Another one for the pros, Head Questions
#31
Originally Posted by Bill.e1
Exactly the point. While designers work "deeper" in the math, effective CR is incredibly descriptive and, within narrow parameters, predicts how static CR, intake pressure and valve timing effect detonation. It is tailor-made for tuning a blower motor.
A quick example that is relevant to this thread. With a twin cam head, you can change cam timing individually. This gives you another tool for tuning. By retarding the intake, you lower the "effective CR" and reduce valve overlap. This allows you to increase the boost without detonation or overscavening (blowing intake charge out the exhaust).
A quick example that is relevant to this thread. With a twin cam head, you can change cam timing individually. This gives you another tool for tuning. By retarding the intake, you lower the "effective CR" and reduce valve overlap. This allows you to increase the boost without detonation or overscavening (blowing intake charge out the exhaust).
Notice how everyone runs higher CR than in the 70's/80's? Valve overlap is bad for emissions so production cars have to close the intake later to maintain airflow. This reduces the "effective CR" so mechanical CR's are bumped to compensate. Obviously other factors also apply. The supercharger's popularity surge is also, at least partially, emissions driven. They allow older designs to make power running emissions-spec cams. They keep exhaust gases hot so converters light earlier. Last but not least, it is faster/cheaper to supercharge an older engine than it is to develop a new one.
Regards,
Bill
Regards,
Bill
Edit: Sorry for the OT,Andy...
#32
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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.
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.
#33
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.
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.
#34
Originally Posted by nine-44
Maybe I'm wrong and most likely don't fully understand but... CR and effective CR is similar to displacement and volumetric efficiency?
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%.
#35
Originally Posted by 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%.
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%.
#36
Originally Posted by Skunk Workz
Obviously he then has no two-stroke experience...then he would know otherwise.
#37
Originally Posted by 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.
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....
#38
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.
#39
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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?
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?
#40
Originally Posted by 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.
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?
#41
Originally Posted by nine-44
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.
#42
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"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!
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!
#43
Originally Posted by 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!
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!
I know it's brain teasing... ...but it makes you think a lot,doesn't it?
#44
Originally Posted by nine-44
I guess the real myth buster would be if CR is measured before or after the valves are closed.
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.
#45
Originally Posted by Skunk Workz
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.