I agree with you Barry, and I hope you, Scott, or Jay (with the magazine motor comment) did not think I was calling you guys out. I considered deleting the post because I really hate passionate forum debates nowadays, but Scott pulled the reply trigger pretty quick so I wanted to clean up my end of the discussion
In the end, SCR and DCR are
both based on physical measurements and movement of reciprocating parts....that's my only argument against those who completely dismiss it, and if one can be used combined with adequate knowledge and experience, so can the other.
Moreover, I think we all agree that DCR is merely one piece to the puzzle, like any other "thing" we can evaluate or change in a build, so we are pretty close.
A 12:1 motor with a 320 duration cam may have good DCR numbers, but it doesn't mean much at some points in the curve, and two 9:1 motors, one with a dome and ton of deck clearance versus a tight quench and flat top could have the same SCR and DCR and act very differently as you point out. To keep it on point, a 360 Ford is a great example of low SCR and DCR but not "right"
Then add different elevations, temperatures and different supporting components, and it will change behavior. I would never argue against that, and to be honest, I do not know of a calculator that can predict anything as dynamic as cylinder pressure, maybe Detroit has that stuff. Pipemax will give you cranking compression at different RPM, but I can't say that it does anything that could predict all the different things that affect actual cylinder pressure in the variety of uses a street car finds itself loading and unloading throughout use
I do believe though, and I am preaching to the choir here....actually blueprinting the engine, building it to suit the use, and using the same DCR calculator without estimating measurements, and then building experience from the results of those numbers, really does help. I mean how did we know 9:1 SCR is
generally streetable, IF a lot of other things are right? ...experience
DCR can allow you to "lean" one way or another based on what you are trying to do, just like you said. Honestly, for me, less than octane tolerance, I'd say it helps me tweak part throttle and idle vacuum in very general terms without going "too far"
Here's something that I would say is a problem with DCR though and what I would say people should "bitch" about...what is the one calculation we will all use for DCR so we can compare results?
I use PKelley's calculator and have about 15 builds on it of varying configurations, all street, more if you count other engines I haven't directly pulled wrenches on. Doing a bench build 390 4V, Streetmaster, 9.72:1 SCR, .050 quench, HMV-272 HFT on 109 (which is actually a 268 degree cam using a standard .006 rise), comes out to 7.85 DCR. All my stuff is measured. I expect that this engine will do fine on pump gas, but could be a little rattly in something real heavy. However, I didn't want to retard the cam and dull idle vacuum, and it's a bench build, so no plan to change any parts, so I am happy with it
Pipemax said the DCR was 9.07 with a cranking compression of 172 psi and Wallace Racing says 7.85 with a cranking compression of 150. Which is right?
Probably none of them compared to each other, but I know a flat top wedge with a decent quench, iron heads and matching parts will be good with PKelley's numbers up to about 8:1. So I made my decision to keep ICL at 109, even though I likely would have wanted a little more idle vacuum. However, if I always used Stan's program, not PKelley, I would likely trust 9.07 just as much. So, IMHO, the biggest issue of DCR is that the
computation of measured values is done so differently by different peopleNow, SCR has only one formula, got it, but people still misuse that too. Think of how many 10:1 390s are out there, except for the fact that they are deep below deck, run an 8554 gasket and the chambers are bigger than advertised
Also, let's take a 360, keep the pistons .090 in the hole and add some dome to get to 9:1 SCR...ugh
If we don't agree, it's OK, nobody loses their birthday
and I have no reason to convert anyone to the dark side of engine build calculators, because in the end, as you said, engines can't read LOL