<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

Here is some food for thought, I know this doesn't exactly relate to the boost question but it may help.
The first three are just some DCR calculators and the last may be helpful or just a pissing match because I didn't get too far into it.
https://ls1tech.com/forums/showthrea...hlight=dynamic
https://ls1tech.com/forums/showthread.php?t=512946
http://www.kb-silvolite.com/calc.php?action=comp
https://ls1tech.com/forums/showthread.php?t=305151
http://popularhotrodding.com/tech/03...press_07_z.jpg

9.5 is way too high for a stock engine to run even w/ premium. There was a thread that said basically the stock LS1/LS6 was around 6-7.5 DCR stock. If I knew the cam numbers at .006" for the stock LQ9 cam I could try and figure it out.

Yep, that's the big problem :)

I also found another problem. I'd done my 408 right before that and forgot to change the stroke and the rod length on the dynamic stroke form lol. Now it says DCR = 8.85 using 230/238 and 117 lsa and icl. That's better.

Why would a preferred DCR for a boosted engine be different than N/A?

I don't think it would be -- that's why I'm partially confused here. There's no way I can adjust for boost and hit 8-8.5:1 DCR at high boost. I'd need a PR of 1.45, so max boost would be 6.6psi? I make waaaay more power at higher boost, so it seems we're missing something here.

The chart Zick posted is also interesting but doesn't seem to account for good intercooling. Maybe that's the difference on the FI side since intake temp affects detonation as much as octane and DCR.

What prompted me to start this thread was trying to figure out the maximum "safe" boost I could run. Maybe I need to be looking at dynamic cylinder pressure...

That is the question that preoccupies our people. :lol:


Seriously though,

https://ls1tech.com/forums/showthrea...36#post4788636
SStrokerAce makes a good point. A FI engine is going to have much higher temps and I'm sure more volume from all the air its cramming into the engine that it may throw off the standard DCR rates.

Or you could skip all the #'s and use the old school method. Make sure you have a good stout short block then push it till mild detonation, then you back off. It has worked for years for many/most. I prefer to stick to the old methods, call me simple minded.
If you push the envelope without good parts then carniage you may have.
Although, I do find the theory of DCR interesting. :lurk:
I just feel there are too many variables to start counting pennies thats all.

It would not be different but I think you're forgetting all the things we do to prevent detonation because we are a higher DCR with boost : race gas, intercoolers, water/meth injection, much less spark advance, 11:1 too rich AFR instead of an optimum 12.8:1, 165* thermostat, colder plugs, etc.

If you plan a boosted engine for just pump gas and driveable then it's 8:1 static CR with a bigger cam and 6-8 lbs boost and it'll be right about 8:1 DCR. If you want more then we have to do the above added things. Also keep in mind that if you plan it for very high boost near an 8 DCR the engine will be a dog while out of boost or at low rpms.

BOOST DCR calculator: http://www.rbracing-rsr.com/comprAdvHD.htm

And another... http://kb-silvolite.com/calc.php?action=comp

People get too caught up on DCR and pump gas. I've looked into this a bit lately and DCR doesn't take into account overlap. Overlap is a key factor when determining what grade of fuel you can minimally run, which is what were all after. More boost, more timing and more power on pump gas.

Another thing I noticed from the posts above is that some of you are using .05" duration and some are using .006" and seat durations. What is the correct one to use? I think that is why some are getting DCR's in the upper 9's while others are getting low 8's and high 7's.