Cam With Dwell At Max Velocity Graph
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Cam With Dwell At Max Velocity Graph
Yesterday we were taking apart an engine for a customer it is an Onan 2 cylinder flathead engine from a Case garden tractor. Anyway I decided that it would be fun to plot the cam profile. It is interesting because it has a lot of dwell at maximum velocity along with very long lash ramps. When is this technique used or is it used at all on modern high performance cam lobe designs?
The engine has .750 diameter lifters and it uses .695 of that diameter. It measures 231 and 226 duration intake and exhaust on a 110 lobe sep. .305 lift on the intake .301 on the exhaust.
Here's a graph of the intake lobe: the exhaust looks just about the same..
The dark blue line is the lift, the green line is the tappet velocity and the light blue line is the acceleration. I left off jerk for clarity. See how the cam has a big acceleration , goes to zero then goes negative as it moves toward the top of the lobe. The exact opposite happens on the way back down. The constant velocity ramp ends at about .020 lift which is also probably the lash point on the cam.
When looking at the lobe it is interesting and it has a shape sort of like a Gothic arch or a tall Gambrel style barn roof.
The engine has .750 diameter lifters and it uses .695 of that diameter. It measures 231 and 226 duration intake and exhaust on a 110 lobe sep. .305 lift on the intake .301 on the exhaust.
Here's a graph of the intake lobe: the exhaust looks just about the same..
The dark blue line is the lift, the green line is the tappet velocity and the light blue line is the acceleration. I left off jerk for clarity. See how the cam has a big acceleration , goes to zero then goes negative as it moves toward the top of the lobe. The exact opposite happens on the way back down. The constant velocity ramp ends at about .020 lift which is also probably the lash point on the cam.
When looking at the lobe it is interesting and it has a shape sort of like a Gothic arch or a tall Gambrel style barn roof.
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Re: Cam With Dwell At Max Velocity Graph
Dave,
What are the valve spring open and seat pressures?
Stan
What are the valve spring open and seat pressures?
Stan
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Re: Cam With Dwell At Max Velocity Graph
It looks like the design technique may have been to draw the velocity profile with lines and parabola, then integrate to get lift.
The peak in the acceleration curve at max lift is probably a byproduct of the manufacturing process such as incomplete clean-up when grinding or improper polishing or a machine that does not reverse the motion of the follower well due to insufficient pressure or bearing issues.
The peak in the acceleration curve at max lift is probably a byproduct of the manufacturing process such as incomplete clean-up when grinding or improper polishing or a machine that does not reverse the motion of the follower well due to insufficient pressure or bearing issues.
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Re: Cam With Dwell At Max Velocity Graph
Figuring a 1.375 installed ht the spring gives 37lbs on the seat and 80lbs open.Stan Weiss wrote:Dave,
What are the valve spring open and seat pressures?
Stan
Re: Cam With Dwell At Max Velocity Graph
Very old school, but I was just looking at some current Mercedes AMG cams, and they're still designed that way.
I bet you're using some amount of smoothing on the graph, and that the acceleration curve is steeper.
I bet you're using some amount of smoothing on the graph, and that the acceleration curve is steeper.
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Re: Cam With Dwell At Max Velocity Graph
Dave,
I think that deal is more about minimum wear and long parts life than getting the most HP.
Stan
I think that deal is more about minimum wear and long parts life than getting the most HP.
Stan
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Re: Cam With Dwell At Max Velocity Graph
There are a couple of cam design techniques being used here. The amount of lift for the duration and the relatively small tappet face requires a dwelled maximum velocity technique in order to control the maximum velocity. If that technique was not used, the maximum velocity would be too high for the lifter diameter. Another technique is used to control the nose radius so it is not too small. That creates the profile shape you described.
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Re: Cam With Dwell At Max Velocity Graph
What exactly are you commenting on that is 'old-school'? Dwelling max velocity? Or the constant velocity ramp?CamKing wrote:Very old school, but I was just looking at some current Mercedes AMG cams, and they're still designed that way.
I bet you're using some amount of smoothing on the graph, and that the acceleration curve is steeper.
Interesting they compromised the valve lift profile instead of putting a bigger lifter in. That amount of v-dwell is going to have some nice Fourier content!
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Re: Cam With Dwell At Max Velocity Graph
The whole design. That's a stupid amount of acceleration for a cam that size.volodkovich wrote:What exactly are you commenting on that is 'old-school'?
It would be very easy to get more area and/or lift at the same duration, without over-riding the kifter, and with less peak acceleration.
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Re: Cam With Dwell At Max Velocity Graph
Not to mention the fact it is symmetrical...CamKing wrote:The whole design. That's a stupid amount of acceleration for a cam that size.volodkovich wrote:What exactly are you commenting on that is 'old-school'?
It would be very easy to get more area and/or lift at the same duration, without over-riding the kifter, and with less peak acceleration.
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Re: Cam With Dwell At Max Velocity Graph
Mike,CamKing wrote:The whole design. That's a stupid amount of acceleration for a cam that size.volodkovich wrote:What exactly are you commenting on that is 'old-school'?
It would be very easy to get more area and/or lift at the same duration, without over-riding the kifter, and with less peak acceleration.
Isn't the point of max acceleration where the constant velocity lash ramp meets the lift curve?
Stan
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Re: Cam With Dwell At Max Velocity Graph
This is one way that the profile could be divided into segments.
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Re: Cam With Dwell At Max Velocity Graph
Here's some more information on that lobe:
Durations
.003 399
.004 392
.006 376
.015 304
.020 266
.040 237
.050 230
.100 197
.150 164
.200 131
.250 94
.300 29
What's interesting to me is how is has a super long slow lash ramp but then suddenly accelerates very quickly. I wonder how much of this has to do with the engine being a flathead vs something with a rocker arm?
Durations
.003 399
.004 392
.006 376
.015 304
.020 266
.040 237
.050 230
.100 197
.150 164
.200 131
.250 94
.300 29
What's interesting to me is how is has a super long slow lash ramp but then suddenly accelerates very quickly. I wonder how much of this has to do with the engine being a flathead vs something with a rocker arm?
Re: Cam With Dwell At Max Velocity Graph
Just screwing around, this is what I came up with with no dwell, .305" lobe lift, and a max acceleration of .00008472DaveMcLain wrote:Here's some more information on that lobe:
Durations
.003 399
.004 392
.006 376
.015 304
.020 266
.040 237
.050 230
.100 197
.150 164
.200 131
.250 94
.300 29
What's interesting to me is how is has a super long slow lash ramp but then suddenly accelerates very quickly. I wonder how much of this has to do with the engine being a flathead vs something with a rocker arm?
.020 268
.040 241
.050 231
.100 196
.150 164
.200 132
.250 95
.300 29
Mike Jones
Jones Cam Designs
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Jones Cam Designs
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Re: Cam With Dwell At Max Velocity Graph
A long constant velocity lash-ramp makes it possible to assemble the engine with parts made to looser tolerances without clearance adjustment.DaveMcLain wrote: What's interesting to me is how is has a super long slow lash ramp but then suddenly accelerates very quickly. I wonder how much of this has to do with the engine being a flathead vs something with a rocker arm?
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