want just a bit more speed

[nevrenufhp]

The older MT643/653 do have a lockup converter. Still not overdrive, but a whole lot stronger than the AT545.

[Will]

Originally Posted by nevrenufhp

The older MT643/653 do have a lockup converter. Still not overdrive, but a whole lot stronger than the AT545.

That doesn't mean it's not scooting out from underneath the torque load. I'll bet it is. This is the same reason every manual transmission version of a pickup truck has a lower tow rating than the automatic equipped trucks. The auto is weaker head-to-head, but the auto can stay out of the way of the engine torque.

[Booyah45828]

They do have a lockup converter, and they are stronger, but they're still only marginally better than the at545. The lockup converter is only part of the equation, with the major part being the gear options.

With a Double overdrive auto trans you can keep the 4.88 rear gear for acceleration, yet still cruise down the highway at a reasonable rpm. Plus, anybody could drive it and you don't have to worry about missing a shift going up or down the mountain.

The only downfall is that you can't full load AND lug an automatic. It's not that they can't handle that much torque, it's that they can't handle that much torque at that RPM. The pump in the trans isn't generating enough flow to create high enough line pressure to hold the clutches firmly. That's why they scoot out from underneath of you.

Also, from what I've read, all you need for the new electronic allisons is a power, ground, and a throttle input. They have they're own speed sensors within the trans.

I doubt I'll ever swap a transmission in my bus, for the sheer fact that where I live is flat, and I don't travel very far away from home. It'd be nice to swap to lower highway rpm, but the cost involved isn't worth it to me, much less the work involved. So unless the engine blows up from being oversped, or the transmission craps out, I doubt I'll ever do it.

[Will]

That's good insight.

Back in the old days of hot rodding you often installed a "shift kit" in your trans. Little spring changes in the valve body would increase the pressure on the clutch and band solenoids to improve the hold and firm up the shifts--plus adjust the shift timing to let the motor spin up more between gears. These things are mostly decided by a computer now, not a bunch of springs.

Problem is, with hydraulics, it's extremely easy to apply enough pressure to hold a clutch--it's not a dynamic system like a hydrostatic drive--it's a simple static system. It's a couple hundred psi at most--nothing hard to get. If they wanted to let you apply 500ft# to their tranny, they could easily do it. 1,000hp, 500ft# dragsters use tiny Powerglide trannies and the clutches hold--at least for a few seconds. Why can't a big old Allison hold that torque?

There may be something I'm missing, but I don't believe it's pressure on the clutch packs. You are saying that the tranny is strong enough to hold the torque, put they can't figure out how to put enough pressure on the clutches to keep them from slipping. I'm saying that it's a static hydraulic problem, like a backhoe, and that instead of needing 3,000psi, they only need 200. It clearly isn't that, in my opinion.

I still cling to the belief that they are building a 300ft# tranny and marketing it at a 500ft# tranny by making it downshift well before it reaches 300ft# of input torque. I've experienced this many, many times, though I have no proof of intent.

[Booyah45828]

I agree that it only uses a few hundred psi to hold the clutches. Some of those shift kits changed the timing of the shift, only the really high performance racing kits significantly changed line pressure that was being used.

I wouldn't use the dragster analogy in this situation. You're right that powerglides hold up in that situation but the situation that a dragster sees and one that a school bus sees are completely different ends of the spectrum. A little off topic but one of my biggest pet peeves is how most manual clutch companies rate there clutches based off of horsepower. The clutch in a 500hp Camaro will not work behind a 500hp Detroit series 60. Even though they're both making 500hp the loading and the environment is completely off.

The reason I mentioned line pressure was because I know of some guys that have shimmed governor springs and throttle valves in order to adjust the way their at545 shifted. They basically wanted exactly what you said, to hold a gear all the way until the torque curve fell off (they were trying to use it behind a 6bt cummins in a pulling truck). Anyways they found out they couldn't get high enough flow and line pressure at that low of an rpm in order to keep from burning clutches up. You also need enough flow to cool the fluid in the converter too and keep everything lubricated. So whether it was lack of pressure, or the fluid was overheated, or some other cause, I don't know. What I do know is that they tried numerous things to increase line pressure at lower rpms and they just couldn't get it. It might have just been an individual problem with that transmission too.

