Car on dyno, is it RICH or LEAN???
02-21-2005, 08:27 AM
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Car on dyno, is it RICH or LEAN??? Even though they put a big fan in front of the car, is the AFR on the dyno rich or lean?
I was told its lean & will richen up while driving at high speeds (track), so tune it slightly LEAN..
Is this correct??
I was told its lean & will richen up while driving at high speeds (track), so tune it slightly LEAN..
Is this correct??
02-21-2005, 09:27 AM
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If the dyno runs up RPM faster than your RPM on
the street, then the fuel side is going to be "left
behind more" on the dyno; that would make the
dyno show lean. By "left behind" I mean that the
MAF is a relatively slow instrument, it has a thermal
lag to it and a fast-changing airflow will always be
represented a bit late. This is why on sudden MAP
changes the PCM reverts to speed-density-only;
on the dyno you have a steady MAP but potentially
(say, on a light roller) an airflow that changes quickly.
On the street however, you have other effects such
as the inertia of the fuel column against chassis
acceleration, which can change (reduce) head-end fuel
pressure by a few PSI since our cars have the regulator
at the back. This would show as a lean bias that is
stronger in 1st than 2nd than 3rd... and no such effect
on the rollers.
Other effects of interest relate to the inlet air temp and
coolant temp; there are significant temp adders to spark
and fuel that roll on at higher temps, and sitting still with
just a shop fan can put you still hot enough to slide into
some of this. A snoutfull of cold fresh highway air vs stale
semi-rebreathed dyno-bay air will be somewhat different.
Personally, I prefer just getting onto the Interstate out in
the county and tuning to acceleration numbers, on the
road. No pretty charts, but plenty of realism and it don't
cost no $100 an hour. Late at night you can stop dead,
and do a full throttle 1/4 mile pull (and more) before any
traffic comes up, around here. More relevant than 3rd
gear only, locked, for us A4 dudes.
On the other hand, if you break something, well, you're
out in the weeds. And leave that radar detector on....
the street, then the fuel side is going to be "left
behind more" on the dyno; that would make the
dyno show lean. By "left behind" I mean that the
MAF is a relatively slow instrument, it has a thermal
lag to it and a fast-changing airflow will always be
represented a bit late. This is why on sudden MAP
changes the PCM reverts to speed-density-only;
on the dyno you have a steady MAP but potentially
(say, on a light roller) an airflow that changes quickly.
On the street however, you have other effects such
as the inertia of the fuel column against chassis
acceleration, which can change (reduce) head-end fuel
pressure by a few PSI since our cars have the regulator
at the back. This would show as a lean bias that is
stronger in 1st than 2nd than 3rd... and no such effect
on the rollers.
Other effects of interest relate to the inlet air temp and
coolant temp; there are significant temp adders to spark
and fuel that roll on at higher temps, and sitting still with
just a shop fan can put you still hot enough to slide into
some of this. A snoutfull of cold fresh highway air vs stale
semi-rebreathed dyno-bay air will be somewhat different.
Personally, I prefer just getting onto the Interstate out in
the county and tuning to acceleration numbers, on the
road. No pretty charts, but plenty of realism and it don't
cost no $100 an hour. Late at night you can stop dead,
and do a full throttle 1/4 mile pull (and more) before any
traffic comes up, around here. More relevant than 3rd
gear only, locked, for us A4 dudes.
On the other hand, if you break something, well, you're
out in the weeds. And leave that radar detector on....
02-22-2005, 03:34 PM
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So which is it?
Tune it for LEAN on the dyno knowing it will richen as it gets driven,
or
tune it slightly RICH on the dyno knowing it will lean out as its gets driven?
OR, tune it 13.0 on the dyno & forget it...?
Tune it for LEAN on the dyno knowing it will richen as it gets driven,
or
tune it slightly RICH on the dyno knowing it will lean out as its gets driven?
OR, tune it 13.0 on the dyno & forget it...?