Author Topic: Camshafts and Timing  (Read 6610 times)

Offline gbritnell

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Re: Camshafts and Timing
« Reply #15 on: June 03, 2014, 10:44:53 PM »
Since I can honestly say I've never considered the valve clearance when I make my cams I thought I would go to the drawing of my recently completed 6 cylinder engine camshaft and do a layout to see exactly how the clearance changed the valve timing.
The bottom drawing is the profile of one of my cam lobes.
The first drawing to the left is the lifter sitting against the cam lobe and the angle from the center line.
The second drawing I moved the lifter up .003 to simulate the valve clearance.
The third drawing I rotated the cam lobe until it touched the lifter.
As you can see it's only around 1.91 degrees or rotational change on the cam or 3.82 of crank degrees, surely not enough to change the engine performance much when you have 20-30 degrees of advance before TDC.
Now these numbers will change a little depending on the design of your cam.
gbritnell
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Offline lakc

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Re: Camshafts and Timing
« Reply #16 on: June 03, 2014, 11:24:01 PM »
Interesting drawing George. If you look at the diameter of the follower vs base circle for the cam, that's almost a "mushroom" tappet in full size parlance. If you redraw that with a roller follower you would see a bigger change.
As you mentioned, it boils down to the overall design of the entire valvetrain.
Jeff

Offline dieselpilot

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Re: Camshafts and Timing
« Reply #17 on: June 04, 2014, 02:32:14 AM »
In the past I have just undercut the base circle.

I came here to say the same. You can reduce the base circle by the desired clearance amount and use a ramp if desired.

Greg

Offline stevehuckss396

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Re: Camshafts and Timing
« Reply #18 on: June 04, 2014, 02:42:02 AM »

Steve, you are using a much bigger opening angle. What angle do you use between the lobes?



110 degrees
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Offline Roger B

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Re: Camshafts and Timing
« Reply #19 on: June 04, 2014, 12:03:45 PM »
Thank you once again for all your replies  :cheers:  Based on what you all say I am fairly happy with my current camshaft but have some good ideas and tips for future developments.
Best regards

Roger

Offline gbritnell

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Re: Camshafts and Timing
« Reply #20 on: June 04, 2014, 12:41:18 PM »
After what Lakc posted about using a roller follower I layed out the cam and lifter for my V-twin engine and these are the numbers I came up with. Quite a bit of difference from the flat tappet lifter. On this type of lifter I would think that just by reducing the base circle you wouldn't be keeping the same timing specs that you expect to have.
gbritnell
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Offline dieselpilot

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Re: Camshafts and Timing
« Reply #21 on: June 04, 2014, 01:12:35 PM »
Normally a roller cam doesn't look like a flat tappet cam. The cam profile is compensated to provide the same lift profile with the roller follower and the cam shape is much different.

Offline gbritnell

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Re: Camshafts and Timing
« Reply #22 on: June 04, 2014, 01:44:34 PM »
You're correct about the profile of a roller cam lobe. If one indeed takes into account the valve clearance when designing the lobe profile it will definitely change the shape of the lobe. With the layouts I presented I was just trying to show how the clearance affected the timing with the lobes I had designed. In the future when I make a roller type follower I will certainly look at the valve clearance. I didn't think it would change the cam angle as much as it did. My roller cam was designed as a symmetrical cam with about 30 degrees opening and closing timing but in all actuality I have lost 8 degrees of that for the clearance. The engine runs fine as is but it would be interesting to make another set of cams with the proper specs to see how it changes.
gbritnell
Talent unshared is talent wasted.