Author Topic: Exhaust Valve Timing  (Read 6499 times)

Offline dieselpilot

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Re: Exhaust Valve Timing
« Reply #15 on: February 08, 2014, 05:42:05 PM »
There is nothing wrong with Brian's cam duration. I did sketch it and verify in SolidWorks. His cam has 246 duration (crank angle) max with zero lash. The Nemmet has 260.

I did see your note here about what I assume is the base circle. If it is not concentric, it will require a large lash and reduce duration as I described on the other site.

Offline John S

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Re: Exhaust Valve Timing
« Reply #16 on: February 08, 2014, 05:49:23 PM »
Well i must be missing something or what's quoted is incorrect.

Brian says , and I quote
"The exhaust valve opened fully as it was supposed to--and then stayed open through most of what should have been the intake stroke as the piston travelled back down the cylinder."

That is not caused by  a couple of degrees or 8 thou error.

Remember we can only go on what we are told.
John Stevenson, Nottingham , England

Offline dieselpilot

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Re: Exhaust Valve Timing
« Reply #17 on: February 08, 2014, 06:16:54 PM »
Right, but he measured 57 between flanks. If you assume the base circle is perfect that determines duration, and I do trust his ability to read a protractor. The only other possibility is as he describes runout on the cam and depending where that error in cam profile ends up easily explains what he's observed. Tolerance makes or breaks assemblies, and cams are sensitive to it. Had the camshaft bore and base circle been made in one setup I'd question it as well. There are, however, several setups each introducing error, fact of machining. .008" error when lash is less than that will cause serious problems in cam actuation. In the few production model engine cams I've inspected base circle error has been on the order of .0002".

Offline John S

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Re: Exhaust Valve Timing
« Reply #18 on: February 08, 2014, 06:38:11 PM »
OK but I still think something is wrong with the information.

Brian earlier says he had 23 thou clearance which would cancel out any 8 thou error and reduce the cam opening duration so where does the long valve timing event come from ?
John Stevenson, Nottingham , England

Offline dieselpilot

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Re: Exhaust Valve Timing
« Reply #19 on: February 08, 2014, 08:51:21 PM »
I think the .023" was observed due to being in a different place on the now known eccentric base circle.

Offline stevehuckss396

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Re: Exhaust Valve Timing
« Reply #20 on: February 08, 2014, 09:25:34 PM »
Just a hunch but if you look at the top picture of the cam lobe it does not look like the point where the lobe meets the base circle is centered on the base circle. The entire lobe appears to be shifted above the centerline of the cam. Visually it appears to have 360 degrees duration because it seems like it would start lifting at 3 oclock to 9 oclock.

If you dont see it I can make a sketch after i finish eating my dinner.
Do not be like the cat who wanted a fish but was afraid to get his paws wet.

Offline Brian Rupnow

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Re: Exhaust Valve Timing
« Reply #21 on: February 09, 2014, 04:11:23 PM »
Very pleased to announce that everything is back together, and this time the exhaust valve really is operating exactly as it is supposed to. Begins to open at 50 degrees before end of power stroke (which is almost exactly 1/8" before piston reaches bottom dead center). Opens fully and stays open during exhaust stroke, and closes fully about 20 degrees into the intake stroke. Even with all of the mating surfaces on the plate framework located with dowel pins the engine has stiffened up a lot after complete disassembly and re-assembly, but  I expected that to a degree. It is nowhere near as stiff as it was after first assembly. Before I try to run it again I am going to make a new adapter to go between the carburetor and the intake manifold. Right now there are two adapters in there, which I will replace with one much shorter adapter. I also want to do something about the routing of my gas-line so I don't have to resort to tying it up out of the way with a string, as seen in the last video.