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Old 12-24-2005, 05:16 PM   #1
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Retarders

Hello to everyone.
I recently purchased my first bus:1968 Carpenter with 401 gas engine and MT40 allison.It has air brakes and a 6.83:1 rear.Seems to run ok but will only run about 53 mph.
It has a pedal marked retarder.Whats this? How do I use it? How can I check it to see if it's working?
Also is their a govenor? Where? How can I disconect or bypass it ?
Glad I ran across this sight. It's awsome!!!
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Old 12-26-2005, 02:03 PM   #2
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Hey, Richard!

I don't know nuthin' about no retarders!

However....I might know why your bus goes 53 MPHs...

1. Measure the diameter of your wheels in inches (not the radius, the diameter), and convert that to feet, and multiply it by pi, 3.14159. (Example: 39.25" = 3.27 feet x pi = 10.27 -- the CIRCUMFERENCE of the tires on my 22.5" wheels.)

2. Take the RPMs your engine is turning at your top speed. (Example: 2800 = the number of RPMs my bus turns at top speed.)

Multiply step 1 x step 2. (Example: 10.27 x 2800 = 28,756 = the RPMs x feet in circumference of my tires.)

3. Divide the result by 5,280, the number of feet in a mile. (Example: 28,756 / 5,280 = 5.446.

The result is the number of miles per minute your bus will travel BEFORE you take into account the gearing of the rear end.

4. Divide step 3 by the rear end ratio of your bus. (Example: 5.446 / 6.5 -- the ratio on my bus's rear end -- = 0.8378.)

That is the actual number of miles per minute your bus will travel. Multiply step 4 by 60 (minutes per hour) to get the maximum speed of your bus.

5. 0.8378 x 60 = 50.27 Miles per hour top speed for my bus. The calculation works out to be about exactly where my bus tops out.

The above assumes that your tranny's top gear ratio is 1:1. If it is something else, you would have to factor that in, as well.

I bought my bus through the NC Surplus Property System from Brunswick County Schools. It had a governor on it that limited the top speed to 45 MPH, but the bus mechanics disabled that for me when I picked up the bus. IIRC, NC restricts (or used to) school busses to a top speed of 45 MPH.

If you have a 6.83:1 gear ratio, I'd bet that your top speed is limited by that rather than by anything else.

Hope this is of use.

Eric
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Old 12-26-2005, 05:44 PM   #3
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a retarder sounds like an engine brake to me.....but i think you only find them on diesels.

it retards the timing of the valves to allow the engine to do a lot of braking for downhill grades and such.
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Old 12-26-2005, 09:22 PM   #4
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Doesn't alcohol have that effect as well? LOL
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Old 12-26-2005, 10:41 PM   #5
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Quote:
Originally Posted by fankhaj1
Doesn't alcohol have that effect as well? LOL
only on the driver......not on the engine
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Old 12-27-2005, 04:48 PM   #6
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Hey Eric,
One minor detail missing from your equation: lockup torque convertor, lockup convertor will reduce RPM 200-300 over a non lockup unit.

I need to find out what gear my bus has, I hear stories of 345 Internationals only running 50 or so, But mine does 60-65 pretty easy, that is right before the secondaries open. Maybe my speedo is wrong. Either way, that is plenty fast for a living room.

Happy New Year everyone!

Brad
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Old 12-27-2005, 10:22 PM   #7
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Quote:
Originally Posted by bdavis441
. . . I hear stories of 345 Internationals only running 50 or so, But mine does 60-65 pretty easy, that is right before the secondaries open. Maybe my speedo is wrong. . . Brad
My bus has a 345 engine and will maintain 75 on straight & level road with no problem. I usually only go as fast as 65 tops though to get better mileage.
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Old 12-29-2005, 06:12 PM   #8
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Quote:
Originally Posted by bdavis441
Hey Eric,
One minor detail missing from your equation: lockup torque convertor, lockup convertor will reduce RPM 200-300 over a non lockup unit.
Is that because the non lockup unit is essentially sloppy, and spinning more than it should? I have an O-L-D Allison AT 540 (I think - that's what the line ticket says), so it's not the most efficient transmission in the world.

Quote:
I need to find out what gear my bus has, I hear stories of 345 Internationals only running 50 or so, But mine does 60-65 pretty easy, that is right before the secondaries open. Maybe my speedo is wrong. Either way, that is plenty fast for a living room.

Happy New Year everyone!

Brad
The gearing for the rear end should be on the line ticket (if that's still around). Mine was stuck to the inside of the hood, next to the grille by the right front fender. I've heard of some folks finding them behind the glove compartment, too.

Happy New Year!
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Old 12-30-2005, 04:48 PM   #9
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Yes, the non lockup convertor is strictly a fluid coupling and cannot be 100 percent effecient. This doesn't give a true 1:1 ratio due to the slightly increased RPM. The lockup unit connects directly via an internal clutch. I think my trans is an AT540 also, I'll check and find out what gear I have.
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Old 01-14-2006, 11:46 PM   #10
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That retarder pedal has nothing to do with valve timing on the engine as another poster indicated. Simply put, a retarder is an added "turbine" like contraption that uses the automatic transmissions oil to push against the direction of rotation of the driveline of the bus. Older units used the pedal like you have. Newer ones use a four position lever on the dashboard. These work O.K. for short duration brake assist. Their primary drawback is that they heat up the fluid tremendously when in use.

The other item that alters valve timing is an engine brake, and is electronically activated. These are primarily used in very large (class 8 truck) diesel engines, where popping the exhaust valve near TDC also creates alot of vacuum within the cylinder. Smaller medium duty engines typically use an exhaust brake - what is little more than a flap that blocks the exhaust just "south" of the turbo.[/u]
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