Rear Axle Ratio Advice Needed

[PorchDog]

I just purchased a 1996 BlueBird International 3800 bus with a mechanical fuel injected 175 HP DT466, Spicer 5 speed, and 4.78 rear axle. (N-175 17,500 Lb with 36 spline axles...) The Spicer is direct in 5th.

The bus is a 34 passenger, about 14.5 feet behind the drivers seat, 21,000 GVWR and has hydraulic brakes.

The rear axle is somewhat noisy-- whine varies significantly from pulling to coast-- so the pumpkin has to come out. The bus has 265/75R22.5 tires, which appear to be 38.2 inches in diameter.

Max torque is at 1700 RPM, and max HP is at 2350. (or 2300?)

Per an online road speed calculator, at 60 MPH I am currently at 2536 RPM.

If I switched to a 4.44 Rear Axle, at 60 MPH I would be at 2356 RPM.

If I switched to a 4.10 Rear Axle, at 60 MPH I would be at 2175 RPM. Also, at 65 MPH-- I would be at 2356-- right at peak HP.

I am really leaning toward the 4.10-- but my concern is whether this is too low-- and maybe there will be hill pulling issues? I "might" tow a utility trailer with two dirt bikes occasionally.

Any input or suggestions would be greatly appreciated.

Jeffrey Fikes

[CHEESE_WAGON]

You may simply have a bad pinion bearing, and some of them are replaceable without removing the carrier unit. But if you would rather change the gearing anyway...

Lower numerically = higher ratio
Higher numerically = lower ratio

Lower ratio improves pulling power and acceleration, but limits top speed. Desirable in hilly terrain, not so much for highway.

Higher ratio drops rpm at a given speed and improves fuel economy, but kills power and acceleration.

Therefore, you must take into consideration what your primary use is going to be.

[Brad_SwiftFur]

I have a slightly shorter bus, T444E engine, same 5-speed transmission. I recently did a gear swap from 5.38 to 3.91. I went from a 55 top speed to ~75 top speed. I find RPM in the 2200-2300 in the 65 range. I kind of wish I'd gone to a 3.78 to get around 80 top end, or more to the point, around 2100 at 65.


The shop I used was not able to reprogram my speedometer, I ended up doing it myself with a Nexiq clone and the ServiceMaxx software. I ended up having to reprogram the gear ratios in the ECM as well as the "pulses per mile" (this parameter actually works the speedometer). I went ahead and changed my "Road speed limiter" and enabled the cruise control while I was at it.

[cadillackid]

whine in Medium duty axles is quite normal.. ever driven by an even newer semi truck that is bob tsiling on the highway and put your window down? alot of the high pitch whine you hear is his rear end gears and not his tires...



I lost a pinion bearing in my Superioer with an RA39 axle in it.. and you **KNOW** when its a pinion bearing.. in fact the gear whine didnt get a lot louder but the rattley and intermittent squealey sound most certainly did get LOUD.. not to mention the punkin got HOT quickly.. while the new Reman unit is much quieter than the broken unit i still hear the gears.. and I checked the tooth pattern and lash before i installed the 3rd member and it was dead on.. I still hear the gears a bit in that bus..



it certainly doesnt hurt to pull the 3rd member, bench it and take measurements and visuals on the gear teeth.. its not a hard job to drop a 3rd.. dont even have to take the tires off..



-Christopher

[cadillackid]

while the DT engine likes to cruise in that 2000-2200 RPM range.. get ready to shift a lot.. you will find that on every small incline you'll need to downshift to maintain speed.. with an autpmatic no big deal it drops up and down for you when properly programmed .. with a stick shift thats on you to down and up shift all the time..



just something to think about

[PorchDog]

I realize this engine is not very powerful, but the bus is sort of small.

I would like to be able to run 60 on an interstate, and not be so "wound out"...

I don't have an ECM, just a mechanical governor inside the injection pump, but I will have to figure out how to reprogram the speedo, or make up a conversion chart-- or use a GPS. Good point there.

Also, the additional shifting to pull hills is a consideration. I may just go with the 4.44's.

I do appreciate the suggestions.

Jeffrey Fikes

[Brad_SwiftFur]

Oddly enough, once I'm on the highway and above 1700 RPM, mine's happy to cruise rolling hills in 5th gear. Then again, I'm in a 14000 pound short bus and my engine is the 190 hp T444E.


Your DT466 will produce more torque than my T444 will. Go on and get those faster gears - unless you plan on hauling a lot of weight (and I mean much more than a couple dirtbikes).

[EastCoastCB]

I'd go 3.90 or 4.10.

My bus has 3.27 and its HIGH but sure is sweet out on the highways.

[EastCoastCB]

Originally Posted by PorchDog

I realize this engine is not very powerful, but the bus is sort of small.

I would like to be able to run 60 on an interstate, and not be so "wound out"...

I don't have an ECM, just a mechanical governor inside the injection pump, but I will have to figure out how to reprogram the speedo, or make up a conversion chart-- or use a GPS. Good point there.

Also, the additional shifting to pull hills is a consideration. I may just go with the 4.44's.

I do appreciate the suggestions.

Jeffrey Fikes

4.44 is going to have you near redline at 60-65. Even with my 3.27 I'd not think twice about pulling any reasonable load on a trailer behind my bus.
my old bus had a 466 with 1:1 4th and 4.44 and it was wound out at 62.

[PorchDog]

My Tach has no notation as far as a "never exceed" rpm. In my mind 2600 is about all this engine should ever see. (For brief periods)

I just went out and drove the bus, and with its stock 4.78-- 55 MPH is 2325 RPM-- and everything "feels good" at that RPM-- like a day at that speed would not be too bad.

