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Date: Thu, 4 Jan 1996 08:41:12 -0500
From: (Pete Read)
Subject: Performance Testing Methods

John Allen asked about a standardized method for estimating the horsepower/torque effects of various engine modifications.

He mentioned both Bowling's Highway Dynamometer program (Web Page and the Metric Mechanic 60-80 in 3rd/4th/5th gear method.

After checking them both out, either method seems fine, but Metric Mechanic (MM) gets my vote. The MM 60-80 mph test is much simpler to do and the results are easier to grasp. Here's a brief explanation of each.

Bowling's Highway Dynamometer Program

Bowling's Web page has a number of interesting programs and is well worth checking out. You plug numbers into input screens and the programs spit out results.

Bowling's Highway Dyno program provides a corrected horsepower and torque readout. Car acceleration is found by running wide open throttle, in the highest gear possible, through the entire engine rpm range without shifting. Elapsed time is recorded at even mph increments (e.g. in 3rd gear, every 10 mph from 30-90 mph). Then the times are entered with specific car information (gear ratios, weight, engine displacement, etc.), and environmental conditions.

Just for fun, I interpolated Evan Evan's 3rd gear 60-80 mph M5 test figures to plug in some numbers (stock 3.96 versus Dinan 3.60 seconds). Assuming constant acceleration, I divided the 60-80 mph time values into four equal parts. Note this test should be done over a wider rpm range and the time between speeds needs to be measured, not estimated.

Time Speed
Sec Mph

0. 60
0.99 65
1.98 70
2.97 75
3.96 80

0. 60
0.90 65
1.80 70
2.70 75
3.60 80

Next the following inputs:

Gear Ratio 1.35 (3rd gear)
Diff Ratio 3.90

Weight       3800 lb (3500 car + est 300, Evan & Son)
Cd           0.40

Front Area 20 sq ft (default)
Tire Press 32 psi (default)

Tire Dia     24.25 inch
Disp         211 cu in (3453cc)
Stroke       3.31 in   (84mm)
Temp         70 F      (default)
Press        30.0      (default)
Elevation    0         (default)
Rel Humid    50%       (default)

The results were: Stock,(Dinan)

MPH RPM Road Hp Flywheel Hp Corrected Hp

60  4379   140,(154)    160,(174)      164,(178)
65  4744   152,(167)    177,(192)      181,(196) 
70  5109   163,(180)    193,(210)      198,(215)
75  5473   175,(192)    211,(229)      216,(234)
80  5838   187,(205)    230,(248)      235,(254)

Metric Mechanic 60-80 mph Test

The Metric Mechanic (MM) catalog is great. It has lots of general BMW information and some rationale to back up their performance claims. They even discuss some downsides to modifications instead of insisting everything is magic! I highly recommend it just for basic information. In future posts, I may report some of the MM information for general M-digest discussion.

The MM test measures 60-80 mph acceleration times in 3rd, 4th, and 5th gears. On a level road in 3rd gear, start at 55 mph and floor the throttle. Start the stopwatch at 60 mph and stop it at 80 mph to measure the elapsed 60-80 mph time. Repeat the test for 4th and 5th gears.

You end up with three numbers (60-80 mph time for 3rd, 4th, 5th) that give a good feel for performance across the useable power range.

  1. 3rd gear 60-80mph (4379-5838rpm) for maximum power
  2. 4th gear 60-80mph (3423-4324rpm) for mid-range power
  3. 5th gear 60-80mph (2632-3509rpm) for low-end power

Evan's before and after tests (3rd 3.96/3.60 sec, 4th 5.13/5.30 sec) indicate that the Dinan chip increases maximum power, but seems to decrease mid-range power. The MM 3rd gear (max power) test correlates well with 0-60 mph and quarter mile times. Evan's 0-60 mph times dropped from 7.00 to 6.52 seconds, again indicating a maximum power gain.

Environmental Corrections

I checked the Bosch Automotive Handbook and tried some different inputs on Bowling's program.

Here's what increases and decreases power.

Increase         Power Change
---------        ------------
Elevation        decrease
Temperature      decrease
Pressure         increase
Rel Humidity     decrease

Elevation seems to be the most important factor. Temperature is somewhat less important, but significant. The elevation rule of thumb is a 1% loss for each 100 meter (328 ft) increase. People in mile-high Denver have 16% less power than people at sea level. Cool outside air temperature helps, but it's heated above ambient temp in the intake passages.

So I'd suggest reporting any significant elevation and the air temperature for 60-80 mph tests.


Bowling's program is great for a dynamometer type readout of a car's power. But, for most of us, it's probably too hard. It's tough to find a 30-90 mph test area and record all the numbers. Then plug the numbers into a program. The final result is a, difficult to grasp, table of numbers.

The Metric Mechanic method produces three numbers. The 3rd gear time correlates well with car magazine 0-60 mph times many people (like me) memorize. The test is easy on your car and repeatable because there's no wheel spin or redline shifting. It's possible to find each test number discretely, in less than 10 seconds, on most highways.

So remember, in 3rd gear, if it goes from 60-80 mph in four seconds it's a real M5, but if it takes five seconds, it's only a 535i.


Pete Read
88 M5

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