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GM LS AFM/DOD question

General engine tech -- Drag Racing to Circle Track

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turbo camino
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GM LS AFM/DOD question

Post by turbo camino » Fri May 15, 2020 4:08 am

I know what happens if you only do one half of the AFM/DOD delete, putting conventional lifters on a DOD camshaft: you get idle misfires because of the loooong clearance ramps on the DOD lobes.

But what if you only do the other half: conventional camshaft and keep the DOD lifters? Does the internal slack/clearance in the DOD lifters cause any noticeable issues? Assuming the lifters are still mechanically sound and DOD is permanently disabled in the tune of course.

No, I'm not trying to find a shortcut, just curious what happens. :)
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A_VAS
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Re: GM LS AFM/DOD question

Post by A_VAS » Mon May 18, 2020 10:43 am

I doubt anyone knows...what sane person would put those lifters back in if they have DOD deactivated via tune anyway?

I've heard many instances of folks around here putting regular lifters on the DOD camshafts, with no issues. The reported misfires and stuff I am not sure if they are internet lore or real.
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turbo camino
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Re: GM LS AFM/DOD question

Post by turbo camino » Mon May 18, 2020 2:48 pm

Curiosity is BAD, amirite?
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GLHS60
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Re: GM LS AFM/DOD question

Post by GLHS60 » Mon May 18, 2020 2:56 pm

Curiosity is good in my opinion.

Helps keep my old brain functioning!!

Thanks
Randy
turbo camino wrote:
Mon May 18, 2020 2:48 pm
Curiosity is BAD, amirite?
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turbo camino
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Re: GM LS AFM/DOD question

Post by turbo camino » Mon May 18, 2020 9:25 pm

So the DOD lifters have clearance inside between the locking pins & lifter body. Because of this clearance, the collapsible section of the lifter is pushed 'up' against the locking pins by oil pressure when the lifter is on the base circle, and since there is no check valve in that part of the lifter (as there is in the actual lash adjuster section above it), the collapsible section is 'down' against the locking pins when the valve is open. The DOD cam lobes have long slow clearance ramps to compensate for the extra lash in the DOD lifters, resulting in the same net seat-to-seat timing for both DOD and non-DOD lobes.

I do know that if you put a conventional lifter on the DOD lobes, the seat-to-seat at the valve is much longer than the non-DOD lobes. Most of the time (no, not always, exceptions to everything I guess) this causes enough of a difference between DOD and non- cylinders that it feels like idle/low speed misfires.

What I don't know, and am JUST CURIOUS ABOUT!, is how much of a reduction in seat-to-seat the lash inside a DOD lifter creates if you put it on a lobe without the original long clearance ramps. Is it enough to cause a noticeable issue? Would it depend on the cam profile - more noticeable with a very mild cam? Or is there a durability issue - are the long clearance ramps necessary to protect the presumably fragile(ish) retractable locking pins? I suppose there would be more shock loading with a conventional clearance ramp...
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