How are limitations of dwell over nose estimated?

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The Badger
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How are limitations of dwell over nose estimated?

Post by The Badger »

Have been helping out with a friends dirt track engine for a few weeks now, just put cam in and he kept on talking about the amount of dwell that he had with the new cam, I can follow along with some of cam talk but I am a novice when it comes to design. Tried searching but couldn't find my answer.

So what does a cam grinder consider when designing cam and making it dwell?

I imagine a lighter valve train means you can run more and jerk won't effect valve train as much, am I on the right track?
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Re: How are limitations of dwell over nose estimated?

Post by Walter R. Malik »

The Badger wrote: Fri Jul 20, 2018 9:02 am Have been helping out with a friends dirt track engine for a few weeks now, just put cam in and he kept on talking about the amount of dwell that he had with the new cam, I can follow along with some of cam talk but I am a novice when it comes to design. Tried searching but couldn't find my answer.

So what does a cam grinder consider when designing cam and making it dwell?

I imagine a lighter valve train means you can run more and jerk won't effect valve train as much, am I on the right track?
Roller or flat tappet makes a huge difference.
Along with lobe lift required and duration;
accelerations and velocities within those boundaries playing a huge part in this, also.
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Re: How are limitations of dwell over nose estimated?

Post by The Badger »

So id imagine that the more duration the softer the transition for the lifter?

What can be done to increase the limits of the acceleration and velocity? Bigger diameter lifter/roller, lighter parts,etc?

Sorry for the most likely simple questions, just wanting to understand this more.
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Re: How are limitations of dwell over nose estimated?

Post by Stan Weiss »

The dwell causes the flat area on the velocity graph and the spick up to zero on the acceleration graph. thes can causes valve train dynamics problems.

Stan
ab-pont-744c-lift2.gif
ab-pont-744c-vel2.gif
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Re: How are limitations of dwell over nose estimated?

Post by The Badger »

Stan could you please explain to me in more detail what I’m looking at with the graphs? Sorry think I’m missing something.
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Re: How are limitations of dwell over nose estimated?

Post by David Redszus »

Camshafts can be designed as single dwell or double dwell.
A single dwell cam has a lift curve with a single point peak lift. It is commonly used in automotive applications.

A double dwell cam has two peak lift points, or a flat top lift curve. It will produce velocity and
acceleration curves similar to what Stan has posted. Double dwell cams are often found in industrial
applications and some motorcycle motors.
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Re: How are limitations of dwell over nose estimated?

Post by Stan Weiss »

These are graphs are from the raw Cam DR data with no smoothing of an older Comp Cams SBF Roller.

Stan
ab-comp161-cam-lift.gif
ab-comp161-cam-vel.gif
ab-comp161-cam-acc.gif
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Re: How are limitations of dwell over nose estimated?

Post by David Redszus »

Stan Weiss wrote: Mon Jul 23, 2018 12:49 pm These are graphs are from the raw Cam DR data with no smoothing of an older Comp Cams SBF Roller.

Stan

ab-comp161-cam-lift.gif

ab-comp161-cam-vel.gif

ab-comp161-cam-acc.gif
Nice post Stan.
Notice at which crank angle peak valve acceleration occurs.
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Re: How are limitations of dwell over nose estimated?

Post by user-23911 »

Acceleration is ALWAYS zero when at peak lift and zero velocity. So due to the sampling rate and lack of smoothing, that part is wrong.
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Re: How are limitations of dwell over nose estimated?

Post by Stan Weiss »

joe 90 wrote: Tue Jul 24, 2018 12:45 am Acceleration is ALWAYS zero when at peak lift and zero velocity. So due to the sampling rate and lack of smoothing, that part is wrong.
It would really be interesting to see graphs or raw Cam DR data from some of your cam lobes.

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Re: How are limitations of dwell over nose estimated?

Post by user-23911 »

The second graph of velocity is wrong too, it should look more like the first one where the line flattens as it crosses the x axis........so because that one is wrong, the acceleration one calculated from it becomes wrong too.
Stan Weiss wrote: Tue Jul 24, 2018 12:59 am
It would really be interesting to see graphs or raw Cam DR data from some of your cam lobes.

Stan

They'll be nothing like these because they're factory developed race cams similar to those which hold records for the most number of records, as in the Paris Dakar rally.
Last edited by user-23911 on Tue Jul 24, 2018 1:20 am, edited 1 time in total.
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Re: How are limitations of dwell over nose estimated?

Post by williamsmotowerx »

joe 90 wrote: Tue Jul 24, 2018 12:45 am Acceleration is ALWAYS zero when at peak lift and zero velocity. So due to the sampling rate and lack of smoothing, that part is wrong.
Lack of smoothing? Why do you need smoothing for zero movement?

That's what you're saying when you have zero accel and velocity.
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Re: How are limitations of dwell over nose estimated?

Post by user-23911 »

Smoothing is to make it look right when there's an insufficient no of samples , particularly in places like peak lift where velocity is zero as is acceleration,



with any sort of A to D sampling / conversion, there's always added errors.
In telecomms it's known as quantisation errors.

https://en.wikipedia.org/wiki/Quantizat ... rocessing)
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Re: How are limitations of dwell over nose estimated?

Post by williamsmotowerx »

As designed you don't you smoothing.

When you measure a cam as ground, you use smoothing sparingly to interpret the profile.

When you have a spike at peak lift it's the cam dwelling. Which causes the valve head to shake like a cold chihuahua. Pop goes the daisy
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Re: How are limitations of dwell over nose estimated?

Post by user-23911 »

The above example has been displayed by sampling a curve using a number of steps. If you don't use enough steps you end up with GIGO.
That's how computers work and it's also how computers get it wrong.
Those wiggly lines aren't real, they're caused by sampling errors.

If you'd studied calculus at school, you'd understand.
But you haven't.
Because you don't understand.
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