Greetings
Ok our new to us u280 hss a pacbrake. Read a little about it. It is similar to a jakebrake?
No where near as strong as a Jake brake. Pac brake has a new model out that is better. Called the PRBX - Diesel exhaust brakes and upgrades for motorhomes, diesel exhaust retarders (http://www.pacbrake.com/index.php?page=prxb-exhaust-brakes-2)
John,
Actually a "not so simple" answer.
First, you have an EXHAUST BRAKE which is very different mechanically from a Jake brake/engine compression brake.
Three things determine the braking HP you can expect:
Back pressure limit set by the engine manufacturer-- generally dictated by strength of exhaust valve springs.
Design of exhaust brake-- the more modern PacBrake PRXB generates 30% more braking HP at lower RPM than a "regular" exhaust brake-- the same braking HP at high RPM
Transmission, and in particular, whether the torque converter is locked up. The Allison 6 speeds torque converter stay locked up in 2nd-6th gears. Allison 4 speeds vary by model.
A call to Cummins or PacBrake can tell you what the max PSI is for the exhaust brake. Obviously, if max is 35 PSI, it will give less braking HP than one with a 55 PSI max.
Which PacBrake do you have-- regular or PRXB?
You may also be able to post that information here and we can answer directly.
Brett
Thanks
I will get it. I know it is older, 6 speed allison. Its a 8.3 cummins.
An excellent drive train.
And one more thing that determines braking HP with the Allison MD3060 (the 6 speed you have). What is the "pre-select gear"?
The pre select gear is the gear the transmission shifts TOWARD (as soon as it will not overspeed the engine) until that gear is reached. The left shift pad window will show this to you-- it will go from "6" while driving to the pre-select gear when you turn on the exhaust brake and close the throttle. The lower the gear you are in (whether because of a low pre-select choice or because you manually "down arrowed" to a lower gear, the more the braking HP.
Counterpoint: I like a 5th gear pre-select because it gives me more choices, but also more use of the down arrow to select the proper gear.
Brett
Ok so i played with it on the way home.
On=5th on trans off throttle at 64mph
Off back to 6
Back to 65 mph on trans to 5th
Slowing to 55 arrow down to 4th really slow can feel it more than 5th
But bot steep hill
No revs above 2200
Are you posting the gear from the LEFT window (pre-select gear) or the RIGHT window (gear you are actually in)?
Irrespective of speed, the left window will give you the exhaust brake pre-select choice.
Just like at a stop light in gear, the left window shows "6". That is the pre-select gear-- the gear that under ideal conditions the transmission will shift to/toward.
We have the older model Pacbrake on our U225 (not the PRXB) and I like it - and use it - a lot. It's not a lot of deceleration but it is enough to keep our coach at a reasonable speed going down the west slope of Snoqualmie Pass (I-90) without using the brakes while motor homes all around us were hitting their brakes. Generally you can barely feel it when I turn it on but it does add braking and it does save wear and tear on the brakes while costing nothing.
Other than routine lubrication with a high-temperature lubricant. Not as good as a transmission retarder but better than nothing. And quiet enough to use when the use of "exhaust brakes" is forbidden.
The combination of the Pacbrake for braking and the Banks package for power make the little U225 perform a lot better than if it were stock.
Craig
PacBrake maintenance: Maintenance (http://www.pacbrake.com/index.php?page=maintenance-4)
I just talked to Pacbrake and they told me the pacbrake would not work with my motorhome because it does not have lock up. That doesn't sound right to me. I have a 1992 U280 with a Cummins C8.3 and an MT647 4 speed transmission. What do you all think about that?
I have a Pacbrake (not the newest model) on my '93 U225 with a Cummins 5.9 and Allison 643. It does have lockup. I really like the way the Pacbrake works on my rig.
Craig
Call Allison with your transmission serial number for the specifics on your transmission and torque converter lock up:
Allison help line 800 252 5283
I will make that call. Thank you for the responses!
I had an 8.3 and MT 647 on my 93 and a PRXB works just fine for 3rd and 4th. You will need to have heavy duty exh valve springs installed. As they weren't installed by cummins until late 93.
Brett
Thanks,thanks! Understand it much better and found your article about it from way past.
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