It'll take some getting used to on braking distance, but I've driven fairly heavy stuff before. My bus is about twice the max weight as my truck and trailer maxed out, but neither is a freight train's weight. I'll be careful about it, but I still want to stay as fast as possible so I'm not losing time or whatnot.

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I could show you some pictures of an accident that happened this fall in this area. No not a bus, tandem axle flat bed truck that probably was grossing 40 K. Brakes were in good condition from what I could see when I looked at it. Was on a grade with a driver that did not have much experience. He found out that going down hill with to much speed not geared correctly is not a good combination. Truck was totally destroyed. Very lucky he came out alive.

One has to ask is that time I think I am saving really worth it?

Tom how many times in your career has someone pulled out if front of you only to see them go out of sight under your hood and you know you are going to hit and there is nothing more you can do only to start to see them coming back in sight? Once was more than enough to make a believe out of me that a few extra minutes is time well spent.

 

The only thing is how OFTEN does that happen, though. I mean, I don't usually see people driving in front of trucks. Behind them, yes, I see that all the time. Another truck in front of a truck, or someone wedged between two trucks, sure. However, someone caped out in front of a truck when the truck isn't deliberately riding their *** is more of a rarity. My bus has loads of cameras all over it, and recordings are saved to the cloud as well as locally to an SD card. So, if someone tries to play the insurance card and I do nothing wrong, then they're caught on cam all around the bus.

If you're seeing something different, let me know. "Accidents" happen all the time all over the country, but true accidents are rare. Usually it's someone acting stupid. While I get cocky in my full size SUV, a 20,000lb bus is another story entirely. Acceleration and deceleration are always going to be slower with something that large. It's just a matter of what speed can I drive at that makes sense for the moment.

I have to admit though, with so many people saying not to do a 3.x axel, I'm probably going to hold off on it and just get the locking torque converter and compatible transmission (MT643) so I can make sure my transmission doesn't burn out. An axel change doesn't need to happen as quickly as a transmission change, as I'll pay the same amount no matter what on the axel. The engine replacement makes a transmission change less expensive, so that one I want done to save some money.

The axel costs about $2,000 + labor, so a budget of $3,000 is there. I do like the idea of doing a turbo, but I don't know enough about it yet to make an informed decision. Presumably that can happen at any time, too, but I'd like to have the engine upgraded right away if it's going to be better.

The thing that worries me about not swapping axels is that a 5.57 gets me about 40MPH top speed, and that's definitely not going to work. Doubling all of my trips is a huge deal breaker. So, if a 3.x isn't going to be the right way to go, I'm open to suggestions on a 4.x speed if that makes more sense. If I want to swap gears later, I could still do that with the right axel, and it'll only cost me about $600 in parts then, and some labor. (Couple hours?) So, maybe $1,000 out the door. However, I'd like to get the gears right the first time. While 85 is a nice to have that presumably isn't reasonable, I'm fine with 75. If I have to start with 60 for the right torque/speed sweet spot, I guess I can try it, but it's still not completely ideal. If that's all I physically CAN get out of it, then I'll need to make peace with it, but accelerating as slowly as a semi to get a higher cruising speed is fine with me. 40MPH is just too low, though.

Thoughts?

 

Here is another direction that you might look into. I have seen it done a couple of times with Alison transmissions although it is not normally done. Install a 2 speed axle under the rear. International Harvester before it became Navistar would build a truck just about anyway a customer wanted. One of the county road districts in the area purchased a truck sit up that way. When loaded they used low range and when running empty high. You are talking about swapping rear axles anyway doubt labor wise be much difference which one you put in other then the wiring to the front and getting the correct speedometer adapter so you would have a correct reading in both high and low ranges.

You will not be able to shift it like you do when using manual transmission. As I recall you had to be stopped and I think be in drive with the park brake applied to stop the drive line from turning so the axle would shift. So on flat lands use high range when in town or pulling a hill shift into low.

The possible advantage to that instead of an over drive unit is you may not have to redo drivelines. Depending on what you could locate it might be a little less costly also.

