Community Forums

Tony Lee · 05-30-2014, 06:13 PM

Quote:

Originally Posted by frankdamp

Modern transmission control computers know what's most appropriate - why bother messing about with the shift?

What they are programmed to do when going downhill is to protect the drive train from damage. The computer doesn't care what speed you hit the bottom at, as long as the engine is running within specs.

The driver has full responsibility for getting up a hill without overheating the motor and also has full responsibility for getting down the other side in one piece.

Google "snub braking". It should be just as relevant for a gas engine with the auxiliary braking valve on the air input as it is for a diesel engine with the valve on the exhaust.

Rich-n-Linda · 05-30-2014, 07:11 PM

Quote:

Originally Posted by frankdamp

Modern transmission control computers know what's most appropriate - why bother messing about with the shift?

Because the computer cannot anticipate anything. It can only react to something that has already happened. By the time the computer finally decides it's time to downshift, it's already too late. You can see grade coming and get the transmission downshifted BEFORE you lug down to the point where you are far below the engine's HORSEPOWER peak.

It is NOT torque that gets you up a hill. It is HORSEPOWER, pure and simple. Those of you who shoot for the max torque RPM on a grade are not operating your engine at the best RPM for maximum performance. You need the RPM up at the horsepower peak for maximum cooling and best performance.

Rich & Linda

Ray,IN · 05-30-2014, 09:11 PM

Quote:

Originally Posted by Rich-n-Linda

Because the computer cannot anticipate anything. It can only react to something that has already happened. By the time the computer finally decides it's time to downshift, it's already too late. You can see grade coming and get the transmission downshifted BEFORE you lug down to the point where you are far below the engine's HORSEPOWER peak.

It is NOT torque that gets you up a hill. It is HORSEPOWER, pure and simple. Those of you who shoot for the max torque RPM on a grade are not operating your engine at the best RPM for maximum performance. You need the RPM up at the horsepower peak for maximum cooling and best performance.

Rich & Linda

**CountryFit** · 05-31-2014, 12:02 AM

a side note - if you have an exhaust brake, it's recommended to lube the brake control unit (air cylinder, rod, hinge etc) at least once a year if you drive your rv regularly or before hitting the road after a long sit. the oil for the lube requires a high temp character, probably near 1000 f. pacbrake uses part# c18037.

Lobstah · 05-31-2014, 05:38 AM

I'm 100% with Frank on this one.
Not sure what the OP has. I have an 8.1 gas in front of an Allison transmission, which is probably one of the most popular, durable transmissions out there, with billions of miles logged on them, tons of research and testing, and they are pretty much idiot-proof, used in RVs, delivery trucks, etc.
Bottom line: Those Allison folks are wicked smaht

...and I'm not going to second guess them. I let the transmission do its job, as designed by the engineers that built it. I've been up and down hills and mountains, and it's never failed to do it's job. It downshifts when it needs to, and the "grade brake" feature is exceptional when going down hills.

I recall a discussion on a forum years ago where someone was going to "reprogram" their shift points, because they were obviously smarter than Allison...yeah, right.

Go drive your RV and enjoy it...find something else to worry about

Jim

Crasher · 05-31-2014, 07:28 AM

The Allison will downshift when climbing hills and you will get to the top. However, I have found that anticipating what is coming and manually downshifting sooner than the trans does, keeps the rpms up for better cooling. With our trailer, we weigh 50,000# and getting the jump on a downshift has made a difference in ease of climbing the mountain. Torque is what gets you up the hill and HP determines how fast you get there. The best cooling happens to be at the best HP and the torque is only about 10-12% less, so I choose cooling.

Another rule of going down the mountain in the same gear as you went up, isn't always true. On the 11% grade going up to Teton Pass west of Jackson, I could climb it with ease in 2nd gear and some places in 3rd with a former coach, but coming down with the exhaust brake and 2nd gear I would be off the side of the mountain even with the brakes. 1st gear was the only way I could go down without losing the brakes completely. We were pulling a 4000# trailer then.

To the OP. Until you get some mountain driving experience, always error on the side of caution. You can always go to a higher gear if you are too slow, but it can be hard on the brakes to slow down enough to shift to lower gear without over revving the engine.

rvjimmy · 05-31-2014, 09:13 AM

1000 horsepower and 10 lbs of torque will do nothing. Torque is where the pulling is, that said it is true more horsepower usually means more torque too. JMHO.

