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FMTV Highway Speed

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DiverDarrell

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Well the taller gears will put the transmission range into where it was designed to operate mph wise (off the shelf product meant for highway trucks), therefore making the lower gears more useable on hill, so in fact hill pulling speeds may increase. Just like when you put bigger tires on the jeep with out a gear change you lose tons of power. do the gear change the performance is back with loss now due to higher wind resistance, tread friction and rotating mass. I am changing gear ratios for the following reasons. First I have a lot of highway travel to get to where I need to use the truck, no doing it for speed, but to get off the redline at 55, high rpm's equal shorter engine life. Second the transmission gear changes are more usable, now closer to the ranges they were designed for. Third the mpg will increase as rpm's are lower and more in the power band of the 3116.

I also will be adjusting the fuel screw, while watching egt's and may change out the turbo to keep temps down as the 3116 builds heat fast. Not looking for a hot rod, but not afraid to look for more performance. If I blow it up, then in goes a C 8.3
 
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DiverDarrell

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It's one grand experiment, that's for sure! Let's just say I won't be getting rid of the other gears right away. Another benefit is it slows down drive shaft speed, which is a known issue for our trucks.
 
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sjohn116

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I cant speak for anyone else here, but in my (admittedly limited) experience with my LMTV, when the transmission is cold and will start from 3rd gear, it gets up and going like it was just kicked in the @XX by a spurred boot. Much better get-up-and-go than when the transmission is warm and starts from 2nd gear. All of this is to say it seems like the 225hp CAT has plenty of power to move 18,000lbs with 3.9:1 gears, and the loss would be marginal with 3.07:1 gears given the super-low gearing of the transmission. At least for on-road driving. My uses for this truck are not centered around off-roading... I drive to Home Depot, the gas station, putting around town, and one day, trudging thru the snow covered roads. I think these uses will be largely unaffected by a gear ratio change, for better or worse.
 
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Bajated

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What is the stock redline on an M1078, 2000 or 2500? I have read that the dump trucks with 3116 engines rev to 2900. Wondering if you could hit 80 MPH with 3.07's and a 2900 RPM governor mod.
 
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Givinup

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Bajated said:
What is the stock redline on an M1078, 2000 or 2500? I have read that the dump trucks with 3116 engines rev to 2900. Wondering if you could hit 80 MPH with 3.07's and a 2900 RPM governor mod.
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I'd be too scared to try. The tires (even without a load) would be too worrisome. Combined with poor balancing techniques, steering stabilizers not build for this speed, overall design not built for speed.

I've read plenty of cruising at 75, which in itself seems pushing it.

I have the high speed gears, but am happy cruising at 62-68 at good rpms.

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FMTV Truck Sales

FMTV Truck Sales

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Belgrade, Montana
We are working with Marmon Herrington on the feasibility of a drop-in replacement transfer case to give us 4x4/4x2, high/low and increased speed.

What is not yet understood is the effect on high speeds in 4x2 mode and whether we can get more than a nominal increase in speed from gearing.

There will be some NRE, they do not have anything COTS that will increase speed so also not sure yet on final price.

Will keep you posted!
 
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pontiac62

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battle creek, mi
My question with these high speed gear swaps. Who has done the swap did you rebuild your driveshafts at the same time or used the driveshafts as is.

Seems like a driveshafts would need to be rebuilt before gear change. Keep in mind I have limited experience with these trucks. Open to being corrected.
Have been doing a lot of reading on this site to gain knowledge.


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M1081 LVAD
 
1951M1078

1951M1078

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Has anyone put a wedge in the front and rear spring mounts to improve the angle on the joints. I don't know what it would do to the front steering geometry ? Maybe it could only be done on the rear ?
 
coachgeo

coachgeo

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1951M1078 said:
Has anyone put a wedge in the front and rear spring mounts to improve the angle on the joints. I don't know what it would do to the front steering geometry ? Maybe it could only be done on the rear ?
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We discussed an appropriate angle to make a wedge block at for the rear but far as I know.... no one has done it. Would have myself but decided to go with a 6x6 front (intermediate) axle instead. Mine should be installed by Feb.?? march?? Waiting for locker to arrive and get installed first. Was mailed out Monday 27th. So that's holding things up and now am having to drive LMTV daily cause the normal daily driver is down with a badly leaking Turbo Oil seal (aka...... best way to cause a run away engine)
 
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coachgeo

coachgeo

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FMTV Truck Sales said:
We are working with Marmon Herrington on the feasibility of a drop-in replacement transfer case to give us 4x4/4x2, high/low and increased speed.
What is not yet understood is the effect on high speeds in 4x2 mode and whether we can get more than a nominal increase in speed from gearing.
There will be some NRE, they do not have anything COTS that will increase speed so also not sure yet on final price.
Will keep you posted!
Click to expand...
IMHO that is not the best route to go. Nothing wrong with having AWD and the existing Granny Gear (labeled as "First Gear")in the existing system. When in true 4x4 "mode" the ratios available (1st thru 5th) pretty much cover everything someone needs when off road. It covers from what would be equivalent to bottom gear of low range in a typical 4wd Tcase via granny/1st gear all the way to an equivalent to high gear in 4wd High; via the 1.2:1 in
5th gear. Added benefit... is when in the existing tranny's 4wd mode you don't have to do anything to reach those ranges... no Tcase buttons or levers to activate when you need a gear that would fall in low range or gears that would fall in high range..... it is all automatic.

What is needed is more alternative ratios above 5th gear in the existing tranny and this may very well be possible via programming changes to the existing tranny computer. Thus, what you really need is someone who has permission from Allison to rewrite the tranny operating software to program the existing tranny to use other physical gear combinations that are already possible via different combinations of the existing physical gears there now. Or someone who can build a second computer to hijack the existing computer and trick it to do these things and not touch Allison proprietary programming. (or a hacker?)

It is also very very possible other physical gears can be put in the tranny ifffffffff software could be written to handle them. Way I heard it there is several of physical gears available that would fit into this tranny cause it is based on a widely used Allison 3000 model tranny which is; via software only and/or physically geared different plus different software, set up for use in various applications) This is straight info from a Tranny rebuilder extremely familiar with these transmissions. He does not know of an Allison programmer though... I asked him. But he has never looked for one either!!!

Tranny ratios for 3700sp and probably for version just before that are

  • First 6.93 : 1 .......... Granny only available in 4x4 mode
  • Second 4.18 : 1
  • Third 2.24 : 1
  • Fourth 1.69 : 1
  • Fifth 1.20 : 1
  • Sixth 0.90 : 1 (not available in 4x4 mode)
  • 7th 0.78 : 1 (not available in 4x4 mode)
NOTE: Italics is all gears available in highway AWD mode.

Example an 8th gear that is 0.5x would be sweet
 
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coachgeo

coachgeo

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Another option.... and may be the easiest.... is put an overdrive between the engine and tranny. Move the rear axle back a bit to accommodate. Add new driveshafts. Turn up the rear axle when you move it back for better alignment with the Tcase.
 
coachgeo

coachgeo

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PS no matter what you do.... offer the best drive shaft the studies suggested (but were not adopted in favor of using the intermediate axle instead) along with tranny braces that add strength to the Tcase housing and the bell housing. And/or kits for using an Intermediate axle to replace 4x4's rear axle as they did with the A1P2 models. (not available to public at this time)
 
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