Rear end ratio with Allison 2500

[Pizote]

Originally Posted by PeteinSF

That is great news.
If I select a ratio where 4th gear matches your 2450RPM=65MPH I will have all the hill climbing power needed in 4th and better economy and quieter in 5th.

I did a quick calc on this - I currently have a 6.17 rear-end. If I changed the rear end to 4.56, I would get 67 @ 2450 in 4th. But 90 in 5th. That seems way too tall for me.

5.29 would be much better. It would give me 78mph in 5th at 2450, and 67mph @ 2100rpm

In 4th, it would give me 50 @ 2100 and 58 @ 2450. Seems to me this would still allow for low end gearing/peppiness and agility in mountain roads without a bunch of searching.

[PeteinSF]

Keep in mind that our DT engines have peak torque @1400rpm, so I would suspect that targeting freeway cruise for 1400-1800 would yield the best result.


5th @ 4.33:
->@1400rpm = 55MPH
->@1800rpm = 70MPH


4th would almost exactly match your current 5th


4th @ 4.33
->@1400rpm = 40MPH
->@1800rpm = 52MPH

->@2350rpm = 68MPH


1st @ 4.33
->@1400rpm = 11mph (not sure if that is good or not)

[Pizote]

Yes we agree on the numbers. It’s jsut a bit too tall for me ;)

[cadillackid]

a 5.13 would probably be perfect if its made for your axle.. not all ratios are made for all axles.. a 5.29 def isnt bad.. it will give you good torque and decent speed uphill. the engine doesnt make any more power after 2300.. thats where it peaks out at.. the only reason to ever run one faster than 2300. is just to extend a gear before shifting.. the jump from 4th to 5th is steep on the allisons.. its 1:00 in 4th gear and 0.75 in 5th.. (if you have 6 gears you'll go down to 0.65 in 6th). 1400 is a little low for RPM's you'll likely end up lugging it alot and thats not good.. you want to stay up in the 1700-1900 range at desired cruise.. if you have a lower spec engine you'll wasntto trend towards 2000.. if you have the 210 HP variant you can get away with a little lower RPM and be OK.
-Christopher

[PeteinSF]

Hmm, are you sure abut the lugging @1400?. That RPM is specificity called out as "peak torque" in the DT466E data sheet and the torque curve starts rolling off @1800...


Maybe my logic is flawed (happens all the time), do I need peak torque @ 45 vs 55mph?

[EastCoastCB]

Originally Posted by PeteinSF

Hmm, are you sure abut the lugging @1400?. That RPM is specificity called out as "peak torque" in the DT466E data sheet and the torque curve starts rolling off @1800...


Maybe my logic is flawed (happens all the time), do I need peak torque @ 45 vs 55mph?

Chris is the smartest dude I've ever met.

The DT's I've owned are happiest between 1800-2200 rpm.

[PeteinSF]

I love smart...


No kidding, my logic may be flawed, that is why i spelled it out. I try to learn something daily.

[Booyah45828]

You want cruise rpm halfway between peak torque and rated hp. That's a suggestion that I've heard forever. If that doesn't work favor going higher in the rpm over going lower.

As far as gearing goes. I always set it up so that at around 55 mph in direct drive (4th gear) you'll be at cruise rpm. Then when shifting into overdrive, It will lower the rpm down to something more fuel efficient.

I was always told that overdrive is meant for cruising along in low power situations, and that if you need the power to pull a hill or pass, you're supposed to downshift back into direct.

My opinion?

Figure out what axle you have, what ratio options were available in that axle, what ratios you have available locally to you, and then pick the best choice available.

You'll end up wanting something in the 4.56 to 5.38 area.

[cadillackid]

Lugging occurs when you push an engine to a paoint you are fueling it and not getting any gain... if you pull the computer program of a T-444E OR A DT-466E you can watch the engine..



these engine computers create a "desired RPM" based on the throttle position.. a calculated factor called 'engine load' is created within the ECM.. it is a perceived point at which the engine can do no more at that RPM.. it is essentially a deviation of desired RPM vs actual.. if I push the throttle down and reach engine load 100%. if i dont see any increase in engine RPM (transmission lock-up engaged) then the engine is currently being lugged... at 1400 RPM its pretty easy to Lug a DT-466 or T-444E..


if you guys want to question it I'll send you my computer and you can pull the data for yourselves... thats why you program your transmission and gear your bus so you dont luig it.. .. pretty simple you go over 100% engine load with the throttle pedal by a few % and you order a downshift.. ..



