Proper DCR Calculation????
TECH Senior Member
Joined: Mar 2004
Posts: 14,620
Likes: 17
From: BFE
I agree with Die Grinder. People are getting their boners waaaay too stiff these days trying to get the "perfect" DCR.
Motors aren't making any more power (especially the 346's) while trying to get perfect DCR than the others who aren't concerned with it. I made 460+ rwhp 4 yrs ago on old parts with a 346 and never gave one crap about calculating DCR.
Unless you're running crazy compression levels, save your time and energy towards something else.
Motors aren't making any more power (especially the 346's) while trying to get perfect DCR than the others who aren't concerned with it. I made 460+ rwhp 4 yrs ago on old parts with a 346 and never gave one crap about calculating DCR.
Unless you're running crazy compression levels, save your time and energy towards something else.
That is the difference between having a combo that puts out 430rwhp (for argument's sake) and the same combo that would put out 470rwhp.
It is all those little bits that you say we should ignore, DCR (mainly related to max compression for fuel used), quench, VEs, dialing a cam in, centering geometry, proper preload, plug range and gap etc......
Building a "proper" combo is not just slapping parts together, it is also how to make those parts simbiotic with one another to maximize desired output in a certain powerband.
Using the calculators also allows the opportunity to trade off between several cam grinds and keep any eye on how much you mill or work the chambers. It also allows one to account for fuel differences that are found in differing parts of the country. While simplistic as compared to an engine simulator, it provides the enthusiast a tool that can be used to examine a combination of components without installing and testing. As I stated in my above post, my new cam has more duration than my current cam at 0.050" yet will provide more dynamic compression due to the faster ramp rate.
Teching In
Joined: Dec 2006
Posts: 39
Likes: 0
Vettenuts, your posts illustrate my position exactly. The very fact that you refer to differing methods of calculating DCR suggest there are serious shortcomings in the calcs. Does the truth really come in three or more flavors? You must have just discovered DCR because if you had previously used it to maximize your combination-you would not be able to use your new cam!
How about using more duration, lowering your DCR and spinning the engine higher. Does that path to increased power no longer exist in the DCR world?
Is the proper term for this calculation Theoretical DCR? Is there such a thing as observed or measured DCR? If it is theoretical DCR, how does it eliminate test and experiment?
How about using more duration, lowering your DCR and spinning the engine higher. Does that path to increased power no longer exist in the DCR world?
Is the proper term for this calculation Theoretical DCR? Is there such a thing as observed or measured DCR? If it is theoretical DCR, how does it eliminate test and experiment?
For those who know how to use DCR calculations I think it can be very helpful in selecting a combination of complementary parts.
Many on this board have seen it and I will definitely consider it when I do my upcoming H/C change. 
I have found the the calculator that uses the 0.050" duration doesn't provide good results because a constant is added. The other calculators I have used (the LS1tech spread sheet and others) provide consistent results and correlate well with David Vizard's methods.
Teching In
Joined: Dec 2006
Posts: 39
Likes: 0
Vette, thanks for letting us into the inner sanctum by telling us that your changing heads at the same time your changing cams!
Please divulge what component of your DCR calculation led you to make more than one change at the same time to complement your new cam. I must admit I'm rather surprised since you said originally that your new cam would increase performance by itself-although you did not say where in the powerband your new cam would help. Perhaps it would help from idle to redline?
What DCR parameters led you to change heads and cam? Did your DCR calcs suggest a different intake or exhaust port cross sectional area? A higher or lower SCR? Did your DCR call for a different intake mach index. Did your DCR call for for longer or shorter intake runners? Did it call for larger or smaller intake runners? Longer or shorter header tubes? Larger or smaller header tubes? Collecter length?
You do know how to calculate intake mach index don't you?
Please divulge what component of your DCR calculation led you to make more than one change at the same time to complement your new cam. I must admit I'm rather surprised since you said originally that your new cam would increase performance by itself-although you did not say where in the powerband your new cam would help. Perhaps it would help from idle to redline?
What DCR parameters led you to change heads and cam? Did your DCR calcs suggest a different intake or exhaust port cross sectional area? A higher or lower SCR? Did your DCR call for a different intake mach index. Did your DCR call for for longer or shorter intake runners? Did it call for larger or smaller intake runners? Longer or shorter header tubes? Larger or smaller header tubes? Collecter length?
You do know how to calculate intake mach index don't you?
Mach index changes from 0.49 to 0.50 factoring in the higher peak RPM of the new cam vs. my current cam. Ultimate goal is a strong powerband with less concern on peak numbers. Static compression will increase slightly due to the thinner gasket I will be using, but the chamber size will remain the same. Port cross sections I am unsure of however port volume will remain the same between the two heads (Dart 205's being removed, AFR 205's being installed).
Last edited by vettenuts; Dec 29, 2007 at 08:58 PM.
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Wow, so many DCR haters...as if any single measure is the be all and end all of cam selection.
For guys driving a mostly street car that want a decent midrange and throttle response it is certainly a figure worth looking at as you evalute your selection.
Comp publishes the .006, .05 and .200 durations for each lobe. It is definitely worthwhile to use the correct figures for the lobes being considered if one uses the spreadsheet I linked.
For guys driving a mostly street car that want a decent midrange and throttle response it is certainly a figure worth looking at as you evalute your selection.
Comp publishes the .006, .05 and .200 durations for each lobe. It is definitely worthwhile to use the correct figures for the lobes being considered if one uses the spreadsheet I linked.
