[cstraub]

We all know torque and HP cross at a given point. That is accepted but have you ever thought about cam duration. The intake side is a depression that we are filling with air and fuel. Early in timing we are the beginning of this and now where near the lift that we reach maximum cylinder fill, therefore depending on the heads intake track flow characteristics at higher lift the duration numbers should be holding on to fill the cylinder.

Now on the exhaust it is under extreme pressures. As soon as the valve is cracked exhaust wants out. The time need to do this happens very quickly at lower tappets heights. Since this is case at higher tappet heights the duration is going to fall off since the majority of the gas has already escaped.

So a cam that is say a 6 degree split at Adv, .050, and .100, .200, .300" and so forth the duration difference never intersects. Why? Why not?

 

[badrad]

It's a different lobe design filling a different requirement. To me it's a YIN and YANG scenario, different but work in harmony for the best performance.

*Re-read the question, pretty sure I didn't answer and not sure I understand the question now.

 

[GRN69CHV]

We all know torque and HP cross at a given point. That is accepted but have you ever thought about cam duration. The intake side is a depression that we are filling with air and fuel. Early in timing we are the beginning of this and now where near the lift that we reach maximum cylinder fill, therefore depending on the heads intake track flow characteristics at higher lift the duration numbers should be holding on to fill the cylinder.

Now on the exhaust it is under extreme pressures. As soon as the valve is cracked exhaust wants out. The time need to do this happens very quickly at lower tappets heights. Since this is case at higher tappet heights the duration is going to fall off since the majority of the gas has already escaped.

So a cam that is say a 6 degree split at Adv, .050, and .100, .200, .300" and so forth the duration difference never intersects. Why? Why not?

Not accurate. Except for very small cams, there is overlap when measured at .050".

 

[cstraub]

Not accurate. Except for very small cams, there is overlap when measured at .050".

?

 

[GRN69CHV]

?

Why the ? ? Point of intersection on a graph representing cam profile is overlap. As you know, Point of intersection on a dyno sheet is always 5252, it's a standard for mathematical calculation of HP. Maybe you need to rephrase your question?

 

[cstraub]

Why the ? ? Point of intersection on a graph representing cam profile is overlap. As you know, Point of intersection on a dyno sheet is always 5252, it's a standard for mathematical calculation of HP. Maybe you need to rephrase your question?

Joe,
Seperate each port as its own path and think about it that way. Don't look at the lobes connected on the camshaft.

 

[steelcomp]

Joe,
Seperate each port as its own path and think about it that way. Don't look at the lobes connected on the camshaft.

Correct...this isn't about where the paths cross, it's about where the numbers cross or where the "split" crosses.

 

[RB69SS396Conv]

Torque and HP only "cross" because the one is calculated from the other, and the constant that relates them is 1/5252.11 (2 * pi / 33,000 .... which is derived from the fact that the English HP was defined by James Watt as 33,000 lbs 1 ft in 1 min, and since torque is ft lbs which are related to linear feet by the constant 2 * pi, it works out real convenient)

Not sure how cam specs are related to any of that?

 

[steelcomp]

Torque and HP only "cross" because the one is calculated from the other, and the constant that relates them is 1/5252.11 (2 * pi / 33,000 .... which is derived from the fact that the English HP was defined by James Watt as 33,000 lbs 1 ft in 1 min, and since torque is ft lbs which are related to linear feet by the constant 2 * pi, it works out real convenient)

Not sure how cam specs are related to any of that?

Really???
It was metaphoric. It has nothing to do with the cam, it was just an interesting way to break into an area of cam discussion that no one seems to be getting.
I wish there was a little emoticon of something going over a little smily-face's head...
swoosh!

 

[novadude]

I agree with RB - not sure where the hp / tq discussion was going?

After re-reading, I think Chris is making the case for a faster acting lobe on the intake, and a "lazy" lobe on the exhaust, since time is a little more important than max valve curtain area when you have all that hot exhaust gas energy helping evacuate the cylinder. The intake sees nothing more than atmospheric pressure. You can't give the intake a whole bunch of time on a low CR engine without completely screwing up "dynamic" CR, so how you use the tima available (valve curtain area) becomes important - need to get that valve open to 0.25 x D ASAP.

Chris - is that what you were getting at?

