Join Date: Aug 2009
Engine: international DTA360
Rated Cap: 72
Re: 1986 International School Bus Need More Speed
I would choose, first, a different axle ratio- 5.57 is very slow. Call a used truck parts place with your axle info, they can probably tell you what they can get in either a third member ( pumpkin, center section, 'chunk' whatgever you want to call it..) I paid $450 for one with a 4.10 ratio. Wish I had picked a 3.91, but will achieve the same thing with larger tires. Not mechanically difficult to change, but it is heavy and awkward. U-joint type and axle spline type must match, also- they vary within a given axle type, and changing either requires more knowledge than installing the differential does.
Secondly, you could install an overdrive transmission. Most automatics and manuals have a 1:1 top gear; the driveshaft spins at engine speed in top gear. Some have an overdrive, and spin the driveshaft faster than the engine. in older used stuff that you're likely to be looking at, I think you'll be limited to a few manuals if you want this.
lastly, bigger tires- in your pictures, it looks like you may have 10r22.5 tires, which might be about 39" dia. 12r22.5 tires are made, I think they're about 43" od. The top speed gain (at a given engine speed) is exactly proportional to the diameter change. (or radius, or circumference change, whatever you care to use)
depending on your mechanical ability, none are really cheap or easy, but well worth it, if you're going to drive very far. My mileage improvement paid for my trans and axle swap in two 8500 mile trips, plus it is much more pleasant to drive, and faster. top speed went from 58 to about 85, but I go 65 for mileage.
Something I calculated, but wasn't sure of until I saw it, is that the bus pulls hills better with the faster gearing. This will vary depending on the torque curve of your engine and what changes you actually make, but your engine makes very little torque or power near it's max speed. My engine makes max torque at about 1700 or 1800 rpm, more than double what is available at 2900. While climbing a hill, the allison auto would downshift at 2300, then be unable to hold 2900 in 3rd, and I'd end up at 2300 or so in 3rd gear. Now, on a similiar hill, speed may stabilize at 1900 engine rpm in top gear ( I now have a 7 spd manual), giving me more speed and better mileage.
People will disagree, insisting that lower gearing automatically gives better hill climbing, but do the math- the 'thrust' at the axle at any given road speed can increase with faster gearing, especially when you're starting point is running an engine at close to double the speed of it's torque peak.
Turning up your governor will make the above situation worse. You'd have to turn it up a LOT, also. If your top speed is now 55 at 3000 rpm, 4090 rpm would get you 75 mph. You might get away with a little, but very much will be very bad, and I bet in any case, your performance in the slightest hill or headwind won't be very good, and either will mileage.
A slower axle ratio (higher numerically) will, in every case, be better for accelerating from a stop, uphill, etc. Your 6.9 might be only 125 to 150 hp, and not that much torque, so might require a slow ratio to perform acceptably in these situations. and might simply not be able to make a bus go 75 mph without a tailwind.