Surprised to find that I rarely run in 6th gear - what gear do you run in?

Topic: Surprised to find that I rarely run in 6th gear - what gear do you run in? (Read 2059 times) previous topic - next topic

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #25 – August 02, 2014, 07:36:32 pm

High idle= with no load on engine. Would apply to going down a hill with exhaust brake/engine brake on. No fueling, the engine is doing no "work".

All three numbers are VERY important on any engine to allow you to maximize economy, performance and engine braking:

Peak torque RPM
Governed speed (maximum engine working speed)
High idle/max permissible RPM (maximum engine speed in overrun/non-working)

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #26 – August 02, 2014, 08:57:28 pm

Very interesting. I have only made one long trip so far (2750 miles). I noticed my shift to 6th occurred at approx 65mph. I could then coast down to 62 and set the cruise. At 62 mph engine rpm is a shade over 1300. The shifting rpm seems to be at 1500. That said, if aggressively accelerating shifting rpm is at 1800.

I have wondered if travelling at say 55mph in 5th, would permit the same or better fuel economy? I guess another question would be; is one better to run in 6th gear at the lowest possible rpm, or run in 5th at a higher rpm? Tom

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #27 – August 02, 2014, 09:09:45 pm

Quote from: crosscountry – August 02, 2014, 08:57:28 pm

I have wondered if travelling at say 55mph in 5th, would permit the same or better fuel economy? I guess another question would be; is one better to run in 6th gear at the lowest possible rpm, or run in 5th at a higher rpm? Tom

In our experience (noted above), for our coach there is no improvement in fuel economy bumping into 6th gear. Wind resistance factors increase as speed increases, and it appears this negates the higher gear. In fact, in a headwind we will intentionally stay at a lower speed/lower gear.

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #28 – August 02, 2014, 10:24:00 pm

In my experience, on all brands of diesels, high-idle has always just been another word for no-load RPM. This means, push the pedal to the floor in neutral and the governor should hold the RPM at 2710 +/- a few.

Also, don't confuse peak torque for max HP. Torque is just a force...HP is power (which is rate of work done, to get back to basics).

To put this in context, on boats or electrical generators, you will rarely see any specifications talking about torque on the prime mover. All that is discussed is power. Torque only comes up to ensure correct driveline equipment sizing (gears, couplings, shafts, etc).

But one must compare the HP curve and the Torque curve and analyze the entire system. How much HP is being made at that torque peak?

What RPM do you want to be at to climb that mountain as fast as possible...max torque rpm or max HP.....think about it.

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #29 – August 03, 2014, 07:17:00 am

Quote from: Dave and Nancy Abel – August 02, 2014, 05:32:49 pm

Howdy Elliot
Boy, ain't it the truth!!! I think I'll go for the 15yr single malt..
Dave A

'morning dave

Pour one for me to !!!

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #30 – August 03, 2014, 09:57:48 am

Quote from: krush – August 02, 2014, 10:24:00 pm

Also, don't confuse peak torque for max HP. Torque is just a force...HP is power (which is rate of work done, to get back to basics).

Torque gets you up the hill, horsepower gets you up fast.

Roland

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #31 – August 03, 2014, 01:08:44 pm

Quote from: Roland Begin – August 03, 2014, 09:57:48 am

Torque gets you up the hill, horsepower gets you up fast.

Roland

I have driven many of the older motorhomes in the industry with more power to weight ratios than my u320.

If memory serves me they all got better mpg.

Obviously the smog standards affected this.

Favorite was the original unicoach with the hotted up series 60 Detroit. Screamer.

The old signature with a m14 at 444/1850 had a strong enough jake to feel like the retarder.

8v92 concept. Fun. What hills.

My m11 is not in the ballpark. Will change out the muffler soon and see.

I think I may have found someone who will not admit publically that they can reprogram the ecu in the engine if I left it off for a few days.

I have had that happen a few times long ago where the shop mentioned I would like the coach better but gave no details.

The m11 works ok just flat on top. I hope that's the muffler?

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #32 – August 03, 2014, 01:23:28 pm

I guess I drive too fast. My last trip across West Texas on I10, I was in 6th for 350 miles. In Houston I drive across town in 6th gear. It's just easier to stay with the flow of trucks.