Booyah has it right.. you want to CRUISE somewhere between max torque and max HP.. you will have the best fuel economy. as well as room to "crowd" (note not lug) the engine a bit without a downshift. but will also prefer the highest gear you can without running up at the top of the RPM range..



im not perfect at this. but ive spent a great deal of time writing an allison program for my red bus that nets me 11-14 MPG at highway speeds.. and I can climb hills as well as a 190HP 444E can do.



anyway carry-on im not here to tell anyone how to build their bus...

[PeteinSF]

Hi Chris,


I was not questioning your advice with my previous "Hmm" only my own logic. This is all new to me other then driving a diesel pickup with bad ratios in the past.


So i understand. If peak torque is 1400rpm and max HP is 2200, half way=1800rpm.


So I should try to find a ratio that gives me 55MPH @ 1800rpm.


Should that target be at the 4th gear 1:1 radio or in overdrive 0.74:1 ratio?


(very educational)

[EastCoastCB]

Heck I'd shoot for that in OD.
My shorty will do 65 at 1800 but its a 5 window with a 190hp Cat and its on the verge of lugging at 55.


I've got a 545 so my top gear is 1:1. doesn't matter if you have 1:1 or OD as long as you get the final gearing right.

[cadillackid]

that little cat likes to run warm! engine and trans. but yeah thats the right idea.. I tend to like more gears because it gives you more options.. but gears are also pricey (to buy a newer bus with 5 or 6 speed or to swap a trans).


EC's shorty seems to be geared nicely for that 3126.
-Christopher

[More Power]

I'm new to the bus community so I'm wondering if anyone ever considers a horsepower/torque increase when considering the best rearend gear ratios. Increasing engine hp/tq will put different rearend ratios within the realm of acceptable performance while still acheiving a higher top speed. Within the range of dt466e engine ratings there are sometimes options to increase the hp and/or torque by only changing programming (other times hard parts need to be changed). It's just an option to consider.

[TJones]

I think you are pretty close with gearing around 4.6 depending on what is available for your diff. I'm planning on swapping my 7.17 to a 4.78 with my 8.3 isc. As painful as it would be to drive it home with its current gearing it would give you an idea of how it would handle various grades.

Ted

[PeteinSF]

Hi Ted,

Thankfully, Pizote just did the same route, with the same engine, transmission, 6.17 ratio on a similar sized bus (we only live 40mi apart).

After his successful trip and report I am not worried about making back home, but I am not sure i will learn much additional info on this topic. (I am sure I will learn other unknowns). We would have caravaned if I had the balls to do it with no plates or CDL (clearly Pizote has enough for both of us )

[cowlitzcoach]

Another thing that needs to be considered is if your engine has enough HP and torque to get your bus going that fast.

A few years ago I was driving a three axle Christmas tree harvest truck that had a DT466 and an Allison MT653 (deep low first gear, direct drive 5th gear). The truck had a bed with side boards that topped out at 13'6" and it had a cabover section that reached forward to the front bumper. Most of the time we cubed out with a total weight of about 25,000 lbs.

Empty or loaded it would cruise at 68 MPH at 2500 RPM's. Even on most of the hills it would rarely downshift. But with all of that frontal area if we got into a headwind we wouldn't be able to get into high gear. I can remember one afternoon when we were going against about a 10-15 MPH headwind and the best speed we could get out of the truck was 50 MPH. The engine just didn't have enough HP and torque to push that much air out of the way.

Later in that season they replaced the solid panel on the front of the cabover section and put in a piece of rolled screen. We only had problems with head winds after that when we had the front section filled with Christmas trees.

My point is that it takes a HUGE increase in HP and torque to push through the amount of air a bus pushes if you want to change your top speed from 55 MPH to 70 MPH. Changing gear ratios may lower your engine speed at 55-60 MPH but there is no guarantee that you will be able to go any faster.

A little more than ten years ago I purchase an IHC S-series/Thomas body bus in Grand Rapids, MI to move treasures from MI to WA state after my M-I-L passed away. It had the T444 and Allison AT 540 and it was geared to cruise at 65 MPH. I discovered as I traveled west that when a semi-truck with a big van body on the trailer went past me if I could draft behind him I could pick up almost 5 MPH. The problem was that on any sort of hill I would drop out of draft and be back at my max cruise speed.

Again my point is that most school buses don't have the HP and torque to go much faster than 60-65 MPH no matter what the final rear gearing might be.