I just find it odd that Allison lists full load rpm at 2400 rpm, which is outside of about every diesel engine's powerband. If it was feasible and reliable to go lower in the power band, I think they would have, because that would have made everybody happy. I wouldn't say it's overrated, because I think the trans would be able to hold it's rated torque at 2400 rpm, however I don't know of a diesel engine that makes peak torque at that high of an rpm. So the rating they use isn't applicable to the situation the transmission is in. So the whole thing is kind of messed up. What would be better is if they would list the torque rating at 1400 rpm, then we could see how mismatched the combo really is.

[syke]

Great discussion guys.

[claydbal]

years ago, i had a buddy that raced dirt track cars. he also built automatic transmissions. he would run automatics on the dirt oval! he would install a lever on the door that he would throw forward to make the car go. i dont know (remember) but it seemed he was controlling line pressure with the lever(valve) and the trans didnt shift. im thinking the trans stayed in third gear and the valve all the way open when racing. the car did not have a shifter!

if this makes any sense, could this type valve work on an allison to maintain line pressure? like once in high gear , throw the valve all the way and keep the clutches tight?

[GreyEagle]

What you really need one of these.........

http://www.vincelewis.net/bigengine.html

[opus]

Yes, yes...and you can have these folks haul it for you: https://www.facebook.com/photo.php?v=10 ... =2&theater

[Booyah45828]

Originally Posted by claydbal

years ago, i had a buddy that raced dirt track cars. he also built automatic transmissions. he would run automatics on the dirt oval! he would install a lever on the door that he would throw forward to make the car go. i dont know (remember) but it seemed he was controlling line pressure with the lever(valve) and the trans didnt shift. im thinking the trans stayed in third gear and the valve all the way open when racing. the car did not have a shifter!

if this makes any sense, could this type valve work on an allison to maintain line pressure? like once in high gear , throw the valve all the way and keep the clutches tight?

Hard to say what he did there for sure. But if he was using a powerglide like most dirt track autos I doubt he controlled line pressure. Mainly because line pressure is also what lubricates the transmission. Most likely he had a direct drive converter in there so that the input shaft was always turning, He would then throttle the hand valve which would apply the high gear clutches giving him a direct drive trans similar to a manual. Some of the guys used glides that actually had a rear pump too so they could push start the car if need be.

All of your clutches and bands are applied via line pressure already. The throttle valve will have some say in how much line pressure is created based off of accelerator pedal position. Under low power situations you don't need max pressure. So the throttle valve will regulate the line pressure down so that it only supplies the clamping force that the clutches need. The problem we faced is that the max pressure that we could produce just wasn't enough to keep the clutches from slipping. If we could have increased the pressure it may or may not have slipped, or it might have exploded the transmission. Or if we could have installed more clutches and steels like they do for car and truck transmissions it might have worked. After all of that they decided to just use a 727 like some of the other guys were doing.

The reason they tried the at545 in the first place was because the chevy guys were using the new 1000 series allisons behind their duramax's and they were doing really well. So the assumption was made that a medium duty trans would work just as good or better. They even got a custom turbo 400 converter with a really low stall to work with the Allison but the whole thing never panned out.

[claydbal]

that makes sense booyah.

[Will]

Well, in my opinion the TH400 is the best auto tranny ever built. Despite my long use of the DTA360 and Fuller 6-speed, my real workhorse has always been my crewcab Chevy with a 6.2 diesel, Banks turbo and a Gear Vendors splitter. I had a 3-axle gooseneck I could always bring the skoolie home on if it ever needed it. The mechanical programming on the TH400 was brilliant--the smoothest shifting non-computer tranny ever. The Ford C6 and the Chrysler 727 deserve some respect, but they could not match the TH400.

I could put a 6.2 diesel and a Banks turbo with TH400 in my bus, and with all of my years of success with that equipment I know it would work. And it would allow my wife to split the driving.

[opus]

Wow, you're the first person I've seen to publicly speak positive about a 6.2, LOL! They were a good engine....if you got a good one. Lots were bad but the good ones were sure good. I'll agree as well about the TH400.

[Will]

It's like when I bought an angle grinder from Harbor Freight 20 years ago. I took it back about 7 times. They guy told me, when you get a good one, it will be good.

It wasn't, but the 6.2 was what he was really talking about.

