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Old 08-05-2020, 11:03 PM   #1
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Can you make a 7.3 IDI highway worthy

I just bought a bus at auction online due to a bit too much bourbon and a Pinterest pipe dream of traveling the world with my kiddo. Well the bus is in Georgia and Iím in Michigan. Another awesome member here on the forum has already retrieved the bus for me and parkex it at a nearby storage yard. Itís a 91 Int. 3700. 7.3 IDI and Allison mt643. I know itís not overdrive and itís not turbo. Brad (other member) told me it pegged out hard at about 55 mph on a flat surface. I gotta get this hoss back to Michigan (unless anyone in Georgia wants to buy my regret mobile from me) and am curious about what mods or improvements could be made to increase the top speed closer to highway speeds. We have time to get things done so let me know your thoughts. Thanks!Adjustments.jpg
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Old 08-06-2020, 12:09 AM   #2
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There might be a road speed governor setting the top speed. If you have a tachometer, what does it read at top speed. If you're going 55 mph and the engine rpm is less than the rated maximum engine rpm you get some more top speed by adjustments.

It doesn't make sense that they would spec a mt643 behind a non-turbo'd 7.3 that is limited to 55 mph. I've seen mt643's behind dt360's, but they at least had turbo's.
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Old 08-06-2020, 08:15 AM   #3
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Well, no offense, but as you said, you're the typical pipedream buyer. You jumped in with both feet not really knowing what you wanted or were buying at the time.

So let us help you out.

The first recommended step in becoming a skoolie is to lay out what your plans are for the bus. People buy buses for all sorts of different reasons. Some of us buy 'em, park 'em, and then live out of 'em. Some of us live out of them but also travel the states year round and are either retired, work remotely, or work seasonally. Some of us, like me, use the bus as a steel tent and camp with it from time to time. Some people use it as a cheap mode of transportation or hauling friends. And some just downright love school buses and keep it just the way the district had it.

So, what are your plans for this?

Once we figure out your plans, then we can decide the best course of action for you and your new purchase.
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Old 08-06-2020, 09:13 AM   #4
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If you're set on using this bus, there are things you can do to make it more highway friendly.

However, the very first thing I would do is a top to bottom inspection on it to make sure everything is in good shape and roadworthy. You're dealing with something that is going on 30 years old, and there's a good chance you'll find stuff broken and wore out.

After you do that, you can then look at upgrades.

Follow what bus_bro said and see if your bus is restricted to 55 by a speed limiter or if it's gear bound. If it's got a speed limiter, you can typically remove some pieces internally to make it non-functional. If it's gear bound, you'll have to swap rear end gears to get more speed, or upgrade to a transmission with overdrive gearing.

Once you get the ability to go faster, you'll need the horsepower to do so.

Hypermax makes a turbo kit for the s-series internationals. It will likely fit your 3700 without much of an issue. Doing that will get you a little power and then allow you to turn the fuel screw up and gain a little more hp on top of adding a turbo. There are also hotter injection pumps being made that can give you 300+ hp out of an idi, but you'll have to upgrade your head bolts to arp studs and also install an intercooler of some sort. Longevity with those hotter pumps isn't well known, so purchase one at your own risk.

Being that you have an mt643(you have inspected it and are sure on that right) you're ahead of the game compared to most that came with the at545.

I have an idi in my bus, and it runs great with no issues. I can do about 70 on flatground with my foot to the floor, but cruising at 55 is much easier on everything, me included. Something I encourage everybody that travels at interstate speeds is to make sure that their bus is capable of stopping from interstate speeds. These older buses weren't designed at a time of 70+mph speed limits. Most folks will do the upgrades to get it to go 70+ but forget about doing the upgrades to get it to stop from 70+, which IMO is more important.

There were a few others on here with full size buses and the idi engine. One of them had a 6 speed manual and a 4 speed auxiliary transmission. He was also turbo'd and intercooled and was pretty satisfied with his end result.

But, I should mention that the money and time he spent creating what he had, he could have put into the initial purchase price and ended up with a top of the line model that came from the factory with big hp, big brakes, air ride suspension, etc.
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Old 08-06-2020, 03:25 PM   #5
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Yea Iíll just sell it.
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Old 08-06-2020, 04:55 PM   #6
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All it needs is regearing.
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Old 08-06-2020, 06:52 PM   #7
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All it needs is regearing.

Wanna buy it?
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Old 08-06-2020, 08:18 PM   #8
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Wanna buy it?
nope. But top speed isn't a product of what engine a bus has. Its the gearing. Since school buses are mostly for stop and go that's what most of em are geared for.

