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**BlackoutSteve** · December 31, 2007, 03:23 AM

Re: Freiburger, DCR, and testing.....

I do see what you are saying, however, I simply see the DCR changing as the VE improves with air inertia as the engine speeds up.
Perhaps this is so hard to explain and grasp because air is so flexible.

Maybe if we express air in units. 10 units squeezing into one creates a 10:1 c/r.
Dynamically, at low engine speeds where reversion exists, one unit gets pushed back into the intake by the up-rising piston, so only 9 units get squeezed. A DCR of 9:1.
At high engine speeds using a tunnel ram for example, 11 units are able to cram themselves in, creating a different DCR of 11:1 in the same engine.

We may be saying how density doesn't have an effect, but of course it does. A denser intake charge equals a higher C/R. If you want to count the air molecules getting squeezed at different engine speeds, perhaps the above example isn't really that far off.

**Saltfever** · December 31, 2007, 04:15 AM

Re: Freiburger, DCR, and testing.....

Originally posted by BlackoutSteve

We may be saying how density doesn't have an effect, but of course it does. A denser intake charge equals a higher C/R. If you want to count the air molecules getting squeezed at different engine speeds, perhaps the above example isn't really that far off.

No it doesn't. You are confusing higher pressure with a change in a fixed ratio which can't change. look at it this way. Take a cylinder and evacuate it out leaving only two molecules of air. Due to the nature of a gas they exert a pressure (although very tiny) because they are flying around bumping into everything. Now compress them into a space 10x smaller. You have decreased the distance they travel bumping into everything, including themselves, by 10x. Their collisions will be more frequent because they travel 10x shorter distances. As a result they produce more heat and pressure. Now use the same cylinder but due to some bad rings on the vacuum pump you couldn't get all of the molecules out and now there are about 1,000 individual molecules of air in there. As you say, it is a denser mix just like ram tuning. They are all flying around bumping into everything. But their flight path is very long and normal so there is no pressure rise. Now compress all of them down to 1/10 the space. 1,000 of them are now flying around bumping into everything, and themselves. They are hot and angry. And since there are 1,000 of them compared to the original two molecules you bet the pressure and heat is greater due to 1000 more collisions in the same tiny space. However, the compression distance (their flight path distance) is exactly 10 times shorter, just like the compression distance of the two molecules. The compression RATIO (the distance their flight path has been shortened) has stayed the same even though the pressure goes up. Folks, my apologies for taking some license with the gas-law problems but you get my drift ;D

**JeffMcKC** · December 31, 2007, 05:41 AM

Re: Freiburger, DCR, and testing.....

Originally posted by JeffMcKC

More keyboard racing, detonation occurs in motors at different times, because of more than just the compression and type of fuel used in it, no matter what the DCR is.

See how much time I tried to save you  :

I hate to agree with DF, but he is right on this one.

" But cylinder filling is not the only variable. With rpm you also have rate of quench and tumble, ignition timing, number of firing events, and TIME. The formula for DCR takes NONE of those variables into account."

I was just glad I did not get sucked into it this time.

**joe_rocket45** · December 31, 2007, 07:41 AM

Re: Freiburger, DCR, and testing.....

it's a new day ;D

Ok, now that everybody has explained what Compression Ratio is...

How do you determine the point the intake valve is considered closed??
That's my dog in this fight 8)

**std** · December 31, 2007, 08:51 AM

Re: Freiburger, DCR, and testing.....

If my car door is ajar, I can't get in or out but it's still not closed.

I say it's when the valve touches the seat.

**squirrel** · December 31, 2007, 09:00 AM

Re: Freiburger, DCR, and testing.....

Originally posted by joe_rocket45

How do you determine the point the intake valve is considered closed??

You hear the "clunk" of metal on metal

Anyways, that still begs the question of when how much of the air slows down or stops moving.

**Eric68** · December 31, 2007, 10:27 AM

Re: Freiburger, DCR, and testing.....

I don't believe I managed to stay out of this one for this long . . . . I felt like Luke Skywalker holding on for dear life trying not to get sucked into outer space through a blown out window in the Death Star ;D Well, I have succumbed to the dynamic pressure (ooooh, bad pun I know) and now I am here with light saber in hand . . . LOL

I really think most of this debate has been over semantics. Especially surrounding what the word "dynamic" means in the context of DCR and cylinder pressure. I think we need to forget about whether the term "dynamic" is right or not -- I don't think any of us really think the term is the best name anyways, but we have to call it something right?

Next I think we need to understand that there are a few different calculators out on the net that all use the term "DCR calculator" to describe them but calculate different things in different ways. Some of them do not take into account atmospheric pressure, like this one -- http://members.uia.net/pkelley2/DynamicCR.html and some do take into account atmospheric pressure, like this one http://www.rbracing-rsr.com/comprAdvHD.htm In addition, some use advertised duration (like the pat kelley calculator) and some use duration @ .050" lift (like the rbracing calc).

The first (pKelley2) calculator I find useful because there is a fair amount of people using it and a fair amount of data supporting what DCR ratios are "safe" with a particular fuel. The number itself is just a figure of merit like Saltfever said. Experience is the only thing that makes that figure of merit useful. Of course other factors like coolant temp, inlet air temp, combustion chamber design, etc. make this method a ballpark type measurement -- but with enough experience on a particular type of engine that runs within particular parameters (ie: fastburn chamber, 170* coolant, 80* air inlet, etc) it can be a useful tool in matching cam and compression ratio to a particular octane fuel.