I suppose what I am saying is before you start spending $$$ on changing gear ratios you need to discover what the baseline performance is on your bus. It may require boosting up the performance of the engine as well as changing your rear gears to achieve the speeds at the RPM's you want.

Good luck.

[cadillackid]

Originally Posted by More Power

I'm new to the bus community so I'm wondering if anyone ever considers a horsepower/torque increase when considering the best rearend gear ratios. Increasing engine hp/tq will put different rearend ratios within the realm of acceptable performance while still acheiving a higher top speed. Within the range of dt466e engine ratings there are sometimes options to increase the hp and/or torque by only changing programming (other times hard parts need to be changed). It's just an option to consider.

Power adders definitely can be a part of that.. me, being the computer guy that I am. plays with that software and the programs of the ECM and TCM..



that said getting the right tools together and the know-how to turen up the power on an engine often makes people go the mechancial route of gearing..



a couple things to think abourt power-adders..



1. dont turn up the software beyond your hardware.. while you can say that navistar made a 250 HP variant of the DT-466e. and may think that opbtaioning the computer program (or another ECM) to crank your HP is easy, you can get in trouble.. I personally dont know the differences between the 175HP and the 250 HP variants.. but I believe there is a different turbo (with a wastegate), depending on the year the injectors are different.. and perhaps other hardware..



the T-444E is a little more straight forward as the low torque (175-210HP) variants all have the same hardware.. the HT or High torque variants (210-250HP). contain a different turbo with a wastegate.. if you are 99.5 and up you have AD injectors on at least 7 cylinders. and likely an AE on #8. ALL vairants contained the same injectors on the 444E.. so if you have a 175. you can safely up your program to 210 without any issues.. but try and punch 230 or 250. into the programmer and you are in non-design territory... will it work? possibly, but not likely are you to get the gains you expect.. also if you have a 466E with 175HP and an AT545, that you punch up to a 215, expect to wreck your AT545 in fairly short order... its beyind design..



2. lets take EGT (exhaust has temperature).. running at too low of RPM and "crowding" or "lugging" creates higher EGTs.. just because you put more fuel in the cylinders doesnt necessarily mean you will go faster.. to a point you'll maske more power.. but if you are geared too high and are running too low RPM then you are likely to run up your EGT.. the factory HP and fueling ratings are safe for the factory installed equipment. if you turn your 175 up to 210, great it *should* be safe too.. however what if the factory never sold a 210 in the same gear / trans config you are running? uncharted territory..





im *ALL* in when it comes to making drivetrain mods.. I love the stuff.. but be smart about it... if you are going to tune, install an EGT gauge. if you are going to approach the rated limits for your transmission be sure to have a trans temp gauge.. build or buy a diagnostic scanner so you can watch your computer.. notably your boost and gallons per hour.. dont ruin your turbo ...



-Christopher

[PeteinSF]

Hi cowlitzcoach,

Great comment, I have noticed the same when I have my tall camper sitting on my 5.9 RAM, wind resistance is the brick wall on speed and MPG...

I found that drafting helps, but the truckers look cranky when you do it.




So to be clear, I have no interest in cruising over 65mph, I just know from my other diesels (ISI7.3, 24v5.9, Yanmar) that the engines are much happier and quieter < 1800 vs 2000+.

A rumbling diesel is calming, a screaming one is unnerving.

[Pizote]

Originally Posted by PeteinSF

(clearly Pizote has enough for both of us )

Haha

Originally Posted by PeteinSF

Hi cowlitzcoach,

Great comment, I have noticed the same when I have my tall camper sitting on my 5.9 RAM, wind resistance is the brick wall on speed and MPG...

I found that drafting helps, but the truckers look cranky when you do it.

So to be clear, I have no interest in cruising over 65mph, I just know from my other diesels (ISI7.3, 24v5.9, Yanmar) that the engines are much happier and quieter < 1800 vs 2000+.

A rumbling diesel is calming, a screaming one is unnerving.

I had this same wind issue this past September going eastbound on I50 in Utah. I had a 20-30 mile headwind and my milage on my Ford V10 dropped from 8.5 to 4.2 or something similar. Couldn't get above 50mph without dropping into 3rd or 4th. It was slow going and brutal.

[Rippinron.ut]

Started out with a 5.57 ratio. Was at 2500 rpm at around 65mph. Switched to a 4.67 ration and now at 65 I run around 2000 rpm. Glad I made the switch. Looks like I have a very similar bus and setup. I just did a 1500 mile trip from utah, across wyo , down through colorado to new mexico and across new mexico back up through utah. I got around 11 mpg and the bus did great climbing the passes.