A (Long) Introduction to Bus Engines, Lengths, Configurations & Biodiesel

There are so many choices out there, so pick the bus that’s best for you!

There are so many choices out there, so pick the bus that’s best for you!

There are many sizes of buses to choose from and each size fits a different need. For me, with my intent to make this my full time residence, I wanted to make sure that I had enough space for me and at least one other person (and my cat). I wanted it to also have headroom and be capable of towing my car. This eliminated all but the 12-window and 14-window carriage. The towing portion led me to search for the largest engine that I could find standard as well so I wouldn't have to worry about weight. Weight is one of those considerations in RVs that leads to compromises in construction and/or materials. I didn't want to make compromises in a structure that I'll be living in that will also hurtle down the highway at 65mph. One other consideration worth making when you're purchasing your bus is that you'll want to try to get one from before 2004 when new environmental legislation required computer systems to maintain/monitor diesel emissions. This can not only hamper the performance of the bus, but can make it very difficult to repair yourself or relatively cheaply at a mechanic.

I've got a 7.3L International Turbo-charged Diesel Allison with a DT466 all-mechanical transmission. This is possibly the most produced and most durable diesel engine in existence. Every workshop has parts on hand and every mechanic trains on this engine. I spent a lot of time digging through school bus driver forums and diesel truck forums and settled on what worked best for me. My advice for most potential bus purchasers, however, is to avoid any standard gasoline short buses. You’ll find they are severely underpowered and they guzzle gas. Unless you’re just getting a weekend warrior, you’ll wish you’d have gotten a bigger bus with a bigger engine.

It’s not the length that matters... Well actually... in this case it does!

Short Bus

Mid-Length (8-10 window)

Long (12 window)

Full Size (14 window)

Typically you can gauge the length of a bus based on the number of windows on a side. The 14-window bus usually comes in at 40ft, which is the legal maximum on length for a bus. At 8ft (and maybe an inch or two) wide, that would give me roughly 320 sq ft of space! BUT not all 14 window buses are created equally! There are three main variations of the full size bus and each of them serves to benefit their owner in different ways.

1. The Dog-Nose Bus

The dog-nose bus is named so because it looks like a dog's nose from the front with the mirrors as ears. The dog-nose bus is probably the most ubiquitous of all models, and for good reason! School buses are workhorse fleet vehicles. They require regular maintenance and easy access to internal components saves time/money. The space inside the bus is also maximized to some extent and they're easier to evacuate if necessary. Dog nose buses are cheaper to maintain if you're a skoolie converter, and what space you do have inside is able to used without engine noise and heat. The pilot's seat and the stairs prevent full use of the space, however. The biggest downside to a dog-nose for me was the inability to use that front 5ft of space because though it is only the engine there and it is outside, but more than anything it means that you're already starting your tiny house conversion with an even tinier 35ft! Because of the location of the turning wheels in front of the driver, the wheel base for a dog-nose bus is spaced further, which means that the turning radius suffers a noticeable amount. This can be frustrating when you inevitably make a wrong turn or need to get out of a tight campground pull-up.

2. The Transit Bus (Puller)

The Transit bus is a flat faced brick that has variation in engine location, so for this first section I'll cover the "puller". The "puller" is named so because it has the engine up front and it transfers power to the dual-tire drive wheels in the rear. For my build, a puller was the best option. Sure the cockpit is noisy and hot when I'm driving, but I'm able to have a blank canvas throughout the rest of the bus carriage. Transit buses can use the full 40 ft of length, unlike a dog-nose bus. The front 5ft of space is used for the stairs, the engine, and the cockpit. The space above can be used for shelving, storage, and other creature comforts for the driver. While this does make maintenance a little harder than for a dog-nose bus, the engine is still largely accessible via a dog-house cover, which is an insulated plastic dome that surround the engine. The wheels in a transit bus are located behind the driver and as a result, the wheel-base is spaced closer and the turning radius is surprisingly concise. Of course, it does take some adjusting to make delayed turns (I discussed this briefly in an earlier post).

This is a puller, which you can tell by the radiator grill at the front.

This is a puller, which you can tell by the radiator grill at the front.

This is the “doghouse,” under which the engine lives. I wish I could find another one that fits in this space, but for heat and noise reduction, I’m hesitant to make a DIY cover.

This is the “doghouse,” under which the engine lives. I wish I could find another one that fits in this space, but for heat and noise reduction, I’m hesitant to make a DIY cover.

3. The Transit Bus (Pusher)

These are “pusher” buses, which you can tell because of the lack of a grill in the front of the bus.

These are “pusher” buses, which you can tell because of the lack of a grill in the front of the bus.

Transit pushers are nearly identical to their puller counterparts, but they do differ in a few potentially impactful ways. The transit pusher has an engine located in the back of the bus. This is a cramped space that gets hot and dirty really quickly. Mechanics will have a tough time getting access to parts and DIY work means extra effort. Having the engine at the back means that the front of the bus is free from heat, noise, and a doghouse cover. With a pusher, the back of the bus, which I had planned to be my bedroom, would no longer have the back wall of windows, which I think is critical to the image of a schoolbus. The pusher also usually manifests as an interior wall, so you'd lose roughly 2-4ft based on how much the engine sticks into the carriage. When driving, the back of the bus is going to be very loud and very hot. If you're going to be driving a lot with someone sleeping in the back, this is not going to be a suitable setup. I can't verify whether it is true or not, but a lot of bus aficionados claim that a pusher is ideal if you're going to be towing a heavy load due to torque strain and weight balancing with the engine in the middle. I don't know if I buy into this line of thinking because it's a medium-duty truck. Trucks like these have been towing things far heavier than cars for decades, and if there were any problems, they've likely been solved in the decades since.

Regarding the biodiesel question…

While I mentioned engine/length specs above, I actually saw a question posted online regarding biodiesel. The gist of the question was:

Should I do a biodiesel conversion? Is it worth it? I’m willing to do it to save money in the long run.

The answer from a mechanical standpoint isn't very difficult: You should do it if you want to. Most people, however, aren't interested in it solely from an engineering perspective. There are other factors, such as being eco-friendly and saving money.

Yes, biodiesel is more eco-friendly and produces fewer CO2 emissions than normal diesel. The biggest challenge with biodiesel is where to find it. At this point, nearly all of the frying oil suppliers to restaurants buy back the used fry oil under contract with the restaurants. McDonalds, Burger King, etc are going to make money off of it instead of just handing it to the hippie driving by in their eco-friendly school bus. Sure you might be able to find some every once in a while from a mom and pop restaurant, but they're few and far between. Filtering the biodiesel and prepping it for use can also be a bit of a chore and requires space that most won't have inside a skoolie. Once converted, a school bus can run off of standard diesel and biodiesel, but bear in mind that as of March 2019, the price of biodiesel is $.20 more per gallon than standard diesel. If you're on a quest to save the earth, I wish you the best of luck, really! From a practicality standpoint, I'm just not sure that the effort will produce the savings I'd want to get from the investment.