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Old 01-12-2013, 11:08 AM
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fully understanding DCR

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

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Old 01-12-2013, 08:02 PM
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Quote:
Originally Posted by prostcelica View Post
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
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.
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Old 01-12-2013, 08:47 PM
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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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Old 01-12-2013, 09:06 PM
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Thanks Vinnie, made me laugh.
Ummmm, that would be girlfriend though, I have no wife.
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Old 01-12-2013, 09:27 PM
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LOL!!! Nice. Ok so what is the maximun DCR a guy can't get away on 91 octane?
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Old 01-13-2013, 09:41 AM
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11:1 CR is a decent guideline for old technology.You need to express this when telling someone that 11:1 compression wont work with out a different fuel. New car technology(BMW,Ford) have engines that are 11 and 12:1 cr. All they need is mid grade gas or better.Mazda has 14:1 CR in the economy car line.How is that possible? Mazda also uses 14:1 CR in their economy diesel? How is that possible?

We as hot rodders have to adapt to newer technology.
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Old 01-13-2013, 10:20 AM
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an example of a crate engine offered in todays market

Horsepower
Torque
Bore & Stroke
Compression Ratio
Fuel Requirement
710 @ 6700 RPM
601 @ 5100 RPM
4.185 x 4.000
12.0 to 1
91 - 93 Octane
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Old 01-13-2013, 12:19 PM
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Quote:
Originally Posted by F-BIRD'88 View Post
THats what I like to call BS.
thats a shame you do that.You could learn if you were interested.Being conservative is good though. Just make sure you state that you choose to be conservative.
The same crate engine seller has many engines at 11:1 CR all for street engines and limited Saturday night specials.
I personally like to learn from those builders
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Old 01-14-2013, 12:08 PM
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Quote:
Originally Posted by prostcelica View Post
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

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.
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Old 01-14-2013, 04:27 PM
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Quote:
Originally Posted by F-BIRD'88 View Post
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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Old 01-14-2013, 04:40 PM
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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.
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Old 01-14-2013, 10:36 PM
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Quote:
Originally Posted by Landshark928 View Post
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.
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.
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Old 01-14-2013, 11:56 PM
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Quote:
Originally Posted by Landshark928 View Post
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.
Understand you theory completely and appreciate it.Thing is when you start calling about "real world situations" we both know full well the mechanics in the upper rpm regions in the real world are far,far,from perfect.Valves that are supposed to be completely closed aren't etc.

Now you can experiment on your wallet all you want with pinging engines.But please for the sake of the rep of the forum,the newbies that come on it for advise,re-frame from your risky behavior advise.

I can without doubt tell you that Tech has a good history with hundreds,no thousands of readers.And I have trailing not far behind him involvement in hot rods agree with what he has said.
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Old 01-15-2013, 10:53 AM
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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!
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Old 01-15-2013, 11:00 AM
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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.
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