Hey all...my 383 motor is together and ready to install. Engine shop got a Static CR of 10.848:1. I am trying to calculate my dymanic CR to double check that she can run 94 octane pump gas without an additive. I have ALL the information, what do you guys need to calculate it? I tried several calculators, one got me 8.9:1 DCR which is GREAT but it neted me a 9.97 static CR, one got my a 10.848 static cr which is exactly it but gave me 9.7:1 dynamic cr....help pls :-) Lemme know what info u need.....

 

try using the advertized duration numbers instead of the @ .050 numbers. i'm almost sure this is your mistake its very common for some reason. what you're actually doing with dcr is calculating the compression assuming bdc is where the intake valve closes completely.

 

its not asking me duration, only thing its asking me when it comes to the cam is...intake ABDC @ .050"

 

use the advertised duration numbers. take the advertised duration and subtract the @ .050 duration from it. take this number and divide it by two. add that final number to the intake valve closing point and that should be the advertised duration intake valve closing point. try calculating the DCR with that.

also, with 10.8 c/r which is only about .4 higher than stock I believe if you have the stock cam you should be able to run pump gas, with a larger cam with a later IVC the DCR will go down which should mean you can run pump gas, no? Id definitely calculate it rather than go by my uneducated rambling there, though

 

I'm having a real tough time accepting the principle behind DCR. I question it's validity. IOW, I question the 'theory' behind it.

 

Seriously? It's pretty simple. No compression occurs until the intake valve closes. DCR is therefore the "real" CR. Static CR is simply the volume above the piston crown at BDC/volume at TDC and does not represent anything meaningful in terms of a running motor.

DCR though is closely related to octane requirement/tendency toward detonation. When you look at a cam catalog, notice how "big" cams will have a specified minimum CR. That because with a late IVC and low compression, the thing would be a dog, due to inadequate DCR.

Rich

 

Hi Rich. Yea, I'm serious. Actually, it was a 'lead in' post. Look out.
Actually, I'm quite familiar how one detemines the DCR, as well as the logic. I'm aware of a couple of different sites that explain DCR. It's just, as noted above, I've a real problem ACCEPTING the logic used to determine DCR, as well as question the validity/usefulness of that logic.
OK, now we are getting somewhere. Is it REALLY closely related to octane requirement, as DCR software interprets it? A couple of points to note. Bare with me, Rich.

OK, focus in on the cam, specifically it's timing. What is the purpose/reason for installing a cam with agressive timing? More torque/HP, correct? More precisely, more top end torque/HP, correct? As will be understood a little later, it doesn't need be, but it is not uncommon for a 'mega degrees of timing' cam to display poor low RPM characteristics, namely poor driveability AND reduced (low RPM) torque, agree? As I interpret DCR, the logic states the more agressive (later IVC) the cam timing becomes, the LOWER the DCR will be, agree? This according to the theory used, will allow a higher SCR, with the SAME level of fuel octane, agree? Why? Because there is lower effective pressure in the cylinder at lower RPM, agree? This theory is implying the more aggressive the cam (which translates into more torque/HP due to more cylinder pressure, BTW), the higher the SCR acceptable with the SAME octane. Why should it? Does this sound logical? What about the effective pressure at the point of peak torque?

The kicker; detonation is most likely at peak torque. Is, or is not (regardless of the RPM at which that peak torque is achieved), peak torque increased with a cam more agressive? My logic states, as peak torque increases, the need intensifies for a higher octane fuel, or a LOWER SCR. That or retard the hell out of it. To add insult to injury, it is tougher to hear detonation at RPM that generates peak torque, than at low RPM during a 'lugging' condition.

Am I missing something here?

