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fully understanding DCR

8K views 29 replies 9 participants last post by  dinger 
#1 ·
Hey guys

I think I know the answer but I thought id ask the guys that will know for sure. Say you had a 13:1 SBC but had a huge cam in it that made your dynamic compression ratio between 8-8.5:1 would it run on pump gas with out detonation. it is to much cylinder pressure that causes detonation right?

PS Im not planning on doing this. I just want to fully understand DCR
 
#2 ·
I have no practical experience calculating DCR for such lofty SCR's, but yes, you're understanding it like I'm understanding it.

Yes, cylinder pressure determines the fuel. Fuel determines the detonation resistance.

Using a 355 Chevy built at 13.00:1 SCR combined with an intake closing point of 70 degrees @0.050" tappet lift generates a DCR of 8.45:1 on the KB calc. Here is the cam I used to calculate this....
Crane Mechanical Roller Camshafts 118471 - SummitRacing.com
Theoretically, the motor should run on pump gas, but like I said in the beginning, I have no practical experience concerning these numbers. If someone could present a valid argument against the results I have shown above, using science and math to convince me, I would yield to their way of thinking.
 
#3 ·
BTW thats techs cam that he uses in his wifes grocery getter. The old wagon has a lenco 5 speed with a Dana rear with 7.50 gears.
Note: its all about the combination,
the Lenco perfectly matches his wifes driving as she lets the clutch out very quickly and forgets to use the clutch when she changes gears
The station wagon is perfectly matched with enough room for groceries,back doors for easy egress for the grand kids,and he even hung a clothes line on the back to dry the laundry.
The 10,000 rpm drowns out the wifes nagging and the grand childrens screaming and other antics on short road trips

its all combination combination
 
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#10 ·
Theoretically, the motor should run on pump gas, but like I said in the beginning, I have no practical experience concerning these numbers.

Exactly. and garbage in equals garbage out.

Some people call it "Junk Science"

Often 10.5:1 is really pushing it on "pump gas". Beyond that the cam timings effect is rather minor, in the overall scheme of things.
I'm always eager to learn new stuff. Please cite your references so that I can read all the particulars of your last sentence. Otherwise, I will consider it just more BS leaking out of your brain.
 
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#9 ·
Built plenty of em this way> Not for the street!

The answer to your question no. Now to answer the question in the terms your thinking of yes and no. It has to do with the SCR and DCR, moreso the realtionship of the 2 terms. The bleed off of SCR by the cam will bring the cyl. pressure down till the design parameters of the overall build come together with the cam being the determining factor in this conversation. Once the cyl. pressuer starts to rise, the engine builds heat and compression, the filling and emptying cycle is reaching designed parameters with RPM rise...better have higher octane in the gas to promote adequate detonation suppression or it will eat itself to death! And if you think just limiting ign. timing will take care of the flame front and travel, it does not work that way!!
I could go on and on with theoretical entropy and enthalpy of a heat engine, gas laws, etc... but that is WAY to involved for here.

In short...it is all about pressure, flow, engine speed minimums and a controlled burning for an intended application. Just scratching the surface on this subject! Its the meaning to the whole.
 
#11 ·
The DCR begins to rise and eventually meet the SCR as rpms increase and the engine approachs 100%+ VE. Above that the bleed off of a large cam doesn't have enough time to affect things as much. Reversion is almost non-existent at that point.

ie, don't go crazy thinking SCR is low enough in a calculator. Real world at high rpms will see a completely different story. Don't forget quench can help or hurt detonation as well.
 
#12 ·
As I've said before, if you are to make the statements you have made here, without citing your sources of information, then it is just gibberish, same as the BS leaking out of F bird's brain. If I say something on this forum that is my opinion, without scientific fact to support it, then I tell the reader that it is my opinion. If I can site a credible source for the info, then it is no longer my opinion, but fact that I am conveying to the reader.
 