All it costs it fuel.

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #33 – August 03, 2014, 04:27:35 pm

My experience is very similar to TheBrays. I had my heart set on a M11 when we were looking for a coach, but it wasn't meant to be. The perfect coach came along with a mechanical C8.3. I don't know why I worried, because it pulls just fine. It shifts into 6th at around 50 mph (1500 rpm). Our coach has a dual display, so there is no wondering what gear I'm in. We drove through the Flint hills (KS) coming home...cruising at 56 -57 mph, and it never downshifted. My wife, who was driving our Jeep Liberty toad - with a 3.7L - shifted out of overdrive long before reaching the top of the hills.

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #34 – August 03, 2014, 06:21:49 pm

After 5000-7500 feet depending on the individual engine all diesels start to lose power.

The mid west and east coast users can use the 8.3 better.

Impatient Rocky Mountain drivers like the bigger motors versus 40mph on all 6 percent grades.

At that point you are in and out of the truck lanes on every hill. Not fun after a while.

I sold many, many ored 300 cats long ago because they would pull all the interstate grades without a car up to 90mph. And get 8mpg wide open. 10mpg at 70-75.

20,000 pound coach is how. Fun to pull everyone up long grades. Evil grin on your face.

Waaaay fun. Miss that.

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #35 – August 03, 2014, 06:55:37 pm

I'm late to this discussion, but we went out for a drive today and I took notice of my 5th to 6th shift point. We have the same engine as Elliott (TheBrays) - C8.3 300hp. Our coach shifts into 6th at 53 mph (mode switch on or off - doesn't matter). Full throttle up-shifts occur at 1800 rpm. Normal shifts are so smooth that I can hardly feel them. Fortunately we have the digital readout to indicate both selected and attained gear. That Allison 6-speed is a SWEET transmission!.

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #36 – August 04, 2014, 06:56:24 pm

I am wary of "lugging my 8.3 below 1400 as the Banks pyrometer starts to shoot up towards 1100 real quick . Truckers and older diesel RV owners alike have assured me that down shiffting and letting her spin over 2000 is much less harmful than seeing how much grunt she can muster down low...RPM's equal airflow , airflow cools exhaust side. Exhaust side is Achilles tendon of these brutes!

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #37 – August 05, 2014, 01:33:18 am

You should be OK at 1100 degrees with an normal range up to 1200. Factory recommended max is 1350 for that engine as I remember.

EGT probe should be pre-turbo otherwise post turbo reading will be at least 300 degrees lower than they actually are depending on the load. Post turbo readings are not really EGT and will give a false sense of security.

Pierce

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #38 – August 05, 2014, 12:53:30 pm

I haven't yet been able to pay attention to the exact shift point, but our 1995 U320 (400 hp Cummins M11) is definitely in 6th gear at 55 MPH under typical conditions.

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #39 – August 05, 2014, 11:55:34 pm

Probe is in manifold directly under turbo. Replacing manifold this week after work. Have to tap new manifold for probe.Have several busted bolts in head. Think its time to invest in a high quality tap set and reverse drills!

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #40 – August 08, 2014, 04:15:47 pm

Since we don't have a pyrometer to detect EGT (450HP M11 ISM) U320 4200, is there any guidelines as to keeping EGT's low? I noticed that if I set cruise to 61mph it will tend to downshift to 5th pretty often. If I set to 62.5 or so it will tend to stay in 6th. But am I doing damage running up hills where it can barely hold 6th?

What guidelines would you follow to drop 5th or go over 2000 RPM to avoid high EGT's and problems they cause?

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #41 – August 08, 2014, 04:41:55 pm

If you are not operating your STOCK engine (we are talking about modern, 4 stroke, turbo, after-cooled engines) below PEAK TORQUE RPM at HIGH THROTTLE POSITION you have nothing to worry about in terms of EGT.

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #42 – August 08, 2014, 05:04:42 pm

I think in my case the transmission may not allow that then so I would have nothing to worry about.