The second calculator I posted (the rbracing calculator) tries to take "DCR" to the next level by incorporating atmospheric pressure and boost into the equation in an effort to calculate cranking compression. It serves as a "sanity check" for me -- especially when you get cams that have goofy profiles or goofy advertised duration numbers (ie: the SBC 30-30 Duntov cam). This calculator is useful because I have found it to be somewhat close in predicting an engine's cranking compression (in PSI not ratio! although this calc does output a ratio number I do not use it because my experience is with the adv DCR numbers.) A lot of people for along time have said certain cranking pressure numbers work with pump fuel so the data is there to make these numbers useful too.

I actually use BOTH of these calculators together (checks and balances) and have done so for a few street/strip type builds of my own.

I forgot who said this, but the comment was basically "the number that a DCR spits out is BS and not useful for determining detonation resistance" well, that is partially true . . . because a DCR calculator does not take into account different filling characteristics (VE) at different RPM during engine operation. Again, that is where experience comes into play. There is enough experience and data out there to know that with a typical NA engine what DCR numbers and cranking cylinder pressures will work with a particular octane fuel (as long as the calculator being used is consistent)

As an example, couple years ago I tried to convince Nick (73Nova) that based on the DCR info for his motor he would be safe to raise his static compression to 11:1 and to use a smaller cam. I think at the time he listened to the nay sayers and built his 355 with a 265-270* @ .050 cam and 10:1 compression (as I recall). Needless to say the engine was soft down low and had no problem with el-cheapo gas. A couple years later Nick goes smaller with the cam (still not as small as I wanted) and up on the compression and it runs better, but still way on the safe side with only 177 psi cranking pressure.

On the other extreme, I personally like to push the envelope a little and found first hand that my old 11.3: static CR 388 (the Drag Week '05 motor) was safe at 8.6:1 DCR and 215-220 cranking psi on pump 93 octane (barely). When we got stuck in Illinois with 91 octane gas on one of DF's cow-path routes, the car ran like a Cummins diesel after I turned off the key. I used that experience to build my new 410 and with the same cam and 11.0:1 compression (it was about 8.5 DCR and 210 psi cranking compression I do believe) the new engine has been fine on every pump gas I've tried (including the Drag Week '06 trip). Since then I upped the static CR to 11.4:1 and the DCR to near 9:1 and am now running E85. I am nowhere near the limits of E85 but hope to go to 13:1 static and a slightly bigger cam in the years ahead.

The bottom line is that a ratio is a ratio is a ratio -- just like DF and Saltfever said. If you compress 43 cu. in. of air into a 4.3 cu. in. area you have a 10:1 compression ratio -- whether you like the term "dynamic" or not a ratio is a ratio is a ratio. Whether you choose to find a way to use that ratio or not is up to you and how much faith you want to have in that type calculator and other peoples experience. Personally, I think "DCR" calculators are useful, definitely not to be used in a vacuum without considering other factors when picking a cam but definitely useful.

Sorry for the long post -- I guess that's what happens when I keep my mouth shut for 6 pages of thread ;D

**Saltfever** · December 31, 2007, 02:16 PM

Re: Freiburger, DCR, and testing.....

Great read, Eric. So, have you put those DCR formulas into your spreadsheet yet? ;)

**Dynoroom** · December 31, 2007, 02:18 PM

Re: Freiburger, DCR, and testing.....

I managed to stay out of it completely :P

**BlackoutSteve** · December 31, 2007, 02:22 PM

Re: Freiburger, DCR, and testing.....

Good post Eric. I was wondering where you got to during our mass-debate. :P
(..and you gotta check this out! http://www.carjunkieforums.com/index.php?topic=891.0)

I would like to correct something I said earlier to DF though..

Originally posted by BlackoutSteve

A DCR in outerspace will always be the same number of zero because you're always pumping nothing regardless of engine speed.

I think in this very hypothetic example, the DCR would be exactly what the formula says it would be and never change. It still represents a certain volume being decreased into another.
In my opinion, the DCR wouldn't change due to the effects of inertia like I believe is does on Earth where actually air weighs something..

**73Nova** · December 31, 2007, 02:30 PM

Re: Freiburger, DCR, and testing.....

Holy fat can of worms batman!!! I just swung back to see if anyone thought it would be a good idea and theres 6 pages. Sorry if I opened up the flood gates.....so....ummmmm, Dave...... any chance of testing this DCR stuff to see how much effect it has and at what point you see detonation with specific octanes???? I was just kind of suggesting that as a cool idea and *BAM*

**Saltfever** · December 31, 2007, 02:34 PM

Re: Freiburger, DCR, and testing.....

Originally posted by Dynoroom

I managed to stay out of it completely :P

You are wise beyond your years!! ; ;D

**std** · December 31, 2007, 03:43 PM

Re: Freiburger, DCR, and testing.....

Originally posted by Dynoroom

I managed to stay out of it completely :P

Just by putting this in means you have some ideas. Time to spill.

**Eric68** · January 1, 2008, 08:10 AM

Re: Freiburger, DCR, and testing.....

Originally posted by Saltfever

Great read, Eric. So, have you put those DCR formulas into your spreadsheet yet? ;)

LOL thanks -- no, that DCR formula would be a bear to program into MS Excel!

**Freiburger** · January 1, 2008, 02:14 PM

Re: Freiburger, DCR, and testing.....

Originally posted by Saltfever

Originally posted by Rendid

So what you need to do is agree on a term for the amount of air being compressed and not the ratio of volume change.

I believe that would be Brake Mean Effective Pressure (BMEP). Which accounts for the density of the charge as well as all the attributes of combustion.

BMEP is on the output side (power stroke) while these guys are trying to define a combination of induction and compression events. That's why I thought BSAC was closer.

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