G-TECH/Pro Horsepower Measurements

Please read this page for information on Horsepower & Torque measurements

The Gtech's ability to measure horsepower and torque is one of the product's most exciting features, but also one of the most confusing and frustrating to use unless you understand how to make the measurements and how to interpret the results.  This page is a collection of links of information that pertain to horsepower and torque measurements.

First some background on shop chassis dynamometer measurements:

• When you take your car to a shop to run it on a dyno, there is a prescribed driving technique.  For example, many dyno shops have you (or more commonly, a shop technician) roll the car through the gears until you're in 4th gear (or whichever gear has the lowest drivetrain loss).  Then in this gear you run your car through the RPM range, and the TQ & HP are measured as the car accelerates through a wide RPM range in this gear.
• With a shop dyno, you should not expect to get numbers that match a manufacturer's published numbers, since published values are typically the engine output at the crank, not delivered to the wheels.  A chassis dyno measures the power delivered to the wheels, as the torque delivered to the wheels is what works against the inertia of the rollers on the dyno.  Typically, a chassis dynamometer measures how the spin rollers are accelerated by the vehicle's torque that is delivered to the wheels, and HP & TQ are calculated from this.  Many dynos can also apply one or more "correction factors" that basically just adjust the shape and values of the curve to account for factors such as temperature, humidity, estimated drivetrain loss, etc...
• Results vary from shop to shop, due to calibration variances, a multitude of correction factors, and the varying levels of expertise and experience at the shop.  For this reason, when doing tuning, it is important that you find a shop that you trust, and then stick with that shop, as the results you get over time can be compared to each other directly (if the shop is worth its salt, you should be able to assume any differences you see from one visit to the next are due to changes in your vehicle, not the shop).

Basically, a chassis dyno measures torque by seeing how vehicle accelerates the rollers, and then (with knowledge of RPMs) calculates HP from torque, since

The Gtech, on the other hand, does just the opposite.... it calculates HP from velocity, acceleration and vehicle weight, and then (with knowledge of RPMs) calculates TQ from HP, using the same formula listed above.

Now that you understand a little about chassis dyno measurements, we can discuss usage of the Gtech for these measurements.

• Just as with a shop dyno, there is a driving technique that we recommend to use when measuring HP & TQ with the Gtech.
• Just as with a dyno, the Gtech numbers should not be expected to match published numbers, as they are measuring net horsepower, not horsepower at the crank.
• Just as with a dyno, variations in driving technique, environmental conditions, etc... will affect your measurements.  Consistency is key.
• The Gtech calculates HP & TQ by mathematical calculations based on the output of its sensors (the accelerometers.)  Therefore, the extent to which the sensors measure the true output of the vehicle is the extent to which your readings will be meaningful.
• The previous item, "in English", means that you MUST take every step possible to ensure that the Gtech is subjected to no external factors that will impact your vehicle's acceleration.  To point:
• There must be no wheelhop or wheelspin during the measurement
• There must be no clutch slip during the measurement
• There must be no headwind/tailwind during the measurement
• There must be no slope / grade to the road or track surface, i.e. it must be level and flat.
• Losses or "slop" in torque converters (for vehicles that have them) can wreak all sorts of havoc.  Just as a slipping clutch will not deliver full power from the engine to the transmission, some torque converters really convolute the power delivery curve.
• As with any type of scientific measurement, the best way to minimize error is to make several measurements and to discard any readings that are statistically "out of range", and then to average readings that are "close to center" to get a meaningful number.  So if you did 6 runs, and 4 of them indicated a peak HP in the range of 150-155 HP, and one reading indicated 128 HP and one indicated 178 HP, you would throw out the 128 and 178 measurements as "bad", and then average the other readings.
• The Gtech relies on an accurate figure for the vehicle weight.  If you are wondering, "what do I enter for the vehicle weight", the answer in conceptual terms is, "Imagine your Gtech on the starting line of a dragstrip, with you buckled in, your car fueled, the toolbox and spare tire removed from the car, etc.... Now imagine at this moment, right there at the starting line, someone slips a scale under the car and weighs the vehicle, i.e. the weight supported by the 4 tires... THIS is the weight you would enter."
• The Gtech measures NET horsepower, which takes aero drag and the rolling resistance of the road into account.  A dyno does not have to contend with aero drag but the Gtech does.  This often accounts for a large part of the difference between a dyno slip and the Gtech results.