My bus only has 190 hp and it will do 80+. I've had a 175hp 444E with a 545 that would do 75.
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Old 08-06-2020, 11:53 PM   #9
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Being the one who picked up and drove this bus and went around 40 miles with it (8 on the freeway), it is *NOT* electronically governed. It's not even turbocharged, it's a naturally aspirated 7.3 IDI. I haven't checked the trans yet but I did hear the faint "transmission whine" the AT545's are well known for in first gear so that's what I'm reasonably sure it has. The problem with this bus isn't just the gearing, it's also that the engine lacks the horsepower to even maintain 55 on even a light uphill slope. I did manage to get it up to 65 on a downhill grade. To answer the questions above, it was turning around 2800 RPM at 55 on flat ground.


The odometer is showing 107K miles and while I'm fully aware the instrument clusters are very common replacement items on these buses, I noticed the gauge needles were a very faded orange on this one, which might indicate it's never been replaced. Perhaps it ran a route for the first 10 years and was a spare bus after that, being driven only occasionally. The paint on the hood is in excellent condition, which surprised me since IH's very commonly have clear coat peeling, there are clues to suggest it's been repainted not too long ago (or replaced, given the LED lights).



It was inspected very recently in June, the sticker dated "Jun 2021" (that's when it expires, the next inspection is due). I found no mechanical problems, tires had sufficient tread (haven't measured but I'd estimate about half worn). I didn't check the lights but I expect they all work (it should not have passed an inspection if they didn't). Coolant temp ran in the 180-190 range, briefly rising to around 195-200 on a hill, the fan engaging shortly after and bringing the temp back down as it should. Oil pressure was around 30 PSI while driving.


As far as I could tell, there is *NO* plywood under the rubber floor, it sounded like steel under the rubber. I'm willing to bet this will be a clean, rust free bus. It was previously owned by Gwinnett County (which is a fairly heavily populated district) and they have only recently retired some late 80's buses! My guess is that it was serviced and inspected for the upcoming school year, the fiscal year rolled over and it was decided to retire the oldest buses for new ones.
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Old 08-07-2020, 12:08 AM   #10
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nope. But top speed isn't a product of what engine a bus has. Its the gearing. Since school buses are mostly for stop and go that's what most of em are geared for.

My bus only has 190 hp and it will do 80+. I've had a 175hp 444E with a 545 that would do 75.

I'll mostly agree with this. The IDI 7.3 Naturally Aspirated engine (at least this one) simply doesn't have enough Oomph to maintain even 55 up a light grade, and regearing it would only exaggerate this further. Sure, mathematically speaking, regearing would give a higher top speed but at what point does one "run outta engine", so to speak?


My own bus, a T444E equipped with a Spicer 5-speed tops out at 55 by the gears (which are planned to be replaced) and I expect mine would cruise 70 comfortably with appropriate gears - but mine's also several windows and several thousand pounds lighter, and my engine has a turbo which I'm sure gives it plenty of extra oomph. (Just in case you're wondering, I currently have 5.38 gears in mine now and considering 3.91's).
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Old 08-07-2020, 12:45 AM   #11
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It is highway worthy

slow down and run with it...... Read up on 7.3 idi engines and you will learn what you can do for more power......

you do it wrong, you will be puttin head gaskets on that engine or more.

careful what you ask for.

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Old 08-07-2020, 01:30 AM   #12
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Well when you think about a sailboat making a passage going about 5 mph, 10 mph they are quite thrilled, going 55mph seems downright blazing.
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Old 08-07-2020, 05:46 AM   #13
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I'll mostly agree with this. The IDI 7.3 Naturally Aspirated engine (at least this one) simply doesn't have enough Oomph to maintain even 55 up a light grade, and regearing it would only exaggerate this further. Sure, mathematically speaking, regearing would give a higher top speed but at what point does one "run outta engine", so to speak?


My own bus, a T444E equipped with a Spicer 5-speed tops out at 55 by the gears (which are planned to be replaced) and I expect mine would cruise 70 comfortably with appropriate gears - but mine's also several windows and several thousand pounds lighter, and my engine has a turbo which I'm sure gives it plenty of extra oomph. (Just in case you're wondering, I currently have 5.38 gears in mine now and considering 3.91's).
I'll bet you this bus has a 545. Why would anyone put a 643 with that engine?
Where is the torque peak on an IDI?

