Search the Community

I am sure most of you know about the Allison HT740 and HT747 transmissions, but the lower-rung MT644 and MT647 are not quite fondly as remembered. These share some of the same parts, such as torque converters. The MT644 was designed for engines up to 300 hp and 780 lb/ft of torque, whereas the later MT647 ups the input torque rating to 950 lb/ft. But for transit buses, these ratings will be different; for now I know the input hp limit for bus engines is 250. The MT648 was the electronically-controlled equivalent of the MT647. Torque converter ratios are 2.21 (TC495) and 1.92 (TC494). The HT740 was designed for engines up to 425 hp and 1300 lb/ft of torque, but in transit buses those ratings are reduced to 350 hp and 1100 lb/ft. The HT747, with its special three-gear pump for better coolant flow, is used only in transit buses and is rated for engines producing up to 350 hp and 1100 lb/ft. The HT741 and HT748 are the respective electronic equivalents of the HT740 and HT747. Torque converter ratios are 3.04 (TC470), 2.21 (TC495), 2.09 (TC499) and 1.83 (TC496, not available for HT747). I also want to know whether or not any buses that originally came with the MT644 or MT647 were changed to the HT747 or HT748 transmissions for durability reasons shortly after initial delivery or during their mid-life overhaul? So far I know a few buses had swapped out their MT644 or MT647 transmissions for HT747s or HT748s: Timmins Transit (Timmins, ON) 1982 Orion I 01.504 52-54 (54 was later resold to Ajax Transit as 2044) Mississauga Transit (Mississauga, ON) 1983 Orion I 01.504 8401-8404 Mississauga Transit 1984 Orion I 01.506 8405-8416 (8416 was resold to the National Research Council shortly after initial delivery) Mississauga Transit 1985 Orion I 01.508 8501 (replaced 8416) Vaughan Transit (Vaughan, ON) (later York Region Transit) 1987 Orion I 01.502 517-518 (original 6V71N engines were also reportedly changed to 6V92TA MUI's) University Transit Service (Charlottesville, VA) 1983 Orion I 01.506 5139-5239 and 5337 ~Ben

Has anyone ever determined which torque converter ratio is best for each engine/transmission combo? Example 1: Allison AT545 transmission with TC290 (1.72:1) torque converter behind Cummins B5.9 190 hp (475 lb/ft) engine Example 2: Allison MT643 transmission with TC360 (2.86:1) torque converter behind Cummins B5.9 210 hp (485 lb/ft) engine Example 3: Allison MT644 transmission with TC494 (1.92:1) torque converter behind Detroit Diesel 6V92TA 253 hp (766 lb/ft) engine Axle torque ratings: calculated input torque to axle, determined by: T = maximum gross engine torque (lb/ft) N1 = lowest transmission forward gear ratio N2 = torque converter stall ratio (2.5 or specific value for automatics, or 1 for manuals) Say you have a Detroit Diesel 6V92TA MUI (mechanical unit injection) engine mated to an Allison MT644 automatic transmission, being used in an Orion I transit bus. Allison recommends the transmission not exceed more than 740 lb/ft of input torque from the engine for transit bus applications. Only two TC (torque converter) ratios were produced for the MT644: 1.92 (TC494) and 2.21 (TC495). As the 6V92TA 253 hp engine has 766 lb/ft of torque, the 1.92 TC ratio would be best suited to the transmission. Otherwise, the Allison HT747 transmission (for engines up to 345 hp/1100 lb/ft) is recommended. 766 x 3.58 x 1.92 = 5,265 lb/ft input torque to axle. If anyone knows what the recommended input torque to axle rating is for a Rockwell R143 series rear axle with 4.63 and 4.11 ratios, please try to answer. ~Ben

Has anyone here heard of the Allison MT654CR 5-speed close-ratio automatic transmission? I know it seemed most popular outside North America; many articulated buses in those countries used it including the Volvo B58 and B10M. Input power: 300 hp max Input torque: 950 lb/ft max Input speed: 1900-3000 rpm Transmission gear ratios: 1st: 4.17:1 2nd: 2.21:1 3rd: 1.66:1 4th: 1.27:1 5th: 1.0:1 Reverse: 10.76:1 Torque converter ratios: TC430: 3.59:1 (crawl ratio: 14.97:1) TC495: 2.21:1 (crawl ratio: 9.22:1) TC496: 1.83:1 (crawl ratio: 7.63:1) TC497: 2.7:1 (crawl ratio: 11.26:1) Crawl ratio: first gear ratio x torque converter ratio ~Ben

Is it true that the MCI MC5C only listed the Allison MT644 as the automatic transmission (for both the 6V71 and 8V71 Detroit Diesel engines), with the HT740 being a special order option not listed in dealer brochures? I know the MT644 certainly was the only automatic that could fit behind the 8V71 in the MC5C without extra clearance, since the 8V71 is a longer engine due to it having two more cylinders compared to the 6V71. The HT740 automatic transmission was both superior to and longer than the MT644, the "longer" part of the aspect being the reason I see all these posts (on Bus Talk and related websites) about the 8V71/HT740 not being compatible in the MC5C due to the overall length of the powertrain, but I did see posts about the 6V71/HT740 being a good fit for the MC5C. That bit being said, even the 6V92TA/HT740 combo, as used in the MC9 since 1981, could make a good-enough fit in the MC5C. Could anyone please shed some light as to whether or not both the MT644 and HT740 were cataloged in the official MC5C brochures? Thank you, Ben (ClassicBusFan81)