A thread in 'tech' addressed an issue with having trouble calculating DCR.
http://web.camaross.com/forums/showt...2&page=1&pp=15

The response I posted:
Is it REALLY closely related to octane requirement, as DCR software interprets it? What is the purpose/reason for installing a cam with increased duration/overlap? More top end torque/HP, correct? This can lead to poor low RPM driveability AND reduced torque. The logic states the more duration (later IVC) the cam timing has, the LOWER the DCR will be. This will allow a higher SCR. Why? Because there is lower effective pressure in the cylinder at lower RPM. The more duration the cam (which translates into more peak torque/HP due to more cylinder pressure), 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? Detonation is most likely at peak torque. As noted though, there are exceptions. THE POINT BEING, peak torque is increased with a cam with more duration. My logic states, as peak torque increases, the need intensifies for a higher octane fuel and/or less timing, or a LOWER SCR.Isn't the point of more duration (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 (or close to) peak torque, is it not? That is also the point of most cylinder pressure, is it not?

In DCR calcs, 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? Although a later IVC causes a drop in the DCR, it is actually causing greater cylinder pressure at peak torque. The point at which detonation is most likely to occur! You've got a higher cylinder pressure, and using the DCR calcs, you are justified RAISING SCR... This will increase cylinder pressure at peak torque/max VE even MORE.

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.I disagree. To me, that doesn't make any sense.

The unanswered question... When, or at what point, is the engine operating at it's true SCR? My reasoning would say at peak torque, or, if different, at 100% (or more) VE. It may have low cylinder pressure at cranking/low RPM, but somewhere the engine is going to start getting efficient, and the engine is going to see whatever compression (SCR) you have. These DCR calculations become less effective as you start to keep (or build) pressure in the cylinder. With enuf load combined with normal timing, it WILL detonate.

I compare DCR to EGT. By itself, EGT is misleading at best, inaccurate at worst. Both are merely a tool, to be used in conjunction with other MORE important data. I do not believe it to be the major tool to determine compression/cam combos, that some accept it to be.

AFAIC, the flaw in the DCR calculations available on the web, is they were not meant for street driven (IOW, pump fuel limited) engines. I believe these calcs were originally intended for race engines, where octane limitations are not a major player. IMO, because of pump fuel limitation, these calcs fly in the face of (my ) logic.

Any input to help anwer the hi-lited question near the bottom, would be welcomed. With any satisfactory answer that would render my reasoning inaccurate, my mind could easily be changed. Until then....

 

OK, I'll take a crack at it. I'd say you don't hit the true SCR unless you're well up over 100% VE. I don't even think you'd hit your actual DCR number until you are at 100% VE.

Generally, I agree wit you otherwise. Just doing a DCR calaculation with cam specs and SCR doesn't tell the whole story of how it's going to work in the real world. I guess the best example would be somebody who makes a horrible miscalculation and ends up with an engine that has a true 13:1 SCR. So they cam the snot out of it until you get the DCR down to where pump gas should work in it, right? Puuhhleeeese. Not gonna happen. That sucker's GOING to have detonation when you get it singing, ASSUMING the rest of the combo matches the big cam and you have a high VE in actual operation. It's gonna make big cylinder pressure when it gets "on cam" and the VE starts to skyrocket. Detonation city here we come.

An open intake valve doesn't bleed off ANY cylinder pressure if air is still flowing INTO the cylinder past it.

I agree that you need to take into account VE in the calculation somewhere, not just DCR. For a street motor with a relatively modest VE hovering around 90% you can probably just keep using the DCR calculation as your guide. For something that's really gonna BREATHE, it's definitely gonna affect the actual cylinder pressures in the motor, and it sure ain't gonna lower them!.

That's my take, anyway. I'm sure the big brains will be in momentarily to sort it all out for us.

 

To me, that makes no sense. What is the definition of 100% VE? If the cylinder is filled to 100%, wouldn't that equate to the actual SCR?Can I interpret that statement as stating you agree the DCR calcs are bogus, ESPECIALLY, for an octane limited (street) engine?Does that sound like ONLY achieving the DCR as the ACTUAL CR, to you? Sure doesn't to me.This would become reality, at a point, such as peak torque/100% (or +) VE.IMO, won't even give you that much slack. ...

 

When, or at what point, is the engine operating at it's true SCR?

Good question, because I've been meaning to post a similar discussion about
boost motors.

Just for an example, my friend swapped an INTERCOOLED turbo from a
stock supercharged 1998 Pontiac GTP (3.8L V6).

