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Topic: Downhill Speed Control - Proper Gear Selection vs Retarder (Read 5141 times) previous topic - next topic

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #100 –
Mark,

The governed RPM is for the engine under load-- read that working HARD.

Touching the throttle enough to cause an upshift (say 5% throttle) is a non-issue.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #101 –
On pulls, 7% and higher, I do keep a close eye of the gauges, but have never had to downshift, the Allison is one smart cookie!
On pushes, same deal, start slowly, hit the PacBrake and toggle it as needed, let the Allison decide.  ^.^d
1993 U-240 "La Villa Grande"..CAT 3116 w/ Pacbrake PRXB...Allison 3060 6-speed..
Previous: 1983 Airstream 310 turbo diesel, 1979 Airstream 280 turbo diesel
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Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #102 –
Quote from: wolfe10 –
Mark,

The governed RPM is for the engine under load-- read that working HARD.

Touching the throttle enough to cause an upshift (say 5% throttle) is a non-issue.

Thank you, that's very helpful info.

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #103 –
We bought our coach for a number of reasons.  One was the safety of a diesel pusher with retarder in those CO mountain descents.  I have not been disappointed and enjoy the ability to descend with both feet flat on the floor and only rare use of the service brake.  The DW commented one time "You know you are going slow don't you?"  To which I replied by pointing out the semi ahead of us doing the same thing.

One aspect of mountain driving I didn't really appreciate was the need to gear down on the climb to limit the transmission from up shifting and getting into too low of an RPM range.  We were heading down to Boulder (having grown up in the midwest north is up, but in CO you have to balance up being north with going down in altitude....thus despite living south of Boulder, because it is lower in altitude it is down) on a hot July day I received an engine temp warning.  Luckily I was at a spot where I could pull off into a business parking lot and let things cool down.

For those of you that have taken Hwy. 285 from Kenosha Pass (9,990') into the Denver metro area, you know that yes you descend 4,000+' but the highway has a large amount of ascents and descents.  I failed to keep the RPM's high enough on the ascents to keep the engine cool.  After reviewing this forum and the Allison manual I realized I needed to gear down to keep the RPM's within 500 of the governed limit (2,100 I believe).

When I got home I was talking with my neighbor who drives truck for the CO Department of Transportation and he said they have diesel trucks with Alison transmissions and they absolutely gear down when climbing Monarch Pass (~11,300') to keep the RPM's high and improve cooling.

Lesson learned!

Tom and Lisa

2003 U320 38' PBBS
Build No. 6217

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #104 –
It was fun to revisit this post from our trip out west last year. We learned a lot and here's my 2 cents. Yes, the retarder works. But I now use it less frequently.

Brett Wolfe, and others, taught me that engine RPM is my friend for braking and cooling. I've even learned it can help with gas mileage. On uphill grades I now downshift as needed to keep RPM above 1800. On our coach Cummins manual says peak power at 2000 RPM.  Even moderate grades on the interstate with cruise on benefit from manual downshifting to maintain speed.

On downhill grades the "run the gear down that brought you up" works before considering the manual retarder. Periodic service brake use....firm and short...benefits from retarder activation from the service brake pressure and keeps tranny temps, especially retarder temps, moderate. 

I'm not saying I don't use retarder, just that manual use hasn't served me as much as exercising engine RPM with manual get changes.

Remember, just my 2 cents.
Randy (N4TDT) and Karen Crete
Sarasota, Florida
SOLD:  2000 U270 34' WTFE Build 5756 "Ole Red"

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #105 –
Randy,

You are a diplomat. Yes, gear down going up to keep EGTs down, especially at higher altitude. Gear down coming down and remember, the retarder is a secondary braking device after both the service brakes and engine braking at higher RPMs.

Not my two cents, just the law of survival.

Tom, when you got that engine temp warning, imagine where the EGTs were at that time. Review the EGT video from a couple of weeks ago. This is why engines drop valves. Since the damage is cumulative, no one can say that they got away with overheating. I don't think twice about running up at the governed limit (2130 RPM) on the way up grades. Better to operate at hight RPM and use less throttle. I usually don't exceed 2000 on the down grades as I don't like to have to concentrate on not exceeding red line but the Jake usually does the job. In Colorado, the pass you describe is just the topography that can shorten a diesel engine's life. Remember measured altitude and density altitude can be several thousand feet different. The high temps and less dense air mean less efficiency for both radiator fan and the radiator itself. Again, less effective cooling, less power at altitude and not keeping RPM way up are the perfect storm for high EGTs. Mechanical diesels are even worse at high altitude.

Pierce
Pierce and Gaylie Stewart
'93 U300/36 WTBI
Detroit 6V-92TA Jake
1140 watts on the roof
SBFD (ret)

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #106 –
This is a great thread - I learned a lot after reviewing the last serval pages of the thread. I enjoy hearing from everybody's experience coupled with the science and mechanics behind the engine, trans, brakes, and vehicle weight. Thanks to all for posting. I will certainly use this info in my travels.

