Engine/transmission braking in gas engines

Alan_Hepburn wrote:

Ivylog wrote:
If the nut behind the wheel cannot reach up and pull the lever down or back up then I guess the tow/haul is a good thing. Any gas engine does a good job of braking but you have to take your foot off the gas and manually down shift to get the rpms up.

The difference being that the computer does some decision-making before shifting: it doesn't take it very long to ask itself "If I downshift now will it cause the engine to over-rev at all?" The nut behind the wheel can also ask himself the same question, but then he has to know the current road speed, gear ratio, and engine speed so he can calculate what the resulting engine speed will be after the down-shift, then decide if that engine speed is within the safe operating range of the engine and reach up and make the shift. He can also over-ride his calculation and say to himself "I don't care what my calculation says - I know my engine has a built-in safety factor and it will be okay". It's also a lot easier to simply put your foot on the brake pedal momentarily rather than to fiddle with the shift lever to make sure you only shift down one gear - or shift back up a gear without pulling it all the way up into Neutral...

The Allison programming makes it impossible for the drive train to be damaged by a shift - it simply won't shift unless it can be done safely.

True however, one should downshift into the gear before starting down the grade to avoid an over rev scenario, although most modern transmissions won't let you downshift anyway if it will cause an over-rev. I read once that a good rule of thumb is use whatever gear it takes to get up the hill, as your gear coming back down. Granted, you would of had to come up the grade at some point to know what that is :)