81vette

Below is a link to a site that explains dynamic compression ratio and it also has a very useful dynamic compression ratio calulator that you can download. The program can also be used to calculate static compression ratio, and cam card specs.

One other thing...when you install the program it may try to install system files that are older than the ones already on your computer. It will prompt you if this is the case, so make sure you don't install over the newer files that are already on your computer.

http://members.uia.net/pkelley2/DynamicCR.html


mark


[Modified by 81vette, 10:21 PM 1/8/2003]

'79ProwlerOrange

:cool:

1979toy

Has anybody checked this program against an engine that they are currently running?
I may be putting the wrong cam in my new engine according to the program, but the recommendations from the cam venders say that the duration and compression ratio are a good mix.
Sure would like to have somebody that knows the specs on thier motor check this program to see if the dynamic compression ratio is anywhere near right.

71roadster

I found this calculator a while back and posted a link in the engine mod section. There is really no way to know if it is accurate by comparing it to your engine specs (tho it did determine my cam timing accurately as compared to my cam card). Based on the definition of dynamic compression starting at the point of the compression stroke where the intake valve closes (ABDC), the calculator is correct. Is this in reality how Dynamic Compression is determined? Who knows, but it sounds pretty viable to me, and I'm an engineer. Not bragging, just making the point that I have a strong math background and that the calulations used are correct for that definition.

With my cam installed at 4 degrees advanced, I get an 8.55:1 DCR. What are you all getting.

Something to consider....the high compression ratios of the glory days were all based on using a really long duration cam to get the DCR into a usable range. Arguements persist as to what a usable range is, but some seem to think about 8.5:1 is as high as you want to go without having to back down the timing.

Chris

1979toy

Using the dynamic calculator my proposed engine has 10.2 dynamic compression on an engine with 10.5 static compression. :eek:
This isn't sounding streetable.


[Modified by 1979toy, 7:59 PM 1/9/2003]

81vette

Steve,

Something isn't right. Why don't you post all of the data your entering, so I can check it for you on this end. Make sure you give me the advertised cam specs, as the @.050 cam specs won't work. When I enter my data into the program using my cam cards suggested static compression ratio I get an 8.4 dynamic ratio.

mark




[Modified by 81vette, 9:09 PM 1/9/2003]

1979toy

The engine is a .060 oversize 400. Bore 4.185
Stroke is stock 3.75
Rods are 5.7 inches
Pistons are flat tops with a 7 c.c. valve relief
Heads are Edelbrock RPM's with 70 c.c. chambers
Head gasket is .041 compressed thickness
Gasket diameter is 4.200
Static compression should be 10.5

The camshaft is/was going to be a Comp Cams P/N 12-677-4 solid lifter cam. I did not take in to acount any of the lash losses from the valve lash setting. I did use the .006 off seat numbers from Comp Cams.
Intake Duration 274
Exhaust Duration 280
Lobe center 110
Overlap 57
IVO 31
IVC 63
EVO 74
EVC 26
Cam was to be install straight up.

81vette

Putting your pistons down the hole .010" I get a static compression ratio of 10.54, and a dynamic compression ratio of 8.57. That may put you on the edge, but in your case if you have some detonation problems you can move the timing to straight up and you'd have a dynamic compression ratio of 8.32 according to the calculator.

I've noticed with this program that to get it to re-calculate the dynamic compression after advancing/retarding the cam you need to press the CALC button on each of the panels in sequence. Cam Timing panel first, dynamic stroke panle next, and then uncheck the check box on the Compression Ratio panel and press calculate. Then re-check the box and hit calculate again.

Make sure when you fill out your info that you only fill out the top section of each panel, and then hit the Calculate button. Let me know if you get it working.

I messing around with the advance/retard. It's interesting to see how advancing the timing could actual produce more HP by increase the dynamic compression ratio on a car with a low dynamic compression.


mark

71roadster

Steve, there seems to be two numbers missing from your description.

intake lobe center. this is different from lobe center of 110. 110 is probably your lobe seperation. I tried the calc at an intake lobe center of 104 degrees. The other number missing is your deck clearence. I ran it assumin you are having the block 0 decked. I came up with a DCR of 8.9.

oh yeah, with that gasket thickness you really need to consider 0 decking the block. any more quench will encourage detonation regardless of your compression ratios.

Chris


[Modified by 71roadster, 2:08 PM 1/10/2003]

1979toy

To update. My block is 0 decked. The intake C/L is wrong. The correct number is 106. I have also found out that the number for the valve relief in the piston should not be a minus number.
With these corrections I now get 8.67 dynamic compression. Darn, still too much by these standards. :sad: