Allison MT-643 vs AT-545

Topic: Allison MT-643 vs AT-545 (Read 3848 times) previous topic - next topic

Allison MT-643 vs AT-545

February 12, 2017, 10:43:47 am

Anyone ever replace their AT-545 four speed with the MT-643 four speed with the locking fourth gear?
From what I have found the 643 is a bit longer so how does one make it fit?
Oh, and those with the 643, how do you like the locking fourth gear?

Re: Allison MT-643 vs AT-545

Reply #1 – February 12, 2017, 02:00:57 pm

Have to check your drive shaft length and angles as axle moves up and down. Too steep and you have to move engine. MT-643R would be sweet, but I think it longer than the standard 643, and you would place additional load on the cooling system.

Re: Allison MT-643 vs AT-545

Reply #2 – February 12, 2017, 04:12:44 pm

I would think that any length difference could be corrected with the drive shaft as the relative positioning of the motor and trans. and drive axle will vary little. I like the MT643 trans. with the locking torque converter on 3rd and 4th gear, acts like a split shifter and performs essentially like a 6 speed.

Re: Allison MT-643 vs AT-545

Reply #3 – February 12, 2017, 04:36:20 pm

Are the output shafts the same spline and diameter? And how longer?

Re: Allison MT-643 vs AT-545

Reply #4 – February 12, 2017, 05:16:01 pm

I haven't a clue, however one would hope that the application parameters ( GVW,HP and Torq) are similar enough. A place like Drive Train Industries or somewhere similar can make up a shaft for the proposed swap.

Re: Allison MT-643 vs AT-545

Reply #5 – February 12, 2017, 05:32:05 pm

Reason I brought it up is James Holder (Tennessee RV service mgr) told me about how much better the 643 is and I would like the improvement. Not sure if he already did his, but have an email in to him as of Friday evening.

Re: Allison MT-643 vs AT-545

Reply #6 – February 12, 2017, 06:17:56 pm

Curious as to why my '81 has the 643 and the '89 has the 545? Does the coach length or GVW come into play?

Re: Allison MT-643 vs AT-545

Reply #7 – February 12, 2017, 06:23:21 pm

Likely engine/HP/torque related.

The DD 8.2 was 210 HP as I recall. The Cat 3208 was 250-300 HP.

Re: Allison MT-643 vs AT-545

Reply #8 – February 12, 2017, 06:34:32 pm

Brett, I've read, somewhere, where there were some DD 8.2L rated at 250 HP. Don't remember just where or what the difference was. Blue painted engines vs some other color.
Guess I am going to have to do some digging in my files.. YUK!

Re: Allison MT-643 vs AT-545

Reply #9 – February 12, 2017, 06:39:45 pm

545 max torque handling ability was 445 ft/lbs. 8.2 torque was about the same, so a good match.

Re: Allison MT-643 vs AT-545

Reply #10 – February 12, 2017, 08:54:13 pm

Quote from: nitehawk – February 12, 2017, 06:34:32 pm

Brett, I've read, somewhere, where there were some DD 8.2L rated at 250 HP. Don't remember just where or what the difference was. Blue painted engines vs some other color.
Guess I am going to have to do some digging in my files.. YUK!

Marine applications. Read that UNLIMITED COOLING.

Re: Allison MT-643 vs AT-545

Reply #11 – February 12, 2017, 09:50:53 pm

My '81 came with the triple nickel Cummins rated at 225. I had it turned up to 250 just a return orifice change.

Re: Allison MT-643 vs AT-545

Reply #12 – February 12, 2017, 10:12:50 pm

Quote from: craneman – February 12, 2017, 09:50:53 pm

My '81 came with the triple nickel Cummins rated at 225. I had it turned up to 250 just a return orifice change.

Had a Wayne bus conversion with an 8.2 DD @ 210hp. I called a DD tuner and told him I needed more power, He told me nothing he could do, it was maxed out at 210.

Re: Allison MT-643 vs AT-545

Reply #13 – February 12, 2017, 10:38:17 pm

What do they have to do to a Cat 3208T to up it

to a 300 hp engine? Is that the most it can put out?

Also mine does not have a boost gauge or a pyrometer

on it. What would be the best to add to keep up on the heat?

Carter-

Re: Allison MT-643 vs AT-545

Reply #14 – February 12, 2017, 10:44:22 pm

Quote from: Texas Guy – February 12, 2017, 10:38:17 pm

What do they have to do to a Cat 3208T to up it

to a 300 hp engine? Is that the most it can put out?

Also mine does not have a boost gauge or a pyrometer

on it. What would be the best to add to keep up on the heat?

