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  #31 (permalink)  
Old 01-20-2013, 08:07 AM
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Some of the experience I bring to this forum is a numinous yrs on forums like this one.Without question every thread reaches a point where it is doomed to repeat it's self.I have I think made my position on this topic clear.

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  #32 (permalink)  
Old 01-20-2013, 08:25 AM
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Her name is Shirley....
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Old 01-20-2013, 09:56 AM
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Quote:
Originally Posted by F-BIRD'88 View Post
Dynamic Compression is rooted in the fact that the intake valve closes after bottom dead center. Since your cylinder isn't sealed right away as the volume above the piston begins to decrease, you do not keep all of the air that is in the cylinder at BDC (Bottom Dead Center). What is compressed is what is left in the cylinder after the intake valve closes. This is why Dynamic Compression is always lower than Static Compression.

this statement is false and misleading.... When the engine is running at speed in its natureal best power rpm range the cylinder pressure is very high. Back pumping is little or non. On a tuned induction system engine.
( a engine with headers, cammed and developed induction ( a powerfull intake) the engine takes in more air , resulting in higher than 100% volumetric efficiently at WOT. cylinder pressure is high.

Back pumping before the inake valve closes only occurs at low rpm, when the engine is running out of its best power band. Not "on the cam".

Real running engine dynamic compression ( when its running "on the cam" @WOT) is not fixed and often and should be higher than 100% volumetrically efficient if you are any good at building and tuning engines.
induction ram effect-exhaust scavedging. A high performance tuned induction-exhaust system engine is not a simple air pump. Cylinder pressure is highest BMEP right around peak torque output rpm.
Air always flows in the direction high pressure toward low pressure relative. and it don't care which way the piston is moving in a cylinder.

In the running engine, running at speed you have a open intake valve, a rising piston and air still entering the engine.
inertia Ram effect.
Thats not possible without some compression. The air keeps moving into the cylinder from the intake port until one of two things happen. 1. the valve closes. 2. the air pressure in the cylinder is equal or higher than the air pressure just under/at the exit of the still open intake valve.
Good point, makes sense..

Duke
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  #34 (permalink)  
Old 01-20-2013, 11:00 AM
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At the risk of beating a dead horse, I'll just say this:

DCR is one of many tools in the engine builder's chest. And like any tool, it works best when used for what it was intended. Most guys know better than to pound a nail using a torque wrench (although sometimes I wonder); using DCR is no different.

For those interested, here is a different approach to the same concept as DCR. I've not used it yet but I'd be interested in hearing how it compares to DCR calculations made w/the "standard" DCR calculators (three are below), should anyone use it. This might be where some of the somewhat incomplete concepts posted in this thread came from, but either way this is a clearly presented case w/the math included. BTW, he uses "supra" a lot. It means basically "there's more info above".

Wallace Racing DCR calculator
Kelly DCR calculator
KB/Silvolite DCR calculator

Last edited by cobalt327; 01-20-2013 at 11:11 AM.
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  #35 (permalink)  
Old 01-20-2013, 11:11 AM
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Very good article. The fine print explains much of what I am saying.
Simplifed, it tells you , its just not that simple.
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  #36 (permalink)  
Old 01-20-2013, 11:51 AM
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Then there's: Spec'ing Cams by Vizard. But the article stops short (by three pages) of telling the whole story. Supposedly an abridged version is in a couple of his Chevy "how to" books. Or you can rent the complete version. Only $100/hr.

Last edited by cobalt327; 01-20-2013 at 12:04 PM.
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