DOD Delete Theory

[j91z28d1]

I hope you do this.. I think it will a 100% work. the dod lifter when locked is nothing more than a longer heavier lifter. if the cam profile results in slightly less lift or duration, the factory tune must be tuned with this in mind. so adding a cam, even a stock non dod should technically be tuned for it.

I'm sure if you measure push rod length you'll know ****** if what they did was just smaller base circle. if you had the engine on a stand and a degree wheel, it would be super interesting to measure the actual lift and duration of a dod cam on the dod cyl and non dod cyl at the valve with the stock setup

be warned there's a ton of different and bad info on measuring push rod length. it's a ugly rabbit hole on YouTube. especially for non adjustable ls stuff. I dislike the center the rocker tip on the valve way and think rocker angle at 50% valve lift is better, gives less sweep area and let the tip fall where it may on the valve. but my logic is left over from the sbc era of adjustable rockers and odd ball angle heads from back then.

BTR had the best deals on random length push rods last time I built anything.

I legitimately hope it works out.

 

[91RS]

They do offer a kit to do it, most DOD delete kits don't include a cam, just the option to add the cam you want; most kits also specify to verify the pushrod length needed after the heads are back on and order the appropriate lenth, don't just assume the stock pushrods will be right. People skip that step but many complain later about noisy lifters as a result.

There is no money to be made with this idea that is not already being made, just an opportunity to save some money and work for those whose cam is not damaged and are not looking for an improvement in performance.

If it works that is, the parts came today so I will see if I have time this weekend to get the heads off and eyes on the cam; if it looks good I will calculate the pushrod length and get them ordered next week.

Show me these kits then? ALL AFM delete kits include a cam or tell you you need one. ALL cam kits say to measure for pushrod length to ensure the valvetrain geometry is correct and center the rocker on the valve.

The lobe shape on the camshaft is different for the AFM cylinders. Make sure you check the compression once you get it back together. The ex-AFM cylinders will probably be 15-25 PSI lower.

 

[SilverSport]

do any of our GMT800s have DoD???

 

[91RS]

do any of our GMT800s have DoD???

They do not. They don’t have VVT either. AFM first came out in the S/T trucks in 2004 if I remember correctly with the 5.3L, then came out in the full-size 5.3L in 2007, and was then added to the 6.2L SUVs in 2010. Now everything has it including Corvettes and Camaros. In 19/21 some of them got changed to DFM and all cylinders can be disabled individually.

 

[SilverSport]

I thought so...I was just wondering why this was posted in this Forum if it does't have anything to do with our trucks...I've noticed several threads with questions about either newer GM trucks or different vehicles all together and just wondered what I was missing...

thanks 91RS

Bill

 

[Alex_M]

OP, I am excited for the outcome on this. I'm with you - I don't see why it wouldn't work. Lots of folks seem concerned about the ticking lifter having caused damage to the cam, but I doubt that would be the case. Lots of hydraulic lifters out there with ticks that are not wiping their cams, even flat tappet stuff.

Can't wait to see what you find. Good luck.

 

[iamdub]

I'm sure if you measure push rod length you'll know ****** if what they did was just smaller base circle. if you had the engine on a stand and a degree wheel, it would be super interesting to measure the actual lift and duration of a dod cam on the dod cyl and non dod cyl at the valve with the stock setup

Or have someone put a stock AFM cam on a Cam Doctor.

 

[j91z28d1]

Show me these kits then? ALL AFM delete kits include a cam or tell you you need one. ALL cam kits say to measure for pushrod length to ensure the valvetrain geometry is correct and center the rocker on the valve.

The lobe shape on the camshaft is different for the AFM cylinders. Make sure you check the compression once you get it back together. The ex-AFM cylinders will probably be 15-25 PSI lower.

I'm still not understanding how the dod lifter adds 15-25 psi if cranking compression.

 

[j91z28d1]

Or have someone put a stock AFM cam on a Cam Doctor.

the cam itself having a bit less profile at the lobe doesn't really matter, it's more about if somehow the dod lifter is adding back that lift and or duration at the valve when a standard lifter wouldn't.

only thing I can think of is if the roller tip is larger diameter, it could effect it. I've not had both in my hands, since mine are still in my truck and hopefully stay that way lol. and can't tell by any pics I've looked up.

but being as my hybrid cam is a completely different grind than anything else. there's no non dod cam off the shelf to drop in as a delete kit. I have to hope this works out haha. cause it's definitely something I would be trying. that or replacing them with a oem afm set, new vlom and turning it all back on for another 100k. kinda my 2 options.

 

[Marky Dissod]

still not understanding how the dod lifter adds 15-25 psi if cranking compression.

