Rear end ratio change.

[bread519]

Originally Posted by Alan N

If you can turn a wrench you can change a gear set.
No need to pay someone to do it.
All you need is a floor jack and some wrenches.
I am sure that there is a video and instructions online somewhere so I won't spell it out here.
But if not ask.
You can do this one.
Save your money.

I just need to figure out where to find the parts needed! Do you think I should rebuild the rear end while in there? That's what my mechanic was saying but I think he's just trying to get extra hours in

[GreyCoyote]

Bread:I'm running the 643's baby brother: an AT545. At 58 mph I get a solid 8-9 mpg with peaks up to 11 with a tailwind.

FWIW It takes between 80 and 90 hp to maintain 58 mph on level ground. That number surprised me, but these things obviously have the aerodynamics of a billboard! Coming across the desert one day I looked at the ScanGauge and saw I was making 140 hp to maintain 55 mph! When the wind switched directions it dropped to 60 hp. Really fun to watch.

[bread519]

Originally Posted by Geo Jeff

They sell rear end gear sets but not speedo gears?

My genesis is dt466 and 643 @22250 or 2300 at 60mph. I would be afraid of getting the gears to tall and not being able to stay in your tall gear? Seems like the slightest grade would kick you out of the gear and then your not able to use it and its not worth a dam. I cant est this myself as i don't have a load on the bus while I'm tearing it down. Also I don't know the HP ratings and what was most common but my sticker on the new engine says 250hp and the old engine was suppose to be 195hp I think. that's got to help hold a gear at high speed. Burn more fuel to trying to get more MPG? its balancing act for sure.

For $2k dollars how many years will it take you to recover that cost. I must be getting old but id rather run 5mph under every one else. You never have brake lights in front of you and the bumps that rip the counters from the walls are so much more timid.

Time wise there is a huge amount to be had at 50 to 70mph. Its to bad we don't have those big 14 liter engines turning 1200 rpm. Manuals are a pain in the ass when your out on vacation trying to hold your bloody marry and match RPMs. But they sure keep that engine right were it wants to be.

What type of fuel mileage do you get? Most speed limits are 70 by us now. I would really like to be able to go 65 at least

[bread519]

Originally Posted by GreyCoyote

Bread:I'm running the 643's baby brother: an AT545. At 58 mph I get a solid 8-9 mpg with peaks up to 11 with a tailwind.

FWIW It takes between 80 and 90 hp to maintain 58 mph on level ground. That number surprised me, but these things obviously have the aerodynamics of a billboard! Coming across the desert one day I looked at the ScanGauge and saw I was making 140 hp to maintain 55 mph! When the wind switched directions it dropped to 60 hp. Really fun to watch.

I'm only getting 8mpg with the full lock up 643 so idk what's going on lol

[cowlitzcoach]

I think re-gearing to a set that is under 4 is going to not be such a good idea. With the RPM's that low, every little upgrade is going to have you downshifting since the downshift point on most DT466's IIRC is about 1800-1900 RPM's.

I would prefer to have the engine running about 2200-2400 RPM's for the ideal engine speed--fast enough to keep the bus going on every little uptick and not so fast that you have to yell to be heard.

One thought just occurred to me: If you fan is turning 100% of the time it is going to eat HP and be really loud. At full RPM's I would bet the tips of the fan blades are going supersonic.

Your friend that is concerned about turning the DT466 too tight has been working on mostly heavy duty diesel engines. Most of them used to have a redline of 2100 and now some of them as low as 1700. Almost all medium duty diesel engines have a power range from about 2000 RPM to 2600 RPM.

Use the engine RPM calculator to find out what will work the best for you.

And remember, nothing you do on a bus conversion will ever be a good $$$ and cents equation. You will be lucky if when you are through with your bus if you get the original purchase price back. And you better forget about any of your conversion costs or sweat equity.

[Geo Jeff]

There is always to much of good thing. I build high MPG vehicles. I have my own that I use to race in the eco fuel races. Its gets 68mpg real average. I use it for every thing it has trailer hitch on it now. i get 35mpg with a 1500 to 2k lb load.

But I have it all mapped out on the dyno and built just for that mapping. If i put a tire on it that's just 1" taller it loses 20 mpg. Virtually takes away every thing I have done to the car from weight reduction to engine mods.

When you want more MPG you have to build more Hp and lower the torque curve. Look at the car manufactures they add more hp every year to the cars and trucks and they keep getting better MPG. I have to build full race engine at wot at 6rpm produces twice the stock engines HP so i can idle that super efficient engine around down low in the rpm to get a bit better fuel burn out of every drop of gas. If you use 100 hp to go down the road and you want it to get actual better MPG you need that 100 hp to show up sooner at lower rpm. So raising that gear ratio or knowing what it will do for you is as easy as trip to the dyno shop to see where your engine is peaking at its shift point rpm. The do the math where the new rpm shift point wil be and then look at your dyno sheet and see how much power you need to have the torque right there. This is when you see your just a few like 50 hp off from ideal. So you take it in and ask the good man to set the engine to that rpm with that hp. No matter how much you move the rpm around you can never get away from the fact it has a camshaft and that's what determines the driving compression and your cruise rpm. Those lobes open and close at a certain time depending on how long there open and peak compression on when they shut both valves and re open them. not to get to complicated but your running compression is what pushes you down the road and the cam shaft shape is how the energy pushes on those pistons and for how long.

