How does the resistance of an injector
02-12-2006, 05:27 PM
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How does the resistance of an injector affect the injector offset table. The stock 26's are 12.0 ohms, 28's are 12.6 ohms and my Racetronix 32's are 16.9 ohms. So from 12 ohms to 16.9 ohms there is a 41% difference in resistance. So should I add 41% to the offset table or should I subtract 41%. I think this may be the clue to working with larger injectors ie (Motron 60's) and especially help with large injectors at idle.
Last edited by HumpinSS; 02-12-2006 at 05:35 PM.
02-12-2006, 07:40 PM
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It's not going to be resistance alone.
What goes on is, you have a coil actuated valve that
has to overcome both return spring and fuel pressure
forces to move off the seat. You have a solenoid force
that is proportional to coil current. So the time you
need to open the valve (just off seat) is the time it
takes the inductor current to hit that force level.
Note the fuel pressure element, that's why you see
that as a dimension in the offset table.
The other dimension is voltage. Current in an inductor
runs up at a rate proportional to applied voltage
(V=L*dI/dt).
Now, the resistor will not begin to seriously affect the
coil voltage until significant current has begun to flow.
The resistance is there to limit the steady state current
and it's going to cap it at about an amp more or less.
Question is, how much current is really required to pull
the valve. I'd expect it to be maybe a couple of hundred
mA (certainly well less the limit value).
So my opinion is that resistance is just the tail, not the
dog, and things like injector spring, valve and coil design
are the big actors.
The other day I got ahold of some injector data and tried
fitting static and 2.5mS pulse flow rates to get an offset
value:
http://hptuners.com/forum/showthread.php?t=4753
32#ers were not represented (too bad, these are what I
have in a box too) but the striking thing about the data is,
there is no rime or reason to the result; offset (as the
zero-flow time intercept) is all over the place (within 0.7
to 1.0mS) and not flow proportional.
If offset is bugging you I guess I'd just back into it by
watching the idle fuel trims and see what brings them
best into line. The shortest pulse width in operation is
what'll be most sensitive. Be sure to push down the
minimum below the offset values too, I think.
What goes on is, you have a coil actuated valve that
has to overcome both return spring and fuel pressure
forces to move off the seat. You have a solenoid force
that is proportional to coil current. So the time you
need to open the valve (just off seat) is the time it
takes the inductor current to hit that force level.
Note the fuel pressure element, that's why you see
that as a dimension in the offset table.
The other dimension is voltage. Current in an inductor
runs up at a rate proportional to applied voltage
(V=L*dI/dt).
Now, the resistor will not begin to seriously affect the
coil voltage until significant current has begun to flow.
The resistance is there to limit the steady state current
and it's going to cap it at about an amp more or less.
Question is, how much current is really required to pull
the valve. I'd expect it to be maybe a couple of hundred
mA (certainly well less the limit value).
So my opinion is that resistance is just the tail, not the
dog, and things like injector spring, valve and coil design
are the big actors.
The other day I got ahold of some injector data and tried
fitting static and 2.5mS pulse flow rates to get an offset
value:
http://hptuners.com/forum/showthread.php?t=4753
32#ers were not represented (too bad, these are what I
have in a box too) but the striking thing about the data is,
there is no rime or reason to the result; offset (as the
zero-flow time intercept) is all over the place (within 0.7
to 1.0mS) and not flow proportional.
If offset is bugging you I guess I'd just back into it by
watching the idle fuel trims and see what brings them
best into line. The shortest pulse width in operation is
what'll be most sensitive. Be sure to push down the
minimum below the offset values too, I think.
