Engines & Transmissions

First off, let's just get this out of the way.

Don't get a gas engine unless you have some really crazy excuse.

Like the bus belonged to your grandpa and you've got a sentimental attachment to it. Or it's a one-of-a-kind bus and you fell in love with this particular one. Other than that, go with diesel. End of story. 

Diesel will get better mpg (usually between 7-12mpg vs 4-8mpg on a gasser) - that means that every 1,000 miles you drive will cost an extra $500-ish. That adds up quickly. Diesels will usually last longer as well so buying a properly maintained diesel with 200k miles on it shouldn't concern you as much as buying a properly maintained gas bus with the same mileage. For those reasons, we won’t go into the details on gas engines, but will focus on diesels. Not that they can't be fine engines, but in the used market a diesel isn't that hard to find, nor much more expensive.

If you’re mechanically inclined, and want to do your own work on it, you might want to try to locate an older pre-electronics engine—that’ll be easier to work on and parts will be cheaper overall. But those are getting harder to find (anything 20yrs old or less is electronic). And the newer engines are cleaner, more efficient, and if you’ll be having a shop do the maintenance on it anyway,  the higher cost of repairs probably won't be that noticeable. 

First two disclaimers:

1. The condition and maintenance of the engine is probably more important than WHICH engine you get. 

2. I don't have personal experience with any of these engines except the Detroit Diesel 6v92 (I'm not a bus mechanic). So this info is just gleaned form forums and online resources. Feel free to correct me where I'm wrong!

ENGINES

Okay, let's jump into some of the common engines you'll run into as you look for a bus:

Cummins 8.3

  • C8.3 first introduced in 1985 / electronic version (C8.3E) introduced in 1996

  • HP: 210 to 300hp

This and the Navistar DT466 seem to be the most desired engines in school buses (especially configured with the higher power rating which has a lot of torque.) Easy to find someone to work on it, easy to find parts, and the cost of parts isn't ridiculous (especially the non-electronic version). Doesn't seem to be very common in FE buses.

Cummins 5.9

  • 6BT 5.9 FIRST INTRODUCED IN 1989 / ELECTRONIC VERSION (ISB 5.9) INTRODUCED IN 1998

  • HP: 160 TO 180 (1989-1998)

  • HP: 215-325HP  (1998-2007)

Most school buses with this engine seem to be in the lower horsepower rating—meaning this would be a somewhat underpowered engine for a full-sized bus. That being said, if you're not planning on doing a lot of driving, this might be fine for you. VERY common to find this engine in buses, which means parts are plentiful (as are mechanics familiar with it). A bit better fuel economy than the 8.3 liter Cummins.

Navistar DT466

  • FIRST INTRODUCED IN 1984 / ELECTRONIC VERSION (DT466E) INTRODUCED IN 1994

  • HP: 165 TO 270HP  (190 & 210 BEING THE MOST COMMON)/ TORQUE: 500-860LBF

This engine has been around a long time, and has a reputation as a rock-solid engine, relatively inexpensive to maintain. One of the most desirable engines you can commonly find, especially if it's configured for the higher power levels.

Navistar T444e

  • 1994-2004

  • HP: 210-275

An okay engine, but most people would choose something else if they had the choice.

Caterpillar Engines

Although some people absolutely love these engines, a Cat engine would probably be my last choice—not because it’s not a good engine, but because I hear a lot of reports of them being more expensive to repair, and really needing to be properly maintained (so if you don’t have any maintenance records on them, be careful). That being said, a well-maintained Cat can be a great engine, so don’t be too afraid of it!

Detroit Diesel (6v92, 6v71, etc)

If you’re purchasing an older bus (pre 1993), you might see some Detroit Diesel 2-strokes in them. These can be fantastic engines, just realize that finding mechanics to work on them can be tricky. These engines were in highway buses for decades, and when properly maintained, will probably outlast any of the other engines on this list (common to see these with 500,000+ miles on them). Read up on 2-stroke Detroit Diesels a bit before you get one of these, and make sure the pro/con list is worth it to you. 

There's a cult following though, so if you do find yourself with one of these engines, join one of the Facebook groups or forums, and you'll have plenty of info at your fingertips.

TRANSMISSIONS

Before we start into transmissions, I want to clarify a common misconception. The transmission is not the main deciding factor on what the top speed of your bus will be. It's the rear differential (although the transmission does play a role). So if you're trying to get good highway speeds, make sure to read the section below on REAR DIFFERENTIAL RATIOS. Okay, back to transmissions.

It’s becoming more and more rare to find a manual transmission in a school bus, unless you’re purchasing an older bus, in which case you're not going to have the option to be very picky about which transmission you get anyway. The manual 5 or 10 speed Eaton Road Ranger or Fuller transmissions are great (although if you have a non-synchromesh tranny it might take you some practice to get good at driving it). On the newer buses, transmissions are almost all automatics where pretty much the only name in the game is Allison.

Allison AT545

This is an automatic 4 speed transmission. The base model, and very common.  Not the greatest tranny for a full-sized bus, but probably fine in short buses with a smaller engine (like the Cummins 5.9). This is one of the only transmissions that *might* be a deal-breaker for you when looking at a bus, since it doesn't have a lock-up torque converter in any gears, which means it's a different animal from the other trannys—especially on mountain driving where it'll generate more heat due to slipping. 

Allison MT643

A medium duty automatic 4 speed, but with lock-up torque converter in the 3rd and 4th gears, making it slightly more efficient, and maybe more importantly reduces the amount of heat generated, resulting in a more reliable transmission.This is a heavier duty tranny than the AT545.

Allison MD3060

I'll list this one on it's own since it seems to be one of the most common of the 3000 series. This automatic tranny is usually setup as a 5 speed (1 overdrive gear), but it actually has 2 overdrive gears (making it a 6 speed). You'll have to have an Allison dealer unlock the 6th gear if yours doesn't have it unlocked. Usually you can only get it unlocked for a RE bus. But even with just the single overdrive, this is a good tranny.

Allison 1000, 2000, 3000 series

These are all 5/6 speed overdrive automatics, and good transmissions. The difference between them is the weight/hp limits. If your bus/engine combo came with it from the factory, you'll be fine.

Allison HT740

This is a heavy-duty automatic 4 speed transmission, not common on school buses, but older Crowns or Gilligs will have it, It has lock-up in all gears, and is a very reliable transmission.

REAR DIFFERENTIAL RATIOS

Since buses are often configured for lower speed driving (stop-and-go in-town driving) if you want to make sure you've got one that's ideal for highway driving you'll want to check the rear end ratio (it's not just the transmission that makes it a good highway rig). 

The lower the rear-end ratio, the higher speeds you'll be able to attain (and the lower the amount of power you'll have at low speeds). Common ratio's are 3.73, 4.10, 4.33, 4.44, 4.56, 4.75, 5.57, and 6.5. If you've got 4.33 or lower rear-end ratio gears, you've got a good highway machine. 5.57... not so much, but if you're not in a hurry to get anywhere it's not a big deal.