Installing a 6 Speed Allison behind a 12v Cummins

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12vhummer

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Dec 12, 2022
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Hey guys, new here. I've been reading up on clutches for high HP builds and from what I've gathered, Alto G3s take the most abuse. Seems like most guys run a mix of GPZ for C1s and C2s and G3s for C2s, C3s, and C4s.

I swapped a 12v Cummins into my H1 and after driving with a built 4l80 for a few months I really wanted the 2 extra gears for the highway. With the portal axles, my gearing is 5.24 on 40s so it feels like it's screaming at 70 mph. I'm probably going to leave the wastegate alone at 26 psi which puts the power around 325 and 800 ft/lbs. If I turn it up to 40 psi, power would be around 450 with 1150 ft/lbs at around 1900-2000 rpms.

My question is what clutches would you recommend being that I won't be able to defuel? As I said, I don't plan to turn the boost up, but if I can get away with building the trans to handle that, I might as well. (Differential explodes instead :D) I don't know how much defueling plays into protecting the transmission, but any feedback is appreciated.


I've ordered the following so far:

TransGo JR kit with Mike L Trim Springs
New bushings
New Gasket and Seal kit
Torrington Bearing Kit
Ratiotek Line Pressure Upgrade Spring
DPC Triple Disk 1600 RPM Stall
Alto G3s C1, C2, C3, C4 (supposed to ship this week)


Appreciate any help, thanks
 
2004LB7

2004LB7

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I'm not so sure any clutch in an Allison will be happy without defuel. I believe the life will be drastically reduced based on the way the Allison shifts
 
DAVe3283

DAVe3283

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This is what my local builder told me when we were looking to put an Allison behind my dad's 24v. He tore his Achillies and couldn't run the clutch. But he healed up quick, so it's still got the manual.

They said they have no issues with the Allison holding up behind the Cummins, especially at low power like this (sub-400 HP). If your clutches are good and the adapter harness/computer is feeding the TCM reasonable torque values, it will shift fine, just feel a bit hard, similar to a 4L80, etc.

That said, I would expect it to shorten clutch life a bit, since it will slip the clutches a bit harder during the shifts. But the shift timing should be the same, and the trans should adapt in best it can.

If you turn things up over 1,000 lb-ft, I'd personally be a bit nervous. If your trans tuning lets it adapt in nicely, it could work. But if not, a tie-up will break hard parts, regardless of what clutches are in there. So you better know someone who can tune the TCM well...
 
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Bdsankey

Bdsankey

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12vhummer said:
Hey guys, new here. I've been reading up on clutches for high HP builds and from what I've gathered, Alto G3s take the most abuse. Seems like most guys run a mix of GPZ for C1s and C2s and G3s for C2s, C3s, and C4s.

I swapped a 12v Cummins into my H1 and after driving with a built 4l80 for a few months I really wanted the 2 extra gears for the highway. With the portal axles, my gearing is 5.24 on 40s so it feels like it's screaming at 70 mph. I'm probably going to leave the wastegate alone at 26 psi which puts the power around 325 and 800 ft/lbs. If I turn it up to 40 psi, power would be around 450 with 1150 ft/lbs at around 1900-2000 rpms.

My question is what clutches would you recommend being that I won't be able to defuel? As I said, I don't plan to turn the boost up, but if I can get away with building the trans to handle that, I might as well. (Differential explodes instead :D) I don't know how much defueling plays into protecting the transmission, but any feedback is appreciated.


I've ordered the following so far:

TransGo JR kit with Mike L Trim Springs
New bushings
New Gasket and Seal kit
Torrington Bearing Kit
Ratiotek Line Pressure Upgrade Spring
DPC Triple Disk 1600 RPM Stall
Alto G3s C1, C2, C3, C4 (supposed to ship this week)


Appreciate any help, thanks
Click to expand...
You won't be able to not have defuel, it's always there in the TCM.

If the company you ordered from actually is shipping G3 C1s then you're a lucky person. Most companies don't have them and haven't for months from what my supplier of trans parts tells me.

For your HP literally any clutch is fine, you aren't making much HP. What are you using to control the trans?
 
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12vhummer

New member
Dec 12, 2022
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Bdsankey said:
You won't be able to not have defuel, it's always there in the TCM.

If the company you ordered from actually is shipping G3 C1s then you're a lucky person. Most companies don't have them and haven't for months from what my supplier of trans parts tells me.