But when I push it to 60 MPH-- the RPM goes to 2550--- and it may be a combination of wind noise along with engine vibration -- but it is not near as pleasant as 55.

The 4.44 would give me 60 MPH with 2356 RPM--5 MPH faster for the same engine RPM.

Sure wish there was someone out there who had done this to the same bus as I have. 175 Horsepower and 430 ft lbs of torque is not a whole lot with a heavy vehicle with the aerodynamics of a brick.

I know y'all are saying go for the 4.10-- but my head is saying whoa-- go 4.44....

[EastCoastCB]

I've had several buses with these ratios and I'd totally go 3.90, man.

[Booyah45828]

Go with the 4.10, and then install a pyrometer and adjust the rack setting in the injection pump. 175 hp is pretty low, and the dt466 in stock form is capable of way more then that. With the spicer manual, you shouldn't have to worry about breakage from more power like those with the 175hp dt466/at545 combo. Hopefully you have an intercooler, the stock turbo is usually adequate for more hp, but without an intercooler you'll be making some pretty hot air.

[PorchDog]

My engine does have an intercooler. I'll have to look into the rack adjustment. It uses a P7100 series inline Bosch injector pump, just like an old 12 valve 5.9 Cummins.

[PorchDog]

I've asked for advice, and most all of y'all (I'm from Alabama) have encouraged me to go 4.10 or even lower. The only negative is from Cadillac Kid and the comment about how much shifting do I want to do?

I also realize that if I want to go really fast-- a school bus is not the ideal candidate. I'm 62 years old, and still work full time. The trips we take in a vehicle could be called "combat touring" to steal a phrase from AeroStitch. My hope is that Bus Travel can be more about flower sniffing, and less about miles per day. I love secondary roads, and miles from nowhere whenever possible.

The bus has no AC-- and I read Cadillac's dissertation about how important it is to have AC on the chassis. That was a downer. I had hoped a roof AC unit with a small generator would solve that issue. Quartzite in winter, and higher elevations in summer.

All this stuff is new to me-- but learning is half the fun!

Again-- thanks for the feedback and input.

[CHEESE_WAGON]

Most of the transmissions I've seen were 400 rpm split per gear and usually yielded 10 mph per forward speed with a diesel.

[sportyrick]

I took out the gas 427 and installed a DT466 with the Allison 3060 that was originally coupled to it. I didn't get to change the rearend for a while and with the double overdrive and a 683 rear gear I ran 65 MPH max. After putting a 4.44 rearend in I run 75 MPH at 1600 RPM. I love it but it does downshift to 5th at hills but i feel that is exactly the way it should be. Cruising at lower RPMs then downshifting and climbing into the higher power range for a hill. I realize you don't have any overdrive so I would recommend if you don't mind doing some downshifting for hills you go as low as you dare. DT466 torque comes in before 1400 so you have enough torque to maintain speed on the flats and grab forth to climb hills, just my thoughts. I also pulled my pump and had it professionally turned up, bigger turbo and pulled my timing up 5 degrees, wow so much power. I get 8.7 MPG pushing it hard in the hammer lane running 80+ and 10 in the speed limit lane.

[PorchDog]

Another vote for going as low (numerically) as possible.

Shifting back and forth between 5th and 4th is not too tough. The reason I chose a bus with this drivetrain is simplicity. Not a computer in sight. Has a lever you pull out on the dash to shut it off. It will not be a daily driver, and I knew it would have to be shifted when I bought it. Now-- that may get old with time---but there is a lot of "peace of mind" as far as reliability is concerned. Not much to go wrong-- and most things that do go wrong can be seen with an eyeball, and fixed with hand tools.

So-- maybe 4.10 is the deal????

[cadillackid]

In my opinion if you go 4.10, install a pyrometer in the manifold pre turbo and then turn up the pump. Your injectors and turbo on that stock are good for a good bit more power,
But it’s really easy to make highway exhaust temps by flooring it at low RPM on a hill climb,

The gauge is a good guide for when to downshift.. you start up a hill and see that number spike up quick and good idea to drop a gear and bring the revs up.

I’m not totally against a 4.10 esp if there is a lot of flat ground to travel or the engine is turned high enough to handle. I always assume the engine will be stock, as very few people here actually turn them up.

[Booyah45828]

I'd normally agree with chris when it comes to turning up the engine. Not only is it harder on the engine, but a lot of buses have the at545, which already produces quite a bit of heat in stock form.

But with this scenario, being that it has only 175hp out of a dt466, your engine will definitely handle more power. Truthfully, the smallest I've seen on a dt466 was 195, so I'm surprised by a 175 number.

And then throw in the fact that you have a 5 speed, and my 2nd concern of transmission longevity is gone too.

Do a 4.10, maybe if you're super confident in only being in the flats would I go with a 3.90. I definitely wouldn't go any lower then that without swapping transmissions for more gears. Honestly, with the 5 speed and 4.10 you'll be slipping the clutch plenty enough to get going, especially if a hill is involved.

Maybe the real upgrade for you isn't in the rear gears, but in going to an HD transmission with one or double overdrives.

I've road in the back of my bus this past year, and my rear axle is also "noisy", but I'm fairly certain the bearings and gears are all okay, so it's just how it is. Unless you're sure you have bad gears or bearings, I wouldn't condemn a noisy pumpkin.

[cadillackid]

I dont say NOT to turn up the engine, I just say to do it wisely.. and with an EGT gauge.. i turn stuff up all the time and Love it.. my 444E is pumped up and gonna go higher... my transmission is designed for 500HP and 1000 ft bs of TQ so I can go a little higher .. LOL