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Allison 6 speed vs 5 speed

Scholioso

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Hey guys.

So I'm doing a cummins swap in my 96 K3500 right now. In the meantime, I'll be running a "level 2" 4l80e. Just to keep costs down. However, in the future I'm looking to throw an allison into the truck. I've been doing research, and pretty much everyone recommends the 6 speed. However, the 6 speed is significantly more money than the 5 speeds. At least in my area (Northern California), that's what I've come to find.

So my main question: Is the 6 speed that much better than the 5 speed?

The truck is lifted 6" on 37's. I don't haul much, if at all. I know the 6 speed just has another overdrive gear, but they are also rated to hold more torque. I'm assuming the 5 speed could be upgraded to handle the same amount as a 6 speed? Still doing research, so forgive me for my lack of knowledge.

Any advice appreciated. Thanks!
 

WarWagon

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Cummins engines are a long stroke 'lower RPM' diesel. So you reach red line around 3200 RPM. This is way lower than higher revving V8 diesels. Further it's annoying to have a Cummins 'wound up' for no reason as several Dodge owners made enough of a fuss when the G56 over the NV5600 was thrown in 'owner of Dodge aka FCA Damler ' changed it back in later G56 model years. The first off the shelf G56 had a different OD ratio that revved the Cummins up more.

You can convert 5 speed Allison to 6 speed with a valve body, expensive, so the difference isn't as big as adding another gearset.

What is the rear end ratio, lift it more with bigger tires, exhaust changes all make a 5 speed tolerable. Stall it a little if first becomes too high with other changes.
 

FellowTraveler

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You must consider the lengths of the Allison's and if the transfer case used as I learned from an old timer out there in CAL will require extensive torsion bar x-member modification to use the IFS suspension. When I say modification I mean welding a reinforcing section to the back of the x-member the cutting out the front center of the x-member within 1" of the keys and boxing areas cut. If I remember right the K3500 has a x-member that is much different that the K2500 has so modification maybe more or less depending on the overall length of the diesel transmission and t-case.

Keep in mind any Allison will require more power to drive it as compared to say the 4L80/85 or a Dodge 47/48 transmission, IMO the Dodge is hands down better choice over the 4L80/85 for torque holding and braking.

OR, do a SAS conversion.

As for your Cummins 1.5k gives a nice return in MPG's and the general rule of thumb is to have a low stall converter that stalls below the peak torque output of the Cummins......
 

Scholioso

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Gotcha. I do know about modifying the torsion bar crossmember. Depending on how much longer the allison is vs the 4l80, I shouldn't have much of a problem. Right now with the 6" lift, the torsion bar crossmember is dropped down. So the transfer case clears easily. I had a local shop "build" me a converter for the 4l80 & that's what he said he put it to was around 1500. Time will tell.

This is all for future research. I would have loved to have an allison now but the budget was getting real tight.
 

THEFERMANATOR

FRANKENBURBAN
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The allison is going to be about 2.5-3 inches longer than the 4l80e(don't remember the exact measurement, been awhile since I had them side by side). Also keep in mind your transfer case choice plays a big role here. The stock transfer cases that came behind the allison are longer and bulkier than say the np241 case. Whatever case you choose, make sure it's one you can get the heavier 29 spline input shaft into.

As to a 5 vs 6 speed, yes you can convert a 5 speed into a 6 speed, but there are several other internal differences between them. Converted 5 speeds when not built see a high rate of c2, and c4 failures because they were never intended to handle the added stress of 6th gear on the clutches.

You also need to keep your tcm choice in mind when choosing. As of right now there is no aftermarket tcm that I know of to convert a 5 speed into a 6 speed in a stand alone application. So this leaves you with running a stock 6 speed tcm calibration.

Personnally I would just start with a Dodge trans and stick with it to start with. If you do go with an allison, research, and plan your swap out. Not all conversion kits have proven reliable.
 
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