[Diy_efi] dyno comparison

[Shannen Durphey]

If you are tuning VE tables, the inertial dyno will tend to cause the
best tuned portion of the map to be  around the top of the table, that
is, where that MAP and RPM are highest.  The hardest areas to tune in
the inertial dyno are low acceleration areas.  Street driveability
tuning centers around these areas.  It's not that it's necessarily
difficult to operate on the dyno in a low acceleration situation, but it
can be tricky to operate there consistently as it requires repeated
acceleration/deceleration cycles.  Without a brake module, this could
take all day.  BTW, a good rule is to _never_ try to use your foot brake
with the inertial dyno.  In most cases they will be destroyed.

Dynojet doesn't offer an eddy current brake for the 248, and they do not
offer power measurement while using the air powered friction brake.  So
a Dynojet with a factory installed brake isn't the same as a braking
dyno.  It might be more appropriate to equate it to an uncalibrated
loading device.

It does serve a fair job of helping to work on the driveability portion
of your tune.  It can be very difficult to modulate throttle, watch
scantool data, and make calibration changes on the street.  The dj brake
can be set to hold a fixed load (called % braking),  a fixed rpm, or a
fixed wheelspeed.  Brake control isn't the fastest, but it can be used. 
The brake is also not substantial enough to maintain a set drum
speed/engine rpm when the engine being used produces large amounts of
torque.

If a person is able to do their own tuning, and they have a calibration
that they feel will be fine on the street, I sometimes mention that it
might save $$ to do as much of the driveability as they can before they
get to the dyno.  In this case it's good to make notes about any
problems or questionable portions of the tune so they can be checked on
the dyno.  

I have done complete from scratch tunes using a dj to do the brunt of
the work, then making final adjustments on the street.  Like I said, it
can get tricky.  Where it can help most is by allowing you to monitor
data in ways which you wouldn't normally be able to do.  For example,
you can fix the rpm and on a distributor equipped car change timing to
see the result on the dj's "braking" display.  It's crude, but it's a
helluva lot faster than trying different settings on the street.

Shannen

Nick Starai wrote:
> 
> So I think we pretty thoroughly discussed the differences between inertial
> and brake dyno's. So other then the friction loses, and possibly the
> software, I don't see a huge disadvantage to using an intertial OTHER then
> the 4 gas exhaust analyzer mentioned several times. (assuming the inertial
> has the LOAD CONTROL OPTION). Now what I'm wondering is if it's a function
> of law as to why you can't use a 4-gas on an inertial or is it that the
> manufacturers simply don't integrate the device into their dyno's. Could
> you simply purchase a 4gas analyzer and datalog it while using an
> intertial with load control and get the same effect? It just seems to me
> that the major advantage of the eddy was the 4gas. Also, I'm referring to
> pump gas, so the extra hp you may will extract from a racing engine
> on the eddy-brake would be negligable on a street car. (for me at least)
> 
> Nick Starai
> 
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