'cause it's smaller by enough compared to the bigger / normal lifter,
to meaningfully inhale less air before the compression stroke than the normal lifter.

 

[j91z28d1]

I'm still not understanding you.

are you saying the afm/dod lifter is smaller so going to take in less air?

if so, the tune must be OK with that because it runs fine in v8

 

[Marky Dissod]

are you saying the afm/dod lifter is smaller so going to take in less air?

No, not quite.
The cam profiles - the parts added to the 'base circle' that push on the lifters -
of the V4 mode cylinders are smaller than the cam profiles of the V8 mode cylinders.
The camshaft itself only has one 'base circle'.

if so, the tune must be OK with that because it runs fine in V8

Yes, yes it is. The VE tables are calculated to be in between the two.
Also, the misfire tables have different values for different cylinders.

 

[RET423]

I'm still not understanding you.

are you saying the afm/dod lifter is smaller so going to take in less air?

if so, the tune must be OK with that because it runs fine in v8

There is no way to compensate in the tune, the lobes lift, duration and separation angle must be the same on all the cylinders in V8 mode; the difference is the smaller base circle before the lifter starts up the lobe.

The longer DOD lifter makes up for this loss in distance so all of the pushrods can be the same length. If you put a standard lifter on that lobe it is too short for the factory pushrod, so the intake valve doesn't open far enough or long enough to get an adequate amount of air into the cylinder on the intake stroke; this results in reduced compression (less air to compress).

A longer pushrod (in my theory) on those cylinders should restore the geometry to what it was with the longer DOD lifters, restoring normal engine performance with a standard lifter in place of the DOD lifter.

The tune would require turning off the AFM just like any other delete and modding the VLOM or using a non DOD valley cover as well; it would just allow the use of the DOD cam and avoid that expense if no performance increase was desired (and save a bunch of labor).

 

[j91z28d1]

There is no way to compensate in the tune, the lobes lift, duration and separation angle must be the same on all the cylinders in V8 mode; the difference is the smaller base circle before the lifter starts up the lobe.

The longer DOD lifter makes up for this loss in distance so all of the pushrods can be the same length. If you put a standard lifter on that lobe it is too short for the factory pushrod, so the intake valve doesn't open far enough or long enough to get an adequate amount of air into the cylinder on the intake stroke; this results in reduced compression (less air to compress).

A longer pushrod (in my theory) on those cylinders should restore the geometry to what it was with the longer DOD lifters, restoring normal engine performance with a standard lifter in place of the DOD lifter.

The tune would require turning off the AFM just like any other delete and modding the VLOM or using a non DOD valley cover as well; it would just allow the use of the DOD cam and avoid that expense if no performance increase was desired (and save a bunch of labor).

oh I understand what you're saying and I agree. I think it will work.

I just don't understand what others are saying as to why it won't.

 

[RET423]

Well, literally every company that sells the parts says the cam must be changed, that's likely why there is skepticism; I just can't think of a reason why it won't work and this is a rig I bought real cheap just for the engine so I thought I would try it just to test the theory.

Nothing to lose but the one cost of 8 custom length pushrods, I think it's worth it just to find out

 

[Donal]

So, did you confirm that the oem pushrods are different lengths yet? What is the length of the dod pushrods? What is the length of the non-dod pushrods?

 

[91RS]

oh I understand what you're saying and I agree. I think it will work.

I just don't understand what others are saying as to why it won't.

The profile of the lobes for the AFM cylinder is different. The lift and the ramp rate are different. This cat has been skinned for YEARS and all his information is out there. If this worked, everyone would sell a kit. It isn’t some money making scam by all the cam manufacturers, because they all tell you that you can use the GMPP AFM-delete cams if you don’t want a performance cam, and they give you the part numbers for you to buy elsewhere.

 

[Alex_M]

Well, literally every company that sells the parts says the cam must be changed, that's likely why there is skepticism; I just can't think of a reason why it won't work and this is a rig I bought real cheap just for the engine so I thought I would try it just to test the theory.

Nothing to lose but the one cost of 8 custom length pushrods, I think it's worth it just to find out

Only 4 custom length push rods, right? Because only 4 cylinders shut down until you get into the LT motors.

 

[Airman68]

Only 4 custom length push rods, right? Because only 4 cylinders shut down until you get into the LT motors.

I think he is saying 4 intake and 4 exhaust pushrods = 8.

 

[Alex_M]

I think he is saying 4 intake and 4 exhaust pushrods = 8.

Oh, duh. I was thinking it only shut down the intake valves for some reason. Long time since I had a DOD motor open.