M923A2 Engine Swap

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ejpolson

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I'm looking into replacing the 6CTA8.3 that my truck came with with a more powerful engine. I found the ISX15 to be an interesting engine and ti comes in a version up to 600 HP. Has anyone done such a swap, and if so what should I expect along the way? Any help with this project would be greatly appreciated.
 

MtnSnow

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From what I have come to understand the Allison trans and the transfer case will be the weak link in a "re-power" so plan on addressing those as well
 

ejpolson

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So..... I guess what I'm looking for is the right transmission to marry to a 500 to 600 HP motor. I would love to see it be a relatively simple exchange without too many modifications to the frame to make it work well. If I use the ISX15, is there a transmission/transfer case that would be a good alternative to what's in it already? I have very good mechanical skills, but don't know much about diesels and their quirks.
 

tim292stro

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I once heard a great quip "HP is just a number, torque is what you feel". For medium/heavy diesels this becomes a point of importance. You will rarely see a transmission rated by the HP prime mover it is attached to, rather they rate it by the torque of the engine. The current 600HP ISX comes in a few flavors (I'm assuming you are finding a rather recent one as a pull-out) - 2050ft-lbs and 1850ft-lbs. this is the number that will help you find the transmission that you want to plop behind the engine. That said, you can de-tune the 600HP block to 400HP and have it put out the HT740 compatible 1450ft-lbs peak torque.

If you want to replace the transmission with something more road-worthy than a 4-speed, there are HD4000 series transmissions around pretty much everywhere. The HD4560 is a common 6-speed automatic transmission found in refuse trucks (unions made the waste companies replace any manual transmissions with autos - makes it easier to place un-trainable people [my opinion, my experience, I used to work for a union]). They are widely available as take outs, and are roughly the same package size as the HT740. The refuse transmissions also often came with a specialty up-rating which allows 1550ft-lbs input - and that would allow you to run an ISX with a 450HP tune.

If you really need the extra power, you can try to find a newer HD4700 World transmission with the refuse up-rating, this can take 1850ft-lbs at the input and would just make the lower 600HP tune on the ISX. If I recall correctly, the HD4700 is also the transmission they use in the HET. they are harder to find (read, you are going to pay more for one in any condition), but if that's what you need you can do it.

The only transmission I'm personally aware of that can take the full 2050ft-lb input of the high 600HP tune on the ISX, is the Eaton Ultrashift Plus VXP - this is an 18-speed standard manual with a computer to run the shifter and clutch. Any fluid coupling transmissions (i.e. an auto with a torque converter) will probably burn up at that power input.

[EDIT:]A brief mathematical excersize regarding "HP just being a number"

HP = ( RPM * Torque(ft-lbs) ) / 5252

600HP = (12,000RPM * 262.6ft-lbs) / 5252

600HP = (1,600RPM * 1,969.2ft-lbs) / 5252

One of those will burn up your bearings if they are not rated for high speed, the other will snap input shafts if it isn't designed for the torque, very important to know what the max RPM and max torque are... [/EDIT]
 
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tim292stro

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Sure thing. That looks like a good match, just from a quick spec check.

[EDIT:]I'm going to admit a memory lapse - I forgot about the 5000/6000 series Allison transmissions. IF you can find one (very hard to find, and REALLY expensive), these are available up to 4000ft-lbs of input torque. Since I normally think of heavy trucks stopping at 80,000lbs, it slipped my mind that Allison has larger transmissions for mining and heavy agricultural purposes. That said, I have absolutely no experience with anything that heavy, and it would be out of my area of knowledge.

The Allison 5600 is available with an SAE #1 hookup (compatible with the 600HP Cummins IXS15), but I saw a picture in the Allison brochure, and it's HUGE. It might actually have trouble fitting in a non-application-specific truck (then again I haven't actually looked at an M939 series frame).

Just for the sake of information completion :)[/EDIT]
 
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74M35A2

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We have an ISX on a test stand at my work if you want any dimensions. My company makes HD starters for the OEM market. It is very similar in dimension to the 855 it basically replaced. 855 became the N14 I believe, then the ISX replaced that, I think. It is an electronic engine, and the ECU is engine mounted, so it is essentially self contained. You'll need a throttle pedal from a semi truck with the same engine, as it is electronic. The trans may be OK, the core question there is how much will you load it up? Most semi trucks pull a 40,000lb load in the trailer, and want the HP to reduce uphill downshifts. I doubt you would blow out the Allison driving the truck empty, even with an ISX. Not a cheap engine (unless you have a good lead on one), but very cool nonetheless.
 

