DOD Delete Theory

[iamdub]

I have a partial update, didn't get as much time on the engine as I wanted this weekend but I did get the heads off & lifters out.

It was what I was expecting, the only lifter that was stuck was the #1 intake DOD lifter, all 16 lifters had no damage on any of the rollers and the cam lobes all look great so this engine is a good candidate for this experiment.

I had 4 busted exhaust manifold bolts to deal with and a bad stud on the exhaust manifold bell to deal with (on the tight Envoy engine compartment the exhaust manifolds must be removed to get to the lower head bolts).

All but the gasket cleanup is done so I am hoping to get the heads back on tomorrow so I can measure for the pushrods, if I do I will post the difference between the DOD cylinder pushrod lengths and the non DOD (if any) and get the pushrods ordered; if there is no difference in the length then that means my theory is a dud.

Hopefully I will know by tomorrow evening

The factory push rods? They're all the same length.

 

[RET423]

Okay, another update; I keep getting sidetracked so this is taking longer than I thought.

I installed the new LS7 lifters, trays and got the heads back on so I was able to check the pushrod length to see if my theory was valid; I appear to be wrong.

This DOD engine uses the same pushrod in all 16 lifters and all of mine were in good shape as were the rockers & trunions.

I used the method of determining the lifter preload by placing each lifter on the base circle, using a known length pushrod (in this case the factory 7.385 pushrod), running the rocker bolt down until zero lash, and then counting the additional turns needed to seat the rocker in the trunion.

There is a formula for computing how far the lifter plunger is depressed by the amount of turns needed but to hit the factory specs you want to see between 1 & 1.5 turns; most high RPM performance builders look for 1.25 to 1.75 turns to promote valve train stability at higher RPM'S. This sets the LS7 lifter plunger between the halfway point and 75 percent of the plunger travel.

On my engine the DOD lobes with the standard LS7 lifters took 1.5 turns to seat the rocker, which is within both the factory spec and the performance spec.

The non DOD lobes with the standard LS7 lifters needed 1.3 to 1.4 turns to seat the rocker with the same factory pushrods, this indicates that the DOD lobes have a slightly larger base circle than the non DOD lobes on the camshaft but so slight that valve train operation should not be effected in any way as all are well within the proper specs.

So this means that the commonly reported issue of trying to do a DOD delete without changing the cam does not work well due to the DOD cylinders ending up with dramatically less compression is not caused by incorrect pushrod length on the DOD cam lobes (as I theorized) but is caused by some other profile difference in the cam on the DOD lobes.

At first I decided I would just start tearing out the radiator for a cam swap but I decided to wait till tomorrow and do a compression check first, mainly because I still cannot imagine a lobe profile that would cause the compression issue with the proper lifter preload geometry (as I currently have).

If my compression test tomorrow reveals the dreaded low compression on the former DOD cylinders I will say uncle and change out the cam but I still will not understand the cause; if I don't have a compression issue then I will finish assembly and see how it runs.

Hopefully tomorrow will shed some more light on the subject

 

[Marky Dissod]

If my compression test tomorrow reveals the dreaded low compression on the former DOD cylinders, I will say uncle and change out the cam
but I still will not understand the cause ...

If you change out the cam, measure ALL the base circles using calipers.
Those measurements will be enlightening.

 

[Donal]

Okay, another update; I keep getting sidetracked so this is taking longer than I thought.

I installed the new LS7 lifters, trays and got the heads back on so I was able to check the pushrod length to see if my theory was valid; I appear to be wrong.

This DOD engine uses the same pushrod in all 16 lifters and all of mine were in good shape as were the rockers & trunions.

I used the method of determining the lifter preload by placing each lifter on the base circle, using a known length pushrod (in this case the factory 7.385 pushrod), running the rocker bolt down until zero lash, and then counting the additional turns needed to seat the rocker in the trunion.

There is a formula for computing how far the lifter plunger is depressed by the amount of turns needed but to hit the factory specs you want to see between 1 & 1.5 turns; most high RPM performance builders look for 1.25 to 1.75 turns to promote valve train stability at higher RPM'S. This sets the LS7 lifter plunger between the halfway point and 75 percent of the plunger travel.

On my engine the DOD lobes with the standard LS7 lifters took 1.5 turns to seat the rocker, which is within both the factory spec and the performance spec.

The non DOD lobes with the standard LS7 lifters needed 1.3 to 1.4 turns to seat the rocker with the same factory pushrods, this indicates that the DOD lobes have a slightly larger base circle than the non DOD lobes on the camshaft but so slight that valve train operation should not be effected in any way as all are well within the proper specs.

So this means that the commonly reported issue of trying to do a DOD delete without changing the cam does not work well due to the DOD cylinders ending up with dramatically less compression is not caused by incorrect pushrod length on the DOD cam lobes (as I theorized) but is caused by some other profile difference in the cam on the DOD lobes.

At first I decided I would just start tearing out the radiator for a cam swap but I decided to wait till tomorrow and do a compression check first, mainly because I still cannot imagine a lobe profile that would cause the compression issue with the proper lifter preload geometry (as I currently have).

If my compression test tomorrow reveals the dreaded low compression on the former DOD cylinders I will say uncle and change out the cam but I still will not understand the cause; if I don't have a compression issue then I will finish assembly and see how it runs.

Hopefully tomorrow will shed some more light on the subject

Thank you for your interest, efforts, and for sharing this event. You may be the Smoky of the LS style engines.
I do want to learn your procedure for determining the location of the mid way point of each base circle. Did you have a degree wheel on the crankshaft? Did you have a degree wheel on each rocker arm hold down cap screw? Are you precharging the lifters? Are you going to block the oil passage to the dod orfices as you assemble the engine for operation?

