Downshift or not to downshift?

[Pinkie]

I was talking to a few people at Aberdeen this past week and I heard someone say downshifting the multifuel motor was not good. It seems the theory is these motors do not handle using the engine compression to slow the truck down. What are your thoughts?

 

[builder77]

Well an engine brake would be doing the same compression thing. Cranetruck has put some miles on his truck since he put on the brake and his engine is.....well.....hmmmm. I have had no problem with downshifting, but don't know.

 

[CanonNinja]

so, my question is, how is it any worse than running up rpms in a gear, then braking without using the clutch?


but to answer too, i've found that its minimal braking assistance keeping the truck in gear... it just doesnt have that much more drag while braking (or i've got a major compression issue)

 

[cranetruck]

Re: RE: Downshift or not to downshift?

Well an engine brake would be doing the same compression thing. Cranetruck has put some miles on his truck since he put on the brake and his engine is.....well.....hmmmm. I have had no problem with downshifting, but don't know.

The multifuel engine is not much different from any other diesel when it comes to engine braking, very little available. In a gas engine, closing the throttle creates a vacuum and it is used as a means of braking via the drive train.
The diesel is always wide open, no throttle, hence no resistance to piston movement...unless a butterfly valve is installed in the exhaust pipe to make it hard for the pistons to "breathe out" (exhaust). The "Jake brake" is more complicated, turning the engine into a compressor, but is not available for all engines.
I have had the exhaust valve on my truck for several years now and it is the greatest, driving on long down hill runs. Used it extensively in the Rocky Mountains last fall with a 9,000 lb trailer to boot. Mile after mile in 5 to 8% hills and hardly ever touched the brake pedal. The higher the rpm, the more braking power, so a low gear is best.

My engine problem is not related to the engine braking or the biodiesel/veggie fuels, it is caused by poor oil maintenance, caused be a leaking main bearing seal among other things.

The military is cautioning you about down shifting because it's very easy to overrev if not down at the right time.
There is an MVM article about all this, check it out.

 

[rdixiemiller]

RE: Re: RE: Downshift or not to downshift?

I downshift when the grade slows me down below about 1600 rpm. I only do so to avoid lugging the engine. I downshift when approaching a stoplight just to keep in the correct gear to accelerate out when the light changes. I too have noticed minimal compression braking with the multifuel engine.

 

[5tonpuller]

RE: Re: RE: Downshift or not to downshift?

Mr. Cranetruck. Is it well worth the money to get one of them ext. brakes for the 5 tons?

 

[cranetruck]

RE: Re: RE: Downshift or not to downshift?

You bet.

http://www.brakesystemsinc.com/wm771.html

 

[cranetruck]

Here is a detail of the installation ( I got mine used and didn't pay much for it, but it really adds a safety factor when driving down hill, well worth the full price of a new one).
The air operated piston moves the butterfly valve from open to partially closed.

Keeping it closed helps warm the engine faster too, a side effect.

 

[Djfreema]

I usually just leave the truck in gear when slowing down to a stop until its just above idle then take it out of gear. I try to keep downshifting to a minumum to save on clutch wear. I figure the brakes are alot easier to change than a clutch.

 

[DDoyle]

The military is pretty emphatic about not walking the Multifuel down through the gears as a brake. In a PS magazine article they specifically state that one should use the service brakes, and not the engine. The article goes on to indicate how downshifting can actually increase the vehicle speed.

Best wishes,
David

 

[cranetruck]

The article David is referring to is in the PS Magazine "Driving" section.
It's important that it is interpreted correctly.
The worst thing you can do going downhill is not to shift down, freewheeling and relying on the brakes alone will end in disaster.
The engine/drivetrain has a resistance to motion, I "measured" it by experimenting for the article in the April 2005 issue of MVM and it's about 40 to 50 hp for the deuce.
I also put together a rudimentary table showing how fast (or slow) you can go downhill without using the brakes at all.

 

[cranetruck]

Here is another set of instructions from PS magazine (1975).

 

[jimk]

There are laws prohibiting coasting in N.

