I've read here and there that what really matters is dynamic compression ratio. This is reflected in the cranking compression pressure since compression can't start until the intake closes. Moving the intake closeing valve event will change both dynamic compression ratio and cranking compression pressure. This pretty much make sense.

Why, when the engine is running at RPM and sonic and intertial effects are in force, would dynamic compression be important? True, the intake is still closing at the same time, but the sonic and inertial effects pack in more air so the VE (volumetric efficiency) approaches (or exceeds) 100%. IIRC, by the definition of VE, 100% means we trapped 43 ci in our 43 ci cylinder rather than the 35 ci that we do when cranking.

 

A couple of thoughts.

Think about what affects cylinder pressure. Instead of thinking about volume of air, think about the density of that air.

 

Carl,

Did you mean "Why, when the engine is running at RPM and sonic and intertial effects are in force, would static compression be important?

Or, did I just not follow you? (and I may not have).

 

Poor wording, I guess. The question is, what do the sonic and inertial effects do to the compression? Can you still assume the dynamic ratio, or is the ratio more like the static ratio as your approach 100% VE? For example, it you have 180 PSI cranking, would you have 180 PSI at 5K?

Prolly should drag out a text book, but thought maybe one of the gurus here could explain it.

 

So the density is greater when the intake closes, in effect packing 43 ci into the 35 ci? So by PV=NRT the pressure is higher at the start of real compression, and the net effect is the same as a higher compression ratio that the dynamic ratio?

 

You guys are trying to make things too complicated. The reason dynamic compression ratio (DCR) is more important than static compression ratio (SCR) because your ultimate compression ratio is determined by the intake valve closing point. You could take two identical motors with 12:1 SCR you could have one motor run just fine on pump gas since it would have a lower DCR (due to larger cam) while the other motor could ping like a **** because the intake valve closes too early and builds too much DCR.

 

LOL!!!!!!!!!!!!!

 

ok, then I have a question... Say two motors have equal DCR, but one has a higher SCR. Would the higher SCR motor build more power?

 

TTT, still curious...

 

I would say no.

John B nailed it though. DCR is affected by closing events. That is why you need more SCR with a bigger cam, DCR goes down.

Now, as for Interntial supercharging. That is another animal.

http://www.truckpulls.com/Tech%20Fil...%20Systems.htm

Intake Runner Length (L)

Knowing that the pressure waves (positive or negative) must travel 4 times back and forth from the time that the intake valves closes to the time when it opens and the speed of the pressure waves, we can now figure out the optimum intake runner length for a given rpm and tube diameter. We must take into account the intake duration, but you want the pressure waves to arrive before the valve closes and after it opens (air wont pass though a closed valve). To do this you must subtract some duration, typically you take off 20-30° from the advertised duration. 30° works well for most higher rpm solid cammed drag motors. So the Formula to figure effective cam duration (ECD) will be:

ECD = 720 - (Adv. duration - 30)

For a race cam with 305° of intake duration it will look like this:

ECD = 720 - (305 - 30)

The ECD of that cam would be 445

The formula for optimum intake runner length (L) is:

L = ((ECD × 0.25 × V × 2) ÷ (rpm × RV)) - ½D

Where:
ECD = Effective Cam Duration
RV = Reflective Value
D = Runner Diameter

If our engine with the 305 race cam needed to be tuned to 7000 rpm using the second set of pressure waves (RV = 2) and had a 1.5" diameter intake runner the optimum runner length formula would look like this:

L = ((445 × 0.25 × 1300 × 2)÷ (7000 × 2)) - 0.75

So 19.91 inches would be the optimum runner length.

Intake Ram Pipe Diameter

This is the easiest to figure out. The velocity in the plenum intake pipe should not be higher than 180 ft/sec at maximum rpm. The formula to figure out the diameter pipe that should be used is for a given velocity is:

D = Square Root of (CID × VE × RPM) ÷ ( V × 1130)

Where:
D = Pipe Diameter
CID = Cubic Inch Displacement
VE = Volumetric Efficiency
V = Velocity in ft/sec

If you're dealing with liters, change CID to liters and the constant to 18.5 so the formula will look like this:

D = Square Root of (Liters × VE × RPM) ÷ (V × 18.5)

An example for a 153 cubic inch 4 cylinder with a 85% VE, revving to 6000 rpm would and a desired 180 ft/sec air speed though the intake pipe would look like this:

D = Square root of (153 × 0.85 × 6000) ÷ (180 × 1130) = 1.96

You would need an intake pipe that has a 1.96" inside diameter to have 180 ft/sec air velocity at 6000 rpm for that engine. In other words the engine would need a little over 3 square inches of intake pipe area.

 

I think the model you guys are proposing is ignoring the inertia of the intake air charge.

Correct me if I'm wrong, but you guys are saying that intake valve duration past BDC is used because at BDC, cylinder pressure is less than the pressure in the port (because of the restriction of the intake valve). So you keep the valve open longer, ideally until the pressure is equalized. So if the port was ideal, you'd instantly clode the intake valve right at BDC?

I don't think this is the whole story. At high RPM, the velocity in the intake port is extremely high, near sonic, right? The air/fuel mixture isn't just going to stop entering the cylinder because the piston starts moving up the bore a little bit. The inertia of the charge can make VE rise over 100%. At higher RPM, the inertia increases, and the time between BDC and intake valve closing decreases. So it would make sense that at higher RPM, the valve would be held open longer to take advantage of the intake charge inertia. So the DCR shouldn't tell the whole story either.

