Skunk Workz

Now it happens to be that we run longer overlap (64 degrees over stock 39 for the car in mention) and seriously long cams (over 60 degrees more than stock) for a lower effective CR to control detonation under boost...lower effective CR,but not any lower amount of overlap. Car passed the smog-testing we have with flying colors,with a little fine-tuning of the CO at idle.

Valve overlap doesn't have to be a problem emissionwise...but usually becomes a problem because airflow velocities are much lower than optimum. Adressing this is a matter of "engine design"...that's why I think effective CR is important. We made them to be optimum in our "old ricer". Combined with EFI,the emissions (as in complying to the regulations we have here at least,which is measured at idle) are not a problem,even with huge cams. Now,running carbs...that's a different story.


Edit: Sorry for the OT,Andy...

nine-44

No prob on the OT, it's still info...

Maybe I'm wrong and most likely don't fully understand but... CR and effective CR is similar to displacement and volumetric efficiency? In a perfect world, a cylinder will displace X ammount of cc of air. In the real world due to losses, it will displace roughly 80% of the actual displacement depending on the engine. I'm seeing it as, from atmospheric pressure to compression, that is the static CR. But due to losses(probably more so in an NA) the compression stroke may begin with a mild vaccume so that part of the stroke is actualy brining it back to atmospheric pressure and then compressing giving the effective CR????? Am I even close on what you guys are saying? In short the starting pressure or vaccume giving the final CR under that condition would be the effective CR. I'm lost but learning.

If I win the lottery, I'm just going to go hang out at Wiesach to learn, lol. I'm a much better hands on learner.

Skunk Workz

Effective CR is the CR you have from the point of actual compression happening....that is,simply,the CR calulated from the volume over the piston being at 60 degrees ABDC (or whatever your cam timing may put your intake valve closing at) instead of the volume of the full cylinder (with the piston at BDC).

ECR gives you a rough guide to what kind of SCR you can run with a spesific set of cams,or,tells you what cams you can (or should,performance-wise) run for any given SCR,among helping control detonation,etc....

VE is VE,not ECR. That's what you get when you start taking mass volume and efficiencies into the equation,like you wrote in your post,Andy.

Geo

Andy, you have it exactly correct.

CR is a static measure. Nothing more. Nothing less.

The rest is VE.

And you don't have to take just my word for it. One of my good friends is an engineer for an OEM and also worked on the TRD GTP engine program. He agees with this 100%.

Skunk Workz

Obviously he then has no two-stroke experience...then he would know otherwise.

Geo

I don't know if he does or doesn't. Wouldn't surprise me if he did (he gets into all sorts of engineering consulting projects). But last time I looked we were talking about 4 cycle engines here.

Skunk Workz

Yes,and if you have a 4-cycle engine and rotate it by hand,you don't start to feel resistance to turning it round when the piston passes BDC...you start to feel it when the intake valves close,and the compression (not the VE...the VE is ECR + "additives" like inertia tuning,wave tuning,etc that doesn't happen at 5 hand-cranked rpm) the engine actually sees,comes into play. And if you rotate it at a really slow speed...then neither VE,compression waves,expansion waves,flow efficiency,inertia tuning or static CR has anything to do with the resistance you feel. All that matters then,no matter the static CR,is when the intake valves close. If you have 10:1 and a short duration camshaft,you will maybe have a ECR of 8.6 :1,and it's heavy to crank by hand...but if you slam in 50 degree longer cams with no alteration to static CR,suddenly the engine is much easier to crank...because your effective compression now is maybe just 6.4 : 1,due to the FACT the intake valves close later,and compression cannot take place in a un-sealed cylinder. Just like in a two-stroke,that,where you have 14:1 static CR but CR measured from the point when the exhaust port close,is maybe just 7:1...

Most engines,both two- and four-strokes, of the last 3-4 decades run an effective CR,that is CR after the valves/ports have closed,of 7:1 to 9:1....but in static CR,they range from 8:1 to 14:1,because the cams/ports determine the actual point in the cycle where compression occurs. And,it's NOT VE... I've had ECR go down and VE up at the same time. And vice versa,depending on if I swapped to or from a long duration camshaft VS the short stock one....