There are lots of 6.2 bashers. It deserved its reputation early on. But it did pass a rigorous regime of testing for the military to accept it--it is not a 350 Chevy converted to a diesel as many people believe. It is a confession by Oldsmobile that they don't know a damn thing about diesels. The engine was designed by Detroit Diesel to fit into the space occupied by a 454 with all of the accessories, mounts and transmissions--though the transmissions have internal differences. My truck is an '88, late in the run, and the problems were probably sorted out.

Another respect often denied to this engine is longevity. This engine is still being produced 33 years on. It's been 6.5 liters for about 20 years, but the parts all interchange. All of the faults have long been corrected.

http://www.amgeneral.com/vehicles/gep/s ... ations.php

In fact, you can buy a new one that will compete with a new Duramax, Cummins or Powerstroke.

http://www.peninsulardiesel.com/WP-PENTEST/?page_id=322

What other engine has a 32+ year history--and still going strong? What a lot of people think of as the worst engine ever is probably the best engine ever. When whatever the current design of your favorite engine is changed--check back and see if the old Detroit Diesel 6.2/6.5 isn't still being produced. It will be. It was a brilliant design that needed a few more bucks spent on the materials.

[opus]

Agreed!

I've never had issue with Harbor Freight "junk". All the electrical tools I've got from them, spare brushes included, have been fine. Well worth the low price, unless you depend on them daily for your trade.

[mtg]

I have an AT542 behind a T444, and drive through the mountains. That trans sucks for my application, and my experience agrees with all of the informative posts in here about it not letting the engine climb hills at peak torque, and the heat that it produces is a big problem.

So, I've considering swapping just the torque converter for one with less slip, or even a different auto with a lock up converter. However, it seems most lock up autos only lock in the top gears, which doesn't fix the problem of intense heat production/power loss when climbing hills.

Now I'm thinking of possibly swapping in a manual transmission. Do you guys have any recommendations? My first thought would be a basic 5 speed from an F350 or something. My bus GVWR is 16,500, btw, so not real heavy.

[opus]

Make some calls and find whatever manual transmission a bus with that engine would use. Might as well have a bolt on setup.

[Booyah45828]

Originally Posted by opus

Make some calls and find whatever manual transmission a bus with that engine would use. Might as well have a bolt on setup.

I'll second, third, and fourth this statement. You're going to have to fabricate/engineer things to get them to work anyways. You might as well put something in that will work with the least amount of fabrication.

Most of your light duty trucks will use a New Venture transmission. Either the 4500 which is a 5 speed or the 5600 which is a 6 speed. If it's a Ford, it'll be a zf6. Although all of these transmissions would work, I'd be leery of using them. They're just not designed for use in a school bus. Plus you'd likely have to get some sort of adapter housing in order to mate the trans to your engine.

*On edit, with the T444E you might be able to easily fit a zf6 in there seeing how they used that behind the 7.3 powerstroke in ford trucks. Might even be able to use a hydraulic clutch. Although they're less desirable, it would make it even easier to mount in a bus.

[Will]

From what I can find the ZF6 weighs 235# and the F5406 Fuller in my bus weighs 360# and they are both rated at close to the same torque. The difference, I'm sure, is duty cycle. A Powerstroke light truck might occasionally test that tranny, but a bus with its massive frontal area lives at full torque. That your bus is lighter might help, but the frontal area is more important in my opinion. I don't know if I've posted a rant about being beaten by semis even though I have double the power-to-weight advantage. The fact is, the weight matters a lot during initial acceleration, but at some point the wind resistance becomes much more of a factor. Having a lot more gear than I have engine, my bus should be capable of close to 100mph, but it can't reach 80. A semi, 4x heavier and with 2.5x the power, will absolutely blow my doors off--unless we hit one of those 6% grades where the weight spikes as a factor--then I can get them.

I think that if you actually got down the the moment of installing that ZF tranny, once you see the comparison in the u-joints when you try to make the driveshaft work, you won't be inclined to do it anymore.

http://www.dieselhub.com/trans/ZF-S6-650.html

http://www.roadranger.com/ecm/groups/pu ... df#page=25

[Booyah45828]

I agree completely with what you said Will. However with his gvwr only being 16500 lbs, I don't think a medium duty trans is necessary, and a school bus would have about the same wind resistance as a one ton truck pulling a tri axle fifth wheel camper, and a ZF can handle that all day long. I'm going to stick with the zf6 because I think it will handle the environment fine and I have a feeling finding and installing one of them will be easier and cheaper for him than any other option. My opinion FWIW