Re: "Dynamic Comp Ratio" doesn't mean ZIP!!!
[Re: BradH]
#2474612
03/30/18 10:42 PM
03/30/18 10:42 PM
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Joined: Jul 2013
Posts: 561 USA
B3RE
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So, what is the magic number to run pump gas? Obviously, if so much credence is given to this one parameter, then someone has to have a number they are looking for. Otherwise, it is just a guessing game.
The fact is, there are engines that will rattle with less than 8:1, and others won't rattle at 10:1 DCR. How do you know what is correct? How can a tool be so valuable if the target is constantly moving, and the real world numbers don't correlate to the formula result? I'm not saying the DCR doesn't have some value if used properly, but when ALL parameters aren't considered, like is often the case, the DCR is almost meaningless.
I think Brad is showing his frustration that he built his engine to run on pump gas, using a DCR calculation, and it didn't work out that way. There are lots of guys out there blowing their stuff up from detonation, when they thought they were safe. And yet, guys are still recommending engines be built using the DCR. That being said, if that's what works for you, go for it.
Mike Beachel
I didn't write the rules of math nor create the laws of physics, I am just bound by them.
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Re: "Dynamic Comp Ratio" doesn't mean ZIP!!!
[Re: dizuster]
#2474728
03/31/18 02:52 AM
03/31/18 02:52 AM
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Joined: Jan 2003
Posts: 15,439 Val-haul-ass... eventually
BradH
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You can't say that static compression ratio and IVC matter to detonation in one breath, and then say Dynamic Compression ratio doesn't mean ZIP in the next... since obviously the two biggest parameters to dynamic compression ratio ARE static compression ratio and IVC! You can try and spin what you said and what you meant anyway you want. In the end you can't un-tie SCR, DCR, and IVC from interacting. I don't have to "spin" anything and I never changed what I meant: DCR doesn't take into account how cylinder pressure changes across the RPM range due to VE and, therefore, is of limited or no value. It doesn't matter that DCR uses CR and IVC as inputs if the output calculated isn't of use. Seeing that the DCRs of two different CR & cam combinations are the same still won't tell you the RPM at which the combination built with a higher CR and later IVC crosses over from less cylinder pressure to more cylinder pressure than the other combination. If you want to know at what point in the RPM range an engine achieves peak VE and how well it carries the VE up to max RPM, you gotta put it on a dyno and collect the data.
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Re: "Dynamic Comp Ratio" doesn't mean ZIP!!!
[Re: B3RE]
#2474734
03/31/18 03:01 AM
03/31/18 03:01 AM
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Joined: Jan 2003
Posts: 15,439 Val-haul-ass... eventually
BradH
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Taking time off to work on my car
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I think Brad is showing his frustration that he built his engine to run on pump gas, using a DCR calculation, and it didn't work out that way.
Naaaah. I knew when the CR ended up at 11.5 compared to the old build's 10.8, while the cam duration & LSA specs didn't really change, that the likelihood of still being able to run straight 93 was pretty slim. I brought race gas to blend with pump gas at the dyno just to play it safe.
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Re: "Dynamic Comp Ratio" doesn't mean ZIP!!!
[Re: BradH]
#2474773
03/31/18 10:16 AM
03/31/18 10:16 AM
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Joined: Jan 2005
Posts: 2,540 Milwaukee WI
TRENDZ
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“DCR doesn't take into account how cylinder pressure changes across the RPM range due to VE and, therefore, is of limited or no value. It doesn't matter that DCR uses CR and IVC as inputs if the output calculated isn't of use.“
DCR= DYNAMIC compression ratio. The term dynamic TELLS you that it’s a changing value. If someone has chosen to use only one value of a constantly changing equation, the results will likely be less effective. Throughout this thread, it appears that the mistake that people are making is they use “dynamic” as a fixed number/ point of reference.
So to me, It looks like (based on the first sentence in quotes) you assumed that dynamic is a fixed number. Maybe that’s the whole basis of this thread???😳
"use it 'till it breaks, replace as needed"
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Re: "Dynamic Comp Ratio" doesn't mean ZIP!!!
