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Old 04-05-2019, 09:40 PM   #1
Mini-Skoolie
 
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IC200 /w VT 365 and Allison 2500, 5.29 rear gears. Making it more efficient

Hi,


I've been trying to get my head around how this works. From what I understand so far:
- Every engine has a point of highest efficiency. I remember reading somewhere (but can't find it back) that for diesels that is 10% above peak TQ.
- Since we plan to do road trips, we want the peak efficiency RPM to coincide with enough tire rotations to move the bus at ~65mph
- Since at highway speed, the transmission should be in it's highest gear, we take that gear ratio and combine it with the gear ratio from the back gears


You stick all that in a handy calculator that somebody linked


Our bus has the A175 configuration of the VT 365, from the manual that engine produces 460TQ @ 1400 RPM (yes, I know TQ is not the unit and max 175BHP @ 2600 RPM. If I got the 10% right, it would the most efficient RPM at 1500 or so.



Our transmission is an Allison 2500 from likely 2004, from what I read that means it's likely a 5 speed. With if I read the spec sheet correctly a 0.74 overdrive gear.


Our tires are 11r22.5, the calculator needs tire height and Google suggests that is around 40" (not at the bus location to check that)


Dealer ran the VIN and we have a 5.29 rear gear.


If I stick the current config in the calculator I get 2143rpm


Reversing that to find the desired read gear I land at a 3.77 gear.


Do I have any mistakes in the above logic?


Second question, from what I read the VT 365 can be bought in different power configurations. All internals seems to be the same, it's the ECM that determines final output. Other configs are:


A200: 200HP @ 2600 RPM / 520TQ @ 1400 RPM
A215: 215HP @ 2600 RPM / 540TQ @ 1400 RPM
A230: 230HP @ 2600 RPM / 620TQ @ 1500 RPM


The Allison spec sheet seems to indicate (but not so sure) the 2500 in school bus design takes max 200 HP and 705 Nm (519 Ft-Lb). Would they spec different transmission to go with the different engine configs?


If the transmission can handle more power then the current A175 config, is there any efficiency gains in getting the engine moved to higher power config. And is more always better? I've asked the dealer if they can change the ECU, and what sort of cost that would be.


All insights welcome!


Bas
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Old 04-05-2019, 10:04 PM   #2
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First off, I think most of those 10% above peak TQ are for inline 6 engines. I’ve spent quite a bit of time playing with these numbers. I have the 7.3 (t444e)z which is the earlier non EGR cousin to your 365.. I swapped an Allison 6 speed in and reprogrammedxmy computer for 250 HP.
in 6th gear at 1550 RPM, the engine will lug.. I have an EGT gauge on it and if I run a transmission tune that holds 6th longer it spikes up quickly. These V8 engines like 1700-1900 to cruise . When I drive 65 and I’m in 5th my engine is happy in that near 1800 range.. ( I have smaller wheels but I’ll be gearing down. )

Allison tends to be real conservative in their input ratings for their transmissions. You can probably bump to the 230 setting without issue. Navistar probably will refuse to do it but you can ask. The 1000 and 2000 were very similar , the duramaxx guys with 1000s were able to run up close to 275-300 hp before they limped and burned up the C5 clutch packs. And a lot of those guys that limoedctheir trans were rapping on those trucks hard by hauling ass foot to the floor on the freeway. I’d have to look but I don’t think there’s an EFRC to get any more than 250 out of that VT . Though I thought there was a 275 HP version. I have all the programming info for those computers I’ll have to look.
I don’t worry about my trans as I had it built to handle 500hp. Not that I’ll ever get that out of a t444e no matter how much I hack the computer. I’m already pushing 14.7 GPH fuel and 21 lbs of boost..
Christopher
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Old 04-05-2019, 10:26 PM   #3
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So I've been in the same boat with the rear end gearing (and the AT545, but you have a much better transmission that will serve you well).

