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Old 12-10-2016, 12:49 PM   #1
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Default Replacing a 9 speed with a Auto ?

I have a 350 Big cam and a Road-master 9 speed. Wife will never drive it ! So where do I go for info on installing a Allison Auto ?
Thanks, Randy
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Old 12-10-2016, 03:44 PM   #2
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What is your rear end gear ratio and is 9th gear 1:1 or over driven (I'm not familiar with the road-master?) I have a truck with an NTC400 and a 5 speed allison that locks up in 2-5 that I'm really happy with. I had to switch the axle gears from 4.33 to 3.07.
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Old 12-10-2016, 03:57 PM   #3
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I can't imagine it would easy or inexpensive. There is one company that I found that you could call since they give no info on their website: Allison Automatic Conversion
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Old 12-10-2016, 10:15 PM   #4
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According to the build sheet , its a CO9700 6 x 4 built for McLane Co. 315 HP at 1800 and 1150 Tq. at 1300. 855 /14L Cummings 350 Big cam. With inter-cooler,air to air. Build date 5/24/1990

Need to go back under and get trans codes. Could be a 1:1 or 1:.75 .

Clutch Housing is a SAE #1. Rear end is 3.42 to 1

And let me say this is all outside my training. I'm a gas guy. But I can learn.

Ideas ? Budget is $4000 and what ever the 9 speed is worth ? Been looking into a HT750. Its used in the big Military truck.
Randy
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Old 12-11-2016, 02:44 PM   #5
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First off...what is a Cummings? Never heard of them....millions of CUMMINS though!

Enough nitpicking....

Being as you have a mechanical injection engine (no ECM's to deal with) it should be fairly simple mechanics wise(no electrical connections or computers to program), pretty much remove old components and bolt in "new", parts wise it could be a little more involved.

You would first need to find a transmission with the right bellhousing pattern(SAE #1 in your case, probably one the most common) to bolt to your Big Cam, as well as determine how the engine/transmission is mounted to the frame (some use the bellhousing as rear engine mounts, some use transmission mounts. When you find the transmission, buy the trans and all linkages/shifter/mounts/etc. You would also need the right torque converter for a 350 Big Cam (the military one probably would have used a Detroit Diesel 8V92(2 stroke, high revving engine) which has a very different torque curve to your inline 6).

Most 9speeds were direct drive back then, but there is a chance it is OD. A used 9speed probably is worth very little, but anything helps.



Here is a link to a parts supplier to give you an idea on ht750 pricing.

https://truckpartsinventory.com/tran...-parts?s=ht750

A $4000 plus 9speed sale budget is going to be tight unless you can snag a good deal or do the labor yourself.

Your best bet is to find a smaller, local HDT repair shop and talk to them about a swap.
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Old 12-11-2016, 02:56 PM   #6
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If you buy an early M915 (before the freightliner/detroit 60 series... which I believe is A3) you get a Big Cam III (400 hp) and the Allison (your NTC 350 is a Big Cam II). It used to be that you could pick up a whole running low mileage truck for $3500. If you contact some of the big surplus guys you could probably get everything you need under your budget. It would also be worth posting something on Steel Soldiers... I saw some older posts about people parting out M915's
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Old 12-11-2016, 03:49 PM   #7
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Here is what I have found.
HT 740 ( 4 speed)
Diff.= 3.42 Tires =41.3"
RPM= 800 1000 1200 1400 1600 1800 2000 2200

3.69 8 10 12 14 16 18 19 21 MPH
2.02 14 18 21 25 28 32 36 39
1.38 21 26 31 36 42 47 52 57
1.0 29 36 43 50 57 65 72 79

In my manual tran's I have 4th. = 3.55
6th. = 1.90
7th. = 1.37
8th. = 1.0

I don't know much about the the converter and stall speed ?
What do you think ?

Randy

And yes I will be doing the work.
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Old 12-11-2016, 04:25 PM   #8
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How much do you weigh fully loaded? How much do you pull? The TQ does make up for a lot of gearing... especially if you're not too heavy.
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Old 12-11-2016, 04:44 PM   #9
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Truck 18,000 and maybe 10,000 trailer.
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Old 12-11-2016, 06:44 PM   #10
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Found a Allison HT740RS. Does anybody know what the "RS" stands for ?
Randy
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