 

[91RS]

If you change out the cam, measure ALL the base circles using calipers.
Those measurements will be enlightening.

All of this information is already out there. All of this has been hashed out for 15 years.

 

[Alex_M]

All of this information is already out there. All of this has been hashed out for 15 years.

Please link - lot of folks have been through this thread already that have obviously never seen it, myself included.

 

[j91z28d1]

interesting.

that would leave somehow the afm lifter when locked is increasing the cranking compression over a stock lifter in the same lobe.

I mean same length it seems since the rods are very close. when the pins are locked, there's no movement besides the standard hydraulic plunger of any lifter. it doesn't seem like the roller is any bigger than the others, since it still pulls up thru the same bore.

if all this info is out there, what magic am I missing? i get they have a difference profile, but byond a softer ramp speed for the extra weight. how is the afm lifter adding compression over a non afm on the same lobe.

 

[j91z28d1]

well fak.. I may have answered my own question. so in a random post on an archived head porting forum there was one post buried in tread where a guy said there's some built in slop in the locks, so they aren't locked solid like I was thinking. the oil pressure holds them extended on the base circle but there's no check valve in that part like the hydraulic part, so as it ramps up it compresses internally as well soaking up duration. so it's not really about lower compression from not opening the valves enough with the normal lifters, it's opening them to long, where you get more overlap and bleeding off comp at low rpm.

if that's true, at the valve you have a long duration low lift cam on 4 cyl after the swap. almost like a Atkinson-cycle engine but only on 4 of 8 cyl.

which is kinda interesting in theory since my hybrid does have late intake valve closing Atkinson-cycle type cam in it. I've always wondered what it be like with a normal truck cam. but the lack of tuning in the ecm, I'd not try it. sounds like if my disabled and bypassed afm lifters ever fail, I'm buying 8 oem afm lifters to replace them.

 

[RET423]

Did the compression test today with encouraging results, test was done with all new lifters that have never ran, no exhaust or intake manifolds installed and no plugs in any cylinder. As I mentioned before all lifter preloads have been verified to be in spec for LS7 lifters.

On the driver side the highest compression I saw was #3 and #7 (one DOD hole and one non DOD hole) these were 225 psi each. The #1 and #5 (again one DOD & one none DOD) each had 205 psi for compression.

On the passenger side I got 205 psi on #2, #6 and #8 (2 non DOD holes & one DOD), but cylinder #4 had only 145 psi (it is a former DOD cylinder)

My conclusion is that with the valve train geometry in spec that DOD cam operates the valves the same way with or without the DOD lifters in place.

The one cylinder that I did find way to low compression on I am not that concerned about, the lifters were not pumped up as the engine had never been started and the DOD cylinders were all coked pretty bad, I assume the rings are sticky in # 4 and the valve faces are likely coked up as well; I expect the removal of the DOD system and a catch can will bring that cylinders compression back up within a couple tanks of gas.

After I run it a bit I would like to do another compression test but if there are no negative symptoms I likely won't, thus engine compartment is very narrow so with exhaust manifolds installed it will not be fun to do a compression test.

But I think it will be fine, if the cam design was going to cause low compression I would have seen it on the other 3 DOD cylinders but they seem on par with the non DOD cylinders.

One note, after I cleaned up the block surface and wiped all the cylinders out to get the dust from the gasket scraping cleaned up, I forgot to wipe some motor oil back onto the cylinder walls before I reinstalled the heads; once the cylinders get lubed up from running I think that 205 psi to 225 psi variation will even out a bit.

I should have it fired up tomorrow, from everything I have seen I expect it to run normal as before except for the number 1 cylinder no longer being dead

 

[Donal]

Did the compression test today with encouraging results, test was done with all new lifters that have never ran, no exhaust or intake manifolds installed and no plugs in any cylinder. As I mentioned before all lifter preloads have been verified to be in spec for LS7 lifters.

On the driver side the highest compression I saw was #3 and #7 (one DOD hole and one non DOD hole) these were 225 psi each. The #1 and #5 (again one DOD & one none DOD) each had 205 psi for compression.

On the passenger side I got 205 psi on #2, #6 and #8 (2 non DOD holes & one DOD), but cylinder #4 had only 145 psi (it is a former DOD cylinder)

My conclusion is that with the valve train geometry in spec that DOD cam operates the valves the same way with or without the DOD lifters in place.

The one cylinder that I did find way to low compression on I am not that concerned about, the lifters were not pumped up as the engine had never been started and the DOD cylinders were all coked pretty bad, I assume the rings are sticky in # 4 and the valve faces are likely coked up as well; I expect the removal of the DOD system and a catch can will bring that cylinders compression back up within a couple tanks of gas.

After I run it a bit I would like to do another compression test but if there are no negative symptoms I likely won't, thus engine compartment is very narrow so with exhaust manifolds installed it will not be fun to do a compression test.

But I think it will be fine, if the cam design was going to cause low compression I would have seen it on the other 3 DOD cylinders but they seem on par with the non DOD cylinders.

One note, after I cleaned up the block surface and wiped all the cylinders out to get the dust from the gasket scraping cleaned up, I forgot to wipe some motor oil back onto the cylinder walls before I reinstalled the heads; once the cylinders get lubed up from running I think that 205 psi to 225 psi variation will even out a bit.

I should have it fired up tomorrow, from everything I have seen I expect it to run normal as before except for the number 1 cylinder no longer being dead

Thanks for providing data.