<TECH ONLY> Dynamic compression...
 

First, this is a <TECH ONLY> thread -- if you're not yet familiar with what that means, please see: https://www.performancetrucks.net/fo...d.php?t=369814 before reading further.

We all know what static compression is, and have probably heard people talk about dynamic compression. It is very important though, once you start down the road of power adders and big boost numbers, as each engine type has a maximum amount of compression it can sustain. Sure, you can experiment your way to a safe maximum boost, but that can get expensive and frustrating. I've also been told (by a guy who builds a lot of race engines around here) to just take the static compression and add the boost psi target to it, but, uh.. that sounds wonky.

I know your camshaft can affect dynamic compression in the following way: wider lsa means lower dynamic compression and tigther lsa means higher dynamic compression.

How can dynamic compression be calculated with more accuracy?

Not sure what you mean by calculating DCR more accuratly since there are a bunch of online calculators that will figure it out for you. Unless you mean what is the optimum DCR with FI, then I can't really help much there.

But anyways, here's a good article for anyone wondering what DCR is.

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

Excellent article Zick!


What I meant by more accuratly was more accurately than adding my boost psi to the static compression, which, after reading the article you posted, makes me think that engine builder was confusing DCR with cylinder pressure.

Zick - good DCR calculator, thanks.

TurboBerserker - to correct for boost multiply the DCR calculator ratio by your pressure ratio (14.7 + boost / 14.7).

For my own example, 8:1 static with a 294* (advertised) cam equals 5.8:1 DCR (from the calculator). At 12 lbs boost = 1.8 pressure ratio; therefore, 1.8 x 5.8 = 10.4 DCR.
With about 8 DCR being the point you can run full timing using only pump gas that means full timing to about 6 lbs (8 / 5.8 = 1.4 pressure = 5.6 lbs)
As Zick's link mentions there are variables, such as temperature, but this gives an idea of what to expect with a specific engine build.

my question is, what are our target DCR's for a high performance application?
would this be a true statement?

"Most performance street and street/track motors have DCR in the range of 8-8.5:1"

This puts my DCR at 5.85 which means adjusting for 18psi -> DCR = 12.87...

Looks like I need a bigger cam on a wide lsa to bring that down.

grippy's question is a good one. It should be noted that my stock engine works out to ~9.66 DCR on the calculator (estimated the advertized cam degrees). It may be that the GEN III's have different DCR characteristics? Or are these more determined by the fuel (Zick's link mentions "gas engines" at one point).

Another question: Do we know the upper limits? I seem to recall a post by F8L Z71 where the LQ9's upper DCR limit was 27 or 28 (but I may be confusing this with cylinder pressure??)

How did you come up with such a high DCR for your stock engine?

Typically for N/A engines, you will want to be around 8.5 max for 93 octane. I'm currently at about 8.6 and w/ 93 I can only pull about 25* at WOT or else I'll get detonation.

So Grippys statement would be correct for N/A engines.

how does boost effect things exactly? in terms of what would be an ideal high performance boosted DCR

I just punched in the numbers into the calculator. The only things I'm fuzzy on are the cam adv. and ICL so I estimated. It's the 10.1:1 LQ9, but still... I plugged in a bunch of numbers for the cam, and it was essentially the same. I never had a number lower than 9.5x