[Re: BradH]
#2474787
03/31/18 10:58 AM
03/31/18 10:58 AM
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Joined: Mar 2006
Posts: 9,835 MI, usa
dvw
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How do we pick a combo? Voodoo? No, we use parameters. Cam, compression, head design, head material, operating altitude, engine temp,fuel type, and more. Miss any one of these factors and the desired goal may not be met. Static compression is just compiled number. Dynamic compression is also just a number. These are tools to point you towards the desired goal. Neither are set in stone. The more knowledge you have, the better chance of meeting the goal. Just like when you ask questions. Compile the answers, throw out what you don't believe in, and make your best stab at it. Doug
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Re: "Dynamic Comp Ratio" doesn't mean ZIP!!!
[Re: AndyF]
#2474797
03/31/18 11:05 AM
03/31/18 11:05 AM
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Joined: Jul 2013
Posts: 561 USA
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DCR only applies at very low engine speeds such as idle and perhaps cruise. Once some inertia ram effect takes place then DCR goes out the window. So if a person is concerned with how an engine will run at low speeds then DCR is something to pay attention to.
I would imagine DCR is a hot topic over in the Tug Boat and Tow Truck section of the board, not sure why it is a topic of discussion in the Race section though. Well said, Andy! I was thinking the Irritation Pump section, but tow truck and tug boat works, too.
Mike Beachel
I didn't write the rules of math nor create the laws of physics, I am just bound by them.
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Re: "Dynamic Comp Ratio" doesn't mean ZIP!!!
[Re: BradH]
#2474799
03/31/18 11:08 AM
03/31/18 11:08 AM
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Joined: Jul 2013
Posts: 561 USA
B3RE
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I think Brad is showing his frustration that he built his engine to run on pump gas, using a DCR calculation, and it didn't work out that way.
Naaaah. I knew when the CR ended up at 11.5 compared to the old build's 10.8, while the cam duration & LSA specs didn't really change, that the likelihood of still being able to run straight 93 was pretty slim. I brought race gas to blend with pump gas at the dyno just to play it safe. My mistake, Brad. I was thinking you built it for pump gas.
Mike Beachel
I didn't write the rules of math nor create the laws of physics, I am just bound by them.
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Re: "Dynamic Comp Ratio" doesn't mean ZIP!!!
[Re: BradH]
#2474800
03/31/18 11:08 AM
03/31/18 11:08 AM
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Joined: Oct 2007
Posts: 4,206 New York
polyspheric
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Gee, why didn't I think of this? Oh, wait... I wrote about it, HERE, 10 years ago.
Boffin Emeritus
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Re: "Dynamic Comp Ratio" doesn't mean ZIP!!!
[Re: dvw]
#2474805
03/31/18 11:16 AM
03/31/18 11:16 AM
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Joined: Jul 2013
Posts: 561 USA
B3RE
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How do we pick a combo? Voodoo? No, we use parameters. Cam, compression, head design, head material, operating altitude, engine temp,fuel type, and more. Miss any one of these factors and the desired goal may not be met. Static compression is just compiled number. Dynamic compression is also just a number. These are tools to point you towards the desired goal. Neither are set in stone. The more knowledge you have, the better chance of meeting the goal. Just like when you ask questions. Compile the answers, throw out what you don't believe in, and make your best stab at it.
Doug This approach is understandable, since quantifiable data can be difficult to obtain for many of the parameters involved. That's where very expensive and time consuming research used by factory engineers, and high end race teams comes into play. The problem is the use of DCR as a singular factor in determining SCR and IVC. It just doesn't work by itself. Again, the engine doesn't care when the intake valve closes. It only sees volumetric efficiency. Get too much charge into the cylinder at too low an rpm, and it will rattle.
Mike Beachel
I didn't write the rules of math nor create the laws of physics, I am just bound by them.
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Re: "Dynamic Comp Ratio" doesn't mean ZIP!!!
