Confusion on published cam specs

[tirwin]

I was looking at some cam specs and I noticed something that didn't make sense.

On pg. 189 of Bruce Anderson's Porsche Performance Handbook, he lists the following specs for an SC cam.

Intake Duration = 229º
Intake Lift = .455"
Exhaust Duration = 220º
Exhaust Lift = .402"
Exhaust Lobe Center = 113º

But pg. 110 he lists the following timing values for engine 930/02:

Intake opens @ 1º BTDC
Intake closes @ 53º ABDC
Exhaust opens @ 43º BBDC
Exhaust closes @ 3º ATDC

My problem is that when I try to convert from one set of data points to the other, they don't jive.

Calculations from Intake & Exhaust Open/Close Values:
Intake Duration = 234º <-- supposedly 229º (from pg. 189)
Exhaust Duration = 226º <-- supposedly 220º (from pg. 189)
Intake Center = 116º ATDC
Exhaust Center = 110º BTDC
Lobe Separation = 113º <-- matches the Exhaust lobe center value from pg. 189
Advance = -3º
Overlap = 4º

Trying to reverse engineer the other direction doesn't work either.

Is my problem:
1) math error (I have checked a couple of times but maybe I'm making a mistake)
2) data problem (1 data set has an error or I'm comparing data from two different cams)
3) something else???

What am I missing?

[Bill Verburg]

Originally Posted by tirwin

I was looking at some cam specs and I noticed something that didn't make sense.

On pg. 189 of Bruce Anderson's Porsche Performance Handbook, he lists the following specs for an SC cam.

Intake Duration = 229º
Intake Lift = .455"
Exhaust Duration = 220º
Exhaust Lift = .402"
Exhaust Lobe Center = 113º

But pg. 110 he lists the following timing values for engine 930/02:

Intake opens @ 1º BTDC
Intake closes @ 53º ABDC
Exhaust opens @ 43º BBDC
Exhaust closes @ 3º ATDC

My problem is that when I try to convert from one set of data points to the other, they don't jive.

Calculations from Intake & Exhaust Open/Close Values:
Intake Duration = 234º <-- supposedly 229º (from pg. 189)
Exhaust Duration = 226º <-- supposedly 220º (from pg. 189)
Intake Center = 116º ATDC
Exhaust Center = 110º BTDC
Lobe Separation = 113º <-- matches the Exhaust lobe center value from pg. 189
Advance = -3º
Overlap = 4º

Trying to reverse engineer the other direction doesn't work either.

Is my problem:
1) math error (I have checked a couple of times but maybe I'm making a mistake)
2) data problem (1 data set has an error or I'm comparing data from two different cams)
3) something else???

What am I missing?

Part of the confusion comes from different ways to measure, ie is duration seat to seat or @1mm lift or what


f0r instance
for an SC cam first used in '76 C3 930.02
seat to seat is
I 236 E 224
but for 1mm lift it is
I 228 E 218

using another spec it is
I 234 E 226 w/ 8 o/l in 1mm o/l timing

over the life of these cams from '76 to 89 the cam advance varied by year from +3 to -3, but that doesn't affect duration or o/l period

[Trackrash]

I've noticed cam spec dependencies as well. If you look at the spec for a stock S cam they are different at WebCam's and Dougherty's for example.

Dougherty Racing Cams Porsche 911, 930 and 964 camshaft profiles
Web Cam Inc. - Performance and Racing Camshafts / Porsche 911 SOHC 12v

[tirwin]

Thanks. At least I'm not crazy.

It looks like most aftermarket cams provide their specs as I/E Duration and LSA.

Am I right in thinking that the factory specs listed in Anderson's book on pg 118 are incomplete? He doesn't list the LSA. Just the exhaust lobe center.

Does anyone have a good list of "normalized" specs for various factory and aftermarket cams?

[Trackrash]

This chart is from BA's second edition. Is it the same as what you are seeing? I went through this recently. It is difficult to compare Web, GE, Dougherty's, and some other cam timing. For example I don't see that Web lists LSA. Most list duration at 0.050" however.

LSA is the offset between the center of the intake lobe compared to the center of the exhaust lobe at peak lift, correct?

[tirwin]

Yep, that's the same on I'm looking at. My version is the 2nd edition too.

Your definition of LSA is correct. Maybe this picture helps.


I think if you have the intake & exhaust duration, the LSA and any advance/retard, you should be able to calculate the degrees that the intake & exhaust valves open and close values.

Conversely, if you have the intake & exhaust open and close values, you should be able to calculate the durations and LSA.

It makes me wonder if the last column is in the book is really LSA. I'm not sure what good having the exhaust center by itself does.

It would be good to know how much overlap there is on various cam choices.

[Bill Verburg]

[Trackrash]

Originally Posted by tirwin

Yep, that's the same on I'm looking at. My version is the 2nd edition too.

