Compression ratios...

Ok, here is a good one for you. Can someone explain the difference between static and dynamic compression ratios, and why there is a difference? Also when talking about an engines CR, which one is normally talked about?

Dynamic compression basically takes into account the effects of changing combustion chamber volume due to the valves being open at certain points in the cycle,thus your cam timing has an effect on this variable.


CR quoted in technical text & manufacturers specs etc are static ratios.

So a dynamic CR is an ever changing number then? What use does it have technically?

I'm guessing that it would be used in calculations at a particular point in the engine cycle.
For what purpose i don't know.


Have you come across the term in your research?

I've heard DCR being thrown about in conversation a few times but never got a satisfactory explanation of it, in laymens terms.

You boys are getting all mathy and techie on us!

Here is a very technical page on DCR.... went right over my head [You must be registered and logged in to see this link.]

Question is, is DCR important enough to care about, given my engine makes the numbers I'm after and drives how I want it to?

Nice to know this stuff though

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I'd say no.Its something to be aware of maybe but I certainly won't be trying to factor this into my build.
Tbh I wouldn't know how.

There was a thread on here about setting up your cam timing etc.
I think this is where this maybe would come into play as at certain rpms the compression ratio will be slightly different than the static value.
But I can't that most peoples engines would be that sensitive to these small changes.
I think it is beyond the scope of our experiences.
I'm sure if i'm wrong someone will correct me