DOD Delete Theory

[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.