I got tired of bouncing between different calculators, so I made my own. Mainly I wanted to see my RPM's at common speeds (10-75 in 5MPH increments).


I plugged in your numbers assuming a cruise RPM of 2200 and a max of 2600:


With 5.29 Gearing: SPEED CALCULATOR


With 3.77 Gearing: SPEED CALCULATOR

With 3.77 gearing and your drivetrain, you'll be running around 2359RPM at 65MPH.
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Old 04-05-2019, 10:33 PM   #4
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Quote:
Originally Posted by geminusprime View Post
So I've been in the same boat with the rear end gearing (and the AT545, but you have a much better transmission that will serve you well).

I got tired of bouncing between different calculators, so I made my own. Mainly I wanted to see my RPM's at common speeds (10-75 in 5MPH increments).


I plugged in your numbers assuming a cruise RPM of 2200 and a max of 2600:


With 5.29 Gearing: SPEED CALCULATOR


With 3.77 Gearing: SPEED CALCULATOR

With 3.77 gearing and your drivetrain, you'll be running around 2359RPM at 65MPH.
You numbers are wrong. The tire height is showing 24" and OP's tires are 40".
All the gear speed calculators I use have his current setup doing 69 at 2300 rpm. With 3.77 his bus would be a screamer at nearly 100 mph at 2300 rpm.
I'd go for a 4.88-ish gear ratio in that bus, OP.
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Old 04-05-2019, 11:06 PM   #5
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Tx for all the info. Good to know I should aim for maybe 1800rpm or so, instead of the 10%


Nice calc geminusprime, will play with it. I had the 11r22.5 as 275/80R22.5. With those numbers it looks a bit different. I changed the transmission gear ratio's, but that seems to reset on each submit?


If I can get the dealer to bump power, is higher always better or is there a downside to being able to produce more power?
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Old 04-06-2019, 12:44 AM   #6
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Originally Posted by EastCoastCB View Post
You numbers are wrong. The tire height is showing 24" and OP's tires are 40".
All the gear speed calculators I use have his current setup doing 69 at 2300 rpm. With 3.77 his bus would be a screamer at nearly 100 mph at 2300 rpm.
I'd go for a 4.88-ish gear ratio in that bus, OP.
Yeah I had to guess on the first number. XXX 11R22.5, I just plugged in 265 for the XXX. My bad.
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Old 04-08-2019, 08:06 AM   #7
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Fuel economy has a lot to do with your driving style as well. Even if you got all your gearing the way you want it, if jackrabbitting and flooring it to pass are your MO then you’ll still be sucking fuel. I drive a boxy Land Rover and a boxy mid 80’s wagon as daily drivers. I can squeeze 18 mpg on the highway out of the Rover if I draft a semi (safe distance of course) and accelerate super softly. We’re talking RPMs not above 2,000 until at cruise. The wagon’s a bit better since it’s lower profile and only rear wheel drive. Drive like you have an egg under the throttle and a pyramid of wine on the hood. That being said aerodynamics are your enemy, but easier you are on accelerating (let the bus slow down on climbs and coast downhill), the better MPGs you’ll see.
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Old 04-08-2019, 11:41 AM   #8
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Originally Posted by brijn View Post
Tx for all the info. Good to know I should aim for maybe 1800rpm or so, instead of the 10%


Nice calc geminusprime, will play with it. I had the 11r22.5 as 275/80R22.5. With those numbers it looks a bit different. I changed the transmission gear ratio's, but that seems to reset on each submit?


If I can get the dealer to bump power, is higher always better or is there a downside to being able to produce more power?
More power makes more heat, make sure your cooling system is up to the task of the higher power output. Remember any gear change upping your top end, will make your bottom end suffer.
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Old 04-08-2019, 12:04 PM   #9
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Hi Marc,


The cooling system is getting some upgrades, it's a bit of a concern area in the the VT365.


I won't go to crazy with the gearing, we like mountains. So going up those is important as well.


tx
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