Tom also suggested a 2 speed quite awhile ago.

 

I just looked, and not only are those expensive, but the 5.75 is even lower than the speeds the 2 speeds provide. (I'm seeing 6.x to 8.x and 5.x to 6.x.)

https://www.ebay.com/p/1841721569

 

@ manwithnobrows, I've been following this post, and I do not want to be rude, but I think you need a wake up call.
This 7.3 engine just has not the power to take your bus to the by you desired speed and to hold it there, other then steep downhill!
Whatever combination of gearboxes and rear axles you hang behind it.
in the end at top speed it all has to be in a 1:1 gear, and the 7.3 can't cope with it.
Please just understand this and adjust your goals.

with the best intentions,

 

more times than i can count.
there are also the idiots that dont follow traffic signs. last spring i was in left turn lane to make the left. triaxle dump and 35 foot trailer behind me with 25 tons of machines on the deck going ot a police station/post office 3 hours away from the shop.
saw a car pull up in the right turn only lane, got along side me then stopped. as the light turned yellow on the cross street, the car moved forward in my blind spot. light turned green for me and i went. about 100 foot later i hear what sounds like a trailer brake locked up. then i see a gold fender moving out from in front of the driver side front of the truck. the dipstick made a left from the right turn only lane, and my passenger front bumper clipped the rear quarter panel sending the car sideways across the front of me. since i could not see the car i pushed it a good 150 foot down the road on its passenger side tires. all while the dipstick was flooring the throttle. driver side front tire was between my radiator and headlight spinning 100mph and left a skid mark across the front of the truck.
when i hit the brakes the car fell to all 4 tires and shot into oncoming traffic almost hitting another car head on, then through a red light going the wrong way on the wrong side of the road causing another accident, and took off down the highway. cops found it three days later all smashed up. it was reported stolen 12 hours AFTER the incident.

 

with your 5.57 rear axle ratio, if i remember correctly the highest gear you can put on that carrier is around a 4:56 or so.
to go to a 4.10 gear you would also need to change the carrier in the rear axle.
if you are seeing 40 mph with that ratio( which i doubt, it is probably closer to 60) going to a 4:56 gearset should see 75 mph top speed.
there are gear ratio calculators out there to take the guessing game out of the equation.
3000 rpm with a 38 inch diameter tire and 4.56 gears will give you 74.5 mph.

 

Thank you. I'll look for an axel in the low 4.x then. Should keep my torque up a bit, too. My current axel is obsolete, so unfortunately I can't just change the gears and need to replace the axel.

Alright, so aside from the underpowered 7.3 engine issues, does everyone think the following is a good game plan to maximize the bus torque and top speed?

1. 7.3 IDI no turbo (Maybe turbo some day.)
2. MT643 transmission with locking torque converter (Just so the transmission won't overheat and burn out. It'll have a temp gauge. It's more efficient than the AT545, but it still does 1:1 max. See above issues with Allison 1000 and higher series.)
3. New axel with gears in the low 4.x (4.1 - 4.5x range)

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Depends on your final goal. If spending lots of money, it is a perfect plan. If making a more bullet proof and faster bus I see very little return on investment.

I would ask myself how many miles will a typical AT545 go before needing replacement. Then I would ask myself how many miles do I plan on driving this bus every year. How is the present transmission acting and do I believe it needs replacement at this time. Then do the math and see how many miles you would have to drive to pay for that transmission swap. I doubt you are going to see very much fuel saving with the lock up converter. Then the big question you will have to ask is does it make any real since to pour money into this transmission rabbit hole.

Not sure why you are worried about the 545 over heating. I do not recall every having issues with them when I worked at the bus shop. Change the fluid and filter every 25000 miles and they went a long time. If you are worried about heating install and aux transmission cooler if it does not have one at this time. Some were equipped with an external oil filter which is a great investment if not all ready on the bus.

Again on paper it looks like dropping lower in numbers for your gear set in the rear end is the answer. I think in real life you are going to find out there are not enough ponies in the stable to make any real difference.