The engine is completely stock other than the turbo and 1.9 ratio rockers.
(stock 1998 GTP s/c camshaft and pistons!)

His best on the supercharger was 13.2@106MPH before recording knock. I believe
he was only able to push 9 PSI using pump gas (94 Sunoco)

NOW...he's boosting to 17 PSI, intercooled, same fuel - no knock!
12.58@116 MPH (street tires, no traction)

 

Stolen from Jon (Oldss) and Kennedy's Dynotune Page

SCR = mechanical ratio (static)

VE% = amount of charge ingested during intake stroke (dynamic)

DCR = theoretical ratio of cylinder volume at IVC over the volume above the piston at TDC (static)

Effective compression = amount of charge trapped after intake valve closes (dynamic).

Therefore 100% VE does not necessarily equal SCR.

 

Damn, I just got one of those tingly feelings in my gut!

Based on the definitions above, if the volume above the piston at IVC is less
with a later closing intake, it would make sense to me that increasing SCR
would pad the lesser amount of charge trapped to give it some punch.

If late IVC are meant to work at higher RPM, then by the horsepower formula
and VE curve, can we conclude that a cams meant for high cylinder filling at
higher RPM should only be used at high RPM where the motor can make more power?

IOW:

Late IVC, low RPM, Low SCR, xx octane = suck
Late IVC, high RPM, Low SCR, xx octane = better

Late IVC, low RPM, high SCR, xx octane = better
Late IVC, high RPM, high SCR, xx octane = best (with proper tuning)

So...that means, we're essentially shifting our same octane fuel to be used
in a higher RPM range with DCR having a TUNED effect on the detonation point by mechanical ratio, correct?

and...

We want to operate as close to our detonation point (safely) to be most
efficient with our given fuel.

 

Amount of charge ingested is theoretically full/complete at 100%. Effective Compresson would then equal SCR, if amount of charge trapped, fills the cylinder, would it not? Therefore, why not?To me, that volume above the piston at IVC, is of little value to me. It is how much ultimately gets in the cylinder before it closes that counts. Why pad anything if the amount of charge trapped is 100% (or even greater on some combinations) of that capable of being trapped in the cylinder volume? "Based on the definitions above...." Based on definition above, I consider DCR calcs flawed. What is your definition of 'proper tuning'? IMO, with pump gas, it means retard the hell out of it. Cuz if you don't, cam timing that promotes additional cylinder filling (higher percentage of VE) coupled with higher SCR, will result in detonation hell, especially at peak torque. That peak torque will be greater with a cam with later IVC, will it not? That translates into greater cylinder pressure, does it not? That is the whole point of a later IVC, to gain more cylinder filling, which is more cylinder pressure, which is more peak torque. Otherwise, why bother putting in that big cam?

AFAIC, what we are doing is raising the octane requirement. Why, cuz we have more cylinder pressure than we did before. Then you want to raise the SCR besides? Sure, great for the low end, but with octane limited fuel, top end presents a challange. That is why I considered the DCR calcs flawed, UNLESS unlimited octane is available. Also flawed, because the more charge trapped (higher VE), the lower the DCR. Does that (calc) make sense? I understand DCR = theoretical ratio of cylinder volume at IVC over the volume above the piston at TDC, but it is basically a meaningless computation, IMO, UNLESS, your intention is to prop up the low end, on a RACE engine, using adequate octane fuel. For any engine forced to run pump fuel, it makes little sense to me.
As I've been stating from post one, sooner or later, that engine becomes efficient.

After my counterpoints, maybe indigestion.

 

Probably is, I'm usually corrected on my understanding of engine physics

My understanding is, the swept volume may reach 100% VE, but due to pressure
variations in the clyinder (before IVC), and pulse direction/reflection, what
comes in BEFORE IVC may not always stay in AT IVC

Next question: Is there such a thing as 100% effective compression?

True, but let's say in a dynamic environment if the volume at IVC = 43 c.i.
and the effective compression is 90%, will a motor with 45 c.i. @ 90% effective
compression need as much SCR?

At peak torque/peak VE, there is no chance of combustion irregularities,
with the combination of SCR/DCR at full potential (I guess resulting in the best torque curve over a window of RPM)

.

Retard the cam timing, or spark timing?

 

The development of racing engines has always held to the dictum of the great automotive engineer Ferdinand Porsche that "The perfect race car crosses the finish line in first place and then falls to pieces."

If it doesn't fall to pieces, it is nice to know there was a little cushion/reserve, for future use. Either way... That makes it a 'win-win' situation.

On topic.... I've explained it about as well as I can. You're not the first to disagree. Won't be the last.

