Static Vs Dynamic Compression, What's the Difference?


#1

I’ve heard those terms but don’t understand the difference between the two. This terminology tends to come up in discussions about cam profiles. So what does it all mean?


#2

This can probably explain it better than I can. Note the first paragraph. Dynamic starts getting complicated and variable; and especially if you’re not familiar with cam overlap, cam timing, and so on…

http://www.enginebasics.com/Advanced%20Engine%20Tuning/Static%20vs%20Dynamic.html

Dynamic doesn’t really come into play for DIYers and mechanics in the field. That’s mostly something for automotive engineers and performance engine builders to be involved with.


#3

I’m not sure I agree that dynamic compression test does NOT come into play for “mechanics in the field”

Granted, it isn’t a test that is performed very often, but occasionally it does come in handy


#4

That link is a pretty good description of the concept.


#5

Excellent link, my friend. It explains the concept better than I ever could! :stuck_out_tongue_closed_eyes:
Thank you.


#6

When the intake valve closes later, not only is the dynamic compression reduced, you could also say the dynamic displacement is reduced, since only the cylinder volume above the intake valve closing point counts as displacement.

The so-called Atkinson cycle engines used in the Toyota Prius uses intake cam timing to control power output, effectively making it a variable displacement engine. Need more power? Increase the displacement of the engine by closing the intake valve sooner, need less power? reduce the displacement of the engine by closing the intake valves later.


#7

Thanks, excellent link provided by OK and informative discussion by the other posters. I understand conceptually at least the difference between static & dynamic compression now.