Out of curiosity- what's your price if you sell it, OP?
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Old 08-07-2020, 07:45 AM   #14
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you could get a 643 in an IDI it was an option.. my MT643 sounds jyst like my 545 did.. faint whine. (the older AT540 and 542. with the straight gears had much louder whines)..



the MT643 shift feels just like a 545 going 1-2 and 2-3.. BUT. halfway through 3rd the lockup is definite surge and the tach drops heavily.. then going into 4th is a heavy ratio drop. and it shifts locked up so you'll see a solid 500 RPM drop going 3-4.. if you dont have that then you likely have a 545.. obviously the easiest way is to crawl under it.. if there is a ring of bolts just ahead of the tailshaft you have a 643.. the 545 has a tapering tailshaft more similar to a light duty rear drive car transmission..
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Old 08-07-2020, 09:03 AM   #15
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If you do have an AT545 then draining and refilling it with good thick fresh new fluid will help a lot

I found this out when I did a trans swap, and I had to re-fill the AT545 when I re-installed it in the donor bus. She got up and went "better" than before... so it seems I was having a lot more slip due to trash/old fluid

she was a T444E and AT545 short bus.. 4.78 rear end... 57mph pegged at flat ground. I imagine it would likely been about 60 with the new fluid if I had kept the trans in the bus.. but still to the floor, and 2500 revs

AT545 just sucks
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Old 08-07-2020, 09:14 AM   #16
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Quote:
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My bus only has 190 hp and it will do 80+. I've had a 175hp 444E with a 545 that would do 75.
What does it take to change up the gearing? My 444e/545 will do 60, it would be nice to do 70 or more.
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Old 08-07-2020, 09:52 AM   #17
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you could get a 643 in an IDI it was an option.. my MT643 sounds jyst like my 545 did.. faint whine. (the older AT540 and 542. with the straight gears had much louder whines)..

the MT643 shift feels just like a 545 going 1-2 and 2-3.. BUT. halfway through 3rd the lockup is definite surge and the tach drops heavily.. then going into 4th is a heavy ratio drop. and it shifts locked up so you'll see a solid 500 RPM drop going 3-4.. if you dont have that then you likely have a 545.. obviously the easiest way is to crawl under it.. if there is a ring of bolts just ahead of the tailshaft you have a 643.. the 545 has a tapering tailshaft more similar to a light duty rear drive car transmission..

I'll check out the trans later today and report back.


Quote:
Originally Posted by RolesvilleMarina View Post
If you do have an AT545 then draining and refilling it with good thick fresh new fluid will help a lot

I found this out when I did a trans swap, and I had to re-fill the AT545 when I re-installed it in the donor bus. She got up and went "better" than before... so it seems I was having a lot more slip due to trash/old fluid

she was a T444E and AT545 short bus.. 4.78 rear end... 57mph pegged at flat ground. I imagine it would likely been about 60 with the new fluid if I had kept the trans in the bus.. but still to the floor, and 2500 revs

AT545 just sucks

The fluid, when checked, was a nice reddish color so if I had to guess, it looked like it had been changed at some point during its career.


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What does it take to change up the gearing? My 444e/545 will do 60, it would be nice to do 70 or more.

You'll need to know what rear axle you have. Often there should be a plate or sticker on it (if it hasn't gone missing over time). The easiest way is to change the "chunk" (carrier assembly) with a rebuilt one having the gear ratio you want.


To do this yourself, you'll start by draining the fluid - probably around 5 gallons of it. Remove the driveshaft. Remove the axle shafts, they do not have to come all the way out. Remove the dozen-or-so bolts holding the carrier to the axle. Be warned - this thing is *VERY* heavy, upwards of 300 pounds. At this point it should come out, the gasket/sealant should be the only thing holding it. Reassembly is the reverse of removal.


Changing the ring and pinion is a bit more involved on some axles, needing to set and adjust (as necessary) gear mesh, lash, preloads and other things I can't remember at this time. Some carrier housings may not fit certain gear ratios. Personally, I'd take this to an axle shop (most big cities have at least one) and swap it for a rebuilt one. I would expect to pay in the 1000-1500 range for the parts.
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Old 08-07-2020, 10:08 AM   #18
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What does it take to change up the gearing? My 444e/545 will do 60, it would be nice to do 70 or more.
Buy a rear chunk with the gears you want. Something like 3.90-4.44.
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Old 08-07-2020, 10:28 AM   #19
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Buy a rear chunk with the gears you want. Something like 3.90-4.44.
How do you source something like that?
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Old 08-07-2020, 11:13 AM   #20
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How do you source something like that?

As I mentioned a couple posts above, either a truck junkyard, or an axle shop.
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