5 ton Top Speed

[Steelreaper80]

I am getting ready to install the pyro and boost gauges on my M925A2. I want to get some baseline numbers before turning it up a tad. My question is this. What is the actual top safe speed for these trucks mechanically. Yes, I know it says 63 MPH on the yellow sign. I am not wanting 128 MPH, just looking for cruising at 65 with and occasional sprint to 70. Will the driveline handle 70? Right now wide open 60 MPH is all I can get on flat ground fully loaded until someone whispers "hill" then down to 45.....or lower. The tires also are rated to 55 but that is if they are loaded way heavier than I am. So is 70 possible safely?

 

[simp5782]

I am getting ready to install the pyro and boost gauges on my M925A2. I want to get some baseline numbers before turning it up a tad. My question is this. What is the actual top safe speed for these trucks mechanically. Yes, I know it says 63 MPH on the yellow sign. I am not wanting 128 MPH, just looking for cruising at 65 with and occasional sprint to 70. Will the driveline handle 70? Right now wide open 60 MPH is all I can get on flat ground fully loaded until someone whispers "hill" then down to 45.....or lower. The tires also are rated to 55 but that is if they are loaded way heavier than I am. So is 70 possible safely?

Yes. You won't get much faster with the Allison.

Goodyear MVTs are rated at 81mph.

My 923 with the small cam. 16spd cat and 395 XZLs would roll 85 like it was nothing when empty. Especially in West Texas

 

[Jbulach]

Stock, the way I got it from the military, mine runs 67 mph at 2100rpm on 14.00’s.
75 at 2400, iirc pushing down hill.
25psi max boost.
Occasionally hit 1300 degrees pre turbo, running heavy, and can force it to 1400 lugging, big, long hills before they force me into first gear.
One of these day’s I’ll probably start with gov springs to get me up to 23-2400 rpm to see if I can keep the shift points up a little and egt’s down to start. Then timing followed by turning up the fuel, then air filter, and way down the list of dreams, air to air cooler, all hopefully with time to collect good data between each...

 

[tobyS]

Putting in gauges is a good thing.
You will probably need to go to the 3000 governor springs. I think the OEM max out approaching 2100. I've read a lot about the 8.3t going higher, but I want mine set at 2300. Road speed then depends on the tires, I'm running 395s, so slower road speed. Mine was set at 1950 and we adjusted to 2100, helped my shifting. I'm looking forward to getting mine in. I read they also allow more fuel through the whole range, so may help when the co-pilot mentions a hill.

 

[tobyS]

Wes.....really....is that the 395 MVT rated to 81 ?

 

[Shirehorse]

Wes.....really....is that the 395 MVT rated to 81 ?

I think it's the MVT Load Range Gs that are rated to 81 if I recall correctly.

 

[simp5782]

G load range. The most common are rated for 81. L Load range are 68

The 335 and 365 G275s are rated at 81mph too

 

[Steelreaper80]

Yes. You won't get much faster with the Allison.

Goodyear MVTs are rated at 81mph.

My 923 with the small cam. 16spd cat and 395 XZLs would roll 85 like it was nothing when empty. Especially in West Texas

Would the 16.00's give much more speed with the Allison?

 

[simp5782]

Would the 16.00's give much more speed with the Allison?

10mph. You won't have the guts to pull that trailer with a load though. You think you are slow now

 

[fasttruck]

Yea you can get it going that fast but will it stop when loaded without overworking the Army brakes ? That is the usual problem with modifying a M35A2 or a M37: it won't stop.

 

[Steelreaper80]

Yea you can get it going that fast but will it stop when loaded without overworking the Army brakes ? That is the usual problem with modifying a M35A2 or a M37: it won't stop.

The brakes on the M939's in my opinion are excellent especially with air brakes on the M322 trailer. I have locked them up before even with the abs going for a tad bit. Honestly, braking is the least of my concerns. I just need some more Go Go juice out of my pump. I am going to adjust the throttle stop tomorrow before I get into the pump. I had a second truck for a bit and it was faster. I think the throttle stop was screwed in more than the truck I kept.

