Jump to content
  • 1
Macktruckman

Eaton behind maxidyne

Question

I have a 1978 R685ST with a 285 maxidyne and the 2 stick 6 speed with 4.17 rears and tall 24.5’s. Truck is getting a new cab this week and paint job. Old cab is rusted out badly. I have an Eaton RTO14909MLL from a wrecked mixer truck with E7, I think has mounts that will work. Now would be the time to swap transmissions if I want. I was thinking it would help lower my rpm at highway speed and give me a little more top end and maybe help me out on hills. I live in an area with lots of hills. Pull a lowboy to move my equipment and pull an end dump. I like the 2 stick but the gap from 4 to 5 is pretty big with all the hills and I can only get about 60 mph or so on flat ground to the floor tached out. Transmissions look to be within 1”-2” of each other in length. U joints different. Would it be worthwhile to swap? Since the cab is off anyway 

thank you 

Share this post


Link to post
Share on other sites

Recommended Posts

  • 0

If you didn't have the hills id say it's be a waste on the maxidynes powerband, they make 90% torque from 1200rpms up. Extra gears are kind of a waste, you're pretty much always in your powerband. And the maxitorque trans has 3 countershafts to handle the torque, the Eaton only has 2. Do you have an engine brake on there? It'll make a huge difference on your stock setup, makes the tall gearing a breeze when you use the engine brake for shifting. I'd start by putting a Jake brake on before swapping transmissions.

Not sure what the top gear ratio comparison is between the two transmissions, that'll tell you if you'll gain or lose top speed.

Share this post


Link to post
Share on other sites
  • 0
1 hour ago, JoeH said:

If you didn't have the hills id say it's be a waste on the maxidynes powerband, they make 90% torque from 1200rpms up. Extra gears are kind of a waste, you're pretty much always in your powerband. And the maxitorque trans has 3 countershafts to handle the torque, the Eaton only has 2. Do you have an engine brake on there? It'll make a huge difference on your stock setup, makes the tall gearing a breeze when you use the engine brake for shifting. I'd start by putting a Jake brake on before swapping transmissions.

Not sure what the top gear ratio comparison is between the two transmissions, that'll tell you if you'll gain or lose top speed.

The top gear in the six spd is direct, no overdrive, and a 4.17 rear with a direct trans means about 60 to 65 at top RPM.    terry:MackLogo:

Share this post


Link to post
Share on other sites
  • 0

Thank you guys for the suggestions. I’m having difficulty trying to post now for some reason 

Share this post


Link to post
Share on other sites
  • 0

Ok I do know the maxitorque has triple countershafts and I like the transmission. We’ve had very good luck with them. But the others are in dumps and mixers. Short haul trucks. This is a tractor. It does not have dynatard or jakes. Never had that on any. So I don’t have a jake available to me. The 9LL is laying here so it would be a very cheap swap. It has a.73 od vs the direct in the Mack. Should bring highway rpms down a bit. The 9LL also has low gears, at least one even lower than the 107 in the truck now, and 3 reverses. By comparing ratio charts side by side the Eaton also has a gear or two between what is 4 and direct in the 107. Rob, I’m a little ignorant about the different engine codes. What is the ETAZ673A and is it still a maxidyne? I haven’t messed with this pump. It doesn’t smoke excessively.  I believe that the puff limiter is disabled. I love the low end grunt these have for crawling around off-road and for the many gravel backroads. But for highway use it could use another gear before direct and after the current 4th gear for hills and then an overdrive for tooling down the long stretches. Thank you guys again 

Share this post


Link to post
Share on other sites
  • 0

If I didn’t live in the hills and if the speed limit on the highways were still 55mph, I wouldn’t touch it. It would work great. The mechanic that’s helping me now is good. He’s great on Cummins and pt pumps. We just haven’t ever tinkered with the Mack pumps. I’m not opposed to that and I’m the only driver of this truck. But I also need it to be reliable and live.  So I’m not interested in tinkering with the pump if it will cause problems down the road 

Share this post


Link to post
Share on other sites
  • 0

I’m 50 miles east of Tulsa and a little ways west of Siloam springs Arkansas off 412. Rose Oklahoma near a little town called locust grove. 

Share this post


Link to post
Share on other sites
  • 0

Rob I don’t wanna lose the maxidyne grunt. It crawls out of the gravel pit loaded heavy with authority. Lol if calibration would cause me to lose the 1,000-1200 rpm power, then that might not be for me. 1st through 4th I’m happy with. It’s long between 4 and 5 and an overdrive would be nice. So I would have to recalibrate the pump to use this 9LL? Couldn’t leave it a maxidyne?

thank you

Share this post


Link to post
Share on other sites
  • 0

As for the cooler, I had it apart about a year ago to replace the gaskets. It was losing boost. So it got a good cleaning last year. 