[Re: TRENDZ]
#2474809
03/31/18 11:25 AM
03/31/18 11:25 AM
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Joined: May 2005
Posts: 1,985 Apollo, PA.
B1MAXX
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If Dynamic comp. doesn't mean zip then put a 290 @ 50 cam in a 440 that had a static ratio of 8 to 1. and see what et you get. Then put a 250 @ 50 and see what et you get. If dynamic comp. does't mean anything why even close the valve? In terms of detonation. Why does my 1988 dodge truck ping, but power tour mountain motors don't? Bet it has something to do with cam selection (along with other things). Detonation is easier to control at higher rpm. there is a point, probably you can stop detonation with rpm. So if we can help control lower rpm detonation with camshaft ivc and the upper rpm detonation with rpm. Sounds like a win win, if you know what your doing. I had to put into this pot
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Re: "Dynamic Comp Ratio" doesn't mean ZIP!!!
[Re: BradH]
#2474843
03/31/18 12:28 PM
03/31/18 12:28 PM
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Joined: Dec 2007
Posts: 12,587 Great Neck,LI,new york
hemi-itis
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I work with effective CR because it's adjustable from 11:1 pump gas and as high as 20:1 if you have steel ones
Last edited by hemi-itis; 04/07/18 02:52 PM.
HEMI-ITIS has no cure. My condition is fully BLOWN!!
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Re: "Dynamic Comp Ratio" doesn't mean ZIP!!!
[Re: BradH]
#2474862
03/31/18 01:31 PM
03/31/18 01:31 PM
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Posts: 570 UK
rb446
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DELETED.
Last edited by rb446; 04/01/18 01:09 AM.
1969 'Cuda 446ci, best 9.96@133.9 in 1990 1971 340 'Cuda, best 11.01@122.8 in 1987
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Re: "Dynamic Comp Ratio" doesn't mean ZIP!!!
[Re: AndyF]
#2474922
03/31/18 03:31 PM
03/31/18 03:31 PM
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Joined: Aug 2003
Posts: 1,313 Charlotte, NC
LSP
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DCR only applies at very low engine speeds such as idle and perhaps cruise. Once some inertia ram effect takes place then DCR goes out the window. So if a person is concerned with how an engine will run at low speeds then DCR is something to pay attention to.
I would imagine DCR is a hot topic over in the Tug Boat and Tow Truck section of the board, not sure why it is a topic of discussion in the Race section though. Most race engine live in the RPM zone between peak torque and peak power and DCR is irrelevant at that RPM. Bingo
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Re: "Dynamic Comp Ratio" doesn't mean ZIP!!!
[Re: TRENDZ]
#2474925
03/31/18 03:39 PM
03/31/18 03:39 PM
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Joined: Jan 2003
Posts: 15,439 Val-haul-ass... eventually
BradH
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Taking time off to work on my car
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DCR= DYNAMIC compression ratio. The term dynamic TELLS you that it’s a changing value. If someone has chosen to use only one value of a constantly changing equation, the results will likely be less effective. Throughout this thread, it appears that the mistake that people are making is they use “dynamic” as a fixed number/ point of reference.
Show me a "DYNAMIC" CR calculator that accounts for the differences in VE that occur at 1000 RPM intervals from 1000 to 7000. If it's in an engine simulation program, then the program probably calculates BMEP, too, which IMO is a more useful data point. What people typically use to calculate DCR isn't DYNAMIC at all because it's a single point-in-time calc that ignores VE completely. THAT'S really a STATIC CR calculator where the IVC event is changed from the default BDC to ### degrees ABDC; nothing DYNAMIC at all about it until it can factor in VE changes under different operating conditions. I'm not the one who is getting confused between the meanings of DYNAMIC and STATIC, regardless of whether the subject calculation is named incorrectly. ///////////////// This has been an enlightening discussion, if for no other reason than I am truly surprised by how difficult it seems for some people to grasp what I think is a pretty basic concept. However, it appears to be headed for a death spiral for that same reason. I'm fine with agreeing to disagree, regardless.
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