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LET'S TALK ABOUT DYNAMIC COMPRESSION, SHALL WE?
#828349
10/12/10 06:25 PM
10/12/10 06:25 PM
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Joined: Feb 2007
Posts: 5,080
organ
maximum entropy
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i did a little on line research to refresh/confuse my memory. any stock/super stock racers out there willing to divulge that little tidbit about their combination? 
Last edited by maximum entropy; 10/12/10 08:12 PM.
for what is the good life if not doing things thoughtfully?
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Re: LET'S TALK ABOUT DYNAMIC COMPRESSION, SHALL WE?
[Re: maximum entropy]
#828350
10/12/10 09:35 PM
10/12/10 09:35 PM
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Joined: Oct 2007
Posts: 4,210
New York
polyspheric
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It only matters if octane isn't high enough.
Even when this is true, it vanishes above about 4,000 RPM and has no effect on peak torque or power.
Boffin Emeritus
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Re: LET'S TALK ABOUT DYNAMIC COMPRESSION, SHALL WE?
[Re: polyspheric]
#828351
10/13/10 05:22 AM
10/13/10 05:22 AM
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Joined: May 2008
Posts: 5,399
Aurora, Colorado
451Mopar
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Quote:
It only matters if octane isn't high enough.
Even when this is true, it vanishes above about 4,000 RPM and has no effect on peak torque or power.
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Re: LET'S TALK ABOUT DYNAMIC COMPRESSION, SHALL WE?
[Re: polyspheric]
#828352
10/13/10 03:57 PM
10/13/10 03:57 PM
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Joined: Feb 2007
Posts: 5,080
organ
maximum entropy
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are you saying that the importance of dynamic compression ratio is overstated?
for what is the good life if not doing things thoughtfully?
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Re: LET'S TALK ABOUT DYNAMIC COMPRESSION, SHALL WE?
[Re: maximum entropy]
#828355
10/13/10 04:54 PM
10/13/10 04:54 PM
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Joined: May 2008
Posts: 5,399
Aurora, Colorado
451Mopar
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Quote:
are you saying that the importance of dynamic compression ratio is overstated?
No.
Dynamic Compression is a fancy term to describe re-calculating compression ratio from the point the intake valve closes. The new compression ratio can then be used to calculate an estimated cranking compression pressure, and the cranking pressure can be used (roughly) to estimate the fuel octane requirements.
If the Dynamic compression is too low you will be giving up some low end power, if it is too high, the engine may need higher octane fuel to prevent detonation.
When the engine reaches a higher RPM where there is no reversion from the upward movement of the piston from BDC the dynamic compression has little meaning to how the engine will perform.
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Re: LET'S TALK ABOUT DYNAMIC COMPRESSION, SHALL WE?
[Re: 451Mopar]
#828356
10/13/10 04:58 PM
10/13/10 04:58 PM
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Joined: Feb 2007
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organ
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i'm really tempted to try a cam that's way too big (on paper) for my combo. just for the hellovit. the old hotrodder's proclivity!
for what is the good life if not doing things thoughtfully?
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Re: LET'S TALK ABOUT DYNAMIC COMPRESSION, SHALL WE?
[Re: maximum entropy]
#828357
10/13/10 05:27 PM
10/13/10 05:27 PM
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Joined: May 2008
Posts: 5,399
Aurora, Colorado
451Mopar
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Quote:
i'm really tempted to try a cam that's way too big (on paper) for my combo. just for the hellovit. the old hotrodder's proclivity!
It's done all the time, just use alot of gear and converter so the engine does not notice the loss of low RPM torque.
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Re: LET'S TALK ABOUT DYNAMIC COMPRESSION, SHALL WE?
[Re: 451Mopar]
#828358
10/13/10 05:50 PM
10/13/10 05:50 PM
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Joined: Oct 2007
Posts: 4,210
New York
polyspheric
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Yup.
Got a smog 400 with a 509 cam and 120 psi cranking?
Use a 9" converter.
Use fade-out point of DCR varies with the build, but (as said above) once reversion stops the engine sees static CR, and perhaps more if VE is above parity.
