[Diy_efi] inertial vs brake

[A6intruder]

I think it's great that you guys are comparing the merits of the two types
of dynos but for most of us we have to do most of our tuning on the street
and then use the dyno for verification and bragging rights.

I did use a local inertial dyno when I first hopped up my 1994 Mustang but
all we could adjust then was fuel pressure and base timing.  Now that I've
discovered the EEC tuner and have a permanently installed WB, including data
log, I am able to tune any time I want.  This is certainly not a same day
process for anything other than the WOT.  The "street" portion of the tune
takes time because I have to let the EEC have miles to add it's own
corrections, then I make more changes to close the gap.

My use is certainly different than a circle track racer.  An inertial dyno
very closely simulates my WOT use.  On the street they frown on sustained
high RPM cruise...too bad.

I think my local guy is fairly reasonable at $60-80/hr but I am not going
back until I have the MAF curve nailed.  Then I will show up with several
files ready changing the WOT A/F on each file.  My use of the dyno would be
to see the difference A/F makes at each part of the RPM curve, then another
series of files to see the effect of timing changes.

I have considered mapping the A/F effects by just measuring the change of
acceleration (from data log) on the same piece of road at the same weight
within minutes of each run.  This could show relative change to help select
the optimum A/F.  But in the end I will want that nice dyno produced HP
curve printed out for bragging rights.

Daniel R. Nicoson
Equipment Exchange Company
Phone:  (814) 774-0888
Fax:      (814) 774-0880

-----Original Message-----
From: diy_efi-admin at diy-efi.org [mailto:diy_efi-admin at diy-efi.org]On Behalf
Of Shannen Durphey
Sent: Wednesday, December 18, 2002 12:00 PM
To: diy_efi at diy-efi.org
Subject: Re: [Diy_efi] inertial vs brake

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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