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #43 – August 08, 2014, 05:33:47 pm

Quote from: piku – August 08, 2014, 04:15:47 pm

Since we don't have a pyrometer to detect EGT (450HP M11 ISM) U320 4200, is there any guidelines as to keeping EGT's low? I noticed that if I set cruise to 61mph it will tend to downshift to 5th pretty often. If I set to 62.5 or so it will tend to stay in 6th. But am I doing damage running up hills where it can barely hold 6th?

What guidelines would you follow to drop 5th or go over 2000 RPM to avoid high EGT's and problems they cause?

Mine and others shift into sixth at 53-54 mph. Either your trans was programmed differently for your heavier 42' or its incorrect. Torque peak is 1350 rpm. Past that the torque goes down and the hp goes up.

Faster using the torque versus the hp.

Or the engine has a low power issue and the trans is trying to compensate for it by staying in the lower gears until enough hp shows up and it then upshifts.

Unless all 99 42's do the same thing from the extra weight?

Defintely needs the dr to look at this I would think.

Check for dirty air filter. The restriction gauges are notoriously in accurate.

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #44 – August 08, 2014, 05:55:34 pm

This is one area where I think our coaches differ. Peak torque on the ISM is 1200 RPM rather than 1300 RPM. Several people here have noted that their shift point for 6th is at 62mph and at that speed it doesn't seem like especially high or incorrect RPM. It feels right. It's going maybe 1450RPM when it upshifts. The bigger engine also has the 4060R transmission and the rear is probably different and geared differently as well.
EDIT: I see you have a 320 as well. interesting. All I know is the coach feels fine and it hauls. It wil happily climb a big hill (say 3-4% grade) in 6th, 62mph 1400 or so RPM pulling a 6000 pound toad. Air cleaner is new, as is oil, coolant, hoses, and other bits.

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #45 – August 08, 2014, 06:00:37 pm

Actually now that I think about it, for the longest time I thought I was in 6th at 55mph. It will not go into 6th until 62mph. Have you ever tried to go faster? You might be in 5th.

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #46 – August 08, 2014, 06:33:27 pm

Quote from: piku – August 08, 2014, 06:00:37 pm

Actually now that I think about it, for the longest time I thought I was in 6th at 55mph. It will not go into 6th until 62mph. Have you ever tried to go faster? You might be in 5th.

No in sixth for sure as I have the dual pad indicator on my 97 320.

You have the same tires and rear end ratio I think. You are shifting past the torque peak it seems which is either to put less stress on the trans by Allison on purpose or its been reprogrammed somewhere along the line.

Need a few other owners to post regarding their shift points to know if (1) it is on purpose (2) any benefit to lowering the upshift points (3) any different acceleration up grades if the shift points are lowered (4) does the raised shift points relate to smog standards on the engine which I doubt.

Long ago Allison mis programmed the shift points several different ways on ored 300's.

I fixed them locally and the project manage called me from Indianapolis asking why?

He was mortified that after all the meetings they got it wrong and some end user sales manager figured it out and had to fix it locally. Normal.

Corporate America at its best sometimes.

Like I said. If the others here are the same either its for a reason or they are all incorrectly programmed.

Maybe the torque at 1200 upshift caused a "jerk" or a noise. I can hear mine squeek a bit as it shift to fourth at the torque peak normally. I had the driveshaft and joints lubed to see if it affected this. A bit maybe. Heavier coach, higher power may have well needed the past the torque peak upshift. Not normal to upshift past the torque peak on any diesel I would think.

That's why I drove every new and used coach as the owners without a comparison had no idea what was right or wrong. I did.

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #47 – August 08, 2014, 07:48:45 pm

I first noticed this shift thing when I was approaching a grade that I thought might cause a down shift to 5th. So just as I'm starting to hit the grade I shifted down to 5th. Wrong, it didn't shift, already there.

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #48 – August 08, 2014, 08:34:02 pm

Well that's very interesting. There are definitely more than one of us in this thread alone. I wonder what the deal is. My coach is heavier for sure, but not that much heavier.

Re: Surprised to find that I rarely run in 6th gear - what gear to you run in?

Reply #49 – August 08, 2014, 08:39:11 pm

On our 03 it shifts into 6 th around 61 mph. When I am running 65 mph my rpm is 1405

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