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Simp's Cat 7155 Swap into a M923A1

simp5782

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Ok, so most people complain about how gutless the NHC250 is even with a turbo. Even with 5 speed or Allison they still just have that gap between 3-4 and 4-5 and then there is the top speed in question based on what tires you have. Bigger tires may help you go faster but you do loose that low end grunt. What about being able to keep the 42 to 46inch tall tires and even duals and still run 75mph down the highway and what I believe is still power in reserve.

I believe the basis for what I use the truck to do means I need gears but I still don't need a calf the size of Texas on one leg, plus the non benefit of driving a 9 or 13spd in city traffic when I want to go out and cruise and the one road has 450,453,043,343 stoplights on it.

So I decided to try to modify a CAT 7155 in place of the Allison MT654CR. The Allison is good and has performed well but I just want those extra gears.

Overall Specs of the Allison are 34" long from bellhousing flush to tail end not including the output yoke. It's rear mount is situated at around 26" of its overall length and weighs somewhere around 650lbs dry.
Gear ratios are as follow 1st-4.17, 2nd-2.21, 3rd-1.65, 4th-1.27, 5th-1.00, R-10.76

The specs of the 7155 are 36" long and the rear mounting studs sit at 31.5" from face. and weighs around 1500lbs I believe. I am not for sure. I posted the gearing specs on it below. I have two different spec sheets for them so but I don't know how to tell the transmissions apart.

There are other 7155 threads over in the Engine/Drivetrain section and a good bit of useful information.

First and foremost. I know I am going to have to move the transfer case back. Simple enough. My plan is that I am going to move it back far enough to replace that driveshaft with the same shaft that goes on a 931/932 tractor. About 4ft long I believe. It probably wont be that much of a move back. BUT I would rather make a longer jackshaft and attempt to change out of the mechanics style joints over to a normal tab style spice like the rest of the driveshafts. Then I would only have to lengthen the rear section of the front driveshaft. Being that I have a center shaft bearing on mine I believe it will be fine. However if it presents itself it might end up just moving that whole support bracket up and adding another one in the rear and lengthen the front section. Only time will tell. I don't use 6x6 much so the slight use shouldn't hurt a longer shaft. I have desplined front hubs so the driveshaft never moves.

I will be sliding the bed backwards and removing the cab all together for this mod so it will be easier to work around. I don't think I have any height to the cab issues on the CAT. I am planning on making a adjustable up and down shifter setup. It will sit in the same area as my transfer case handle and driveline brake area. I am planning on a scissor type setup so it can go up and down.

The transmission has to have good clean air. As well as a regulator and mister. I ordered both of Parker and should be here Monday. They are what is pictured below. I also added 2 larger than M939 air tanks just below the drivers door. It may look like I have a lot of room but I am planning on putting an air dryer and the regulator/mister in that same area. I will run a T off the compressor main line to the rest of the system into that giving it it's own system. I am going to put a T in as well on the feed line into the spring brake system. in the even of an emergency or a line break on the primary air side I can reroute air to the spring brake system allowing some brake power or even release.

Air tanks were done today. I believe the 10/12 gallons they are should be enough. If Not I can always add one on top of the mounting bracket on the top tank to the outside. I plan on installing the dryer on a plate between the tank and the steps and the regulator mister near the lower tank. The bottom tank fit right between the steps and bolted to them. It has about a 3inch drop below the step itself.

Mod please move to Mods and Hotrods. I kept getting a proxy server flash when I would try to click on it earlier.
 

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tobyS

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You might want to consider one or two in-line check valves after your T so it can't drain the air back in case of a leak or having used the air from one system.
 

gringeltaube

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I can only imagine that you are doing all this because swapping engines (to a healthy; turned-up 6CTA) would destroy your Allison in no time?
 

simp5782

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I can only imagine that you are doing all this because swapping engines (to a healthy; turned-up 6CTA) would destroy your Allison in no time?
Allisons are giving me the most trouble. Blown one. Now another has an output vibration. Plus even then with a 65k gross load that 6cta and Allison isnt going to make much difference going up and down mountains. It may pull but the engine can only do so much. If i don't like the way it runs i could easily swap the BCiii in or I have a C13 available.

I could always turbo the 250 since i have a aluminum radiator with intercooler available but there is no telling how long it would last.


My 250 has alot of power for a NA truck. Ran against csmdavis the other day in his 931a2 and it was a fender race to 60. I would say its comparable to a 290 easily.

Plus the 250s powerband is small. 1800 to 2200. Being able to jump from that low band rpm back to max with just a split second should drop egts and maintain road speed. We all know how bad it drops speed below 1800 to the 1600 shift point.

My only concern is using R1 even in high range and not blowing the t- case. I may even be able to use R2 in low reverse.
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simp5782

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You might want to consider one or two in-line check valves after your T so it can't drain the air back in case of a leak or having used the air from one system.
The transmission will have a shut off valve on its side and will only be open if the other side has a leak. And i need to isolate the spring brakes. The transmission tanks will have a 1way air valve as well.

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silverstate55

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You might want to consider one or two in-line check valves after your T so it can't drain the air back in case of a leak or having used the air from one system.
This is exactly what I meant to say yesterday via text, when I mentioned the trailer air tanks draining the truck's tanks.....

Thanks Toby!

[thumbzup]
 

infidel got me

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Simps- put one of those dukes of hazard automatics in it... 72 forward gears.... that thing shifted 7 times in each turn.. He cant drive a clutch, he needs one hand to keep adjusting the rainbird---- baaahhhaaaaa
 

simp5782

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Simps- put one of those dukes of hazard automatics in it... 72 forward gears.... that thing shifted 7 times in each turn.. He cant drive a clutch, he needs one hand to keep adjusting the rainbird---- baaahhhaaaaa
I drove a m39 series truck for awhile and i dont miss a clutch at all.

I would rather listen to this than clint grinding gears.

https://youtu.be/3yRtgjAA9JI

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74M35A2

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I'd rather have a turbo manual than an auto natural, but I do hope it works well for you.

Thread should be in 5t mod.
 
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simp5782

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I'd rather have a turbo manual than an auto natural, but I do hope it works well for you.

Thread should be in 5t mod.
You aren't a gear jammer like Cletus Snow

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patracy

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I'm interested in this too. But I'd probably just stick with the nifty 250 I have in my truck already (has a turbo I added). It'd be kinda spooky to think about the road speed that would be possible. I have "tested" Odin up to 68mph.
 

simp5782

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Well. I did some more measuring today. Since the BC I bellhousing is not the same as the NHC250 I have to get one of those. I am unsure of its depth. I am hoping it is near the NHC250 one so it doesn't make things too difficult. With that and the 7155 not using a spacer ring it is the same length as the MT654 overall. This may allow me to leave the transfer case in the same spot IF I can find the right output yoke for the transmission. This would alleviate a few days worth of work on shifting the transfer case back as well as two driveshaft mods and builds.


Right now I am replacing springs till before the safety critics get upset.
 

JDToumanian

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I'm fascinated by the 7155... It's one of the reasons I've been looking to replace my current tractor with a M920. I have a 7155 question: How do you think it would behave behind a completely different engine than the Cummins 400? Let's say the Detroit 8V92TA from a HEMTT at 445 hp. Horsepower is similar, but the Detroit RPMs rise and fall more 'snappy' and it would have different torque characteristics. Would it still shift fine? Maybe the Cat's valve body would need to be tuned/recalibrated?

Jon
 
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