Change Transmission?

[ben384608]

I have an 88 suplerliner with a 350 mack and 9 speed trans. I had the fuel turned up and the truck runs much better. My biggest gripe is that whenever I have to climb a hill (not just big ones) the truck dies off. If I changed my trans from a 9 to a 12 or 13 would it make much of a difference? I know I won't have more power but it just seems like a few more gears would be nice. Just a rookie and any help greatly appreciated.

[theakerstwo]

That would help to get a closer ratio top gear but only if you pull your injection pump and have recalibrated. I just went thru this. The E6 350 is some what a maxidyn. Meaning it is a hi torque rise governor which it in a stock setting pulls better at 1400 rpm or has more torque at 1400 than at a 1800 or 1900 rpm range.So if you take your pump in and tell the man to cut the torque rise down so you move the torque range up higher to 2100 than at 1400. This way higher the rpm the more torqe you have and on a 13 speed trans you dont work it at 14400 but close to 2000 or 2100. Hope this makes sence and if some one knows better feel free to make a statement cause i have not stop learning. I did my pump that way and it pulls better at top rpm than when set stock.

I cant say that you would get from point a to point b any faster but with the 9 speed with the .71 o/d top gear you should be running at a 1500 to 1600 rpm range more in a day than at 2100 rpm so that way you need a hi/torque rice governor or other wise you would not have enough power at mid range.But that works good for a cross countery operation but for a in town like a dump truck you need a low torque rice pump set close to 2100 and a close ratio trans like a 13 speed.This way your rpm is and can be run closer at 2100 more of the time.

[Lmackattack]

a 350 with a 9 was a good combo for over the road truck. THe 12 speed was both on and off road and had all the gears that you wanted when loaded. Bonus was you could drive it like a 5 speed empty, you had 5 reverse gears, a extream low low. However there may be an issue with your current Rear end ratio. you may need a faster ratio to go with the slower ratios of the 12 speed?

the 9 speed has a fast OD and if I recall the rear ends used with it were commonly in the 4.42-5.02 range. With a 12 speed you would want something in the 3.87-4.42 range to get the top end speed and good pulling RPM. All of this needs to be calculated to see what your RPM split is and what the top end will be.

[Mack Doctor Jr.]

I understand what u all are saying and have very good points. But I have also had this experience. One other option is running a low pro tire. We tried this on our truck and it worked great. We lost 7 mph on top speed but it did pull a lot better. I do know u just cant change tires unless they are wore out too.. LOL Its just something to think about.