Read the sticky...
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From: Commerce Twp, MI
Read the sticky... I read the sticky, but I just wanted to make sure. Are there any tables that need to be changed besides the IFR table when swapping injectors?
PS - The Excel sheet looks like it'll be extremely helpful, so thanks to all who contributed.
PS - The Excel sheet looks like it'll be extremely helpful, so thanks to all who contributed.
The injector offset table might need to be changed
if the injectors are of a different construction, they
can vary in opening/closing time. That's something
you'd have to back into, once you think the main
flow is about right, based on low-pulse-width fuel
errors observed (like idle fuel trims movement).
If you're going way up in injector size you may also
need to lower the minimum pulse width, if that was
anywhere near the idle / decel minimums you see in
driving. If you leave that alone and step up (say) to
42s from 26s, your minimum squirt will be about 60%
fatter and this might make it impossible to idle clean
(can't trim past a hard stop) unless you take out that
bind.
One thing I'm not sure if the spreadsheets address,
is fuel fade. As you go up in injector size you would
flow more fuel and expose more of the pump and
plumbings' flow/pressure effects. The IFR table
embeds an assumption of working head pressure
(the table index is BARO-MAP, but injector pressure
drop is rail+BARO-MAP). If there is significant fade
(made worse by more delivery) the straight-line-to-
sqrt(dP) fit of the stock table, won't be all that
good a fit to reality. You probably want to verify
the rail pressure at idle, WOT/loaded and maybe
some midpoint, see what the deal is and whether
you will need to massage the IFR table (though I
believe usually this is left, to be "crutched" by the
PE tweaking at the end of it all).
if the injectors are of a different construction, they
can vary in opening/closing time. That's something
you'd have to back into, once you think the main
flow is about right, based on low-pulse-width fuel
errors observed (like idle fuel trims movement).
If you're going way up in injector size you may also
need to lower the minimum pulse width, if that was
anywhere near the idle / decel minimums you see in
driving. If you leave that alone and step up (say) to
42s from 26s, your minimum squirt will be about 60%
fatter and this might make it impossible to idle clean
(can't trim past a hard stop) unless you take out that
bind.
One thing I'm not sure if the spreadsheets address,
is fuel fade. As you go up in injector size you would
flow more fuel and expose more of the pump and
plumbings' flow/pressure effects. The IFR table
embeds an assumption of working head pressure
(the table index is BARO-MAP, but injector pressure
drop is rail+BARO-MAP). If there is significant fade
(made worse by more delivery) the straight-line-to-
sqrt(dP) fit of the stock table, won't be all that
good a fit to reality. You probably want to verify
the rail pressure at idle, WOT/loaded and maybe
some midpoint, see what the deal is and whether
you will need to massage the IFR table (though I
believe usually this is left, to be "crutched" by the
PE tweaking at the end of it all).
Last edited by jimmyblue; Jan 13, 2006 at 08:55 AM.
