Sale ends todayGet 30% off any course (excluding packages)

Ends in --- --- ---

Dynamic compression ratio

Off Topic Discussion

Forum Posts



Tech Articles

If it's not really about tuning or wiring. Then it belongs in here.

= Resolved threads



Is there any course/webinar done regarding dynamic compression ratio?

What happens (oem 9.0:1) after installing aftermarket camshafts? (N/a, forced induction)

There are plenty online dynamic compression ratio calculators that will give you general idea of how it is related to the camshafts.

In truth, there are (at least) three "compression ratios" - or at least the way I look at it, others will disagree ;-) - but it may make an interesting discussion topic?

The first is the nominal one we're all used to - (swept + unswept)/unswept.

The second is theoretical, but often referred to as "dynamic" with the swept volume taken from the closing of the inlet valve and not the bottom of the stroke, because the piston would have pushed the air out the open intake valve and not actually have compressed it.

The third is what is more accurately called dynamic, IMO, as it's what you get in the running engine - this is directly related to the volumetric efficiency (the real meaning, not the forced induction rubbish) as there is potential, by using port velocities, tuned induction lengths, etc, to get more than the nominal 'volume' of air into the cylinder. With racing engines it's possible to get up to 150% filling of the cylinder, which means it is going to be effectively compressed 50%.

So, for a race engine, the nominal may be 14:1, the theoretical may be 10:1 and the dynamic at peak efficiency may be the equivalent of 21:1. That last bit may help you understand why some engines may suffer from mid-high rpm detonation when the timing is the same.

I'd agree with Gord's post - Most of the discussion on 'dynamic compression ratio' ignores the fact that the cam design optimises cylinder fill across a certain rpm range and hence the actual air mass trapped in the cylinder is significantly higher than what you'd achieve if you had a cam profile where the valve opening and closing events occurred at TDC/BDC which would theoretically give you the same static and dynamic compression ratio. In all honesty it's not something I give much consideration to as a metric.

+1 for Gord’s explanation.

we need to consider that any fluid has inertia and by calculating dynamic compression ratio as the point of which the IVC we are ignoring physics. It is a good way to visualise how cam characteristics effect compression ratio but a much more practical example would be to consider the pressure ratio as this is something we can easily validate with in cylinder transducers or the Plex kit for example

In many articles I have read about this topic it says that people often get confused between DCR and cylinder pressure which are two separate things. DRC is nothing but one of just many engine properties whilst cylinder pressure depends on many factors not only DRC.

We usually reply within 12hrs (often sooner)

Need Help?

Need help choosing a course?

Experiencing website difficulties?

Or need to contact us for any other reason?