Engine tranny swap from 2000 TopKick

[run96k3500]

I saw on Facebook today, a 7.4L with the CNP setup and an Allison transmission from a 2000 TopKick 7500 for sale. Would this be a difficult swap if they are providing all the harnesses, ecu, etc?


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[someotherguy]

A swap into your 96 K3500? Well the Allison will be a 2WD transmission and also likely won't fit in your transmission tunnel without a body lift.

Richard

 

[run96k3500]

A swap into your 96 K3500? Well the Allison will be a 2WD transmission and also likely won't fit in your transmission tunnel without a body lift.

Richard

Good call on the Allison being 2wd. I do have the body lift. But not getting ride of the 2wd.


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[someotherguy]

Thing is your 4L80E is already a pretty stout transmission, plus has the benefit of already being 4WD and compatible with your truck since it came with it.

I'm guessing you're looking to replace a 5.7? An L29 Vortec 7.4 out of a regular GMT400 truck would be an easier swap, IMO. I don't have the figures handy and someone will probably come along and correct me but I wouldn't be surprised if that L21 7.4 from the Topkick is lower HP/TQ than the light truck version. This is pretty common when these engines cross over to the medium duty platforms; they drop compression or at least spark advance to make the engine sturdier under the heavier expected loads. The only real benefit I see to the CNP setup is getting rid of that rotten plastic distributor, but you can do that with an aluminum aftermarket unit anyway.

Richard

 

[run96k3500]

Thing is your 4L80E is already a pretty stout transmission, plus has the benefit of already being 4WD and compatible with your truck since it came with it.

I'm guessing you're looking to replace a 5.7? An L29 Vortec 7.4 out of a regular GMT400 truck would be an easier swap, IMO. I don't have the figures handy and someone will probably come along and correct me but I wouldn't be surprised if that L21 7.4 from the Topkick is lower HP/TQ than the light truck version. This is pretty common when these engines cross over to the medium duty platforms; they drop compression or at least spark advance to make the engine sturdier under the heavier expected loads. The only real benefit I see to the CNP setup is getting rid of that rotten plastic distributor, but you can do that with an aluminum aftermarket unit anyway.

Richard

I already have a 7.4L motor in my 96k3500. Was looking for. A donor motor to build and install. But based on what you’re saying, that’s a different motor?


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[someotherguy]

There's not a lot of info out there as the application of that engine (originally) is pretty limited, and some of the info is contradictory.

The L29 is 290 @ 4000/410 @ 3200, and while some sources say the L21 is "lower power" than the L29, Scoggin-Dickey has the L29 listed at the same specs. SD also says it's got a cast crank where some others claim it's forged...

If you're already a Vortec 7.4 and just looking for a rebuildable core engine, I'd still stick with the same engine as you have. Reduces chances of any weird findings and also I'd skip that Topkick engine. It's likely seen more abuse than any light duty truck could endure.

Richard

 

[vr1967]

The only differences I found with the L21 and L29 have been the CNP and it’s oil pump drive /CPS and the throttle body is DBW. The accessory drives, and just minor external differences.

The few L21s and about 1/2 dozen L29s I’ve pulled apart have all been the same long block and intake.

I believe I have found an L21 pullout locally from a wrecked Tipkick. If we can work up a deal, I plan to play with the tuning on it, as I believe you can do a segment swap in the ECM for the 4L80, if not, will try one of my 0411s that has a 7.4/4L80 tune and just go to DBC throttle body