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Old 03-15-2015, 09:08 PM   #1
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Join Date: May 2014
Location: West Ohio
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Year: 1984
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Chassis: International 1753
Engine: 6.9 International
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od trans vs lower rear gear or?

What kind of drivetrain mods have you guys done to lower cruise rpm? I'm not looking to go faster, I'm just tired of being redlined when I want to drive on the highway anywhere. Anyways, I'm either looking at either changing the at545 out for an o/d 6 or 7 speed manual. Or swapping rear gears and maybe a tighter torque converter. My local craigslist stinks when it comes to these parts so I'm struggling to find anything. Anyways, let me know what you guys have done and what it cost you if you don't mind.
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Old 03-17-2015, 12:29 PM   #2
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Chassis: International 36ft
Engine: DT466 w/ Allison MT643
Getting rid of the at545 for either a lockup automatic or a manual will improve your economy and cruising speed a bit. I'd do that before changing the rear end personally. A tall overdrive wont reduce your rpm as much as a drastic rear gear change, but taller gears with an AT545 is asking for trouble and poor performance. Put in a good tall overdrive manual and you might be happy without needing a gear change. Just my opinion based on what I've read about that trans and my test drive of a bus with one. So sluggish!
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Old 03-17-2015, 03:19 PM   #3
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I agree with Dalez.
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Old 03-17-2015, 07:40 PM   #4
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Alison auto transmissions are built very tough.

The AT545 is a very good trans. The best choice to to improve the engine operating rpm's, is to change the rear axle ratio. To drive at a lowered rpm, there is a need for more horsepower. Frank
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Old 03-22-2015, 11:48 AM   #5
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Engine: DT466 w/ Allison MT643
The AT545 is a fine transmission... for a smaller, lighter vehicle. In a full size skoolie behind a DT466, it's entirely inadequate. Just looking at the SAE adapter going from 2 to 3 the transmission looks small. Not only that, but the dang thing doesnt even build enough hydraulic pressure to fully engage at full load until the revs are high. It'll never let your engine build torque down low, it just shifts out of the way. Performance on hills, especially loaded down heavy, is terrible vs my MT643. I can mash the pedal and everyone and everything inside not tied down gets thrown to the back of the bus as the torque converter grabs with authority, and the turbo is spooled up and the engine making torque by 1200rpm. The dt360 with AT545? I had to rev it halfway up just to get the bus to *start rolling*, with the engine not acting like it was really loaded till ~2200+rpm. A real slushbox, the AT545.

AT545: 267lbs, 200hp, 445ftlbs, GVW 30,000lbs
MT643: 510lbs, 250hp, 640ftlb, GVW 73,280lbs

That said, I still want a Fuller RTOO9513 "triple overdrive" in my bus
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Old 03-22-2015, 12:16 PM   #6
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For a manual tranny, I would want the spicer 6+1 that I drove in medium trucks in 94 and 95 overdrive and synchronized, DT466 has a wide enough operating range that a 13 speed is overkill and I did enough double clutching in 13 years of truck driving to last the rest of my life
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Old 03-22-2015, 03:08 PM   #7
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Join Date: May 2014
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Engine: 6.9 International
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With a naturally aspirated idi, I doubt I exceed the power level of the trans. The at545 shifts good in mine. But I did go through it and adjust the shift points and the throttle cable. The converter it has in it seems like a 2000 rpm stall. If I could lower that I think its be alright.
I found a guy with a spicer 6543 that seems reasonable. Have to find a donor truck/bus to get clutch pedals out of and get a flywheel.
An rtoo9513 would be sweet, most of the gears would be unnecessary. But it has a really low overdrive that's desirable.
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Old 04-04-2015, 08:52 PM   #8
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Is the 6.9 all mechanical? And is the MT643 an electronic trans? Fitting an electronic trans to a mechanical engine would be tough to get right. I would say Allison MD3060 6 speed swap because that is what my bus has, I love it, and it is overkill even for a fully loaded bus with a more powerful motor, but getting that to work with an old mechanical engine would be difficult. Not to mention the unit is real expensive. I would try a manual box swap. I recall putting bids on an international bus with a 7 speed manual (could be the same that Kubla is referring to) which would be a nice box, but just a quick ebay search showed $2800+ for a reman one and $1000+ for a used one. I saw a few 6 speeds for cheaper. But then you have to make sure bellhousings match, get a new flywheel that fits, get the balancing on the current flywheel matched to the new one, get the right driveshaft yoke, modify the shaft if the trans is a different length, fabricate mounts, and it just ends up being a whole can of worms.

What is redline on that motor, and what speed is redline at? I do get your frustration with that, I just recently got the 6th gear enabled in my bus which dropped engine speed to 2000 rip-ems at 65 mph. Haven't measured fuel consumption yet, but it is hella quieter and feels less stressed.
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Old 04-04-2015, 08:56 PM   #9
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643 is not an electronic transmission.
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Old 04-04-2015, 09:29 PM   #10
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I just found a 95, 40 foot TC2000 with a mechanical 8.3 Cummins and the allison MD 3060.

So what was being said about mechanical engines and electronic auto transmissions?

I'm currently working a good deal on it. I will post pics when I bring it home from the shop.

This will be the drive train for my Four Season Prime.

Nat
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