Carter-

Believe 300 hp had turbo and aftercooler.
What is your CAT 3208 a NA or a Turbo - Wanderlodge Owners Group

Re: Allison MT-643 vs AT-545

Reply #15 – February 12, 2017, 10:48:14 pm

They typically put out 375 hp in a boat. Too much for reliable life in a motorhome. Here is a good but long article. The 3208 is featured about mid article.

Google Docs - create and edit documents online, for free.

Pierce

Re: Allison MT-643 vs AT-545

Reply #16 – February 12, 2017, 11:49:02 pm

Sorry, this should be a shareable link: Comparing Diesel Types - Google Docs

P

Re: Allison MT-643 vs AT-545

Reply #17 – February 13, 2017, 02:26:08 am

Beaver had a lower end '3500" model with a 230hp 8.2 Detroit and a 643 trans

Re: Allison MT-643 vs AT-545

Reply #18 – February 13, 2017, 07:08:46 am

Quote from: Texas Guy – February 12, 2017, 10:38:17 pm

What do they have to do to a Cat 3208T to up it

to a 300 hp engine? Is that the most it can put out?

Also mine does not have a boost gauge or a pyrometer

on it. What would be the best to add to keep up on the heat?

Carter-

No you can reduce your pumping losses which decreases the intake temp from the turbo due to higher mass flow. Low restriction exhaust, low restriction air filter from a larger more common otr truck. Lots of 1-3 percents increase here and there and when you start adding them up it can become significant. The inlet and outlet conditions at the turbo are very critical and are often overlooked, in an effort to make a compact and inexpensive engine. Fuel consumption will not decrease as much as performance increase, as it still takes x amount of fuel to travel a mile at x speed .

Re: Allison MT-643 vs AT-545

Reply #19 – February 13, 2017, 08:44:25 am

Quote from: Old phart phred – February 13, 2017, 07:08:46 am

The inlet and outlet conditions at the turbo are very critical and are often overlooked, in an effort to make a compact and inexpensive engine.

Could you elaborate?

Re: Allison MT-643 vs AT-545

Reply #20 – February 13, 2017, 10:30:43 am

Quote from: jcus – February 13, 2017, 10:14:46 am

Some small increases can probably be had, but given equal size, design engines, [such at 3208 nat, turbo and aftercooled turbo]. Meaningful increases only come from increase of fuel and oxygen. The turbo on the engine compresses the intake air and fuel is injected. This results in 250 hp. To reach 300 hp, Cat has to compress the air even more to supply more oxygen.This works well, but because of the increase of compression, the temp. of the air is now too high for good combustion.
Now the intercooler or charge air cooler cools the air, making it denser and better for combustion.
Not sure of diesels, but believe the limit on most gas engines to 4 or 5 psi before intercooling is necessary. I would image the Cat 3208 TA 300 hp runs 20 to 30 psi.
I have always liked to have a aft turbo pyrometer and a boost gauge. Very good indicators on engine condition.
Most newer electronic controlled stock engines do not need them because ecu will adjust engine accordingly and not let you damage engine.

Re: Allison MT-643 vs AT-545

Reply #21 – February 13, 2017, 01:25:09 pm

Got an email from James Holder. His 1984 40' GV came with the 8.2L DD V8 AND a MT-643 Allison tranny.

Re: Allison MT-643 vs AT-545

Reply #22 – February 13, 2017, 01:47:04 pm

Texas guy I had an ORED 250 cat customer show up with a complete marine Diesel twin turbo top end bolted on to it.

95k miles on the install. Only got on it in locked fourth on the trans.

Stock bottom end 3208.

Spectacular drive. What hills?

Said you needed to watch the temp gauge and pyrometer but used carefully had had no issues. Got better mpg also.

Boat buddies put it on in Galveston

Some increases are available from adjusting the fuel system if memory serves me.

Smoked more turned up.....

Re: Allison MT-643 vs AT-545

Reply #23 – February 13, 2017, 01:52:05 pm

Quote from: Texas Guy – February 12, 2017, 10:38:17 pm

mine does not have a boost gauge or a pyrometer

Excuse the heck out of me, but it blew me away when we got into looking at FTs and saw no b&p gauges! Doing some checking, I was told, "There is no reason to have gauges unless the engine is modified for more h.p." That being said, we've pulled some killers and had problems zero, but I miss them!

Re: Allison MT-643 vs AT-545

Reply #24 – February 14, 2017, 09:20:18 am

Quote from: Chuck Pearson – February 13, 2017, 08:44:25 am

Could you elaborate?

attached is a pdf printed from the Loren Cook Cookbook, who is a leading manf. of fans and still does R&D, and after all the turbo is basically a interlocked pair of fans. BTW they have a very nicely appointed corporate jet for client dog & pony shows. Furthermore, and this gets extremely technical, if the air is swirling (twisting) at the inlet its far better that the swirling motion is opposite (counter) the rotation of turbo wheel.

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