For your HP literally any clutch is fine, you aren't making much HP. What are you using to control the trans?
Click to expand...
I have been reading they have been tough to get on and off over the past 2-3 years. I saw posts from 2019 talking about the G3 supply issue. The sales guy claimed they are all G3s. With that said, I've read posts saying they can feel too aggressive at times. Would you suggest GPZs in the C1 and C2 or just run the G3s and rely on a good tune? I was going to reach out to Jason at Transmission Tuner.

I thought defuel was something only Duramax's could do as the ECU and TCM would be working together. My understanding is it would cut fuel to reduce some torque as it shifted.
 
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12vhummer

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Dec 12, 2022
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DAVe3283 said:
This is what my local builder told me when we were looking to put an Allison behind my dad's 24v. He tore his Achillies and couldn't run the clutch. But he healed up quick, so it's still got the manual.

They said they have no issues with the Allison holding up behind the Cummins, especially at low power like this (sub-400 HP). If your clutches are good and the adapter harness/computer is feeding the TCM reasonable torque values, it will shift fine, just feel a bit hard, similar to a 4L80, etc.

That said, I would expect it to shorten clutch life a bit, since it will slip the clutches a bit harder during the shifts. But the shift timing should be the same, and the trans should adapt in best it can.

If you turn things up over 1,000 lb-ft, I'd personally be a bit nervous. If your trans tuning lets it adapt in nicely, it could work. But if not, a tie-up will break hard parts, regardless of what clutches are in there. So you better know someone who can tune the TCM well...
Click to expand...
Yea coming from 195 hp and 400 ft/lbs with the 6.5 turbo, to 300 hp /800 ft lbs it's more than enough power for wheeling and overlanding. This truck doesn't tow and with the aerodynamic properties of a brick, it's not winning any races.
 
JoshH

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Bdsankey said:
You won't be able to not have defuel, it's always there in the TCM.

If the company you ordered from actually is shipping G3 C1s then you're a lucky person. Most companies don't have them and haven't for months from what my supplier of trans parts tells me.

For your HP literally any clutch is fine, you aren't making much HP. What are you using to control the trans?
Click to expand...
Brad, there is no way a TCM can make a mechanical diesel like a 12 valve Cummins defuel on shifts.
 
Bdsankey

Bdsankey

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JoshH said:
Brad, there is no way a TCM can make a mechanical diesel like a 12 valve Cummins defuel on shifts.
Click to expand...
Maybe that's not the word I'm looking for. I know at work we have a specific algortithm that Allison uses behind non-CAN/non-electronic engines that is designed to optimize strategy. Our CR trucks speak with the Allison TCM so thus they are defueled but some of our other programs/older vehicles are not.


While fuel quantity doesn't drop it certainly doesn't shift like an Ally with no defuel.


Let me go back to finishing my coffee, I don't function well without it these days as can be seen here..............
 
Chevy1925

Chevy1925

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Long ago, there was a fella on here that put a an allison behind a 12v. i cant remember his screen name though. it was "LB7 something" iirc. it took him a long while but he got the allison to work. defuel was and will most likely always be an issue though
 
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PureHybrid

PureHybrid

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An acquaintance local to me is running a 6spd behind a Ppump 24v in his Dodge. Trans is setup with all G3 frictions, he tore the trans down to do a bunch of billet parts but he's 650+hp. Jason at Transmission Tuner has tuned the TCM and apparently is working great. There's a thread over on CompDiesel about it and other guy's experience doing the swap.

If anyone REALLY needs a G3 pack for the c1 module I have a brand new stack
 
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Bdsankey

Bdsankey

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PureHybrid said:
An acquaintance local to me is running a 6spd behind a Ppump 24v in his Dodge. Trans is setup with all G3 frictions, he tore the trans down to do a bunch of billet parts but he's 650+hp. Jason at Transmission Tuner has tuned the TCM and apparently is working great. There's a thread over on CompDiesel about it and other guy's experience doing the swap.

If anyone REALLY needs a G3 pack for the c1 module I have a brand new stack
Click to expand...
Is it a 5spd (14 friction) or 6spd (16 friction) setup? If you have a 6spd setup I desperately need them for my close friends race truck trans build.
 
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12vhummer

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Dec 12, 2022
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Can anyone confirm what brand of clutches these are? They came in Alto boxes labeled G3. This is a photo of the C2 and C4 clutches. The C3 are the slotted GPZs, and the C1 are G3s

BC6CF358-34F2-40EC-984F-1C6113974169.jpeg
 

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