Which Rear Axle Ratio? (31ft MH, 7.3idi turbo, Allison 1000)

[Clemens Kruse]

Hello,

I'm slowly coming along with my project, but in the near future I have to decide which rear axle ratio will be installed.

My Setup: 31ft Airstream 310 Motorhome Max. 14500lb., Dana 70HD Axle, 7.3idi with Hypermaxx turbo. (Rebuilt is planned, I will consult the forum, right now the plan is do go to the stage 1 injectors, stage 1 cam from r&d, 90ccm injector pump), and an Allison 1000 6 speed transmission.

Goal: don't create a traffic jam, doesn't matter where I drive but be able to cruise with good fuel economy and low noise.

I have attached an excel file with the gear rpm calculation. I still would think a 1:3.54 rear axle is possible. The maximum torque will be around 1300-1400rpm. In my understanding torque is good to hold the speed?

 

[The_Josh_Bear]

Oh man that's pretty cool! I think between the 6th gear and the weight of that motorhome you'll be best suited with 4.10's.

I had 4.10's for a long time and recently switched to 3.55's. I love the setup now that I have a solid turbo and charge air cooler, but the most I tow is 4,500lbs plus a 2,200 cabover camper. That puts me pretty close to your max weight. If I had that 6th gear like you will, I wouldn't have swapped gears. You'll get out of everyone's way faster with the 4.10s and still be in the torque while cruising for sure.

One big thing to note about your charts is that they are for a stock turbo setup, which was barely turned up at all. The RD90 pump will be pushing way more fuel and the cam brings that power curve up much higher. So peak torque will be more like 1800 or 2000 rather than 1400. This is due to the turbo adding 8-10 pounds of boost by 2000(or whatever your numbers are).

On my rig I can cruise easily down to about 1800rpm towing the above weight but it gets smokey below that with too much throttle, as an example. And your 90 pump has much more fuel than my stocker.

Hope that helps!
Joshua

 

[Garbage_Mechan]

With double OD’s in the Allison I would want to err slightly to the too low (4.33?) side. You are nearly into a mid range box van truck with a 31’ motorhome. One thing I had to watch spec’ing new trucks was driveline rpm. With double OD (.64) IIRC the driveline is 1.36 times faster than the engine. U joint and driveline manufactures have published charts.

 

[Garbage_Mechan]

Another resource is the regional Allison rep. His job is to help folks spec new eq correctly. He can run an Allison SCAAN report which will show speeds in gears, shift points, torque converter ratio, grade ability in gears start torque etc.
You would find the rep by asking a HD ( not pickup) Allison dealer. Be polite but persistent. The rep for about 7 western states is Ray Loose. I may still have his contact.

 

[Garbage_Mechan]

I have only ran in to the driveline speed limit issue once designing a truck for Bitterroot Disposal in MT. Truck would only see occasional highway but many side canyons and gravel roads. We wanted max gradebility and retarder performance. I went one cog too low in the rear end ( on paper) and SCAAN kicked it back because at road speed governor driveline rpm was over the limit. Had 2 choices raise gear ratio or lower governed road speed. Raised gear ratio and resubmitted. Last I heard the truck was a rock star. Great climbing and descending !

 

[Garbage_Mechan]

I plan to put an Allison in my crew cab 7.3 PSD

 

[no mufflers]

that 90cc might be a little big with just a stock hmax turbo.

 

[Thewespaul]

Those hypermax turbos do take a good bit more rpm and fuel than the other turbos to get spooled so I would say your power range is going to be more above 2000 rpms, you likely will hardly be making any boost below there, the 90cc pump will help bring it in sooner but you’ll probably have an egt issue below 2000 because the turbo hasn’t taken off yet. I think 4.55s or 4.10s would be your best choice.

 

[Clemens Kruse]

that 90cc might be a little big with just a stock hmax turbo.

Even with a wicked wheel 2?

 

[Black dawg]

With that turbo, I would gear for above 2k for sure. If you are set on keeping a lower cruising rpm, I would go with a wastegated turbo.

 

[u2slow]

I would not go 3.54 with the auto. Too heavy a beast IMO. The Allison seems to have some tall OD ratios. It may have a lockup point dictating a certain rpm, not sure though.

http://www.duramaxhub.com/allison-transmission.html


What tire size? 30-31"? Usually you get into 33" with 19.5" rims. I'm thinking 4.10 or 4.56... with a Dana 80 you have 4.30 as an option.

 

[no mufflers]

Even with a wicked wheel 2?

might just have to keep the revs up a little like wes said. i had a banks with a 90cc and really had to watch the pyro.

 

[Rusgo]

R&D's smallish turbo will bolt up to your hypermax mounting bracket. Will have to do a custom downpipe, but it's a vastly superior turbo than that hypermax dog. Spools much earlier. With factory fuel settings you won't get over 15psi. So overboost isn't a big issue.

Never looked into the Allison 1000. How do you like it?


Sent from my SM-G975U using Tapatalk

 

[Clemens Kruse]

The problem is that I got the hyper max new. It’s not even installed.

 

[IDIBRONCO]

My opinion is that since you have a new Hypermax, keep it. If you do go with the 3.54 gears, you won't be using 6th gear while pulling the camper. You may not be able to use 5th a lot either. You just won't have enough RPMs to build boost. You'll have to keep the RPMs up in order to build boost. Although it still won't have much boost, the set up you're talking about should be a good highway cruiser while empty. Either you can have a good tow rig or a highway cruiser (empty). You can't have both. It's all a compromise. Are you going to be pulling the camper a lot? If not, you may get along alright with your set up. If you do plan to pull the camper a lot, especially in a bunch of hills, you should rethink your rear end gear ratio. You'll be a lot happier. Some day, I'll find a truck to use as a tow rig. it will have 4.10's in it. I'll keep the 85 F250 with 3.54's and a ZF5 for a highway cruiser to get better mileage. That will be my compromise.