I often downshift in the -higher- gears when slowing (or controling speed). I am careful to match input/output speed by 'double clutching' though I often skip the first clutch by unloading the drivetrain and slipping in into N (make'n me an outlaw I suppose). Holding the clutch down for long periods works the throwout bearing (and much more so at hi-speed as opposed to idle). It also puts side load on the crank thrust bearing.

Running at 2600 uses more fuel,offers less torque, makes more noise, increases ring wear...The engine seems efficient at 2000. I'll upshift for this zone (+/-300) if practicable.Or perhaps slow down a bit for it. JimK

 

[Djfreema]

I guess I should clarify that I downshift when coming down hills to help keep the brakes cool and use as much engine braking thats available to help maintain a safe speed, but when Im just driving in regular traffic/flat areas, I dont. When driving these trucks I am always thinking about brake failure and planning for it just in case. An exhaust brake is definately in the plans for my truck.

 

[alphadeltaromeo]

I have always down-shifted each vehicle I owned that was manual. With the deuce I also down-shift, but am careful when doing so. I don't see any issue with using common sense when down-shifting. Most of the time I'll down-shift 4th and 3rd, but rarely do I go to 2nd...it's generally too slow at that point. I found that when towing the 6 ton semi, that down-shifting proved helpful to my braking.

 

[madsam]

used to drive OTR full semi trucks. The last thing you wanted to do under load is faid your brakes. Worked for an moron who had bad trucks and lost 90% of my brakes coming into Steamboat springs co with 45000 lbs of potting soil. (Full 80klbs) On a 14 mile 8% grade hill. The cat was doing about 3500 rpm because the engine brakes did not work on this truck too. I wasn't about to try any shifting at that time. I have no idea what kept the engine together. (Devinne intervention.) The only time I ever thought about using the run away truck ramp. At the bottom of the hill I just coasted to a flat spot and killed the engine without setting the brakes. If you set the brakes when they are hot, they can stick. Smoke was coming off them. Turns out this guys trucking company had the worst safety rating in Colorado. I quit this guy the next day.

I have seen several trucks burn up at te bottom of I70 hill coming into Denver from the mountains. The brakes got so hot they started the duals on fire.

My 2 pennies? If you are pulling a load down a hill slow down and down shift, making sure not to over rev. In town at slow speeds unloaded, no real need, but you can. as long as you don't over rev. Loosing brakes on a hill with enough weight on your back to kill you, sucks. (The old song 'Wolf Creek Pass' went through my head. )

 

[JasonS]

Just one point of correction. A gas engine doesn't really have any more braking ability than a diesel. Once the throttle butterfly is closed, it isn't pumping anything.

 

[cranetruck]

Just one point of correction. A gas engine doesn't really have any more braking ability than a diesel. Once the throttle butterfly is closed, it isn't pumping anything.

Well, actually when the throttle is closed, there is resistance to breathing, a vacuum is created, which provides braking power.
The diesel has no throttle and there is no resistance to "sucking" air or exhausting unless a valve is added in the exhaust (=exhaust brake). The Jake brake is different in that a controlled modification of the valve action temporarily turns the engine into a compressor.

You could also add a "throttle" in the diesel intake, which would also serve as an emergency shut down in an engine runaway situation, but it is not as effective.

Other driveline brakes include the electric and the hydraulic types.

 

[FormerNewMVGuy]

I downshift my deuce all the time. I mean i dont get crazy with it and over rev the engine, but my foot is off the brake pedal as much as possible. If you start to slow your truck well before the stop sign you should be able to walk it right up to the sign without useing the brakes till you get there. some body posted about not downshifting because of clutch wear? I must just be an old trucker, but i ussually only use the clutch to start and stop. you would be surprized at how nice these trucks shift without it. At least mine does.
I have a banks exhuast brake on my 2001 dodge3500 with cummins5.9 I pull a 33Ft camper with it and i dont think i could be without the exhuast brake once i had it. I never thought about it on the deuce but now I'm sure i'll have to have it. Do you know the average cost of that model? Mr cranetruck

 

[cranetruck]

Go here for details:

http://steelsoldiers.com/index.php?name=PNphpBB2&file=viewtopic&t=3465&highlight=exhaust+brake

For a new one, expect to pay $400-500 (Blue Ox?).