4104 driving advice.

[Harry6674]

I seem to spend a lot of time at speed limits of 25-35-45mph. When I'm at 25 my bus is 2000 rpm in second or 1300 in third. It works out the same at 35 and 45. The speeds are with a Garmin the speedometer is off quite a ways. So which is best 12-1300 or 2-2100? First longer (150) mile drive since I've owned it and found major oil leak in the front of the engine somewhere. The fan is throwing it everywhere. Still more work to do.

[bevans6]

I would say you are about ready to shift to third at 25 mph so I would run in third at that speed.  I would probably run in third up to around 50 mph, then shift into fourth.  You can run the engine at 1300 rpm just fine if you are loping along at a steady cruise and light throttle, it's asking it for full power at lower rpm that is lugging it.  From the old stories some of the old buses were governed to 1800 rpm for fuel economy.

Brian

[HB of CJ]

It is complicated and depends upon the drivers perceived load on the 671.  We were taught to drive the 4104, (not the 4106 which was different) with lots of "free" engine rpm, especially in the few higher gears. 

What this translates into is that if you had a choice of running in either 3rd or 4th, pick 3rd.  If you had a choice between 2nd and 3rd, pick 2nd.  But...sometimes between 1st and 2nd, maybe either one.  Depends.

If on the flat and level like in town or just going slow, then the higher gear and lower rpm would be best.  But...if climbing or placing a load on the engine, then high rpm is the way to go.  You only have 4 gears.

HB of CJ (old coot)  My old Crown Supercoach's RTO910 had 10 forward gears, all quite close together.  At 55 the coach could take either 8th, 9th or in some conditions, 10th.  Just the opposite from the 4104.

[Harry6674]

Thanks guys.

[gus]

The 671, like all DDs, likes to run fast. Never lug it, 1300rpms is too slow under any kind of load.

This is the nature of the two stroke, totally different than the 4-stroke Detroits.

[TomC]

Under low load situations, my automatic will shift with the rpm coming down as far as 1,000rpm. As long as you don't feel jerking, or rumbling or see black smoke, letting the engine down to 1,000rpm won't hurt it. But I would suggest you use 1,200rpm as the rule of thumb. Coasting through a town at low speeds, around 1,200rpm is fine. The only time 2 strokers need rpm, is when you're asking them to work.

My first truck had a mechanical 8V-92TA with 9A90 injectors. I had a tune up at Delaney and Ahlf in Bakersfield, Ca. I wanted to get better fuel mileage. They said to up shift at 1,800rpm and down shift at 1,400rpm. Not only did my mileage go up, I actually pulled the hills better since I was down in the torque range. I ran one time with a guy that had a Cummins NTC400Big Cam and another with a Cat 3406B. Out pulled both on the hills keeping the rpm down.
But-non turbo and non aftercooled Detroits like to be revved. Driving a 6-71 up shifting at 2,400rpm and down shifting at 1,900rpm used to be the norm. Good Luck, TomC

[zubzub]

I would encourage you to use your ears and other senses....I have no tach and no speedo on my '04 so just shift when it feels right....and if it is not smooth somehow I compensate the next time and it all works.   Sometimes all the info can distract from the moment.
Still I wish I had a tach and speedo....almost there.

[gus]

On a climb, if it won't accelerate with added fuel, I downshift.

Upshifting I always run it up to the gov when traffic permits.

[TomC]

If you have a working speedo, the suggested ways to drive the 4spd is accelerate up to the governor, shift, then notice what speed you're doing. That will be your down shift point. It's too bad a 6spd wasn't made for the Vdrive-the extra lower gears and having the gears closer would have been nice. Good Luck, TomC

[John Z]

I agree with you Tom about a lower gear for the 4104. Camping over the weekend I was backing into a tight spot and got off the "driveway". I could feel that it was rolling pretty hard. Had to work pretty hard to get out of there. After getting parked on the hard stuff, I went back to look. The driver side wheels had sunk over 4". I would pay dearly for a granny gear.

[gus]

JZ,

This is the real advantage to an AT:)

[John Z]

Hey Gus. Yeah, I wish the '04 was easier to do those kinds of upgrades to.