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-   -   want just a bit more speed (http://www.skoolie.net/forums/f37/want-just-a-bit-more-speed-8553.html)

nevrenufhp 07-24-2014 10:25 PM

Re: want just a bit more speed
 
The older MT643/653 do have a lockup converter. Still not overdrive, but a whole lot stronger than the AT545.

Will 07-27-2014 11:35 PM

Re: want just a bit more speed
 
Quote:

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 07-28-2014 09:31 AM

Re: want just a bit more speed
 
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 07-28-2014 11:45 PM

Re: want just a bit more speed
 
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 07-29-2014 02:13 PM

Re: want just a bit more speed
 
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 07-29-2014 06:13 PM

Re: want just a bit more speed
 
Great discussion guys. :D

claydbal 07-30-2014 05:24 AM

Re: want just a bit more speed
 
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 07-30-2014 07:36 AM

Re: want just a bit more speed
 
What you really need one of these.........

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

opus 07-30-2014 09:34 AM

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

Booyah45828 07-30-2014 03:02 PM

Re: want just a bit more speed
 
Quote:

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.


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