cp1969 said:
But the trend is toward fewer gears in Class 8 transmissions. Lots of 7 and 9 speeds; Mack used 5 speeds for years (may still for all I know). It had to do with the useable torque range of engines getting much wider. In the old days, with only a 200-300 rpm range of useful torque, many gears were required. With a wide torque range, you don't need all those gears.
We used to call them constant torque or high torque rise engines although I don't recall if that was proper.
I drove Mack's with the 237 and the 300HP ratings with the 5 speed. I also drove a Diamond Reo with a 270 Cummins set up the same way though that one had this wierd Spicer 7 speed in it. I also drove a Mack Cruiseliner with a six speed but the extra gear was an air switched deep reduction gear for off road use.
I do not recall the RPM's that these engines worked at but you could and did lug them down. You did have to get used to holding them in say fourth gear as you pulled a hill because fifth would be too tall.
The RPM's did not readily drop off so they were slower to shift. An engine brake was a godsend to shift on a hill. You could use it to drag the RPM's down or else you did not have time to shift and would end up more or less dead in the water.
Shifting these babies to some practice. That 7 speed Spicer I mentioned? I got "replaced" on that job and the first day the new driver took it out and promptly blew the tranny.
I ran that Cruiseliner two years throughout the northeast. With it's little 300, it was weenie by today's standards but overall, not bad to live with. Certainly, it was easier than say a 318 Detroit with a 13 speed that you never stopped shifting.
By the way, I also drove an R model Mack which is still on the road with a five speed but it was not the constant torque engine. It had an auxillary tranny with reverse, high and low range but low was only for off road. I only drove it a couple of times but I think it was rated around 300HP.
I think that one was a seventy something year. The truck is in use and somewhat restored actually. I have driven 5 and 4's as well as some old B models with twin sticks but this one was different for sure.
FWIW, the truck I drive daily is a KW T800 with a 465 Cat and a 8speed EF. Not my choice but I can pull most of the hills with just one drop and never more than two. With those wider ranges, you need to watch the torque load on the drive train.
Sorry to deviate on the main topic but I thought some readers might find this interesting.
Or driving those heavy kids to school.