Your definition of LSA is correct. Maybe this picture helps.


I think if you have the intake & exhaust duration, the LSA and any advance/retard, you should be able to calculate the degrees that the intake & exhaust valves open and close values.

Conversely, if you have the intake & exhaust open and close values, you should be able to calculate the durations and LSA.

It makes me wonder if the last column is in the book is really LSA. I'm not sure what good having the exhaust center by itself does.

It would be good to know how much overlap there is on various cam choices.

I think there remain some unanswered questions.

How do you define lobe center? Is it the point of highest lift or is it the center point between the opening and closing point of the lobe? Cam lobes aren't necessarily symmetrical.

LSA by definition is the difference between the lobe center of the intake and exhaust, so there is only one number. BA referees to it as lobe center, I assume.

So I don't think you could automatically calculate LSA from the opening and closing values. Yes the duration, but unless the cam lobes are symmetrical, which I doubt is the case, the LSA remains unknown.

Now if CAMGRINDER pipes in with some of his proprietary info we may now for sure.

Of course, if Bill figures out how to do his measurements, we will know for sure.

[tirwin]

Gordon,

You raise some good points. In my reading, I think there are some assumptions being made. Maybe they aren’t always valid.

Here is what I think. If I am missing anything, please feel free to correct.

Lobe center is the peak opening point of the valve. It is the centerline of the intake and exhaust lobe.

I suppose it is possible that a lobe could be asymmetrical. The peak value won’t change though. It would open and close the valve at different rates. I am trying to imagine what that would accomplish. I think duration (the area under the curve) is more important than slope. Maybe an asymmetric lobe would affect the time required to charge or discharge the cylinder? I thought lift and overlap had more to do with that.

LSA would be the degrees between the intake and exhaust centerline (lobe center). As Bill noted, as long as the measurement points are the same between the intake and exhaust, the duration should be the same.

You could not determine the exact opening values by only knowing the duration and LS. For example, there could be a few degrees of advance or retard from TDC. But the opening/closing values should be the same relative to one another.

[Trackrash]

Perhaps I am splitting hairs. I have read that the cam's profile needs to take into consideration the acceleration of the valve when opening and closing. Therefore depending on how the cam was engineered there probably is some difference between the profile on the front of the lobe compared to the rear of the lobe. This could result in the peak lift not being in the exact center of the lobe. Does that make sense? That may not make any difference in LSA, but may complicate actually measuring it's location. That said, I generally agree with what you are saying and perhaps I am overthinking it.

It would be interesting to hear from a real expert or engineer with experience on cam design who could clarify our assumptions.

[stownsen914]

Here's how Webcam describes it

Web Cam Inc. - Performance and Racing Camshafts / Degreeing In Your Camshaft

[tirwin]

Originally Posted by stownsen914

Here's how Webcam describes it

Web Cam Inc. - Performance and Racing Camshafts / Degreeing In Your Camshaft

Yes, this is one of the sources I was reading from. The approach outlined in that link would not work if the cam lobe was not symmetrical.

The positions of the intake and exhaust lobe centerlines don’t have to be symmetrical relative to one another though. By that I mean they can be at different angles. That should not affect LSA though. In fact that is what defines LSA. So if you know LSA and either the intake or exhaust centerline angle, you can find the other.

[tirwin]

Originally Posted by Trackrash

Perhaps I am splitting hairs. I have read that the cam's profile needs to take into consideration the acceleration of the valve when opening and closing. Therefore depending on how the cam was engineered there probably is some difference between the profile on the front of the lobe compared to the rear of the lobe. This could result in the peak lift not being in the exact center of the lobe. Does that make sense? That may not make any difference in LSA, but may complicate actually measuring it's location. That said, I generally agree with what you are saying and perhaps I am overthinking it.

It would be interesting to hear from a real expert or engineer with experience on cam design who could clarify our assumptions.

I see what you are saying. And I agree that it would be nice if a cam designer/grinder could confirm.

I have read that the equation of the lobe parabola is y = ax^2 + b. If that is true, a would determine the width of the curve and b would determine the height. Then knowing the diameter of the cam (a) and the lift (b) would let you plot the curve.

If the curve is asymmetrical or the parabola equation is different, then the curves would obviously be different.

If the peak lift point on the lobe was asymmetrical that would mean that the opening or closing time would be longer. I don’t see what that would accomplish. Not saying it doesn’t accomplish something - just that I don’t understand.

But even if the lobe parabola equation is different or asymmetrical, the opening and closing points have to be symmetrical because they are the endpoints of the diameter of the lobe base circle.

[stownsen914]

I'd guess that for ease of reproduction, cam grinders may have the lobe shape symmetrical so they can use the same master cam profile to grind the left and right side camshafts.

