| Author | Message | ||
| Peter Broadribb (Madbrit) (208.186.36.187)
Rating: N/A |
I been told of an MC-7 bus with a 6 speed Allison behind a 450+ hp 8V92. The shifter has the following pattern on it: 1-2,3-4,3-5,3-6,N,R. Anyone have any imput on this tranny? I am assuming that it only uses the 1-2 as a crawler and that possibly 2 is lock-up? I looked on the Allison website and found nothing about it. Thanks. Peter. | ||
| Mark R. Obtinario (Cowlitzcoach) (204.245.250.157)
Rating: N/A |
The HT-70 was the transmission used before Allison developed the HT-700 series of transmissions. I think the reason they stopped making the HT-70 is it never wore out or broke unless you really abused them. The HT-70 was a much larger version of the old Powerglide/Jetaway transmissions GM used in their cars and light trucks. Where the Powerglide had one set of sun and planetary gears and the Jetaway had two sets, the HT-70 had three sets. One problem I have found with them is there is no gear selector posistion to lock the tranmission in the lowest gear. Most of the time the bus starts in 2nd and almost never uses 1st. When you need 1st, you have to feather the throttle to keep it down in the low hole. Like the Jetaway, the transmission is very smooooooth shifting. Unless you are listening to the RPM's change you can hardly feel the shifts. It isn't surprising you didn't find any information on the Allison website on the HT-70 since it has been almost 40 years since they stopped making that transmission. If you go to an Allison dealer I would be surprised if you could find anyone that knows anything about them any longer. I think the only reason Allison stopped making the HT-70 was they never made any more money once the transmission left the factory. Would I retrofit an HT-70 into a bus? Probably, if I knew the history of the transmission. If I didn't know the history I would be a little cautious since parts and service are getting into the collector car parts price category. Mark O. | ||
| Peter Broadribb (Madbrit) (208.186.36.187)
Rating: N/A |
Mark, Thanks for the very detailed report on the tranny. Does one normally just start in the 3-6 and drive it like an auto and only use the lower selections if traffic requires it and only use the 1-2 if starting up a hill or with a heavy load on? Is this an actual 6 speed or is one of them the lockup convertor, does it even have a lockup or is it too old for that? Thanks. Peter. | ||
| Henry R. Bergman, Jr. (Henryofcj) (63.224.197.10)
Rating: N/A |
If memory serves, all 6 gears were forward gears. I DO NOT remember if this particular 6 speed had a lock up clutch. 6th was an overdrive. Yeah, one usually left the shifter in the 3-6 position and just pushed on the throttle. If you wanted some more engine braking you... ...just used the 3-5 position and so on. If you were pulling out of a hole loaded, you used the 1-2 position which only gave you the.... ....first 2 gears which were kinda low and slow, then you shifted up to the 3-4 to 3-6 position. Does the 8V92 have a Jake? If it does, then your 6 speed Allision has a lock up clutch. If the Detroit does not have a Jake.... ....then we are not sure either way. I remember coming down some very steep grades without a Jake, instead using the mill instead locked in a lower gear. These older Allisons were very large and heavy. With a proper oil cooler, they last forever. Do NOT know either if the 8V92 is too much mill for this box. Good luck. Henry of CJ | ||
| Peter Broadribb (Madbrit) (208.186.36.187)
Rating: N/A |
Thanks Henry, The motor has a Jake model #71 and a Jacket Water Aftercooled (JWAC) cooling system, whatever that is? Peter. | ||
| Henry R. Bergman, Jr. (Henryofcj) (63.224.197.10)
Rating: N/A |
Hello Peter; I am soosssss old that in high school they actually still taught industrial drawing using pencils and tee squares and stuff. As a project we drew a cut-a-way shaded view of the then very new Allison 6-speed which then was way cool. Remember also drawing the lock up clutch in the torque convertor of the tranny. Now I do not remember the... ....particular model of 6 speed this was. They made a MT verson and a HT verson. If the 8V92T Detroit's Jake works, then you PROBABLY have a functional lock up clutch in your 6 speed tranny. NOT sure if a water to oil tranny cooler will be enough cooling for your tranny. Maybe a dedicated oil radiator could be a good idea. Bet your potential bus really hauls. Good luck. Henry of CJ | ||
| Henry R. Bergman, Jr. (Henryofcj) (63.224.197.10)
Rating: N/A |
Also I think now that the JWAC dohickie thing may be the turbo aftercooler and that your mill may not have a radiator mounted air to air turbo intercooler. Sneak around the bus's rear end and sees if there is some sort of oil radiator dedicated to just the tranny. Bet there may be. Also wonder just how... ....fast the coach will go with that overdrive 6th gear? He he he.  Henry of CJ | ||
| Ron Womack (213.78.98.81) Rating: N/A |
I have that same tranny in a 4501 with standard 8v71 and Henry is right it will haul. I have no trouble cruising at 75-80. The only problem I have had is the kick down is to slow so I usually force shift by dropping one notch on the shifter before I lose rpm on a grade. | ||
| William R. Graf (Billfrombrazil) (206.158.10.42)
Rating: N/A |
Our Crown also has the HT-70 and I have a complete manual on it, allthough I haven't had to refer to it except for a shift link adjustment just after we got the bus. I have the same experience, runs up to 75 or 80 MPH but does pull down to very low RPMs for a downshift so usually shift through the gears manually when in the mountains. That shift link adjustment is important to get it to shift at the right points. The tranny is equipped with a retarder which can be used on the long downgrades or in traffic also but you must watch the transmission oil temperature closely because it heats up very quickly. I have a cooler integral with my radiator which seems to be sufficient in all situations for our bus. We are grossing a little over 35,000 pounds with the toad on behind with a Cummins 250 and everything runs cool, calm and collected over the 6% + grades. I leave the tranny in one of the lower gears in traffic, as suggested in the manual, and use engine to slow down as well as avoiding the up and down shifting. Have fun with it. Bill | ||
| Mark R. Obtinario (Cowlitzcoach) (204.245.228.228)
Rating: N/A |
My HT-70 is in a 40' 10-wheeler Crown with the Cummins 262. It shifts too quickly up and too slowly down for my taste. If you leave it in the top hole it will shift into the top hole by 30 MPH. It won't downshift until about that speed as well. At 30-35 MPH, every time you let up on the throttle it will shift up to 6th and every time you step on the throttle it will shift down to 5th or 4th. It is most annoying! I always start in the 3-4 hole and then hand shift to the 3-5 about 35-40 MPH. I shift into the 3-6 hole about 50 MPH. The 1-2 hole is good for almost 30 MPH. With 11X24.5 tires, the top hole is good for almost 80 MPH @ 2150 RPM. My cooler is plumbed through the radiator and I have never had problems with overheating. I think it has a lot to do with the fact the Crown engineers knew most of their coaches were going to be working hard in hot places. The pyrometer goes up into the red zone long before the water gets too hot. The only time it started to be a problem was going up the grade from the Columbia River towards Goldendale, WA on a 105 degree day. 10 miles @ more than 7.5+% grade with a full load is tough on any vehicle. I made it all the way in the 3-4 hole at less then 190 degree water, 900 degree pyrometer, and the trans temp in the green. Even going down the same hill with the same load on an equally hot day the trans temp didn't even approach the yellow zone using the retarder. Mark O. |