#14 ·
Example of adiabatic compression

Let's now look at a common example of adiabatic compression- the compression stroke in a gasoline engine. We will make a few simplifying assumptions: that the uncompressed volume of the cylinder is 1000cc's (one liter), that the gas within is nearly pure nitrogen (thus a diatomic gas with five degrees of freedom and so
= 7/5), and that the compression ratio of the engine is 10:1 (that is, the 1000 cc volume of uncompressed gas will compress down to 100 cc when the piston goes from bottom to top). The uncompressed gas is at approximately room temperature and pressure (a warm room temperature of ~27 degC or 300 K, and a pressure of 1 bar ~ 100,000 Pa, or about 14.7 PSI, or typical sea-level atmospheric pressure).

so our adiabatic constant for this experiment is about 1.58 billion.
The gas is now compressed to a 100cc volume (we will assume this happens quickly enough that no heat can enter or leave the gas). The new volume is 100 ccs, but the constant for this experiment is still 1.58 billion:

so solving for P:

or about 362 PSI or 24.5 atm. Note that this pressure increase is more than a simple 10:1 compression ratio would indicate; this is because the gas is not only compressed, but the work done to compress the gas has also heated the gas and the hotter gas will have a greater pressure even if the volume had not changed.
We can solve for the temperature of the compressed gas in the engine cylinder as well, using the ideal gas law. Our initial conditions are 100,000 pa of pressure, 1000 cc volume, and 300 K of temperature, so our experimental constant is:

We know the compressed gas has V = 100 cc and P = 2.50E6 pascals, so we can solve for temperature by simple algebra:

That's a final temperature of 751 K, or 477 °C, or 892 °F, well above the ignition point of many fuels. This is why a high compression engine requires fuels specially formulated to not self-ignite (which would cause engine knocking when operated under these conditions of temperature and pressure), or that a supercharger and intercooler to provide a lower temperature at the same pressure would be advantageous. A diesel engine operates under even more extreme conditions, with compression ratios of 20:1 or more being typical, in order to provide a very high gas temperature which ensures immediate ignition of injected fuel.


Here ya go guy's...the science behind it. Theory works to get you something useful in practical applications but stray too far from theory and the practical won't last too long!
 
#15 ·
Look up Adiabatic process. The wikipedia one is ok, it is accurate enough for this conversation. If that is not enough for you look up the thermodynamic laws and the heat engine. This is a great start to understanding the how it is and why it is then take it from there.
 
#19 ·
Ignoring carbs for the momment
we can see how tbi will not tolerate as high of compression as direct port injection.

and there are many many reasons for fuel to not ignite just because it reaches an ignition temperature that is just so far beyond any even caring,when some of us cannot even figure out the displacement of our engines when we bore and stroke them.
Im very happy I have Tech here to do the simple DCR math for me.

I contacted 76 with a couple ,what I thought was simple questions about fuel/octane etc,and they offered to fax a simple answer to me. I got around 70 sheets of formulas from them. I understood less than 1/2.
 
#20 ·
Then slap components together to see if it lives and for how long. Make up theories and excuses after the fact to support your failure or triumphs. The people that could have told you that your thinking is either good to go or hmmmm, got a problem are really just trying to help. Whether you believe them or not is your choice.
I am done with this subject....knock yourselves out!
 
#25 ·
I know I said I would not say anymore on this subject but I can't contain myself. :D

To the guy's that say it is ok to run "pump" gas ( of any grade) on an engine with 13-1 and compensate with a large duration, high overlap cam is good to go then why would one use a better composition of gasoline to run this same engine at the track. If there is no need then why do it.....cause it smells good?

To the guy's that were like me back in the early 70's building hot rods and high compression engines (even over what you could get already from the factory) to cruise the blvd and race out on the lonely road we definitely understood (even before we were off to college for higher learning) the need. Sunoco 260 was our friend!!

To the stubborn crew that does not want to listen to experience?......go ahead and try your combo on the street with the quality of gas these days. Let us all know of your real world experience learned. Bet I know the answer if your feedback is honest!!

To the guy's that do it successfully.....kudo's for the knowledge and understanding that there is more to it than just huge domes, small chambers and wild camshafts!!!!
 
#26 ·
I don't think any of us are willing to risk good money on a "what-if" of this kind. This was originally a question posed by Vinnie and I chimed in to put my 2 cents worth in. I've never tried the combination and never will. Advancements in the sport are not made by those who sit back and are comfortable with the status quo, they are made by those who buck the system and try different things. Again, like I said though, I'm not foolish enough to try this, it was just an exercise in fantasy land.

You seem to be a level-headed fellow and I appreciate all your input. Thanks.
 
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