What's it good for?
How happy you'll be with a low compression engine - the DCR numbers tell you how "soft" it will be at low speed, and a guess at if it will knock.
JM2¢? I think it gives a fair comparison of torque around 1,000 RPM to estimate how your new cam will affect your stall speed: multiply DCR × displacement at intake closing. If the number is down more than 10%, the idle will be worse and the converter may not do as much.
The reverse: if the cam is selected, tells you how much static CR you need to make it stronger off-idle.
Again: don't just pick the psi #, which is only the knock value. It's also acting on a much smaller cylinder.
Boffin Emeritus
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Re: LET'S TALK ABOUT DYNAMIC COMPRESSION, SHALL WE?
[Re: maximum entropy]
#828364
10/14/10 02:21 PM
10/14/10 02:21 PM
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Joined: Jan 2003
Posts: 16,123
Grand Haven, MI
patrick
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Quote:
Quote:
General question....
So can you raise or lower your dynamic compression with a camshaft change? Is that what happens with a more or less aggressive cam?
Michael
yes. that and changing the static compression ratio. earlier intake valve closing, and/or higher static compression both raise the dynamic cr, and vice-versa.
don't even have to change the cam...just advance or retard it to change the intake close point....a low buck trick on stock 400's is to advance the cam 4-6 degrees, it boosts DCR and helps with low end torque by closing the intake valve sooner.
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Re: LET'S TALK ABOUT DYNAMIC COMPRESSION, SHALL WE?
[Re: patrick]
#828365
10/14/10 03:17 PM
10/14/10 03:17 PM
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Joined: Nov 2004
Posts: 6,890
North Alabama
Monte_Smith
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Dynamic ratio, is mostly controlled by cam timing events and is VERY important number to know, for certain applications. I mostly deal with the cranking number, but is all relative. For what I do, nitrous motors, I need to know this. If I have two pretty much identical motors and they both have static CR of 14.5:1, but one has a cranking psi of 160 and the other has 200, because of cam events, that will very much effect the way I tune the nitrous.
Monte
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Re: LET'S TALK ABOUT DYNAMIC COMPRESSION, SHALL WE?
[Re: maximum entropy]
#828366
10/14/10 05:07 PM
10/14/10 05:07 PM
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Joined: Dec 2008
Posts: 1,200
UK
602heavy
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Quote:
.the more the cylinder can ingest the higher the dynamic/cylinder pressure...
true, but don't confuse cylinder pressure with dynamic compression ratio, which, like static c/r, is a calculated number that doesn't change.
Dynamic CR is calculated much the same as how static CR is calculated , knowing static CR will help net you a bench mark on what type cam the engine will use , dynamic CR #s WILL change throughout the rpm range of the engine , many have the misconseption DCR is a constant , if this were so to what benefit would it be? , DCR is a calculation of swept volume when intake valve is on it's seat , the higher the VE climbs the higher the DCR , or should i say ECR. 
Last edited by 602heavy; 10/14/10 05:35 PM.
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Re: LET'S TALK ABOUT DYNAMIC COMPRESSION, SHALL WE?
[Re: BradH]
#828368
10/14/10 10:48 PM
10/14/10 10:48 PM
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Joined: Feb 2003
Posts: 12,675
Columbia, CT
moper
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Because I deal with street engines I use the DCR fairly often. I think if you have a certain fuel type (meaning octane) to run, not taking into account DCR means you're overlooking a crutial ingredient. It's a much better way for me to predict the low speed potential for detonation. I also agree that as rpm rises it's importance is reduced. If you can run any fuel type you want, I dont think it's as critical because you can always use the fuel to stay away from the problems too much pressure for a given chamber brings.
Well, art is art, isn't it? Still, on the other hand, water is water! And east is east and west is west and if you take cranberries and stew them like applesauce they taste much more like prunes than rhubarb does. Now, uh... Now you tell me what you know.
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Not on paper 
I don't want to brag, if your not testing by thrashing and developing new combinations your getting left behind 
