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Old 01-28-2008, 08:34 PM   #11
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Re: best trans for dt466

There is no overdrive option for the MT643, but it has a lock up converter which is a step in the right direction from an AT545. I have an AT545 with 4.56 gears and 10.00R20 rubber.

Here's the breakdown...
2600 RPM redline
1:1 ratio in high gear
4.56 rear end gears
41.3 inch tall tires

That would net me a top speed of 70 mph. In reality I do about 63-65 depending on terrain. I CAN in fact go faster than that, but it's only briefly and only on downhill stretches.

What am I combating?
1. no lock up on the converter-there will always be a small amount of "slip" as coupling phase still doesn't assure 1:1 impeller and turbine speeds without a mechanical clutch

2. the tires are not actually 41.3 inches tall by the time you get a load on them and whatever other variations there might be.

Basically I am looking at a 7-8% loss over total theoretical top speed. If I did the same calculations, but with a 4.33 rear end gear the theoretical top speed becomes 74 mph. If you factor in the loss you'd actually be looking at a 68.82 top speed, a gain of 3.82 mph. With the governor turned up to 2800 RPM you could see 73.47 mph realistically, but you're going to be thrashing that motor. That also doesn't take into account exponential losses due to the wind resistance. Still, I think 70 mph might be doable consistently with 4.33's and a 2800 RPM redline.
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Old 01-28-2008, 10:45 PM   #12
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Re: best trans for dt466

I tried some calculations from my bus and I am surprised at the difference. I have an MT643 and was curious about the difference between it and the AT545.
  • ~2300rpm governor
    1:1 in 4th gear
    4.33 rear end
    12R22.5 tires (used 42.5" diameter though that seems optimistic)
That calculates out to a top speed of 67mph. In practice, I actually have observed a top speed (verified with GPS) of around 68mph on long flats. It could be that my tachometer is wrong (or that I am reading it poorly - it's not exactly a precision display) but anyway, I am seeing right around the predicted top speed.

Does the lock up converter really add that much more performance?

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Old 01-28-2008, 11:38 PM   #13
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Re: best trans for dt466

The lockup converter will add about 5% across the board for all practical purposes. It might end up being more or less, but that's going to depend on the specifics of your bus. The AT545 is always going to be multiplying atleast a little torque. Just like with actual gear sets, a fluid torque multiplier has to give up something to get that multiplication out. In this case it is going to be output speed.

Food for thought...before creating overdrive transmissions General Motors messed around with the idea of a lock up converter for the 3 speeds. While the TH-350C isn't exactly a shining moment in GM's history it IS evidence that they thought it was a step in the right direction. Heck, go way back and you even find variable pitch converters. A torque converter has to do a lot and it expected to perform at peak efficiency through all of it. This is not possible. Enter the mechanical clutch to essentially "eliminate" the converter from the equation.

Now here's a question...how much more horsepower does an MT643 suck down as compared to an AT545? It is common knowledge that the TH-400 is a pig compared to the TH-350, albeit a rock solid pig.
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Old 01-29-2008, 07:42 PM   #14
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Re: best trans for dt466

would the 4.33 ratio sacrifice too much hillclimbing/starting drivability?
what about tire size with this new rear end comes the larger diameter tires and mine right now has 10R22.5 s i believe and the new look to be 24.5's but haven't confirmed. what will the tire size do?
thanks
i really appreciate you guys and your feedback
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Old 01-29-2008, 09:32 PM   #15
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Re: best trans for dt466

The new tires should account for another 5% gain in speed. You might lose something in hillclimbing, but really you have the power you have. Basically I would expect the torque converter to uncouple and/or the transmission to downshift an extra gear as compared to something with lower gearing, but if your final drive ratio accounts for it...well. You might see a slight lose in performance, but I doubt it will be that much. I guess ultimately you wouldn't be able to climb as steep a hill, but we're talking about stalling the torque converter out completely and losing all forward speed if that's the case. I have yet to meet an object that will stall my bus out. Sure, it might need to wind up a bit, build some boost, and get that torque converter slipping, but it will climb anything I reasonably want it to.

Long story short...I would go for it if the price is right and you plan to do a lot of long treks.
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Old 11-06-2008, 12:24 PM   #16
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Re: best trans for dt466

Regarding MT-643 allison transmissions what kind of reliability , or length of time between rebuilds behind the DT-466 engine
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Old 11-07-2008, 01:45 AM   #17
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Re: best trans for dt466


Don't even think about using an AT545 with a DT466. Specs here: http://www.skoolie.net/forum/viewtop...p=12668#p12668

An other advantage of the MT643 is the protection against excessive temperature. The biggest generator of heat in an automatic tranny is torque converter slippage. The MT643 eliminates slippage at all times in 4th, and part of the time in 3rd. Thus less heat.

Given my druthers, I'd certainly take a stick shift, and I love the 13-speeds in some of the big trucks at work. But admittedly, a stick shift is not for everyone. I don't know about prices, but there should be a lot of late-model 9-speeds and 10-speeds out there in the big-truck wrecking yards. The Volvos use a hydraulic clutch linkage.

An other option ought to be the medium duty stick shifts that come with such engines as the DT-466 in medium trucks. Six speeds? Seven speeds? Anybody?
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Old 11-27-2008, 06:33 PM   #18
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Re: best trans for dt466

I have the 9 liter International Harvester V-8 engine coupled to the AT-540 or AT-545 (whichever gis the older and less desireable of the models.) My problems are two-fold, lack of horsepower and rear-end ratio. And the tranny leaks over time, so probably I need a new one. BUT ANYWAY...

My gearing from engine to pavement gives me a maximum possible speed of 50MPH, and on a slightly downhill stretch of road, I have attained that speed WFO at 2800 rpm. I have a 6.5:1 rear end ratio. The bus could probably pull a pine tree out by the roots, but it won't do it quickly.

My engine has a Dyno tag on it certifying 180 hp at rebuild. That ain't a lot of ponies to move 19,000 pounds around. I can't make better than about 30 uphill if it's a long hill.

I think a lot may have to do with the tranny leaking so badly. I have to add much fluid when I get the bus out to drive it every year or so. I can just imagine it must be all slip and cavitation inside the case when it's running.

I want to get an MT643 for the bus, but that's a bigger expense than is absolutely necessary right now.
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Old 11-27-2008, 11:33 PM   #19
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Re: best trans for dt466


Do be careful with the engine redline on those downhills. I bent the valves in my first bus that way.
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