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Old 12-22-2006, 06:38 PM   #1
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Basic Question: Oil Pressure, Trans. Size, Governor.

First time buyer. Looking at a '89 BB All-American 8.3L 40ft. (see more in 'conversions' posts).

What should the oil pressure be upon cold start (in warm climate)? When running and warm?

What is the best/ideal size trans. for this bus and what do each number of the trans. stand for? (i.e. AT 545 or MT643).

This bus has a governor that will allow it to run only 55mph max. Comments Please.

Thank you all!
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Old 12-23-2006, 04:41 PM   #2
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Hi! We meet again!

I can't remember the rule-of-thumb on oil pressure; it was posted here (somewhere) not too long ago so maybe one of the engine tech guys will chime in here (I think it was 10 pounds per 500 rpm). Mine runs about 50 to 60 psi at cruise speed and maybe 25 or so at idle (as I recall at the moment anyway!).

Again, we've got some pretty savvy mechanics here at the can go in depth on the transmissions but from a user standpoint...

The first letter designates the general use; that is the "A" is (relatively) light duty, an "M" is medium duty, and an "H" is heavy duty. I believe the "T" is just transmission or perhaps the designation for all automatics. The first number is the series...500 series, 600 series, 700 series, etc and goes along with the A, M and T. So you'll see AT5XX, MT6XX, HT7XX and such. The second number is the number of gears, and the last number is... ...I can't remember (I think it's the design level number or indicates a particluar feature of the transmission). So the AT545 is a light duty automatic with 4 speeds; the MT643 is a medium duty automatic with 4 speeds, the HT750 a heavy duty automatic with 5 speeds.

The AT545 does not have lockup on the torque converter, meaning that when you're cruising on a fairly flat road at a steady rpm the torque converter does not lock and therefor there's always some slip (and therefor lower mileage). The MT643 does have lockup (feels like you're shifting into 5th gear).

The transmissions are rated for use with regard to horsepower, GVWR (gross vehicle weight rating), and engine torque. Generally school districts would go with the AT545 unless one (or more) of the numbers above got exceeded. Since school buses really normally fall into the "light duty" category with regard to vehicle weight and since they're typically used in or near town a lot of buses got relatively small engines and the AT545 tranny. But sometimes an engine choice was made that required the use of the MT643 transmission; that's considered a "good" thing by a lot of folks since even in the most severe school bus application that transmission is considered under-worked.

There should be a placard on the engine that lists the max rpm; if the governor is set for that rpm there isn't much you can change with regard to that. If you can get full rpm (that is, the governor hasn't been turned down) then the speed is being limited by the rear-end gearing. The Allison AT545 and MT643 have a 1:1 final drive ratio so that doesn't affect the road speed (in the sense you're asking about). There is usually a tag on the rear end that lists the ratio of the unit. There are lots of (free) online speed/rpm/rear end ratio calculators; once you know the stats you can figure out the rest. For instance...if you know what tires are on the bus (size-wise) you can look up the diameter for that tire (Google search)...or just measure it on the bus from the center of the hub to the ground (times two of course). If you then know that the bus is hitting top rpm you can input the rpm, the tire diameter and road speed and get the rear end ratio. If you know the rear end ratio, tire diameter, and rpm you can get road speed. And so on and so forth. In some cases you can change the tires for those with a larger diameter and get a higher speed but most of the time you'll need to change the rear end ratio to get the speed you want.
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Old 12-23-2006, 05:47 PM   #3
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Thanks. That was helpful, clear and pretty much answered my questions.
So, for a 40ft. full conversion schoolie, what might be the ideal rear end ratio to get it up to 70mph on occassion yet have a comfortable rpm?





Les, by the way, what do you plan to do with your 79 International?
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Old 12-23-2006, 06:14 PM   #4
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10 psi per 1000 RPM is the generally accepted rule. Obviously more is just fine. You would have to have some SERIOUS oil pressure for a motor to die from it being too high.

Just a few more comments on the transmission choice. When calculating your speeds, use a correction fator of 15% if you have an AT545 due to the slipping of the torque converter. Basically, just multiply the RPM by .85 to get what the actual diriect drive RPM is for a given speed. The MT6XX's lock up converter in 3rd and 4th gear will eliminate this.

The pros of the MT643 include a stronger tranny, better top speed, and better mileage.

The pros of the AT5XX include the fact that it is extremely common making finding parts in a podunk town easier. It is also far cheaper than an MT6XX because the torque converter is not a wear piece in it. Prices vary om area to area, but $1500 give or take say $500 seems like the norm for a remanufactured unit. An MT6XX is going to be easily double that.