Geo

I'm sorry, but effective CR is bogus. Yes, there is compression. There is cylinder pressure for which there are other measures). There is VE that affects the cylinder pressure. All the "additives" and stuff you talk about are still VE. VE is how effectively you fill the cylinder. All the little things you talk about affect VE, but it's still VE. And cam timing affects VE. Sorry, I agree with you about a lot of things, but saying the change in pressures due to altering cam timing is not VE is simply incorrect.

nine-44

Honestly guys, it makes sense either way. I can see it either way or it being read either way.

My comparison wasn't direct between VE and ECR, just something that allowed me to get an idea of the differences. Just saying that, kinda in a way, the difference between CR and ECR was like the difference between displacement and VE. Same but different, lol.

I guess the real myth buster would be if CR is measured before or after the valves are closed. If the CR is measured from BDC to TDC with the valves partially open for a period of the stroke, definately not going to be the actual CR. If it's measured from the point of closing of the valves to TDC then it would be accurate.

Whatever you call it, it's definately something to consider!

Anyone care to get into VE? I think I understand it and I see it as the relationship between measured displacement and what it actually see after losses or gains?

Skunk Workz

VE is how effectively you fill the cylinder. Agreed. When you crank it by hand,you have 100% VE all the time,because you have all the time in the world to fill the cylinder,the short time-frame,restrictions and badly tuned pulses that usually gets you below 100% VE is not there. One atmoshere outside,one atmosphere in the cylinder at BDC. Heck,you can even leave the piston at BDC for 5 minutes,and then it will have 100% VE. Right?

But when you then start cranking it slowly by hand,the first 60-ish degrees of crank motion doesn't compress anything ,it just blows the air you had in the cylinder back out the intake port until the valves close. THEN you have compression of the gas. Measure the volume you have left in the cylinder at that point,and calc the CR. You will see a CR of somewhere between 7:1 to 9:1,effective CR. Then measure the full cylinder,calc,then you'll have a completely different CR,most likely the CR listed in the car's user's manual.

Cams affect VE,agreed. But tell me then why cams that give a better VE usually needs higher static CR to work,especially at idle? Tuning,waves,all is the same...changing the cam doesn't change the intake lengths and flow,it doesn't change the port sizes or flow...All the same,yet you still would like a higher CR. Why?

Skunk Workz

CR as calculated/stated in manuals etc is from the full volume of the cylinder. Not from when the valves close.

nine-44

"But when you then start cranking it slowly by hand,the first 60-ish degrees of crank motion doesn't compress anything ,it just blows the air you had in the cylinder back out the intake port until the valves close."

Would boost or resonance tuning effect this or possibly resist the air from being pushed back out through the intake when the engine is at speed? With that being the case, boost or resonance vs RPM would change ECR at various RPMs. What a brain teaser!

Skunk Workz

The valves close at the same point,so ECR would be the same,just like your static,measured CR is the same... ECR is just a number,a measure or a tool, just like Lobe Center Angle,to compare actual setups. But as the actual mass of air you have inside to compress is affected when the engine is running by,mostly inertia,then flow,then pulses...which gives you ECR+ restrictions+pulses, then you get a different VE.

I know it's brain teasing... ...but it makes you think a lot,doesn't it?

Geo

Andy, CR is a purely static measure. The talk about when valves open and close are simply the effect on VE.

Stop and think about how CR is measured. Volume of the combustion chamber is volume of the combustion chamber and it's always with all valves closed. VE does NOT entered into CR. It DOES affect cylinder pressures which is the crux of the ECR argument.

Geo

And therein lay the confusion here. The reduction in volume of gas is a reduction in VE. The CR is a static measure and remains the same always. The change in volume of gas in the cylinder (reduction in VE) is what changes the pressure in the cylinder, but CR is static and constant and VE changes the volume of gas and the pressure under compression. The volume of the cylinder didn't change, just the gas, ergo, VE.