IMO, YMMV. I really need to buy Engine Analyzer Pro and play with it...

JRod, I believe what you are refering to isn't inertial supercharging, its more acoustical (sound waves). I guess some people call it inertial supercharging, but isn't that a misnomer?

 

John I guess I'm a little confused about some particulars here. Isn't the DCR # in fact just another static # which simply takes the intake valve closing point into account ? It was your comment "intake valve closes too early and builds too much DCR" that kind of threw me. I think you're referring to cylinder pressure, not DCR #'s here since, while the DCR #'s imply a certain cylinder pressure there are other things such as actual cylinder filling percentages that affect cylinder pressure that the DCR #'s can't predict.

Correct me if I'm wrong, I'm just trying to learn more than I currently know without blowing up too many synapses

 

Yeah, we seem to be dancing around the issue.

Let me restate the real question ...

If cranking compression is 180 PSI with a DCR of 9:1 and SCR of 12:1, cylinder pressure just prior to ignition at cranking speed should also be ~180 PSI. What is the cylinder pressure at 4800 RPM just prior to ignition? Is it still ~180 PSI, or is it higher because of inertial and acoustic effects?

 

Since DCR and SCR are both calculated with the piston at TDC, and ignition occurs BTDC typically. For that reason alone cranking pressure "should" be less than 180 psi just prior to cylinder firing. If it actually is or not I don't know.

So I can't tell you what it will be, just that it will definitely change from idle/cranking rpm's to 4800 rpm. I suspect it will be lower unless you have the perfect head/intake/exhaust setup that enables greater than 100% volumetric efficiency, which to my knowledge is only rarely achieved on racing applications.

I would love to hear a few more opinions from the guru's however

 

Ok my take on it. DCR is based on the "dynamic compression" brought on by a motor in motion, and influenced by valve events.

DCR and SCR change once the actual combustion process is in place. So, that 180 psi you had at rest is not valid once the actual combustion process is taking place. This is similar to what a head flows on a bench vs how it works on a motor. But, by observation you know that x psi and y DCR don't detonate. So, by virtue of that you can make an informed decision.

As for the effect of inertia supercharging. That would be something to take into account if the runner imparted supercharging at some RPM. With an ls series manifold the RPM this takes place at is VERY low. My calcs show that the runner length need only be around 5" for first order inertia supercharging @ 6800 rpm on a LS1.

This is my opinion on the subject, but I would be interested in how others feel.

 

Remember that ignition occurs before TDC, so the compression wouldn't quite reach the same peak as it would during a compression test. Then you have to account for the compression tester's effect on CC volume....

Hopefully its higher though, from inertial and acoustic effects. An accurate MAF sensor and some number crunching would tell for sure. The actual cylinder filling divided by the engine's displacement is known as the volumetric effeciency, and it usually gets above 100% at certain times. Volumetric effeciency is more or less directly proportional to torque.

 

Boy I guess my "understanding" of it is way off from some others then.

I might be completely wrong here, but it is my understanding that SCR and DRC are both just #'s derived from measuring existing mechanical relationships. SCR is simply the ratio of the volume that exists between the piston top and combustion chamber when the piston is at BDC versus TDC, assuming the valves open and close at exactly BDC and TDC respectively and nothing more. If so then this cannot change just by changing rpms

It is also my understanding that DCR is simply SCR with the closing point of the intake valve mathematically taken into account to reflect the actual "effective" compression ratio?? Technically you could call this the "effective static compression ratio" or some **** If this is true ( and it may not be ) then neither one of these things can change with changing rpms correct?? Somebody help me out here, if I'm wrong then NOW is as good a time as any to find out

I'll stick my neck out just a little bit further here ( something I know I'm good at ) and say that the way it seems to me is that the only thing that's "dynamic" about this whole thing would be cylinder pressure, which changes with nothing more than an rpm change, or a timing change among other things. I guess I could've made this a whole lot shorter by summing up my understanding of it this way

1) SCR can only change by changing piston dome volume, bore size, or combustion chamber volume

2) Assuming the same SCR, DCR can only change by changing the cam, or advancing or retarding the same cam

3) cylinder pressure can/will/is change/changed by every little friggen' detail that you can possibly come up with

OK it's time to go pet my dog for awhile to let my brain rest

 

I agree with you racehead. Dynamic is a little more useful than static. Volumetric efficiency is more useful than dynamic or static. On the other hand, volumetric efficiency is the hardest to figure out.

 

Yes, I apologize for the confusion. The DCR remains a calculated value based on intake valve closing and cylinder pressure is the variable due to potential ram or supercharging effects.

 

I agree pretty much. 5" seems a bit short for 6800, but I don't want to do the math right now.

The question is based in reality. I have more compression that I wanted (shop cut more than I asked) and I have to run high octane gas. I don't want to ruin quench with a thicker gasket, so the options are limited. Moving the IVC event out would drop the DCR, which, in theory, should help lower octane requirements. But, I got started thinking about it and wondered if it really would. On the one hand, you have emperical evidence of guys who say they run 13:1 with 93 octane with a 250* at .050 cam. On the other hand, intuition says that 100% VE implies a full cylinder and therefor high cylinder pressures. But we know that higher RPM reduces the tendency to knock because there is less time, so maybe a bigger cam will drop octane requirement, but who knows how much per degree of cam??? My head started hurting so I thought maybe one of the gurus here knew the answers. There is probably an SAE paper that addresses the issue, but I don't have it.

In any case, we have some interesting theories here and food for thought.