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By Greg Banish
Having tuned lots of cars over the years, I've had the opportunity to do
so many different ways. When I first started, my only choice was to drive
the car on the street and make an attempt watch the wideband in between
glimpses at lane lines, traffic and pedestrians in some cases. While this
worked great for confirming the "real world" performance of the
vehicles I was testing, the conditions (and safety) were less than ideal.
Enter the chassis dynamometer, that wonderful piece of equipment that
every tuning shop must have these days to be "state of the art."
The chassis dyno allows for easy recording of actual WOT power and usually
incorporates wideband air/fuel, engine speed, and a host of other
measurements into the data array for the calibrator. The ability to record
and later look back upon the data makes WOT tuning a snap once the tuner is
no longer preoccupied with the prospect of jail time or worse resulting
from his testing.
Today's performance enthusiasts are becoming even savvier though. They
want good horsepower and street manners out of their cars. Luckily, most
EFI systems have the capability to deliver this IF they are properly
calibrated. The real trick is to build not just a wide open throttle
characteristic of the engine, but an entire map of engine performance
accurately. This means that at any given speed and load point for the
engine, the ECU must know how to properly control things.
Building this map for an engine is a lot like driving from Detroit to
St. Louis. Along the way, we might need to stop in Chicago or Indianapolis
(even if it's just to top off the tank!) before reaching our final
destination. Without a road map, this journey is more difficult and we
might even get lost. But if we read a map that shows us exactly where to
turn and how far it will be until the next landmark, our travels are much
smoother.
EFI Engines operate the same way. They use maps to tell them where to go
on their way to WOT or redline. The more accurate these maps are, the
smoother the journey.
So where does the dynamometer come in? A load bearing dyno gives the
calibrator the ability to hold the engine at one location while he refines
the map of the surrounding area. Inertia only dynamometers freely
accelerate as the engine makes more power. This makes it difficult to hold
the engine steady at all the necessary map locations and build a detailed
map. Load bearing dynos have the ability to hold
the engine steadily in a much wider range or map locations in order to
properly tune these areas. The more accurate these measurements can be at
each point, the smoother the engine will run. So a dynamometer that can
allow the tuner to make accurate measurements at each individual point on
the map gives him the potential to make the engine run smoother in these
areas as well. Keep in mind that this applies not only to fuel delivery,
but the spark map as well. Finding MBT timing at part throttle requires
instantaneous torque feedback at steady state that can only be done with a
properly load controlled and instrumented dynamometer.
The added benefit to a load bearing dynamometer is that when it comes
time to test dynamic conditions, the rate at which the engine sweeps across
the RPM range can be adjusted to match exactly what happens in the real
world. This means that the amount of time it takes to complete a run is the
same between the track and the dyno. We have a better idea of just how much
heat and load the engine will see on the street or track. In today's world
of handheld flash tuning devices and "canned" tunes designed to
fit a wide array of cars, testing under these real world conditions can
make difference between "That should have been fine, I don't know why
it broke at the track," and "Everything performed just like we
expected" when the car comes off the dyno.
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