M809 to M939 compare

[ericp351]

Hi all. Very general stuff. I Reviewed how the A1's, with the big Cummings works compared to the newer A2's, like mine. In general, how do the newer M809's stack up to the A2's in civilian "work life"? Any major differences in low end pull, very different top road speed, or the like? Not out to disparage a 5-ton, just curious.

 

[NDT]

The 250 Cummins in the 800 series is essentially the same as in the 900 slick and A1s, axle ratio is the same, so the big difference is 5 speed vs Allison. Top speed is mostly about tire size. Transfer case in either has it’s own pros and cons.

 

[KN6KXR]

Hi all. Very general stuff. I Reviewed how the A1's, with the big Cummings works compared to the newer A2's, like mine. In general, how do the newer M809's stack up to the A2's in civilian "work life"? Any major differences in low end pull, very different top road speed, or the like? Not out to disparage a 5-ton, just curious.

Hmmm. Well I have a 1991 M936A2 and a 1972 M817 and both are original configurations. It's really sort of an "apples and oranges" comparison between them. There's an incredible amount that's different. For starters the entire cab is way bigger on the 939 as the 809 shares the same cab with the M35. So the battery tray, hood, fenders, glass, wipers, etc, etc.. are all totally different. Top speed on the 817 is 52mph maybe 55mph and it's basically screaming at that. It's a MUCH louder truck generally. Besides being a manual 5 speed with an unsynchronized first gear, the 809 series is air over hydraulic brakes and an almost completely different technique to brake with that and 939 with air brakes.

The 939 is a much better "civilian" truck. That said there's stuff like the lack of adequate compression braking that's annoying. Not an issue on the 809 it's a manual. I like the 809 because I feel I have better control. Also the 817 model I have is pretty short (for a dump truck) and I can get that thing is some pretty tight spots. If you asked me to pick one to drive for a few hundred miles it would be my M936A2 hands down. Safer, more comfortable, less noise, easier to work on the engine bay, etc... The 817 has it's purpose but general driving around isn't one of them I only drive that in parades or when I am using it as the dump truck it is.

 

[HDN]

The 939 is a much better "civilian" truck. That said there's stuff like the lack of adequate compression braking that's annoying. Not an issue on the 809 it's a manual.

Can't you downshift with the gear selector to do the same thing? I do that with my M35A3, shifting down to 3rd gear to engine brake. The operator's manual for my truck says this is okay to do. I have an Allison AT1545P locking 4-speed in my truck.

I'd like to have an M809 and a M939, though ai can see myself driving a 939 more since I'm more comfortable with the safety of the air brakes system.

 

[ericp351]

Thanks guys. I guess they operate more different than they look! Good stuff to think on when Wind Chill is wicked in north Pennsylvania! Are the newest Cab-overs more practical yet, despite the smaller engine? One popped up in a local auction, but bidding was over my head for a 2nd truck.

 

[M813rc]

Most of the differences were well covered above.

My M813 on 14.00s has a top speed near 70, however personally I do not feel comfortable driving it that fast, even though it is nice and stable. The truck itself seems to like 57 as a cruising speed, and I have no reason to push it.
My two M931A2s on 14.00s have about the same speed, but don't seem to like going fast, they too seem to prefer around 57, and again I feel no reason to push it. I do feel more confident with the air brakes on these trucks.

Cheers

 

[KN6KXR]

Can't you downshift with the gear selector to do the same thing? I do that with my M35A3, shifting down to 3rd gear to engine brake. The operator's manual for my truck says this is okay to do. I have an Allison AT1545P locking 4-speed in my truck.

I'd like to have an M809 and a M939, though ai can see myself driving a 939 more since I'm more comfortable with the safety of the air brakes system.

You're right you can downshift the automatic. For me it seems that it's not very effective with the 8.3 motor. You actually NEED to on steep downhills to keep the revs up, drive the air compressor faster, keep the brakes ready. Using the "stab braking" technique to keep the drums from expanding uses a lot of air. My M1078 has the 3116 motor and an automatic but the shift points and compression braking are amazing. It's doesn't have a PacBrake either. Some folks I've seen refit the PacBrake to their M939 to get better compression braking out of it.

Also consider this: my experience with the 939 platform is just with the M936A2 which is the heaviest of the bunch at 37k pounds. It's loaded everywhere it goes. So my opinion might be skewed. A cargo truck is likely a lot better.