Hi all. New to group. Hoping to start a conversation about static compression vs. dynamic compression and how it relates to cam selection and even degreeing in a new cam.I always measure everything during a build and cc head chambers to calculate the exact static compression ratio. In the past, I’ve used that static compression number to select cams based on how I want to use the motor. As I learn more about dynamic compression ratios, I can see how my old method was essentially shooting from the hip and it explains why sometimes I got fantastic results while other times, I’ve been disappointed.

Now that I understand a little more, I take my static compression ratio, along with other information like intake valve closing point, rod length, etc. AMD plug all this information into an online dynamic compression ratio calculator by Wallace Racing. This is where I’m starting to run into some confusion.

The Wallace calculator states to use the advertised or ‘seat to seat’ intake valve closing point but when I do this, it spits out a ridiculously low dynamic compression ratio number. When I use the .050” closing figure in the same calculator, I get a very believable number.

As an example, before reading about dynamic compression ratio, I put a nice Hughes cam in my 340 that had 10:1 static compression. Runs like a beast and I love it but I get some pinging. I’ve been able to work around that but I’m trying to learn from my mistake by inputting the data into the Wallace calculator to see where my dynamic compression really was.

If I use the .050” intake closing number in the calculator, I get my dynamic compression ratio in the low 9s. As I understand it, anything above 8.2 is likely to ping on pump gas. Since it pings, the number generated by the calculator seems believable.

If I use the actual seat to seat intake closing point, that number is not available but I know it’s a number far greater than the .050” number of 33 degrees. This value in the calculator shows the dynamic compression ratio would be in the 7s. No way that can be right.

Example 2: I’m currently swapping a cam in a low compression 440. Static is 8.4. The cam I selected is a mild factory hp cam from Elgin. .050” intake closing figure of 31 if I advance it 8 degrees, which I intend to. That would put my dynamic number right at 8. If I use the seat to seat number Elgin supplied, the calculator shows dynamic at about 6. That cannot be right.

So am I doing something wrong? Are there better calculators?

Some suppliers provide .050” valve numbers only. Some are .006” only. Some show nothing at all on their websites. It’s frustrating.

If there are serious engine builders on this site that can break it down for me in technical talk, I’d appreciate that. I don’t want to be told what to do by armchair warriors. I want to actually understand so I can build better motors.

I know YouTube has stuff but I’ve watched hours of useless videos that never get around to explaining what I am looking for. There is a never ending stream of videos that will spend days over explaining simple concepts like valve lift. Or I have to listen to 15 minutes of warm up talk only to find the video doesn’t have what I need.

 

Dynomation-5 I think is what you are looking for…

 

This is a street build, running 91 e10 and I'm choosing my compression ratio for the pistons I'm going to pick and got me thinking about cams and DCR. My static compression ratio choices are 11.5-1 or 12.5-1 but the cams I have lower the dynamic compression ratio of both down to 10-1 and 11-1 So all that said, which compression ratio is the more important one? snaptube vidmate

 

Aluminum or cast iron heads it makes a full point difference in compression in what is tolerable maybe more with E-85.