Arnie,

Not all engines have peak VE at torque peak. Intake and exhaust tuning, among other things determine where max VE occurs. Brake torque (and hp, of course) is the result of indicated torque (actually produced inside the engine) LESS friction and pumping losses which are increasing with rpm. That's one of the reasons most torque curves eventually fall off.

Detonation also doesn't necessarily occur at peak torque. If you are trying to get max output from an engine from say 2500 to 6500 and peak torque occurs at 5000, and peak hp at 64-6500, you need to be very careful of ignition timing in the 2500-3000 range, or you can get some rattle there. This is especially true if you built the engine to have a fat torque curve in that area.

There is a tendency for folks to confuse DCR and something we might call "effective compression" (EC). The engine only knows how much mixture got into the cylinder and when it is ignited. Part throttle is a good example of VERY low EC and VERY low VE. At a 2000 rpm cruise, say 70 mph, you car might require less than 40 hp to maintain that speed on a level road. That is about 105 lb-ft. Your engine may be capable of 210 lb-ft at wide open throttle (WOT) at 2000. Lets say the VE at 2000 WOT is 70%. At the cruise condition it's actually only about 35% because that's all the mixture the engine needs to crank out 40 hp. So, the EC might be about half of the DCR at that condition. That's why old distributors had vacuum advance, to get the max they could out of the very little charge in the engine under part throttle.

As others have said, DCR is a more reasonable way to look at what the engine wants/needs than just SCR. That we're even discussing DCR means we've come a LONG way in understanding what goes on. The caveat is that the more we know the more understanding it takes of how an engine really works, both mechanically and thermodynamically. We can easily get lost in the forest.

If you recall when Winston Cup restrictor plate engines didn't have a mandated maximum SCR, the used something like 17:1 SCR or even more. The "plate" restricted the high rpm (7500 or so back then I think) VE so much that the EC at power peak was probably down around 10-11:1, or what ever the gas would handle. If you looked at calculated SCR and DCR they were off the chart, but at power peak the engine didn't feel that. What it did feel was EC. One of the problems was that below about 5000 the plate could pass enough air that WOT would cause severe detonation and engine destruction because the EC approached the calculated DCR. The driver had to be a little careful at pit out!

FWIW, generally cam "aggressiveness" doesn't refer to duration but rather to how quickly the valve is lifted. No offense intended. Aggressiveness does relate to area under the lift curve, and the amount of lift you can get for a given duration, so it does affect the cylinder filling, and therefore torque/hp. If we are calculating DCR at .006 lift which many use to define total duration, you'll get the same DCR with a lazy cam as with an aggressive one. However, the EC might be different. Clear as mud?

Whatdayathink?

Jon

 

Wow, now thats a good post. I may get lost at some points but I take great pride in knowing that if I don't understand what is said there are people that will explain it so efficiently that even a novice can understand.

 

If that is the case, then MY reasoning is incorrect. My reasoning/logic figured at peak torque (100% VE... or more), it would be more like the true SCR, not the DCR. IOW, DCR applying to lower RPM, and at peak torque, it would be more like the SCR.

 

Ya have to "forget"(not really) about static,it is just numbers on paper and a guide and numbers to figure your DCR. The DCR is the number to pay attention to.

 

There may be exceptions, but as a rule....
That may be true, no arguement. But as a rule, peak torque and detonation...
Understood. VE was so low, compensation with more SCR was tolerated.
What are you implying by "off the chart"? The numbers did not compute, as in flawed DCR software logic?
Or IOW, 100% VE or there abouts?
OK, if I used the incorrect description, I'll accept that, providing you understood my intent here.
Sometimes, less pain, when I don't.

As I stated to Larry, my reasoning was peak torque and/or 100% VE was indicating something along the lines of true SCR. This thinking in sync with a long duration/big overlap cam, with it's low VE, could tolerate a much higher SCR. If that reasoning is incorrect, I stand corrected, Jon.

So, if I am on drugs, when, or at what point, is the engine operating at it's true SCR? Again, my reasoning would say at peak torque, or, if different, at 100% (or more) VE.

 

No problem,I just hope I explained it well enough,and with Old Strokers eloquent post(wish I could do that) everybody may learn something.
Never have been one of them Philly lawer's when it comes to documenting things,I just understand the mechanical things and formulas.

 

That you do, dude!

arnie, I'm not sure that you really want to change you mindset on this topic and it doesn't look like dude and I are doing much agreeing with you. Just about anything we say now will be a rehash.

We all sorta hijacked Nick's thread, but he did get his answer early on...

NVetro..cool screen name, Nick. Is that a description of your job?

 

Thanks to you and Larry for making an honest attempt at convincing me of the merits of DCR.

Has little to do with wanting or not wanting to change my mindset. If I don't agree, and I don't agree with the logic, and the posts have not been very convincing, in regards to satisfying my quest for answers. With my latest question having gone unanswered, and as these posts have (as noted) hijacked ths thread, I've decided to start a dedicated thread in the interest of finding an answer. Thanks again, guys.

 

Thanks guys, yea you all answered my question. And NVetro username I use, stands for Nick Vetro. Vetro is my last name :-D

 

THANKS for the complement. Coming from the Old Stroker it means a lot.

 

Had to chuckle. I read it as "In-vetro", more correctly "In-vitro" as in IVF.

http://www.fertilityich.com/ivf.html

My mind wanders around a lot...

 

I get that all the time, i blame my parrents, ***'s hahaha