Jim

tropical36 · 05-31-2014, 12:19 PM

Quote:

Originally Posted by snickers1

Know it might seem foolish but new to Rving and need to know when to shift out of OD to Drive and lower gears? is there a speed, RPM? Going to leave NM and will be driving first time in CO mountains and do not want to burn up engine or brakes. Thanks

Things will pretty much take care of themselves and with your transmission, going up, but going down, you should downshift to the point of not having to use your brakes and if so, sparingly. Forget this business of going down in the gear you came up with, as you didn't go up that side in the first place and Donner's pass is a very good example of that. Brake hard and long if needed with no riding or pumping. Shut off your rear views and don't be concerned with what's behind you. Having said that, pull over when you can to let traffic by.

snickers1 · 05-31-2014, 03:40 PM

Someone said to keep RPM/s up going uphill, what is good level to keep them at, I thought it was best to keep up 3000 rpms?? thanks for the advice.

tropical36 · 05-31-2014, 06:16 PM

Quote:

Originally Posted by snickers1

Someone said to keep RPM/s up going uphill, what is good level to keep them at, I thought it was best to keep up 3000 rpms?? thanks for the advice.

Depends on what you have with HP and torque peaks that vary by the engine. With the later model gas engines, the big block GM V8 will develop this at a lesser RPM than say the smaller Ford V10 that might sound like it's screaming for mercy when running at peak efficiency. Diesels however do best around 2200 - 2500 RPM if I'm not mistaken.
Just a suggestion, but making your rig's description as part of your signature will get you more and better responses.

Rich-n-Linda · 05-31-2014, 06:31 PM

Quote:

Originally Posted by rvjimmy

1000 horsepower and 10 lbs of torque will do nothing. Torque is where the pulling is, that said it is true more horsepower usually means more torque too. JMHO.

Jim

Nope...sorry. Horsepower is horsepower, no matter what torque or RPM values are used to generate it.

Horsepower is developed and measured by using a combination of torque and RPM. The formula is TORQUE x RPM divided by 5252. For an engine to develop 1,000 hp at only 10 foot-pounds of torque would require that engine to turn almost 500,000 rpm. By the time you geared that RPM down to something usable, say 1,500 RPM at the tranny, the torque multiplication due to the gear reduction results in 3,333 foot pounds of torque at the transmission input shaft. That'll work!

To illustrate this the opposite way, how about an engine that only turned 10 RPM, but could develop 10,000 foot-pounds of torque? That's a whopping 19 HP, and that won't get you out the driveway, let alone up a hill.

Rich & Linda

cc8375 · 05-31-2014, 06:47 PM

I use Tow haul almost ALL the time for almost all driving needs.... hills, flat etc.

Here is a good thread explaining Tow Haul
http://www.irv2.com/forums/f23/tow-h...ch-154461.html

Fasteddie08 · 05-31-2014, 07:12 PM

CDL training will tell you to go down the hill in one gear lower than what you climbed it in. That is the proper gear. Use both service brakes and engine brakes (when needed) and you will be fine. Stick to this rule and you will not have any problems.

GA Navigator · 05-31-2014, 07:20 PM

Quote:

Originally Posted by Rich-n-Linda

Nope...sorry. Horsepower is horsepower, no matter what torque or RPM values are used to generate it. Horsepower is developed and measured by using a combination of torque and RPM. The formula is TORQUE x RPM divided by 5252. For an engine to develop 1,000 hp at only 10 foot-pounds of torque would require that engine to turn almost 500,000 rpm. By the time you geared that RPM down to something usable, say 1,500 RPM at the tranny, the torque multiplication due to the gear reduction results in 3,333 foot pounds of torque at the transmission input shaft. That'll work! To illustrate this the opposite way, how about an engine that only turned 10 RPM, but could develop 10,000 foot-pounds of torque? That's a whopping 19 HP, and that won't get you out the driveway, let alone up a hill. Rich & Linda

I agree that horsepower is just a measurement derived from a formula. I think some car manufacturers, due to marketing, from one year model to the next will increase the max rpm redline so this formula will result in a higher horsepower rating from the same engine.
Locomotive engines make 2000-3000 or more horsepower at very low rpm so the torque must be enormous. Dragsters probably make the same horsepower at 10,000 rpm but probably wouldn't last long pulling a train.
I'm sure there are people on here much smarter than me that could explain the relationship in great detail but the discussion has probably gone further in detail than what the OP was looking for.