Regarding driving technique for HP and TQ measurements:

• Consistency is  key.
• Before doing any runs, you must decide in your head which gear you are going to use for all your measurements:
• A general rule of thumb is that you should use the lowest gear where you won't experience any wheelspin/wheelhop/clutchslip under full throttle acceleration.  For many unmodified cars, even sports cars, 2nd gear is fine for this.   You will have to see for yourself.  Some higher powered vehicles might need 3rd gear to get clean, smooth acceleration throughout the entire RPM range.
• Unlike a chassis dyno, it is not necessary to do a run in 4th gear.  It is more important that you use the same gear for each run.
• The higher the gear you use, the lower your numbers will be, due to aero drag.  You are going much faster at 3000 RPM in 4th gear than at 3000 RPM in 2nd gear.  The higher the speed, the greater the aero drag, and thus the greater power loss to aero drag.
• Automatic transmission cars will usually have to be manually shifted to obtain the proper RPM range.
• Don't launch hard like you do for a full-out quarter mile... launch hard enough to trigger the Gtech (which is not very hard), but no harder.
• Shift at a fairly low RPM until you enter the desired gear at a reasonably low RPM.  For example, if you decide to do your measurements in 3rd gear:
• Launch fairly gently in 1st gear and make sure the Gtech triggers
• Shift into 2nd gear at perhaps one-third to one-half of redline, thereby landing you in 2nd gear at perhaps one quarter of redline.
• Accelerate until you are at about one-third to one-half way to redline, and then shift into third gear, so that once in third gear, you are at a low RPM.
• Press the accelerator pedal to the floor and keep it there through the entire RPM range.  You must keep the pedal pressed to the floor to measure full engine output, and you must cover the entire RPM range that you want to measure.
• When you reach peak RPMs in the desired gear, shift briefly into the next gear (which would be 4th gear in this example), and then let off the throttle.
• This example used 3rd gear as the "desired gear", but many users will want to use 2nd gear instead... in this case, just short-shift from 1st to 2nd gear, and then nail it.
• One difference (advantage?) of a 2nd gear measurement is that the numbers will be pleasingly higher than 3rd gear because of less aero drag.
• Another difference is that you won't have to reach a very high speed in 2nd gear at peak RPMs as compared to 3rd gear.  For example, many sports cars peak out at 55-70 MPH in 2nd gear, and between 85-110 MPH in 3rd gear.
• We strongly recommend doing your testing at a dragstrip, where you have access to a flat, level surface and a controlled environment.

If you are getting strange results, the result in all likelihood is one of the following:

• incorrect vehicle weight
• mounting system not tightened, or cracked/broken (lots of vibration)
• slope/grade in the road
• severe vehicle vibration
• wind (head or tailwind)
• clutch slip, wheel slip, wheel hop, torque converter slop, bumps on the road
• incorrect driving style

Additional links: here are some other resources regarding the Gtech and HP measurements:

• Here is a technote that describes factors that influence the Gtech's HP & TQ readings.
• Here is a technote that explains how aero drag affects horsepower measurements.
• Here is an interactive aero drag calculator.
• Here is detailed information on the Gtech's horsepower/torque measurements.
• Here is some information about interpreting PASS results.
• The RR, SS and Competition all have the same sensos and accuracy, and measure HP & TQ identically.  But to provide a single resource for information on these measurements, please download the RR manual here, and then refer to pages 102-106 regarding HP measurements.
• There is a section in the Gtech FAQ that discusses Horsepower and Torque.