 

I'm not disagreeing...trust me, I don't know enough about this to make
such a statement. I'm trying to understand you and the concepts at this point.

What I've gathered from you is:

If my engine is going to meet and exceed 100% VE, why am I increasing SCR
to the point where reaching or surpassing SCR once the engine becomes
efficient will only promote detonation.

My questions are:

How come a 1998 Grand Prix GTP engine can eat more than twice the charge
and still have room for more with 94 pump gas? How much can you cram
into that cylinder before the lights go out?

If 100% VE = filling the cylinder fully, after IVC is there any chance of having
all of that charge remain...or increasing through resonant tuning?


My thought/position is:

100% effective compression is tough to achieve, therefore SCR is increased
to make the fuel more efficient resulting in a higher Torque peak, and more
torque across the band which makes more power at peak RPM.

EDIT (Another question):

max DCR would be valid only at peak VE/peak torque. Anything above and below
that point is safer to operate.

If we made all of the calculations based on max VE @ RPM, would our engine
be able to trap enough charge to become an issue?

If not, wouldn't increasing SCR help power output?

If yes, then SCR would need to come down.

In a naturally aspirated motor, with the piston rounding BDC and the intake
valve closing late (at resonance), is there enough pressure in the intake
runner to overcome the cylinder pressure and trap 100%, or higher?

As the cylinder gets closer to capacity, it's tougher to move more inside, and
keep more inside.

 

Isn't that the idea of having a cam with more duration? If you weren't getting closer, you wouldn't be attaining more cylinder pressure. If you weren't getting closer (or attaining it), the big cam would be pointless.When cam comparos are done on a dyno, is the SCR automatically raised to see an improvment, and/or validate the test? Wouldn't that be changing more than one variable?What I'm having a problem understanding, DCR goes down, as the VE goes up.That is the crux of my arguement.That is not the issue. That is a given (to go in that direction), otherwise why bother with the cam.
Again, that is not the issue, AFAIC. The issue is as we get closer, the SCR should be lowered, not raised.

Manipulate the DCR calcs to achieve (or something close) a DCR that equals the SCR. How efficient is that engine at peak torque? NOw you could raise the SCR. You'd probably detonate under a lite load, at low RPM, though, wouldn't you?

 

Arnie,

I'm going to sit back and watch some more replies come in.

Just a thought about your last statement:

That's going to change the peak torque RPM...so I don't know how to answer
that. It's like we're buying the saddle before the horse?

In any event, I'm going to play with some DCR calculators and try to find the
errors that you see. At least then, I can be more effective with my debate.

RE: the volume remaining once the intake valve closes.
In an extreme example, if the IV closes when the piston is half way up the bore,
even if the trapped charge was 150% due to tuning, the amount of charge
to squish is relatively small when compared to the SCR.

I'm thinking there's a fine line between the volume @ IVC vs. the trapped
charged percentage...and this is where SCR can be varied to get the most
of out the power stroke.

Correct my logic: SCR augments, or attenuates the torque curve but doesn't
directly change the torque peak RPM?

 

Forget about the RPM. At issue is the peak torque/cylinder pressure ACTUALLY achieved. Just some FFT.

 

Stop posting for a little bit so I can read all this and grab the correct points and bad points and give a better explaination of what is going on.

Bret

 

Ok......

Preface BMEP = Brake Mean Effective Pressure

To start with here, basically Arnie your problem is that your statement of "Detonation is most likely at peak torque." is not true most times. It's true in a Pro Stock motor where they have to reduce their SCR because they have so much cylinder filling, but this is with the best VE% seen in NA combustion engines. In fact in street motors it's less likely to occur at peak TQ where VE is highest but at lower RPM where VE is probably closer to 70-75%. WHY? Well this is usually due to the time the motor has to burn the charge in the cylinder, remember timing is measured in degs not in secs, where the speed of the flame front is measured in secs. Plus you have the dynamic condition of the piston and it's piston speed effecting the combustion process, the piston is in the same point at 2000rpm and 35° of total timing as it is at 7000rpm and 35° of total timing but the flame front isin't nearly as developed at the 7000rpm 35° later @ TDC.

Next.... "Although a later IVC causes a drop in the DCR, it is actually causing greater cylinder pressure at peak torque." not really.... in almost every high end motor out there (Cup, F1, Engine Masters) the peak TQ per cube or L is about the same, but this really depends on the rules and limitations that the motors have in their respective classes. If peak TQ per cube is the same then esentially the peak BMEP is about the same. The other problem here is that compression ratio is directly related to BMEP, not the case. Compression ratio is linked to the Max Cylinder Pressure, and that's where the problem lies. Your theory is right when you look at Max Pressure, but not Brake Mean Effective Pressure which is directly related to TQ output. You can have the same TQ output but less Max Cylinder Pressure with a lower SCR motor. Here lies the problem.... we need to establish a vocabulary here for this discussion and clarify some things. Max Cylinder Pressure does follow the same trend as BMEP but it's also related to the STATIC Compression Ratio.