Don't let the bad rep the AT5XX seems to get turn you off from it. The reason there are so many documented failures is because there are so many of them out there. It was a 30 year transmission series used in A LOT of vehicles. At the same time, I think it is the general consensus that the MT6XX is a better tranny. Choose based on your needs. PErsonally, I would want an MT6XX behind an ISC Cummins.
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Old 12-23-2006, 09:19 PM   #5
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Quote:
Originally Posted by greyguzziart
Thanks. That was helpful, clear and pretty much answered my questions.
So, for a 40ft. full conversion schoolie, what might be the ideal rear end ratio to get it up to 70mph on occassion yet have a comfortable rpm?
That's not an easy question to answer without assuming a lot of the other values involved. You've got tire diameter, engine rpm, rear-end ratio and road speed; so you have to have at least 3 pieces of information (or assume them) to come up with the other.

I have 11R22.5 tires (listed as 41.5" diameter) on my bus with a 4.56 rear-end ratio; at 60 mph I'm turning 2200, at 65 it's 2400 and at 2600 (max rpm) I'm at 70 mph. I use this online calculator since I know all my numbers:

http://www.csgnetwork.com/rearendgearcalc.html

So...if you know the tire diameter (or size then you can get the diameter off the Web), the rpm you want to run and the road speed you'd like to have at that rpm you can plug all that into the following calculator and it will give you the ratio needed:

http://www.csgnetwork.com/gearratcalc.html

Obvioulsy, just like in a pickup, if you go to too tall a gear you can't pull your own shadow. By the time you get to 4.10's on a school bus you're talking fairly tall gearing and it will take some ponies to make that work. My 3208 Cat is rated at 250 hp and I don't get the sense that I'm screaming away from a stop (although it's deceptive since the bus is so quiet) and I don't feel like there's a lot of acceleration out on the freeway so going from 60 to 65 isn't just a blink but it's not a slug either so I think they got the gearing down pretty well. I certainly wouldn't want a numerically lower ratio for my setup and I'll only truly know how good my 4.56 rear-end is when I get the bus loaded and head for some western mountains.

Quote:
Originally Posted by greyguzziart
Les, by the way, what do you plan to do with your 79 International?
It's going up for sale; I just decided to wait until all the Holiday hubbub was over before doing that. And I was excited to get started on the new bus and spent time on it rather than getting the Blue Bird pulled out, cleaned up, washed, and photographed.
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Old 12-23-2006, 10:25 PM   #6
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Rear end ratio, tire size.

On the bus in discussion, owner says the rear end ratio is 4:36

Tire size 11r 22.5
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Old 12-23-2006, 11:13 PM   #7
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Here is a good sheet on all the Allison transmissions. You can read about the MT643.

On another note the "T" in the model number is to designiate that the transmission is for a T drive configuration. Some models (v730, v731) are for a V drive configuration about only found on a transit bus.

http://www.industrialautomatic.com/html/onhiway.htm
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Old 12-24-2006, 12:16 AM   #8
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I agree, 10 pounds per 1,000 rpm @ operating temperature is the standard for oil pressure.

10-15% slippage is normal for the torque converter (non lockup)
1:1 transmission ratio is standard unless otherwise specified

5280' in a mile

60 min/hr, this makes 60 mph an easy speed to work with-1mile/minuite

engine speed, peak power? or peak (governed) RPM? or peak torque?

axle ratio, distance in revolutions driveshaft turns for 1 tire revolution

tire circumference, piD or measure the od and convert to ft.

I would use 90% peak power rpm to compensate for 10% slippage @ the torque converter, your tire size is fixed, and their should be enuff extra speed in the engine(governed speed to get to 70 mph. Now all we have to do is write the rite equation
5280/tire circumference=tire revolutions per mile,
driveshaft rev/minute / tire rev/mile=axle ratio @ 1mph

The hard part is finding the correct power curve for your engine and then compromising to end up where you need to be the first time because gears aren't cheep.

happy holidays
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Old 12-24-2006, 11:53 AM   #9
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Re: Rear end ratio, tire size.

Quote:
Originally Posted by greyguzziart
On the bus in discussion, owner says the rear end ratio is 4:36

Tire size 11r 22.5
Well...I'll go out on a limb here.

I don't know if it's a 2400 or 2600 rpm engine (those are the two numbers I found associated with the 8.3) but if we assume is a 2400 rpm engine then it should top out around 68 mph with that rear-end and those tires; you should get a 60 mph cruise around 2100 rpm.

If it's a 2600 rpm engine the numbers are even higher; almost 74 mph at 2600 rpm. That seems too high so I'd guess the former rather than the latter but who knows until you get to see the plate on the engine.

It doesn't seem to me that you'll need to do anything to the dirve train at all (changing-wise that is). Find out what sort of governor the bus has got, disable it and you should be good to go.
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Old 12-24-2006, 11:58 AM   #10
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Quote:
Originally Posted by busone
Here is a good sheet on all the Allison transmissions. You can read about the MT643.

On another note the "T" in the model number is to designiate that the transmission is for a T drive configuration. Some models (v730, v731) are for a V drive configuration about only found on a transit bus.

http://www.industrialautomatic.com/html/onhiway.htm
*Slaps head*

I knew that! I forgot about the "V" transmissions in the transit buses. [And I was looking hard at an RTS bus!]

Thanks for the info and the link.
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