add air to the cylinders one at a time and listen. If the cylinder is good you hardly hear anything, just a slight hiss but if the cylinder is bad at all you can really hear the leak. The motor will also turn over each time a cylinder is hit with air pushing the piston down to BDC. A poor motor will not turn over.
Le Mans Master
Joined: Apr 2000
Posts: 6,341
Likes: 1
From: Shelton CT
Re: Compression Test Values - SBC (norvalwilhelm)
I am confused. Is there a way to calculate compression ratio?
I did a comp check on my car earlier this year, and all were between 162-173psi... :confused:
I did a comp check on my car earlier this year, and all were between 162-173psi... :confused:
Race Director
Joined: May 2000
Posts: 12,712
Likes: 2,270
Re: Compression Test Values - SBC (MrNuke)
There are no magic formulas for cranking compression pressure. It's a function of static compression ratio, valve timing, and cranking speed.
Using 1963 Corvette engines as an example the two medium performance engines (250 and 300 HP) with 10.5:1 CR and a mild hydraulic cam have a OEM specified cranking compression pressure of 160 psi. The SHP/FI engines are only specified at 150 psi despite their higher 11.25:1 CR because of the overlap and late closing inlet valve of the Duntov cam.
If all cylinders test at no more than ten psi max variation between highest and lowest, then your engine is in reasonably good internal condition. Most GM service manuals specify maximum variation of 20 psi. If it is more than this, corrective action should be taken.
It it normal to test compression when the engine is at operating temperature. Remove all spark plugs, block the throttle wide open, disconnect the ignition and injector power on EFI engines. Cranking RPM should be about 250.
Duke
[Modified by SWCDuke, 7:25 PM 10/21/2002]
Using 1963 Corvette engines as an example the two medium performance engines (250 and 300 HP) with 10.5:1 CR and a mild hydraulic cam have a OEM specified cranking compression pressure of 160 psi. The SHP/FI engines are only specified at 150 psi despite their higher 11.25:1 CR because of the overlap and late closing inlet valve of the Duntov cam.
If all cylinders test at no more than ten psi max variation between highest and lowest, then your engine is in reasonably good internal condition. Most GM service manuals specify maximum variation of 20 psi. If it is more than this, corrective action should be taken.
It it normal to test compression when the engine is at operating temperature. Remove all spark plugs, block the throttle wide open, disconnect the ignition and injector power on EFI engines. Cranking RPM should be about 250.
Duke
[Modified by SWCDuke, 7:25 PM 10/21/2002]
Melting Slicks
Joined: Jun 2001
Posts: 2,429
Likes: 14
From: CA
Re: Compression Test Values - SBC (MrNuke)
mrnuke- The only way to know the compression ratio is to know what pistons & heads you have. I guess there is a way you could do it by filling the cylinder wih liquid with the piston at BDC and TDC, & measuring the difference, but I wouldn't do it. Your motor already has good compression. 170psi is a good target for a street motor.