 

[Enigma]

What he's asking is: said cam is listed as 285/[email protected]/[email protected]/[email protected] blah blah. At each checking point there is a spread between intake and exhaust duration. Why is there no checking point in which there is no spread? [email protected] lift.....why aren't the durations the same? The answer is two-fold. 1: you're trying to relate two things together that really have nothing in common beyond looking the same. The task required of the exhaust lobe is vastly different than that required of the intake lobe. Beyond looking the same, and living in the same neighborhood, they haven't much in common. Or......shouldn't. Until you get into low dollar cams in which lobes aren't designed to be intake or exhaust only. 2: look at lifter velocities and accelerations. Imagine how catastrophic it would be to try to slow one down/speed one up enough to catch the other, given the usual 8-10 degree split at advertised figures. When dealing with .00025/*/*........takes a lot of ramp to make any drastic changes to that without creating havoc within the valvetrain. Now look at how you cam say.....a race rite head on a BBC with something like a 14 degree split. The exhaust lifter would have to stop and take a smoke break for the intake to catch it in that situation. The lobes look a lot alike, but don't have much in common beyond that. Certainly there are cams in which the spread balances, but on most TRYING to balance them at any checking point would cause tragic jerk values and probably catastrophic failure in short order.

 

[RB69SS396Conv]

Sorry, I'm just a mathematician and physicist; something as complex as a .... car .... is beyond me I'm sure. After all, those are the very pinnacle of technological mystique, the very latest cutting-edge technology, that only a rarified few that have experienced alien abduction or something, can possibly comprehend.

Just not "getting" how the formula for calculating HP from torque, has any connection to the valves, or calculating the functions of their motion; or, where Chris is going with his "duration intersects". Although his point about the difference in the pressures available to move the cyl charge through the exh valve compared to the int valve is certainly true, as novadude says. Since the pressures on the gas are different across the 2 valves, it stands to reason that the lobe shape - not just the dimensions - might want to be different as well, at least in some situations. But trying to connect that somehow with "HP = torque * RPM * 2 * pi / 33000" is sort of obscure.

 

[cstraub]

Sorry, I'm just a mathematician and physicist; something as complex as a .... car .... is beyond me I'm sure. After all, those are the very pinnacle of technological mystique, the very latest cutting-edge technology, that only a rarified few that have experienced alien abduction or something, can possibly comprehend.

Just not "getting" how the formula for calculating HP from torque, has any connection to the valves, or calculating the functions of their motion; or, where Chris is going with his "duration intersects". Although his point about the difference in the pressures available to move the cyl charge through the exh valve compared to the int valve is certainly true, as novadude says. Since the pressures on the gas are different across the 2 valves, it stands to reason that the lobe shape - not just the dimensions - might want to be different as well, at least in some situations. But trying to connect that somehow with "HP = torque * RPM * 2 * pi / 33000" is sort of obscure.

I see to many cams that the duration split is parallel through all the lift points. They never cross. We are talking about engines and I used the HP/Torque as an example of something related to engines that crosses/insects each other.

 

[cstraub]

What he's asking is: said cam is listed as 285/[email protected]/[email protected]/[email protected] blah blah. At each checking point there is a spread between intake and exhaust duration. Why is there no checking point in which there is no spread? [email protected] lift.....why aren't the durations the same? The answer is two-fold. 1: you're trying to relate two things together that really have nothing in common beyond looking the same. The task required of the exhaust lobe is vastly different than that required of the intake lobe. Beyond looking the same, and living in the same neighborhood, they haven't much in common. Or......shouldn't. Until you get into low dollar cams in which lobes aren't designed to be intake or exhaust only. 2: look at lifter velocities and accelerations. Imagine how catastrophic it would be to try to slow one down/speed one up enough to catch the other, given the usual 8-10 degree split at advertised figures. When dealing with .00025/*/*........takes a lot of ramp to make any drastic changes to that without creating havoc within the valvetrain. Now look at how you cam say.....a race rite head on a BBC with something like a 14 degree split. The exhaust lifter would have to stop and take a smoke break for the intake to catch it in that situation. The lobes look a lot alike, but don't have much in common beyond that. Certainly there are cams in which the spread balances, but on most TRYING to balance them at any checking point would cause tragic jerk values and probably catastrophic failure in short order.

Your on the right track. Seperate induction and exhaust and for now forget the lobes are on a common shaft. Think about how these systems function.

