Interpreting Dyno Runs
Temple of VTEC World would like to discuss our ideas on how best to interpret
dyno runs. We feel that dyno runs are the best means of assessing the
effectiveness of an aftermarket modification. However, analyzing the
implications of a dyno run may not be straightforward.
One of the most important characteristic of a dyno run is what it measures.
This has often been an area of confusion. The most popular dyno facility in the
aftermarket at the moment seems to be the Dynojet machine.
Dynojet measures power output at the wheels. Power at the wheels means
the engine's flywheel power minus losses incurred from the transmission and
attached peripherals (power steering pump, air-conditioner compressor, etc).
This means that actual measured power will always be lower than the
manufacturer's quoted specification. To further complicate matters, losses
incurred from the transmission are different depending on the type of
transmission - automatic transmissions that uses torque converters incurs very
high losses, typically 30% whilst those from manual transmissions are much
lower. However, there are also different types of manual transmission which
incurs slightly different amount of losses, typical figures ranging from 12 to
15% off engine's flywheel power. Measuring power at the wheels is advantageous
because it shows exactly how much power we are getting to run our car
A typical dyno run measures power and torque developed at the wheels over a
portion of the operating rpm of the engine. This means that a dyno run will not
measure wheel power from 0rpm to the engine's rev-limiter but rather, power and
torque starting from a certain rpm, typically 1/3 of the engine's operating rpm
range till the red-line (typically never up to the rev-limiter).
When looking at the result of a dyno run, what is usually looked at is the
measured maximum power. However the maximum torque and the specific RPMs at
which this and where maximum power develops are also important. At any
particular instance, best acceleration usually means shifting to the lowest
possible gear provided the engine's power curve complements it.
Therefore, TOV-W feels is the most important aspect of an engine's
performance is the shape or profile of its power and torque curves.
The profile of these curves takes into account the power/torque of the engine
over its entire rpm range. Unless we are using the newest Honda Multimatic
wonder-CVT transmission, we will never be really concerned with the power or
torque at a fixed, single rpm point, but rather that over an rpm range. A dyno
run with a very high maximum power but very bad curve will infer an undriveable
car, for eg, one with an engine with a very peaky race-cam and thus no useable
power in the mid-range and below.
TOV-W feels that the best power curve is one where the power steadily and
smoothly increases over the whole rpm range. This infers a constantly increasing
torque up to the maximum torque rpm and acceleration at WOT (or maximum
throttle) will be good all the way to the redline. Of great concern would be any
dips or spikes in the curve because this will induce a feeling of flat-spots -
areas of the rpm band where the car feels lethargic or feels like bogging
Following this, TOV-W feels that the interpretation of the effectiveness of
an aftermarket modification will need to take into account its impact on the
power and torque curves. This will frequently also reveal the reason for
discrepancies between manufacturer claims and measured results. For eg, a quoted
improvement may be for a certain rpm point and maybe not in peak power. A
mod may give say 5hp improvement but at a rpm somewhere in the mid-range (say
5000rpm) but only 2 or 3hp in the peak power of the engine. Other mods may give
high peak power gains but lowers the power curve in the lower rpm range (eg
exhausts with over-large diameters).
For day-to-day use, the best mod would be one which gives improvements over
the whole power/torque curve. Sometimes, an even more desirable factor would be
if it can smoothen out a power flat-spot, where the torque curve takes a dip.
However, mods that gives increases in the mid-range will make the car more
driveable even if it reduces high-rpm power when the driver seldom revs the
engine up. For racing uses however, a mod which increases power in the high-rpm
portion of the curve will be more desirable, even if it were to impact power
delivery in the low-RPMs.
TOV-W feels that there are no hard and fast rules for subjective
evaluation of a dyno run result. The subjective value depends on the individual
and his or her needs and application environment. When evaluating the usefulness
of a mod, bearing this in mind, particularly if you do not have any prior
expectations beforehand, will be very helpful.
©1998 Temple of
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