jdknech

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why not go with a Cummins Big Cam 400, and tune it up some.. you can pull up to 600-700hp out of one, and still be streetable.. and the upside is it is a 855 block.. (what the A0 and A1 version of your truck came with) so buch easier to swap in..
 

ejpolson

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Back again. I'm settling on the Allison 4500 RDS, but don't know which engine to use yet. I would like something that would be as straight an exchange as possible with the 6CTA8.3. I've seen some comments about the Cummins 400, but all are pretty vague. Again, I'd like to get something that would be a "simple" exchange, that is the motor mounts matching... I can pretty much make any other modifications needed, but I don't want a project that will take forever. Can anyone tell me if the Cummins 400 (model unknown at this point) will be the closest exchange? I'm willing to get a "rebuildable" and go through it, if necessary.
Thanks.......
 

jdknech

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Back again. I'm settling on the Allison 4500 RDS, but don't know which engine to use yet. I would like something that would be as straight an exchange as possible with the 6CTA8.3. I've seen some comments about the Cummins 400, but all are pretty vague. Again, I'd like to get something that would be a "simple" exchange, that is the motor mounts matching... I can pretty much make any other modifications needed, but I don't want a project that will take forever. Can anyone tell me if the Cummins 400 (model unknown at this point) will be the closest exchange? I'm willing to get a "rebuildable" and go through it, if necessary.
Thanks.......
the M939A0 and A1 came with a 250(HP) small cam cummins.. this uses the same block that the 400(HP) big cam(BC) cummins used.. you can find BC-350(HP) engines pretty easy, and can do a compleate rebuild (to 400HP specs) for $1000-$1500

But I have to ask, Why dont you build the 8.3 to 350-400hp? they are like the "dodge 12v cummins" of the larger truck world.. very easy to pull more power out of..
 

Jim Timber

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JD, what's needed to get that much more power out of the 8.3?

Also, at what point is the transfer case going to grenade? We already know it can't handle reverse in low range.
 

tim292stro

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Just a question - what is your motivation for juicing the HP, and what is your end-game?

I'd actually stick with the 8.3 unless you have a terrific reason to not do so. The 8.3 is rebuildable using an in-frame kit (new liners gaskets, pistons, rods, etc...) for about $1,100, and parts are everywhere. You can as at least one poster has mentioned uprate your engine by turning up the pump. If you are not planning serious military environment exposure (deep desert at 110+*F at altitude), without that abusive Private Ledinmyfut, or Quartermaster Cantadd, you won't need the extra margin they have spec'd in to the engine. Recall that the military COTS mentality is to take a commercial-off-the-shelf parts and make them work in severe circumstances, and that is done by de-rating the heck out of everything and over-building everything - it's also why the trucks weight more than a similar capacity recreational truck does (think about the gigantic spring packs as an example).

The P-Pump used on the 8.3 is the same style as the better 5.9 P-Pump (I believe it's the P3000 rather than the P7100), and quite a few of those parts are interchangeable (I have swapped a boost compensator from a P3000 to a P7100), making power adds rather easy. If you follow the normal power-add recipe of "Fuel, Air, and Lower-resistance", it works the same in smaller engines as it does in bigger ones - engines are, in their most fundamental concept, just air pumps. Bigger intake filter for less resistance, larger or compound turbos for more boost (ball-bearing if you want less turbo lag and shaft drag), intercooler or (bigger one) to cool the air charge, more fuel to match the higher air charge (same fuel/air ratio for complete burn), and a larger pipe out the exhaust for less back-pressure - and there you have it.

They do the 8.3 in the 500-600HP range in marine applications, the big thing there is the cooling (boats have an ocean as a cooling system). Just remember that any diesel engine is between 35-45% efficient, the rest of that is waste heat (cooling water ~30%, exhaust ~20%, ambient radiation is the balance). Diesel has about 128K-BTU/hr/gal energy content, assuming the absolute best efficiency (45%) you are using 1,144,995.11BTU/hr (~9GPH) for the power output at 450HP, but an additional 1,399,438.46BTU/hr (~10.9GPH) is just waste heat. Pay very close attention to the size and condition of your radiator if you plan on running close to your rated power at high duty cycle no matter what engine you settle on (de-rating an engine eases the cooling needs too). This doesn't take into account the parasitic loads like the radiator fan (15-25HP), or the drive train losses.

One last thing, take a good look at your transfer case before you take the plunge on the Allison - my recommendations are based on the engine to transmission interface. Assuming the transfer case is "divorced", it should be a drop-in, but if it is "married" be prepared for some work to adapt the new transmission to the old transfer case (I would hope in this case that you have long-term access to a machine shop).
 
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