You may just need a different cam shaft as i suspect we all do. When i use to work for VW /benz most people don't realize a gas German engine is the same as a diesel is with just a different head. For instance the rabbit diesels until they got to TDI had the same cam as the gas engine. No one cared and was looking for more power. Take 1.9 cam profile and stick in a old 1.5 rabbit and you can feel a difference. So don't over look actual performance because its your friend when you need more HP and more MPG there one in the same.

having your cam reground is cheap and you don't have to know cams at all just go a new modern day style like from a 1996 VW tdi Canadain model. Tacoma cam grinders can do one in week return mail.

Running a rv cam (same as your chevy truck) profile in a non turbo diesel is a great way to go as well. so if you have NA 3208 cat in a old beast this would be a great upgrade. helping you make that tall rear end gear really work for you. I use to do this to the 4cyl benz non turbo and rabbit.

what i don't understand is why my 6 speed 350 hp c7 cat in my 40ft RV only gets 6 mph. i need to check those gears because this thing might beat a small import honda racing around town its brutally fast on the tae off and holds the pass at any speed you want but never gets better or worse than 6 mpg. I may drive it off a mountain pass soon and see if i can get a bit more MPG in free fall. I just had it scanned and dynoed and they say its 100% at 70k miles.

[Geo Jeff]

Originally Posted by cowlitzcoach

I think re-gearing to a set that is under 4 is going to not be such a good idea. With the RPM's that low, every little upgrade is going to have you downshifting since the downshift point on most DT466's IIRC is about 1800-1900 RPM's.

I would prefer to have the engine running about 2200-2400 RPM's for the ideal engine speed--fast enough to keep the bus going on every little uptick and not so fast that you have to yell to be heard.

One thought just occurred to me: If you fan is turning 100% of the time it is going to eat HP and be really loud. At full RPM's I would bet the tips of the fan blades are going supersonic.

Your friend that is concerned about turning the DT466 too tight has been working on mostly heavy duty diesel engines. Most of them used to have a redline of 2100 and now some of them as low as 1700. Almost all medium duty diesel engines have a power range from about 2000 RPM to 2600 RPM.

Use the engine RPM calculator to find out what will work the best for you.

And remember, nothing you do on a bus conversion will ever be a good $$$ and cents equation. You will be lucky if when you are through with your bus if you get the original purchase price back. And you better forget about any of your conversion costs or sweat equity.

all great advice and really how it should be done.

[Asetechrail]

Bread519
I'm running the same bus 1993 Amtran Genesis DT466, Allison MT643.
Full size 40ft bus. I don't run over about 62mph. That's turning 2426rpm with the lock up converter and 41" tall tires.

I'm comfortable running there most of the time but would prefer more top end also. If I did a gear change it would be from 4.78 to 4.11. That gearing would put me at 72mph with the same 2426 rpm.

72 is faster than I would really need unless on the interstate. I continue to check mpg every trip and get a consistent 11-12. Unless I were to find the gears and do the swap myself, I would just leave it alone.

I run 55mph at about 2200 rpm most of the time to get that mileage. 4.11 gears would put me at 65mph which would make me a happy camper!

[Geo Jeff]

It probably comes down to if your running the mountains on the west side or east side of the country. picking up just 2 mpg is 20% and that seems like so much the builders would have done this.

If it works I'll do it as long as i don't have to go over the pass at wot at 35 mph. I was going to put in a black WMO system and that would get me 50% more mpg but with the added headache that comes along with doing it. For $1000 i can install a gear set and not buy the stuff fro WMO system. I'd rather get 20 percent and do nothing than 50% and have to manage that system. plus I like to cruise slow any how so i may even get more.

Whos out there with scale weights and how fast are you going and what are you getting now. If i got 3mpg from slowing down and 2mpg for gear change id be in heaven i just don't see it. The if your greedy a WMO set up at 50% mix your in the adjusted zone of 22ish MPG? that's a empty new power stroke truck mpg.

[GreyCoyote]

In a DT466, dont bother messing with the cam or compression. These are tricks for gassers and NA diesels . Regrinding is fruitless (many tractor pullers use the stock cam) unless you are radically changing the operatingrange of the engine. And the engine simply cant use any more compression.

The DT engine is already capable of breathing far beyond the amount of fuel you can throw at it. Need more flow? Add PSI and more intercooler.

If you want more power/mileage, look at the injectors, turbo, fuel timing, etc. The only real difference between a 180 HP and a 350 HP DT466 is the ECM, the injectors/pump and the turbo. (And pistons, but only in the material used. The higher HP variants use a 2-piece cast iron piston. Lower HP variants use a 1-piece forged aluminum design. Both have the same cup and quench design). These components are the areas where huge gains can be made if you have the time and budget.

I'd also be remiss if I didnt point out the DT530. Its nothing more than a DT466 with an extra half-inch of stroke. A 530 would build big power at 1900-2000 revs and could seriously use the gears you are contemplating. Dunno if the MT643 would survive the experience, but hey...

