"dfroggatt".......Our previous coach was a 36' Monaco Diplomat with a 400 ISL that was chipped to 450 HP. We could climb those grades at 55+ while towing a truck. The only problem was going down the other side with just an exhaust brake. (I don't agree with down the hill in the same gear you climbed the hill, not always true in higher HP engines).
I never feared those grades, but I didn't like that I had to work so hard to keep the coach slowed down. Here is what you need to learn. There is a speed that your exhaust will basically disengage and do nothing. Mine was 63 mph in 5th gear. On hills steeper than 5%, my exhaust brake would not keep the coach at a reasonable speed.
There is a speed where your coach will downshift and hold speed. Mine would downshift into 3rd at 43 mph and hold just about any 6%-7% grade. The only issue was the trucks were driving down at 55+ and running up my rear. I hated this, but eventually said the heck with them.
Soooooo.....with a little trial and error, you can find a speed where you shift, probably into third, and can hold your speed. As someone noted, you can use the emergency lane in Oregon, but your subject to debris that can cause a flat.
Lastly...when you start down the hill, it doesn't do any good to start at 55 and hope to slow as you go, it won't work. The Allison will upshift the minute it sees a certain rpm. So, crest the grade and immediately drop your speed to that which you find will hold your coach back, probably about 45 mph. The mountains through Lake Shasta will be good practice before the Oregon grades. If you still need some braking, stab the brakes as recommended.
We'll be driving that route in three weeks enroute to the Country Jamboree in Sweet Home, Oregon. But this time we'll be driving our new coach with a two speed engine brake. The two speed is nice, but what is really great is that the engine brake turns on and off while in cruise control to maintain our set speed.
Don & Mary
2014 Newmar Dutch Star - All Electric - 450 ISL
2016 GMC Canyon 4WD Crew Cab