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Compression test...

17K views 24 replies 8 participants last post by  BillyGman 
#1 ·
Does anyone know where I can find some kind of conversion chart that will tell me the compression of my car if I am doing a standard compression test with a lbs gauge.

Or does anyone know what kind of reading I would get say if it were 10:1,11:1,12:1,12:.5,etc...

Thanks
 
#2 ·
That totaly depends on the cam/duration etc ,you can have 11:0 comp with with 150 psi cranking compression and 9:5 comp with 180 psi cranking comp depending on the cam.

Just curious,what size motor/what heads/what pistons/and what are the cam specs?

Scott
 
#3 ·
You cannot calculate what the cranking compression will be (PSI) from knowing what the static compression ratio is due to their being too many variables, however I have seen an online DYNAMIC Compression ratio calculator which tells you what the cranking compression of your engine will be, but you have to punch in the exact camshaft specifications, and if you don't know what they are then it won't work right. Furthermore, if you perform a compression test on an engine that has worn rings, worn or improperly seated valves, then your PSI readings on the compression guage will be reduced, and that will make it more difficult to accurately determine your actual compression ratio from such a test. There really are a lot of variables. When I get home, I can post a link to a dynamic compression ratio calculator. Although there are a number of them on the net, I've only found one which gives you the PSI reading of what the cranking pressure should be.
 
#7 ·
i had 11.7 to 1 bbc 396 with a solid ZL-1 cam specs 262/273 @ .050" it was cranking out 150 psi in the cylinder.

396 bbc with 7.7 to 1 compression and a 272 crane cam it too was 150 cranking psi

454 with 50 cc dome pistons 14.5 to 1 compression and herbert solid roller cam 254/268 @ .050" it was 270 psi.

little off beat 350ci with 11.4 to 1 compression and 284 crane cam had 220 psi.

thought i would just throw those out there to show some examples of some of the engines i have put together.
 
#9 ·
Here you go.... thelink below will bring you to a site where you must punch in all the cam info as well as the bore and stroke of your engine. You can then leave the "volumetric efficiency" box at 94 and the boost at 0 as well as the altidude at 0 and then click the box right below it that has the flames in it. A window will pop up with your "Dynamic" compression ratio and also the dynamic cranking cylinder pressire in PSI. And the second link below is a good calculator for static compression ratio in case you don't know what it is in your engine. You're going to need the exact static compression ratio figure for the the first one for finding your true dynamic compression ratio.

http://www.speedwaybids.com/DynamicCompressionRatioCalculator.php

http://www.rbracing-rsr.com/compstaticcalc.html
 
#10 ·
Here you go.... thelink below will bring you to a site where you must punch in all the cam info as well as the bore and stroke of your engine. You can then leave the "volumetric efficiency" box at 94 and the boost at 0 as well as the altidude at 0 and then click the box right below it that has the flames in it. A window will pop up with your "Dynamic" compression ratio and also the dynamic cranking cylinder pressire in PSI. And the second link below is a good calculator for static compression ratio in case you don't know what it is in your engine. You're going to need the exact static compression ratio figure for the the first one for finding your true dynamic compression ratio.

http://www.speedwaybids.com/DynamicCompressionRatioCalculator.php

http://www.rbracing-rsr.com/compstaticcalc.html
Thanks BillyGman!
 
#13 ·
BTW,FWIW, the famous David Vizzard claims that engines with dynamic cranking pressures of less than 200 PSI will run on 93 octane pump gas without pinging, and when you go above the 200 PSI mark, you're usually getting too close to pinging on pump gas, and race gas is usually required to avoid it. I hope this helps.
 
#15 ·
Yeah I did a dynamic pressure test today, came up 220 psi.

I am thinking of just running race fuel and re-adjusting the timing because obviously it's retarded in order to run on pump gas.

she still runs like an animal though, without pinging or any noise.
 
#18 ·
That would be awesome if I could run mine on pump gas. A little scared to try it though...
Are you sure you have 12.8:1 static compression ration in that motor? Did you do the math, or are you just going by what the piston manufacture said? There are too many variables to be doing that.

Yeah I did a dynamic pressure test today, came up 220 psi.

I am thinking of just running race fuel and re-adjusting the timing because obviously it's retarded in order to run on pump gas.

she still runs like an animal though, without pinging or any noise.
I'd be careful if I were you. If you have a loud exhaust, then it might ping on pump gas without you even hearing it...... Not good.
 
#16 ·
Buy some kemco real lead bosted so you can run the full/correct timing curve for best perf,why build a perf motror just to detune it to run a pimp fuel.

Race is more epxensive then buying cases of kemco at the discount price which inc free shipping too.

1qt kemco to 18 gals 93 fuel + approx 97.5 octane and with the doscoing price thats approx 50 per gal more when trsated with kemco,it should do the trick and its less $ then race fuel ,give it a try,its workes good for me.

Scott

Go to their site to buy it.
 
#17 ·
Buy some kemco real lead bosted so you can run the full/correct timing curve for best perf,why build a perf motror just to detune it to run a pimp fuel.

Race is more epxensive then buying cases of kemco at the discount price which inc free shipping too.

1qt kemco to 18 gals 93 fuel + approx 97.5 octane and with the doscoing price thats approx 50 per gal more when trsated with kemco,it should do the trick and its less $ then race fuel ,give it a try,its workes good for me.

Scott

Go to their site to buy it.
Thanks, I will try it.

Yeah I agree with you on the building an engine with high compression not to use it. I bought the car this way and I am still trying to figure out the best way to manage it.

Drew
 
#22 ·
The only problem I have with the theory behind the "dynamic" compression is that it does not account moving mass (of the air/fuel mixture) and the overfilling action of the engine cycle. The speed of the starter is certainly not fast enough to show you what happen at 6000rpms. So you may see 190psi (or whatever your number is) at cranking speed but I guarantee you as soon as you get to the "opperating range" of your cam, you'll be right at your static compression of 12.5-1. You'll detonate less with a very large cam at low rpms (with a light load) because you are indeed not filling (and compressing) as much than if you had a smaller cam (again efficiency range of the cam), but, I think one should alway be cautious, as this is never black or white far as what can works and what doesn't.

Also, the theory behind the 200psi max cranking for 93 octane goes out the windows because it depends on your cooling efficiency. LS2 engines are 10.9-1 cr from the factory and they don't ping. Marine engines also are less prone to detonation and can use more compression because they have a "lake" or "ocean" as their radiator and can be maintain at a very cool temperature (but do sustain higher loads, so it's a give and take relationship)
 
#23 ·
Also, the theory behind the 200psi max cranking for 93 octane goes out the windows because it depends on your cooling efficiency. LS2 engines are 10.9-1 cr from the factory and they don't ping.
The 200 PSI max for 93 octane pump gas assumes that your cooling is right around 180-190 degrees constant once the engine warms up. As far as the LS2 engines, we would need to take a look at their cam specs, because if the duration is long enough, then the 10.9:1 static CR will still yield less than 200 PSI on the compression tester gauge. But quite frankly, I don't know what the cam specs of those engines are. If your camshaft is big enough, you can run an 11.5:1 static CR successfully on pump gas too without pinging. ;)
 
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