I suspect that in some cases there may not be as much precision in cam grinding as we'd like. I had a set of 911 cams ground about 20 years ago by one of the major respected grinders. Being young and enthusiastic, I decided to degree them while I was assembling my engine. I found the intake and exhaust opening and closing events to be very repeatable from one cylinder to the next. But the lobe centers were off on the left bank vs. the right bank by several degrees.

Scott

[tirwin]

By the way, one of the reasons I’m interested in this is to understand the overlap. That is not always published as far as I can tell.

My understanding is that on a street engine, you generally have little overlap — especially if emissions testing is a consideration. Larger overlap helps because the open exhaust valve creates additional suction to charge the cylinder. The problem with overlap is that unburned fuel can be sucked out the exhaust side which raises emissions. Not a problem for race cars, but certainly an issue for street cars that have to be tested.

The reason I started in on this was I was thinking it would be nice to visualize the differences in various cam choices.

[tirwin]

Originally Posted by stownsen914

I'd guess that for ease of reproduction, cam grinders may have the lobe shape symmetrical so they can use the same master cam profile to grind the left and right side camshafts.

I suspect that in some cases there may not be as much precision in cam grinding as we'd like. I had a set of 911 cams ground about 20 years ago by one of the major respected grinders. Being young and enthusiastic, I decided to degree them while I was assembling my engine. I found the intake and exhaust opening and closing events to be very repeatable from one cylinder to the next. But the lobe centers were off on the left bank vs. the right bank by several degrees.

Scott

Interesting!

I think the reason the opening and closing events were very repeatable is because they are the endpoints of the diameter of the cam base circle.

[tirwin]

Originally Posted by tirwin

Interesting!

I think the reason the opening and closing events were very repeatable is because they are the endpoints of the diameter of the cam base circle.

Actually, now that I think about it I’m not sure that’s true. Then the opening/closing angle would always be 180° and that is clearly not the case.

[tirwin]

Here's another reference I found:
High Performance Math

This says the intake cam degrees is equal to valve duration divided by 2.

Then it calculates "theta" which is angle from the x-axis to the point at which lift begins. Theta = 90º - the intake cam degrees.

Nothing I have come across ever raises the possibility that the cam lobe could be asymmetric.

[tirwin]

Let me go back to the beginning.

If I use the intake/exhaust open/close values from BA's book for an SC cam profile, here is what I get using the method posted on the Web Cam site.

SC Cam:

Intake Open: -7 TDC (BTDC is -, ATDC is +)
Intake Close: 29 ABDC

Exhaust Open: 49 BBDC
Exhaust Close: -3 TDC (BTDC is -, ATDC is +)

Intake Duration = open + close + 180, so -7 + 47 + 180 = 220º
Intake Lobe Center = ( Duration / 2 ) - opening, so (220 /2) - (-7) = 117º

Follow the same logic for the Exhaust except subtract close from Exhaust Duration if ATDC and add back the close from the Exhaust Duration for BTDC and the results:

Exhaust Duration = 226º
Exhaust Lobe Center = 116º

Lobe Separation Angle (LSA) = ( Intake Center + Exhaust Center ) /2 so:

LSA = (117 + 116) / 2 = 116.5º

Those numbers don't match with what is on pg. 189 of BA's book. They are close though. ~4% variation.

Intake Duration:
Calculated: 220º
Book: 229º

Exhaust Duration:
Calculated: 226º
Book: 220º

Given that you can calculate the estimated peak torque speed from the intake numbers and the estimated peak HP from the exhaust duration, I'm just wondering why there is a difference and if that difference is significant. The calculated numbers would suggest a lower peak torque RPM and higher peak HP RPM. You start using one number that's off and multiply that by another number that's off and pretty soon you have a worthless result. I just don't have enough experience to know if these differences are meaningful or not.

Edit: The open/close numbers in Bill V's post above are significantly different from BA's book. Using Bill's numbers, the intake duration matches perfectly with pg. 189 of BA's book but the exhaust duration is still off. Strange.

[Trackrash]

"Ramps are the parts of the lobe where the lifter is either moved up or allowed to drop. Every lobe has two ramps-an opening ramp and a closing ramp. In performance camshafts, the curve of the ramps changes several times, which is a tool the cam designer uses to fine-tune the speed and acceleration of the lifter.

An asymmetrical lobe refers to opening and closing ramps that are not identical. In order to maximize both valve speed and control, the lifter must be raised in a different manner from which it is lowered. For example, in performance applications the valve is generally opened as quickly as possible, but the speed of the valve slows significantly as it nears maximum lift to keep it from lofting. But on the closing side, the valve must be seated relatively gently to keep it from bouncing. An asymmetrical lobe design allows this."

Taken from here.... The Science Behind Camshaft Design - Hot Rod Network

And this appears to be the way to determine LSA.