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Gtech 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
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Torque * RPM |
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HP at a given RPM is |
equal to: |
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5252 |
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:
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