[turf]

maybe i missed the reason in the thread, but do i understand right that 55 mph is your top speed?

i have a 190hp cummins 5.9, AT545 and 4.78 rear. i can do 67 mph @2500 rpms.
i have no tach, but the tire size, gear ratio and speed, give me the rpm in the calculator. all the math lines up beautifully. a swap to a a 4.1 rear would put me at 75 mph. i have not done the math for your number but thats got to put me over 80.

whats the difference? why are you at 55 when i do 67?

[Versatile]

Originally Posted by bread519

I'm really wanting to change my ratio of my rear end from 4.78 to 3.73 if I keep my bus to make highway speeds more attainable. I found a remanufactured rear end for 1500 with core deposit. The local shop said it would only be about 3 hours of labor plus fluids. So out the door at 1900. Does this sound reasonable? It seems a little silly as I probably wouldn't see a return in the investment for years. But would make driving much more comfortable.

What does everyone else think?

Bus is 1993 Amtran Genesis DT466 MT643

Do it. Because you said.

But would make driving much more comfortable.

[EastCoastCB]

Originally Posted by turf

maybe i missed the reason in the thread, but do i understand right that 55 mph is your top speed?

i have a 190hp cummins 5.9, AT545 and 4.78 rear. i can do 67 mph @2500 rpms.
i have no tach, but the tire size, gear ratio and speed, give me the rpm in the calculator. all the math lines up beautifully. a swap to a a 4.1 rear would put me at 75 mph. i have not done the math for your number but thats got to put me over 80.

whats the difference? why are you at 55 when i do 67?

His 195hp DT466 does around 60-65 top speed. Similar power and gearing, seems you may get a couple more mph. Is yours a 40 footer?

[turf]

i have a 30' BB. 21k lbs gvw. and i believe 4th gear is 1:1 on both transmissions (my at545 and his mt643)
i get pretty good mileage at 55mph, but my norm of the foot to the floor its in the 7-8 mpgs.

i wouldnt fear 75 on the interstate cruising with the 4.1 rear, but 80 might be more than the bus or i could take.

if you DIY, post lots of pics

good luck

[EastCoastCB]

Originally Posted by turf

i have a 30' BB. 21k lbs gvw. and i believe 4th gear is 1:1 on both transmissions (my at545 and his mt643)
i get pretty good mileage at 55mph, but my norm of the foot to the floor its in the 7-8 mpgs.

i wouldnt fear 75 on the interstate cruising with the 4.1 rear, but 80 might be more than the bus or i could take.

if you DIY, post lots of pics

good luck

So- with the same amount (roughly) of power your bus which is ten feet shorter will do a couple mph more than Bread's 40'. Seems reasonable.

[bread519]

Yup seems about right to me. I'm pinned out at 2550 RPM and 65 MPH 42 feet bumper to bumper

[EastCoastCB]

I think my governor may be limiting my speed a tad. It will do 62-63 with the pedal to the metal. But is happy driving that way 800+ miles. 195 hp. 4.44 rear.

[bread519]

Originally Posted by EastCoastCB

I think my governor may be limiting my speed a tad. It will do 62-63 with the pedal to the metal. But is happy driving that way 800+ miles. 195 hp. 4.44 rear.

If you're a 4.44 and you have 11 r 22.5 like me, you should be right at 2316 RPM according to the calculator. I'd take that all day

[Alan N]

I have 5.29 rear end gears in my bus.
Changing the ratio wasn't practical for several reasons.
I swapped my 5 speed transmission with no overdrive for a 10 speed with a .78 overdrive.
It is The Best modification I have done to my bus so far.
Now it drives like it should.
I have a Cat engine with a little bit more than 250 hp and 11r24.5 tires.
I had the old crate up to 80 once, but that's too fast for a 40 year old school bus.
Do your swap.
I think you'll be happy.

[ElspethFlynn]

Hello!
I am new to this (and wondering if I have just wasted way too much money on a bus that will not work for me). I bought an auction bus for 2k.. while driving it home I blew out a tire (they all ended up needing replaced because they were capped and no good and cost around 1,100). On top of that, the bus would not go over 40 to 45 mph (the speedometer, when working, said 55, but my cousin followed me and confirmed I wasn't going over 45). Anyways, I made it back to TX (finally) and took the bus to shop #1... Shop number one did minor things like flushing fluids and replaced an axle seal, oil change, brakes, etc. (which cost about 800). They said the fuel injector module was bad and recommended another shop. So, shop #2 ended up replacing all of the injectors, fixing the wiring harness, and doing a couple other minor things (ended up costing 4,000). Sigh. So, now I have over 7,800 sunk into this bus and they tell me it won't go over 55 MPH. They lifted the governor to 70, but they said that it was not capable of going over 55 and the woman on the phone said that it was not built to go that fast because the engine would implode?!?! WTH? Has anyone ever heard of that? I am wondering if I should bail out now, or continue on and convert it and just deal with plodding along at 55 MPH, or if it is possible to get it moving a little faster... Thank you for any input!

Sincerely,
El