It seems that you are very determined to find out so I would suggest that you spend the money and then report back to us what you have learned.

 

My mechanic is getting a price quote on a turbo kit. I'll probably do that and skip the axel. If I still want to change that later, I'll do so. Probably best to maximize the input first and see what happens, as that seems to be the biggest issue from what I'm hearing.

 

Things to think about.

If you add more fuel to the combustion process what do you get? HEAT. What do you need to do when adding a turbo to an engine? Turn up the fuel which makes more heat.
I have no idea on this series of engines and I am not going to bother looking so I will speak about engines I am familiar with. The DT300 and 400 series of engines when the turbo was added they also added more piston oil coolers, usually had larger radiators, depending on application more fan blades all to control the increased heat that the turbo added to the engine.

These people who add turbos to pick-ups can get away with that up to a point as for the most part they are only loading the engine to the max for shorter periods of time. I get the impression you are wanting to get behind the wheel, put it in gear, pedal to the metal and go as fast as the bus is capable of going and do that for hours on end. In other words 90 percent load or better for hours on in.

The couple of buses I was around that had the 6.9's in them did OK because of the way a bus is used. Mostly stop and go driving and seldom speeds over 50 or so. Yes the were some what slow getting up to speed but in that application who cares? Point is the engine had plenty of times to cool back down as they were no where close to full power for hours on end. Average out I doubt they were doing much of 50 percent full power application.

You are asking a fair amount out of a 6.9-7.3 to begin with to move 22K and they did quite well at it if you did everything correctly. They do even better in a pick-up application where usually they are running in the 30 percent power range. Turn that up into the 90 percent range, well something you will have to determine.

Just some thoughts for you to work through as you are reengineering your project. Have fun.

 

I ordered the MT643 today to add locking.

On the 7.3 heat issue, let's see what the kit includes. I doubt it'll have a new radiator, but maybe it'll have some cooling tech for it.

I'm also going to look into it I have an oil cooler for the MT643, or what it'll cost to add that. Not sure if it matters, and I do have a temperature gauge so I can see if we do get close to hot. If it's not cost prohibitive, I'll grab one.

EDIT:
My bus already has a transmission cooler, so no need there. My mechanic also says there's no engine heat increase we need to handle with the turbo kit.

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with a turbo it is very important to add an exhaust temperature gauge.
and keep exhaust temps below 1,100 degrees at all times.

 

It comes with one. https://bankspower.com/products/turb...6-transmission

EDIT:
The Banks kit they said wouldn't work. I went with the S-Series Hypermax turbo kit which is designed for bus. I have an automatic transmission, so I got the automatic transmission version.

https://www.gohypermax.com/ProductDi...7-0e06877bb507

For those of you following along to upgrade your 94 international 3700 skoolie, the I got a panel Mount from Auto Meter along with an EGT, boost, and transmission temp gauge. Should help me understand more of the bus' dynamics as I drive.

Also, as I understand it, if it's too high on the EGT or transmission, I just need to let off the throttle. On a hill, that might be difficult if it's really high up. (Ex. Mountains) I figure if it's possible to pull off to the side of the road, I'll do that for a moment. The problem then becomes how much of a problem it'll be to move the 20,000 pound bus up the incline as it gets going again.

Any ideas on how to handle this aside from "just avoid mountains"?

EDIT 2:
The MT643 does not need a SAE 2 to SAE 3 adapter to couple with the 7.3L IDI. I had to return mine. Plus, it already had one to couple it with the AT545 that was on there.

The Hypermax team is really nice and goes the extra mile for people. (No pun intended,) It might need a little work to get it to fit on the engine while in the engine bay.

If you buy a Jasper reman 7.3L IDI, it comes with a water pump. You don't need to buy one separate. I had to return mine as well.

I'll let you know more as the project goes on.

EDIT 3:
My mechanic bailed on the transmission swap, as we couldn't locate the needed flexplate. It might not even exist anymore if it even did. So we kept it at the AT545. If we need to change it later, I'll get another mechanic to install one. My biggest concern is if going uphill will be an issue or not.