HORSEPOWER WHOOIE...

[Archive]

About a year ago I upgraded my "Dr. Performance" pump from a "Phase III to the more powerful "Phase IV," and went with a new set of matched injectors at the same time. I paid for the difference in the advertised price of the two pumps, but wasn't charged for any additional labor to swap the pumps out.

Smith Diesel, of Carlisle KY (Everett Smith -- Owner -- 606-289-7892) did the install and performed a valve adjustment at the same time. I was (and continue to be) very happy with the performance of my truck, but remain somewhat skeptical about an accurate method for determining the horsepower of our diesel trucks.

As a longtime TDR member and reader, I have attended local club events, here in Atlanta and elsewhere, and listened to all the discussions about effective ways to measure horsepower. I've tried the "G-Tech" accelerometer, and gotten all sorts of off-the-wall and varied readings. Being told to "average" the results in order to "insure accuracy" makes the scientist in me cringe in pain...

This past weekend I took my truck back to Everett for a "tune up," during which he found (after 30,000 miles) that the truck's exhaust valves had gotten pretty loose. The truck felt much more powerful after the valve adjustment. Everett has a measured quarter mile (nicely marked on the pavement) in front of his shop -- a benefit to living way out in the country. We toed the truck to the starting stripe (with him driving...) and let her rip. The first run produced an ET of 17.4 seconds @ 83 mph. This was the result of simply stepping hard on the accelerator and letting the truck "do its thing." The truck went through all the gears very quickly, and dropped down into overdrive "lock up" at about 66 mph. Next, we turned off the overdrive and made another run. This time, with Everett shifting "manually," (I have an automatic) the results were a more impressive 16.8 seconds and a 90 mph top speed.

We used the "math" to calculate horsepower, relying on the formula in the latest TDR (page 27). The quarter mile elapsed times vs. top speed calculations were not even in the same ballpark. My truck has everything but the kitchen sink hung on it, including a very heavy tool box, lights, brush guard, WARN winch, and so on... I put my 2500 over the truckers' scales on I-75 coming home from KY, and it weighed (without Everett's 200 pounds) 7400 lbs. By my calculations (using top speed of 90 mph and weight of 7600) this truck is pushing very close to 400 HP. The top speed for the three runs was verified using Magellan's GPS ("Neverlost") which is a $2000 "hardwired" unit with roof mounted antenna. The three runs recorded top speeds of 83 mph at the end of the first run, 90 after the second, and 93 after the third. Everett had another customer waiting or we'd have played with the truck a while longer, and would have continued to "learn" the truck's characteristics, netting better performance.. (or trashed the tranny, first...)

My question relates to the disappointing discrepancy in the methods of calculating (using "math") horsepower. First, when relying on the ET method, (using 16.8 seconds) returned a horsepower value in the low 300 range. But the top speed method (using 90 mph) yielded 389 horses. Which one to believe??

The problem I have with this method of calculating horsepower (particularly the ET route) is that "driver skill" is the controlling factor. I'm convinced that a skilled driver, after spending some time with my truck, could produce elapsed times in the low 16s, or high 15 second range. That makes a huge difference in the calculated horsepower rating, obviously. Our tests were also affected by the surface of the road (slightly damp and oily) and other conditions (wind, etc.) which could have been a factor. My truck runs 285-75-R16 B.F.Gs and has 3.54 gears. Even with these tall tires, however, it breaks the rear end loose (truck gets "sideways" under too much off-the-line throttle...) and barks the tires with every shift (when shifting "manually"). I wonder how much time was wasted spinning the tires and going nowhere at the starting line...?? Would using my smaller "stock" tires have made a difference??

I took the truck to the local ("Cummins South") dealer for a dyno run during a TDR event. They used the old style "water break" dyno, and the guy who did the testing (without realizing it) ran the whole test with the truck in overdrive. Even so, he recorded 315 hp. When he tried to redo the run with the overdrive off, he couldn't keep the truck from jumping the chocks, so he stopped the test after recording a peak reading of 355 hp.

If anybody has a better suggestion for estimating horsepower, I'd like to hear it. Thanks.

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97 2500 4x4, club cab, auto, 3.54 limited slip, JRE 4" exhaust, Dr. Performance Fuel system (370 HP), "Twister Turbo," Geno's guages -- Boost, EGT, Tranny & Rear -- MAG Hytec covers, SunCoast Converter/Transgo shift kit, Brite Box, tons o'chrome under hood, Prime Loc, EZ Drain, Seat covers, wood dash, Rancho Suspension, Warn M12000 Winch on Warn Brush Guard, Warn driving and fog lights, Hella twin back up lights, 285/75/R16 B.F.G. ATs on Mickey Thompson rims; Linex bed liner, BD exhaust brake.

[This message has been edited by Sasquatch (edited 10-02-2000).]

[This message has been edited by Sasquatch (edited 10-02-2000).]

[This message has been edited by Sasquatch (edited 10-02-2000).]