 

[simp5782]

Yea you can get it going that fast but will it stop when loaded without overworking the Army brakes ? That is the usual problem with modifying a M35A2 or a M37: it won't stop.

They actually stop better loaded than unloaded.

 

[Steelreaper80]

10mph. You won't have the guts to pull that trailer with a load though. You think you are slow now

Even with fuel turned up?

 

[simp5782]

Even with fuel turned up?

No.

 

[charlesmann]

Id be happy with 60, 65 would be better. My 936a2 is bout as gutless as a runned over tx jackrabbit or governor. Im trying to catch back up and find the manual xmsn swap into someone’s (i think 923a2) and start gathering the parts needed to throw a 10 or 13 spd in my donor 936a2. But i do good to hit 55 on the kinda rolling ant mounds here in central tx. Not sure how much power gain ill get by fixing the cooling fan from engaging full time but i cant think it’ll be enough.

edit, 13 spd swap, which i just found the thread.

 

[Steelreaper80]

Id be happy with 60, 65 would be better. My 936a2 is bout as gutless as a runned over tx jackrabbit or governor. Im trying to catch back up and find the manual xmsn swap into someone’s (i think 923a2) and start gathering the parts needed to throw a 10 or 13 spd in my donor 936a2. But i do good to hit 55 on the kinda rolling ant mounds here in central tx. Not sure how much power gain ill get by fixing the cooling fan from engaging full time but i cant think it’ll be enough.

edit, 13 spd swap, which i just found the thread.

Mine rolls at 60 at WOT fully loaded, it is just slow as molasses on a cold winter morning on hills. I really want to just be able to run 65 with a 70 sprint. Honestly, my goal is not only to get to where I am going faster but to hug the rear of another truck and let them push the wind and save some fuel.

 

[Lukes_deuce]

Mine rolls at 60 at WOT fully loaded, it is just slow as molasses on a cold winter morning on hills. I really want to just be able to run 65 with a 70 sprint. Honestly, my goal is not only to get to where I am going faster but to hug the rear of another truck and let them push the wind and save some fuel.

With 53s you can cruise at 65 and push it faster if you wish. I wouldnt want to hug the rear of another truck to save a tiny bit of fuel. The brakes are good on these trucks, but stopping distance on a m939 is not a high-light of the truck.

You will notice a power loss with 53s. I love them on my 923a2 but it comes with draw backs. I occasionally run heavily loaded locally, and she does fine. On the hills is where you see the lack of power. Last road trip I did, I weighed in at 33k gvw, truck weighed 27,600 and trailer axles were 5600. I was in third on any decent sized hill. On flat land, she will cruise along at 65 weighing in at 38k-39k.

Also to add to Wes' response, you can turn up the fuel to get some more power, but with a heavy load and the fuel turned up, you will get hot quickly. Loaded up, I live by the EGT gauge. Empty I can get away with goosing it but I can still get hot. Best bet is to tune the truck to your situation and how much you want to keep an eye on the temps.

 

[Steelreaper80]

What is the normal operating EGT of these trucks and what is the max sustained EGT before getting too hot and damaging the motor? I have the gauges, just need to install them.

 

[Lukes_deuce]

What is the normal operating EGT of these trucks and what is the max sustained EGT before getting too hot and damaging the motor? I have the gauges, just need to install them.

There is no real normal, just an operating range. A normal depends on the truck, mainly pump settings and load on the truck. My 923a2 always ran strong and would hit 1200 pre turbo with some weight on it. My 931a2, was a slug and you couldnt get it above 1000F no matter what, but it was lower on power than my 923. I suggest installing the gauge on a stock motor and get used to how it reads. Then you can see how your truck is behaving.

As for max sustained, I have lived by the rule of 1200F max sustained pre turbo. Thats not just on military trucks, but all my diesel trucks/equipment. Ive never melted down a motor so thats what I stick too. They can handle a short burst of higher, but I keep it to a couple of seconds and then let off completely to cool down.

 

[Jbulach]

I don’t know how hot, how long it takes to toast the motor, but I no longer get too concerned until around 1300 for any period of time. I can hit 1200 empty, running hard from a stop, but my truck has always run very well the way I got it, see post #3.