 

DCR is what the piston actually will "see" when running.
The reason the higher static with a big cam is to keep the amount of DCR up to a point that the engine will run up to it's potential.
Example: take an 8-1 static motor and put a 75*IVC cam in it and it will reduce the DCR to the point of maybe not even cranking and it definitely won't run right and all sort of tuning issues with tuning at low RPM's.It just won't burn the fuel correctly.When ya get in the upper RPM's it happens so fast it's Ok with the low DCR but still won't put out like it's suppose to.
On the flip side to high and it will detonate and quench will play a big part in that picture.DCR is just a guideline.
The engine can't start making compression until the intake closes.If it closes late then the less DCR ya have with a given static compression.
There are cut off points for every octane level 89-116 and the DCR tell's ya what fuel the engine will tolerate.

The best I can explain it.

 

Isn't the point of more aggressive timing (among other valve events, later IVC) to increase VE at some point in the RPM range, taking advantage of high RPM mixture speed/momentum? That point, (max VE) is peak torque, is it not? That is also the point of most cylinder pressure, is it not?

 

Nick, all you had to do was ask.... 8.06:1 DCR for your setup.

Bret

 

Your point is???

 

Trusting you agree with my wording in above post, the point is..... Try to focus in on peak torque, or IOW, max VE.

The later IVC point causes a decrease in the DCR. This supposedly justifies an increase in SCR. Why? Why would you want to do that? Altough a later IVC causes a drop in the DCR, it is actually causing greater cylinder pressure at peak torque. The point at which detonatin is most likely to occur! You've got a higher cylinder pressure, and you (with the DCR software logic) justify RAISING SCR, which will increase cylinder pressure at peak torque/max VE even MORE. Tell me this does not add fuel to the fire. Pun intended.

 

The static and DCR never change,thus it can't "increase" at peak TQ.
Raising static only puts the DCR in an acceptable "window" of the octane in the fuel ya run.
Raising the DCR to a calculated point only puts the engine in a position to make the HP it is capable of at the given fuel octane rating ya run without detonation at a given timing setting.Removing either will make it down on HP.
Like I said in an earlier post 8-1static + 75* closing makes for a lazy engine.

 

I'm not sure we are on the same page, Larry.

I'm not disputing the result/product of any given data, that is inputed into DCR software. What I'm stating is... I do not agree with the logic built into the software, or the product of those numbers that are 'crunched'.

When IVC is delayed, the cylinder pressure is increased at peak torque/max VE. If it wasn't, there would be not point to installing a more aggressive cam. We agree on that much, anyway. With that understood, (my) logic states the SCR should be reduced not increased, PROVIDING we are concerned with the possibility of detonation at the RPM that generates peak torque. IF, we don't care whether or not detonation rears it's ugly poose at peak torque, but are more concerned with proping up the low end, then yes, we should increase the SCR, as you stated. Both of us are aware of the tactics used to prop up the low end of a 'cammed' engine. But if that is the case... detonation probabilities at the RPM most likely to be an issue... be damned. Also, if that is the case... the octane of the fuel, is no more important than the amount of spark timing we pull at RPM of peak torque, in order to keep engine from self destructing. I take it, we disagree. OK.

For low RPM improvement, aggressive cam timing combined with increased SCR = good.
For high RPM detonation prevention, aggressive cam timing combined with increased SCR = bad.
Sorta similar to reducing SCR for a FI setup. We reduce SCR to allow for the increased cylinder pressure generated by the blower/turbo, at a higher RPM.
Is my logic flawed? Anyone else with any input?

 

Don't know if your logic is flawed or not,but ya ain't thinking right about DCR.
The cyl pressure for a given static or DCR doesn't change either unless ya change the cam,piston's,gasket. A 9.0 DCR is 9.0 at peak torque,it may be less at 1500RPM's and detonation won't occur if the DCR or the fuel is right for the given DCR.Cyl pressure can only go as high as the chosen DCR let's it. Otherwise where does any increase come from?
Ya raise the static for a late IVC so the engine don't fall on it's face.That's why cam companies spect ya a cam to your compression.They already know what the DCR will be.That's why the don't recommend a 260-270 cam with 9 or 10 to 1 static.They generally go low to cover all situation's rather than push the limit's.