A Trip to the Dyno

[BrianE]

From time to time the subject of whether a dynamometer test would be a good idea when going through the pre-buy stages of purchasing a Prevost. While I've never spoken to anyone who has actually done this, I've always been curious about the validity of the procedure. It turned out, with the proper equipment, a trained technician, and thanks to DDEC and a good computer program, the test should be very accurate.

When our new to us Royale had just turned 100K miles I contacted the previous owner and learned the bus had never had a valve adjustment. Since we were having the rear closet re-carpeted anyway and would soon be leaving for the Oklahoma rally, it was a good time to get the job done. All well and good however during the trip it seemed we had lost some power as a result of the adjustment. Add to that the fact that Mango's Mothership easily out climbed our bus on the trip to California Coach, I decided a trip to the dyno shop would be in order.

Last week I made an appointment at Pacific Power Products (a large west coast DD-Allison dealer) in Eugene, OR and had the job done on Thursday. PPP was very cooperative when I explained I'd like to take some photos for a show and tell and the tech was also enthusiastic when he learned that some Prevost owners are actually interested in the greasy stuff.

In the first 2 photos you can see the rear of the bus is restrained by a large chain shackled to the frame, the purpose of this chain is to keep the rear of the bus straight. The front chains actually keep the bus from charging ahead off the roller.

The drive wheels are shown on the dyno roller in the third photo. This dyno is normally used to test tandem axle trucks, hence the two rollers. The rollers are approximately 6' in diameter and are 15' long. Water is pumped into the rollers to create the rolling resistance. This particular machine was rated at 800 horsepower, a fact I wasn't made aware of until we ran the test. More on this later.

The 4th photo shows the tech holding a specialized DDEC reader. He will control engine throttle position directly from the reader, manual application of throttle is not necessary. Shift selection however is made on the shifter keypad. There is a laptop computer sitting on the dash behind the right windshield.

The 5th photo is a laptop page showing the engine configuration and basic parameters.

Ran out of photo space, continued on next post.

[BrianE]

Continued:

The first photo on this post is the laptop instrumentation page. It shows the engine at idle, 0% throttle with 37% load. The 270 alternator and maybe the compressor use some horsepower.

The second photo is a max speed check, level ground, 6th gear. This bus has a 4.56 final drive ratio and tops out at 77mph.

Third and fourth photos show the maximum load. The bus is in 4th gear (highest non overdrive), the dyno has loaded the speed down to 40 mph, boost is 27.9lbs, ECM tach is 1532 and horsepower to wheels is 394. It was here that I was priveleged to see what the normal Prevost customer doesn't get to see. THE BUS WAS STALLING THE DYNO. Max torque is listed at 1200RPM. In order to not risk a shift to third gear, the target was a torque/horsepower reading at 1350RPM (too close for comfort in my estimation).

Since this dyno is rated at 800 HP and is a dual roller dyno, each roller will only withstand 400 HP without stalling. A good lesson was learned here. Before having a dyno check, make sure it is capable of handling the 500 HP your bus is probably programed for.

The good news however is seen in the last photo. The middle line represents horsepower. As seen above, the line ends at 394HP/1532RPM. If you continue the line to 1350 RPM you would see well over 400HP. According to the folks at PPP, anything over 80% of rated horsepower on the dyno is normal. In other words, up to 20% of rated power can be lost through the transmission, driveline, differential and accessories.

Let the fun begin.

[truk4u]

Great post Brian... So it seems after the valve adjustment your engine is doing it's job, even though the Mothership smoked you on the hills. Is it possible the Mothership has a different rear gear ratio? Your Royale was probably much heavier than the Liberty as well, since Mango abandoned it years ago for the H.

Hmm, Beverly was probably saving fuel in the Royale while you had the hammer down spending Mango's money! No wonder the transmission needed serviced enroute...

[Jon Wehrenberg]

I'll bet the difference was weights. Mango's bus was empty and Brians bus has his stuff in it.

I think at our next rally we need a campground with a wide quarter mile road so we can set up drag races.

[tdelorme]

Great post Brian. So, the results show that the perceived loss of power after the valve adjustment was only imagined?

[JIM CHALOUPKA]

Very interesting Brian!

Wondering what your usual mpg is on the flat with no excessive winds.

JIM

[Kevin Erion]

Brian,
All that info looks good, the one thing I don't understand about the dyno. Even though trucks have dual rear drive axles, only one does all the work until you select the switch on the dash to lock in the 2nd drive axle. But that should not change the HP reading at the roller, I think? I wonder if the dyno was rated for 400HP, but that also is strange because most over the road trucks now are higher than that. I must be confused, an easy thing lately.
Looks cool!

[JIM CHALOUPKA]

It would have been interesting to know what the Dino read out was before the valve adjustment as well!

[BrianE]

Tom, Agree that the difference in performance is probably weight related. I suspected the Mothership might have had a different gear ratio although never verified whether it did or not. The Prevost mtc manual lists 4.56:1 as the standard for motorhome shells.

Jon, The whole excercise was interesting but didn't prove much in this case. Would think that it would be a good prebuy tool or would be useful if you definitely sensed a decline in performance. At $450.00, it's not something you'd normally do without just cause.

Ted, Yup, imagined. I wouldn't have thought there'd be much of a weight difference when you consider the boatload of mirrors installed in a Liberty.

Jim, You probably noticed the DDEC reader gave an instant MPG of 2.1 under max load. Since this is a DDEC read out it is the same info that appears on the Prodriver. Sad to say however, I didn't note the number while on the dyno.

Kevin, For reasons not asked or explained, the dyno check is run with the axles in lockup with a truck tractor. The power is divided between both rollers in this configuration. Each roller has the capability of 400HP.

[Jim Skiff]

Here is the link to Brian's Dyno Article.