PB

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #107 –
Quote from: Pierce & Gaylie Stewart –
Gear down coming down and remember, the retarder is a secondary braking device after both the service brakes and engine braking at higher RPMs.

I see this differently, here's why.  I agree, the service brakes are the primary braking, but I would prefer to exhaust my secondary braking first before getting heavily into the service brakes.  In practice, keeping RPMs up and retarder typically meet 95% of braking needs, (Rockies, Cascades, Bighorns) and an occasional stab on service brakes is used to keep RPM/retarder/speed functions within limits.  I would hate to be in the position of having exhausted my service brakes and relying on the retarder to haul it down.  Load shedding would be a problem....
"Not so  long ago we were a nation of risk takers, riding five million pounds of  thrust straight into space."  Joe Gresh
Chuck Pearson
1996 U295
2018 Can Am X3 TurboRS

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #108 –
Chuck,

That is not where I was going at all. Trucks and buses came with service brakes and gear boxes long before retarders came along. Using the gearbox/engine to keep speed under control is what professionals have been doing since they put diesels in trucks and buses. Too many RVers rely on the retarder to do most of the braking and have no real knowledge of how to use the service brakes in conjunction with the gearbox to keep downhill speed under control. So, when the yellow retarder light comes on, they don't have the knowledge or experience to know how to continue a safe decent. Sure, use the retarder but don't count on it to solve every grade decent encountered. I took many trips, in the Rockies with my old Greyhound. No retarder, just drum brakes and a Spicer gearbox. The grades never were a problem. Sure, I would have welcomed a retarder or Jake but I had experience on how to make the descents with only what I had. My point is many new (or old) drivers skipped the basics and see the retarder as the #1 braking system.

Same with the knowledge of altitude, temperature, barometer and humidity. Yes, even humidity. It's like learning to be a detective. Until you have this knowledge, you can't understand how the environment effects each and every trip you take. That's why the PO's driving habits can catch up to their old coach thousands of miles after they sold it.

It's no accident you see RVs pulled over part way up long grades. 90% of drivers can't really see the difference between driving their family SUV and the big rolling apartment they are behind the wheel of other than it's bigger and uses more fuel. Fortunately, this forum introduces the newbie to the safe operation of their pride and joy.
Pierce and Gaylie Stewart
'93 U300/36 WTBI
Detroit 6V-92TA Jake
1140 watts on the roof
SBFD (ret)

Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #109 –
Quote from: Pierce & Gaylie Stewart –
The high temps and less dense air mean less efficiency for both radiator fan and the radiator itself. Again, less effective cooling, less power at altitude and not keeping RPM way up are the perfect storm for high EGTs. Mechanical diesels are even worse at high altitude.

Pierce

And to determine Density Altitude you really should have an altimeter, outside air temperature and an E6B to do the math. I have two out of three and I've experienced density altitude in an Ercoupe leaving Orange, MA in the summertime 2-up.
1992 Foretravel Grand Villa
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Re: Downhill Speed Control - Proper Gear Selection vs Retarder

Reply #110 –
I've not used my E6B for ages but it brings back a lot of memories. With 85 hp, your Ercoupe needed lots of cool air if you were anywhere close to gross.  I had a Emigh A-2 Trojan 8324H pictured in the link. emigh trojan for sale - Google Search It had the same engine but the ribs on the outside of the wings. Really had to be careful out in the Mojave, especially in summer.

OK, you don't need your E6B any longer for most stuff. Here is a density altitude calculator much like the one I use. You first look at  your GPS for the altitude, then you go to Windy: Wind map & weather forecast and zoom in to the state where you are. You make sure the "airport" on the bottom right is turned on and then just click on the closest airport. Up comes the info in terms used by airmen, but if you click on the airport, a left hand column comes up with everything in plain English. The humidity will be called the dew point and is in Celsius so you have to remember that when pugging in the values in the density altitude calculator: density altitude calculator

Pre-flighing an RV trip is almost as important as doing it for a flight, especially if extreme weather may be encountered. Heading across the Southwest, you can check the current winds with velocity and then use the slider at the bottom to see when they may weaken or stronger and which direction they are coming from. Several years ago, I posted photos of an RV and big rig on their sides within a few miles of each other. Same with temperature. Just a lot of information in an easy to understand visual presentation. So, Windy: Wind map & weather forecast or others like Ventusky - Wind, Rain and Temperature Maps. I find windy is easier to check the local airports for info.
Pierce and Gaylie Stewart
'93 U300/36 WTBI
Detroit 6V-92TA Jake
1140 watts on the roof
SBFD (ret)