[Diy_efi] inertial vs brake

Shannen Durphey shannen at grolen.com
Wed Dec 18 16:56:04 GMT 2002


It's harder to get consistent back to back tests for comparison by just
pushing the throttle "part way."  The mechanical stop would be one answer. 
Logging MAP vs RPM while doing this can help to fill in VE table entries. 
But it's tough to work on low speed driveability and around town type
operation using a strictly inertial dyno.  

With VE tables set up in 2 or 3 axes, you want to find a method to work
through the table in an ordered fashion.  A good method is to vary rpm at a
fixed MAP and/ or TPS, or to vary MAP and/ or TPS at a fixed rpm.  The
problem is the unbraked inertial dyno can always accelerate.  Once the
engine rpm stabilizes, fixed throttle gives much less load than normal
(very little, actually).  You cannot maintain a specific rpm and put
significant load on the engine to represent steady state cruise.  You need
to work against the fixed mass  provided by the drum, and you need to
accelerate that mass to create a load.

The question for driveability is "what are you tuning for?"  A minimum
requirement would be acceptable throttle response, maximum fuel economy,
and the perception that the engine's running correctly.  If the vehicle is
subject to emissions regs, then emissions will be first and foremost in the
list of tuning goals, and the 4 gas analyzer and load type dyno will
probably be used for testing.  For economy, one can watch power output and
indicated AFR at a fixed tps to get closer to maximum efficiency.  Since
the drums are accelerating, response or throttle crispness can be roughly
estimated by watching the time it takes to go from one rpm to another at
that fixed throttle angle.  This type of tuning would take a significant
time and when all is said and done, the calibration would probably need
more tuning on the street.  Still, if I were paying for dyno time, and had
some time left over, and felt that I had most of my WOT and high MAP tuning
done, I might try a few runs with a throttle stop installed to at least
rough in the bottom of the tables.  

I feel it's easier to judge the result of the driveability tune by watching
for reduction in TPS or by watching fuel consumption at a given power
output and rpm, or by watching the power required to maintain a specific
load at a fixed throttle angle and rpm.  In other words, it's easier to do
it with a braking dyno than an inertial dyno. 

Shannen

Erik Jacobs wrote:
> 
> A question -- couldn't one test at less than WOT on an inertial dyno
> (Dynojet) to get different load conditions?  Would this be possible?
> Beneficial?  You could "guesstimate" constant throttle position by looking
> at the output of something like a TPS sensor and trying to hold that as
> constant as possible... you could also probably jury rig something on the
> throttle with some kind of electromechanical device to get a constant
> throttle position (servo or something).... am I talking nonsense?
> 
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