Share this post


Link to post
Share on other sites
  • 0

I know a few people that swapped in a Over drive Eaton be hind a maxidyne to get down the road faster and at a slower RPM. It worked for them but if does change the driving style of the truck.  the low end grunt was there but the gears were shorter. most skiped gears when empty. when on the highway as long as the engine cruised around 1600 and above EGT temps stayed cool and was happy. the engine was built for low RPM grunt but it liked upper rpms to stay cool.

Share this post


Link to post
Share on other sites
  • 0

I have a sister in Norman. They don’t have hills. I’m about 30 minutes from the Arkansas line so we’re in the foothills of the ozarks here. This is the only truck I’ve had with the tip turbine and the water/air charge cooler. So I need to pull that aluminum top off and clean the back side of the cooler fins?

Share this post


Link to post
Share on other sites
  • 0
17 minutes ago, Macktruckman said:

I’m 50 miles east of Tulsa and a little ways west of Siloam springs Arkansas off 412. Rose Oklahoma near a little town called locust grove. 

loaded turkeys outside Siloam Springs one time,interesting trip down, was following another truck that thought he knew a short cut,I don't remember the highway # we got on south of Joplin,but we spent alot of time in the wrong lane to keep from knocking top row of crates off the trailer.

 

Share this post


Link to post
Share on other sites
  • 0

Lol. There ya go superdog 

you know the kind of the roads I have to travel then. Kinda sounds like highway 10 out of tahlequah 

Share this post


Link to post
Share on other sites
  • 0
2 minutes ago, Macktruckman said:

Lol. There ya go superdog 

you know the kind of the roads I have to travel then. Kinda sounds like highway 10 out of tahlequah 

we loaded west of Siloam Springs,turned of the highway a little ways and then turned onto basically a solid rock trail up to the turkey farm ,right beside a cannery of somekind.

Share this post


Link to post
Share on other sites
  • 0

51 highway. That cannery is not far from westville. Mine and my wife’s family farmed long before I was born and they both took produce to that cannery

Share this post


Link to post
Share on other sites
  • 0

Rob, where do I find the specs and directions for pump adjustments?

Share this post


Link to post
Share on other sites
  • 0

Or anyone who can direct me for that matter and thank all of you guys again 

Share this post


Link to post
Share on other sites
  • 0

Let us know how it turns out once you have it running.

Share this post


Link to post
Share on other sites
  • 0
8 minutes ago, Macktruckman said:

Or anyone who can direct me for that matter and thank all of you guys again 

It will take a fuel shop to work adjustments on the pump. I will say to not touch it yourself as it is a very precise calibration procedure to go through for accurate and balanced delivery. Should you elect to do this, pull both the pump, and fuel injectors after marking which injector is from which cylinder and send them in together. Mack always specified to match the injectors to the pump they were to be used with for proper balance. They are calibrated as per 1000 strokes on a very precise machine, (test stand). I've used Thompson Diesel in both Tulsa, and OKC a few times but that has been a while so don't have any recent history.

Share this post


Link to post
Share on other sites
  • 0

Ok. I have done business with Thompson in Tulsa several times. They would be where I would go. Was a guy there named Paul and he’s sharp. 

Share this post


Link to post
Share on other sites
  • 0

I wish I could stick that 14909 in there and not touch the pump 

Share this post


Link to post
Share on other sites
  • 0

Nothing says you can't. I feel you can do better than that however but it will take a bit of time and unfortunately, $$$. With the "Maxidyne" tuning and gears closely spaced the engine will not perform as designed but will still run well. As mentioned most find the gearing "short". That was the problem I ran across with most conversions is the pump is not worked with.

Share this post


Link to post
Share on other sites
  • 0

I don’t know then. Maybe I should abandon the transmission swap. I’ve spent a small fortune on this already buying a cab and a good hood and then some interior parts from other places, plus paint and all the supplies to ready for paiy. And I’m putting 8 new drives on at the same time. I have the transmission and thought if it didn’t cost me much more to swap it in then it might help me out on the road. But pump work means more money and time. And it’s been down over a week already so it’s getting about time to get it hauling again and get a paycheck. Lol

Share this post


Link to post
Share on other sites
  • 0

I suppose I can try it and see. It wouldn’t be a big deal to pull the pump and injectors later and have them done. After my bank account healed 

Share this post


Link to post
Share on other sites
  • 0

If it were me that truck would need to be earning it's keep before I modified if mechanically sound as it is. Not a lot is saved by converting the transmission and driveline without the cab on the frame over doing it when the truck is all together. That power platform is very robust if taken care of and I'd get some work out of it till the budget allows working with it more.

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Answer this question...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...