So take a Engine Masters motor like Sherman's 2002 motor. 530ft lbs of TQ @ 5000 (actually 529.7 since I have the real dyno sheets), it also had a 101.3 VE% at that RPM and a BMEP of 199.3. This motor given it's RPM, Parts, and Development Time constraints still makes about 1.45 ft lbs per cube at peak TQ. Now if you go to a Cup motor at 540 ft lbs peak it's around 1.50 ft lbs per cube, and this is about the same for a F1 motor as well. Now the fact that these motors use something from a 92 Octane pump gas, to a Spec Sunoco 116 octane race fuel or a Shell highly engineered race "gasoline" and compression ratios from 11.7:1 to who knows on a F1 motor I don't think that you can make that assumption considering that the BMEP is probably not more than 5% different here and varies in RPM from 5,000rpm to 16,000rpm for TQ peaks. BMEP is not a determining factor of octane requirement from anything that I have seen, but the Max Cylinder Pressure is. Problem is it's hard to measure, but I have some good guesses down below.

Now the really cool thing is that the Cup motor (12:1 SCR) and the EM motor (11.7:1 SCR) could have the same BMEP at peak TQ but the Cup motor would have LESS DCR. You could run pump gas in those things without any problem if you treated them the same way as a EM motor with temps etc.... (I realize that's a stretch, but who knows unless you try it)

"effective compression" is a good idea, but with the VE% changing with RPM and the burn speed relative to RPM it's not that easy to say X "effective" compression can be used with X octane gas. That's why we use DCR, but different cooling, different burn speeds (due to chamber shapes, fuel atomization etc...), intake port temps, different max cylinder pressures..... all have tons to do with it.

Another big part of detonation that the Forced Induction guys know is intake air temps, or air density. The cooler the charge the less likely the detonation is going to be. With that comes the fact that on a NA motor which sees anthing over 80% VE is going to have some intake tuining pressure to help that, but with increased intake tuning pressure (and VE) comes a decresse in intake port temps. On a street motor the 1100-1500rpm cruise intake air temp can be 120°F but at WOT and highest VE it can be in the 60°F range. Hence less detonation.

Ok so how about the situation where a big cam is added to a combination and then you add SCR to get the DCR back in line? Well take Shermans EM setup... AFR 215RR's, a 234/234 108LSA cam, 11.7:1 SCR (8.5:1 DCR)and a Super Vic... Now lets throw a 250/250 108LSA cam and 13.1:1 SCR (8.5:1 DCR) and no other changes at it. Your going to see basically the same average HP from both motors with more peak power out of the larger cam motor, but less average TQ so you would have a lower score in the EMC.... but our look is at octane and detonation on this same combo.

BMEP and Vol Eff % on these two setups are basically shaped like the TQ curve, so we have less power, VE% and BMEP down low and below peak TQ on the larger cam and compression motor while they are all higher above peak TQ.

The smaller cammed motor will have lower intake air temps, and better intake pulse tuning over the larger cammed motor. So the smaller cam motor helps itself in the detonation department.

With all of that, the higher compression ratio (13.1 vs. 11.7) even though the DCR is the same the motor will detonate more at ALL RPM than the lower compression motor. So it's not linked to VE%, BMEP, or DCR ALL the time, but it is linked to the SCR and Max Cylinder Pressure. Even though the VE% and BMEP is close to the same the Max Cylinder Pressure on these two engines is greatly different. While BMEP is within 1% at the highest readings the Max cylinder pressure is 10% higher on the higher SCR motor while the SCR is 12% higher.

What does that tell you? Well you get your best street performance on a smaller cam, higher DCR, lower SCR motor on pump gas than you do with a high SCR, big cam, higher DCR (same DCR) motor on pump gas.... STATIC compression ratio still counts for something since it's directly related to Max Cylinder Pressure. OTOH the higher overlap of the larger cam, the lower idle vacuum and the lower TQ at low RPM would make the car less driveable and less of a street car anyways so it's much more likely to run race gas.

Sounds like some mean street motors out there to me, Mindgames is one of them I think fits all of these rules for a high compression high DCR pump gas high output street motor, it's a good example to follow on what to do.

Bret