 

[Enigma]

I'd be inclined to think that, given what is trying to be accomplished with each lobe, a completely parallel (IE: 6 degree spread at each and every checking point) would be a strong indicator of a very cheap "generic" cam, in which the lobes are designed neither intake nor exhaust specific, but are simply lift curves. Which is happening far to often, these days. Much like having a bucket of lobes, reaching in and grabbing one for the intake and one for the exhaust. Even single pattern cams should have a duration interim between the intake and exhaust that grows and shrinks at different checking points. As I said, the fact that lobes kind of look the same......and are glued to the same stick, confuses people into thinking that they have anything in common. And they don't. Or, rather, shouldn't. The fact that people are infatuated with finding (or creating) ratios doesn't help the situation any. If one were to cut up a head, pull the intake port off and put it in one room, and the exhaust port off and put it in a separate room.......then look at each one with an eye towards what that port/lobe is trying to accomplish on it's own........the picture will become more clear.

 

[cstraub]

I'd be inclined to think that, given what is trying to be accomplished with each lobe, a completely parallel (IE: 6 degree spread at each and every checking point) would be a strong indicator of a very cheap "generic" cam, in which the lobes are designed neither intake nor exhaust specific, but are simply lift curves. Which is happening far to often, these days. Much like having a bucket of lobes, reaching in and grabbing one for the intake and one for the exhaust. Even single pattern cams should have a duration interim between the intake and exhaust that grows and shrinks at different checking points. As I said, the fact that lobes kind of look the same......and are glued to the same stick, confuses people into thinking that they have anything in common. And they don't. Or, rather, shouldn't. The fact that people are infatuated with finding (or creating) ratios doesn't help the situation any. If one were to cut up a head, pull the intake port off and put it in one room, and the exhaust port off and put it in a separate room.......then look at each one with an eye towards what that port/lobe is trying to accomplish on it's own........the picture will become more clear.

I'm guilty of using ratio's but use them as a reference. If a customer comes to me for a cam because what they have is not running. I will ask what heads or if they have flow numbers. Say the cam they have is a 6 degree split, but the ratio between I/E is 60%. I can then explain to them the engine is not cammed correctly and why it isn't. It's a quick way for me to determine what direction is needed to correctly cam the engine.

 

[Enigma]

I'm guilty of using ratio's but use them as a reference. If a customer comes to me for a cam because what they have is not running. I will ask what heads or if they have flow numbers. Say the cam they have is a 6 degree split, but the ratio between I/E is 60%. I can then explain to them the engine is not cammed correctly and why it isn't. It's a quick way for me to determine what direction is needed to correctly cam the engine.

That's understandable, and expected. But you've got close to two decades in doing what you do, and have developed a sense of "feel" and use I/E ratios as a metric to better serve yourself and your customers. And, as you say, explaining your logic using I/E ratio is far easier than giving them an entire volume on gas dynamics. Ratios exist all throughout an engine, simply by virtue. But people are trying, these days, to rely to much on them because doing so is easier than learning and understanding the physics behind the ratio. I just fear the day in which we are choosing our carbs based on the fan pulley to oil pan bolt count ratio. Do know that I meant no disrespect in my prior post. I respect you AND your accomplishments and am just adding my two cents to what seems an interesting topic.

 

[cstraub]

No disrespect taken. I'm just glad the thread is getting some looks and some reading. And your right, it has taken me several years to be able to do what I do and I don't have time to explain in detail how I do what I do. It's September and since Monday I have 11 cam orders. Unheard of this time of year.

Buying the cam is cool and I just see to many times guys get a cam then they buy cylinder heads!!! UUURRRHH! The cam should be the second to last thing you buy, pushrods being last.

 

[Enigma]

To those who took the time to read it, AND absorb it, that was an excellent dissertation on how you do what you do that you posted several years ago on the yellow site. But I still contend that you pointedly left out one tiny, seemingly insignificant sentence, that protected your bread&butter. And rightfully so. Still great reading, though.

 

[cstraub]

To those who took the time to read it, AND absorb it, that was an excellent dissertation on how you do what you do that you posted several years ago on the yellow site. But I still contend that you pointedly left out one tiny, seemingly insignificant sentence, that protected your bread&butter. And rightfully so. Still great reading, though.

You are 100% correct and the day I retire I will turn over my "black book" to whoever wants to do this in the future.