When to shift

Discussion in 'Questions From New Drivers' started by Taytay051, Jul 11, 2019.

  1. Taytay051

    Taytay051 Bobtail Member

    Mar 31, 2017
    So Firstly, I apologise if this is the wrong place to ask, but I wasn't sure which part of the forum to post this.

    So I have been driving a COE Freightliner (Argosy) now for 4 months or so, with a DD15 and Eaton Autoshift, and I've been getting numerous different opinions from drivers and trainers about shifting it.

    So this is an Automated manual transmission, it runs in auto or one can manually selected the gear.
    One of the pieces of advice I've heard is to just leave it to do its own thing, However on a downhill piece of road, I do not trust it to select a gear for me, and I don't think I ever will.

    On the other hand, I know plenty of the drivers use them solely in manual, as they find the auto "Dim-Witted"
    I get the impression from my time in them, that it seems to mimic the way its driven, eg: if you drive it manually up the hills and hold it at a higher rpm, then it tends to over-rev when its in auto on flat ground.

    This is another piece of confusion for me, I know guys who pull up the hills at around 1700-1800rpm, because thats peak power, But Im also told the torque curve for the DD15 is between 1000-1500, so that seems to be working it far too hard. In my exploring with it, I find it works hardest up the hills between 1400-1600 rpm, But then am I better letting run lower? Will I see better fuel economy working it at lower rpm?

    I was also told, by our driver instructor, that the Jake works at best between 1800-2000 coming down a decline, with the engine fan on for extra drag. This seems ridiculously high to me, But at the end of the day I'm the rookie. Just curious where others like to let them sit.

    Finally, around town the auto has this strange habit of starting in 3rd, revving its self up to 2000rpm, changing up 3 gears. It does this up until 9th or 11th depending on how heavy we are, before then taking two gears or even singles. This does get you up to speed pretty quickly, but again, opinions on that? seems like high rpm for no real reason?

    My Trip is about 8 hours all up, I go through a variety of mountain sections and flats, Motorways and backroads and then spend a bit of time in the city.

    All opinions welcome.

  2. uncleal13

    uncleal13 Road Train Member

    Apr 9, 2009
    Humboldt, Sk
    All sounds about right.
    Best fuel economy under 1400 rpm.
    Best power up to 1800 rpm.
    If you are jaking and put the transmission selector into L, it will rev it up to 2000-2100 rpm. Nothing wrong with it. But 2300 is the limit, 2500 some have come apart.
    I’ll let it lug down to 1100 on small hills but on the long climbs I keep it between 1300-1500.
    Mine is also programmed to default start in third gear. But if I’m starting on a hill I’ll select 2nd or 1st.
    MACK E-6 and x1Heavy Thank this.
  3. x1Heavy

    x1Heavy Road Train Member

    Mar 5, 2016
    White County, Arkansas
    whoa whoa whoa .. easy there.

    I'll work backwards.

    1- In town the Auto has one overriding commandment. Save fuel. So it will make the largest shifts up into a gear that barely turns over while you creep around town at a low RPM. Unfortunately it's excessive shifting will wear it out. And you end up binding the entire drive system with a loaded down form of lugging that it will NOT like once you try to get on it.

    Engine fan is just a Accessory load. If you had say a 500 Detriot, you are going to parasite the potential 500 horses with Alternator, waterpump engine fan, air conditioning, heater, Air compressor and so on. Your drives, transmission and shaft etc leeches more horses. You end up with about a 390 to 410 horses at the drives ready for you.

    You come up and go down the mountain in the same gear. So. If you are upgrade in manual mode, hold the RPM's somewhere above torque at all times but not past the high horse RPM. For example a engine gives you a 1200 to 1650 shift range. 1500 being the sweet spot. Redline at 1950. You will be climbing that hill somewhere in the 1400's As long your pyrometer, your oils (Axle, transmission, engine oils) are pressure happy and relatively warm but not lifethreatening hot. And that your water temperatures is holding steady and so on all the way up.

    You do lug your engine down to about 1200 before you down shift. If you do it right she will settle at around 1650 for our hypothetical driving upgrade problem and then slowly surrender RPM's if the load is heavy until you need either a downshift again or you meed the required reduction and power to finish the pull comfortably.

    Your trainer talking about engine fan extra drag. Well... There are certain places around the USA where temperatures are life threatening and your truck is hurting so you turn off as much of everything you can to reduce the load on the engine and pull really easily like you got a egg between your foot and hammer. One rpm all the way up.

    Your fuel economy is not what we worry about when lifting 40 tons up and over a stiff mountain ridge, grade or whatever. You are actually burning gallons per hour. Some of the worst grades Ive had was with a 330 or so Cat back in the early 90's and it comes out to roughly 34 gallons per hour all out vs a three mile straight up 20+% grade that took a hour to pull at slower than walking pace. ATS gave me a tractor that only had one fuel tank. I had loaded in Beecher Falls VT at the border with about 62 gallons remaining out of 145. It's aggravating. And to see the kitty drink half of that again up and over... it's a problem.

    Revisiting 1200 Torque to 1650 high horse power rating on your engine chart with a redline of 1950. Anything beyond high horsepower is a curve down, meaning that engine gets weaker and cannot accept a ever increasing pull. You are already all out or in the old Engine talk "Balls out" regarding a governor in the days of steam engines that drive factories. You cannot get any more work out of it.

    The engine going uphill with a load will fall in RPM and you down shift at 1200 properly until you find a correct gear ratio at which the 18 wheeler will be able to lift up and over the mountain at a relatively calm RPM, say 1450. Sweet spot. You can pull all day any mount on earth at that level provided you have fuel, cooling and nothing breaks.

    Anyone can run up hill. It's the getting down that seperates boys from men.

    Referring to the rule that the gear you came up is the gear you go down or one or two lower. Slow and easy does it.

    Not 5 mph slow with 80 mph traffic slamming into your trailer. Throw in the Jacobs Braking. Get it to where it's close to redline above high horse. 1850 or so for me for a 1950 engine. Older Iron would rev to 2300 and I take it all I will redline that Jacobs at 2300 Provided the engine pressure is steady, oil temperature is steady agreeing with transmission and axles relatively equal around 220 degrees etc If the grade is balance going down, you touch no brakes, no shifting no thing except your coffee now and then keeping your brakes and drums really really cold and ready for battle.
  4. Trucking in Tennessee

    Trucking in Tennessee Road Train Member

    Mar 19, 2018
    Engine brakes are great these days. Leave it alone and let the brake work. I never go above 1500.
  • Draft saved Draft deleted