Apart from running roughly on cold starts, a bit of hesitation getting off the line, and an exhaust that is trying to spray paint my rear bumper black, is the overfueling at low revs going to cause me any engine problems?
If the 1800 injectors are providing too much fuel at low revs, will they also be supplying too much at full throttle? why will reducing the pressure so its okay at low revs cause a lack of fuel at high revs.
I've seen quite a few cars on the net running Sebrings with 440cc injectors(1800's are 230cc). I think most of them had LINK computers or similar + intercooling & greater boost. Are these aftermarket computers able to give a much greater drop in fuel pressure at idle or will they have a similar idle to me. Will the AFPR only give you a set increase percentage (ie. max fuel will always be 50% greater than min fuel), and does that mean that the AFPR is set up specifically for use with 1600 injectors, whereas the 1800 injectors would work better with an AFPR that gave, say a 60% increase. That would mean fuel flow could be low enough at idle to make it easy to live with, and still have enough at full throttle to avoid leaning out.
ps- I have read several articles on the net about the RX7 airflow meter that claim its not an easy swap as the factory computer doesn't read the signals from it the same as the standard air flow meter.
From MattGunn@newstalkzb.co.nz Fri Apr 27 16:09:43 2007
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Remeber
All colours are equal , but some are more equal than others..
cheers
MADMXV
re[2]: Overfueling
-
Fletcher Blades
re[2]: Overfueling
Ok, you've got a few different questions in there...
Your basic fuel injector is just a nozzle like your garden hose
nozzle... that can be switched on and off by the engine computer
(ECU). And just like your garden hose, the amount of liquid that can
get through it is proportional to; a) How big the nozzle is (cc
rating of the injector), b) how long it is held open for, and c) the
pressure of the liquid behind the nozzle.
Your basic MX-5 (or most any-other fuel injected car), already has a
fuel pressure regulator that changes the fuel pressure depending on
the vacuum of the intake plenum (so you get less fuel through the
injector at idle, with the throttle closed making high vacuum, than
you would get for the same injector on-time when the car is under load
(less, or no vacuum)). They need to do this because injectors have a
minimum on-time (because of the mechanical opening/closing of the
valve), and a maximum on-time specified as a duty-cycle. (depending on
who you believe, injectors should not be on more than 80-90% of the
time). Most any car requires more fuel at full-power, compared to
minimum requirements (idle) than can easily be covered by injectors
running at a fixed pressure. ie. for a given flow rate, you couldn't
open them for a small enough time at idle, or a long enough time at
max-power to give the proper air/fuel ratio.
A super/turbo charger or other power adder puts more air in the
combustion chamber, so you need to have more fuel to get the correct
fuel/air ratio...
You can a) make the nozzle bigger, b) increase the injector on-time,
c) increase the fuel pressure, or d) a combination of a,b, and c.
The Sebring supercharger and Greddy turbo use a fuel-pressure
regulator that leaves the fuel pressure where it was set by Mazda when
in vacuum, but increases it during boost. This is elegant because you
dont need to change the injector on-times (so you can use the stock
ECU).
A aftermarket ECU like a Link, Tec, Haltech, Motronic or others,
leaves the fuel pressure alone, and changes the injector on-times...
But, the stock injectors are already running near their duty-cycle
limit in the normally aspirated set-up, so you need bigger injectors
as well...
Why not just put in bigger injectors?
Lets say you put in Injectors double the size of the factory ones...
The factory ECU has no idea what size injectors its running, do it
just keeps firing the injectors for the same time it always did... At
full power, this may be just right... but at idle, you're now getting
twice as much fuel as you wanted... This is why you need a
programmable ECU with the bigger injectors, to get the idle and
partial power settings correct.
The adjustable fuel-pressure regulator method is a simple and elegant
but slightly imprecise method of achieving your aims. There are also
practical limits to how high you can raise the pressure. An ECU is
fully adjustable for injector on-times over a "table" of RPM and load
(vacuum/boost) levels, and is a more expensive, but more complete
solution. You adjust the settings to suit your car, so if its
over-fuelling, its because its programmed wrong.
Other topic...
The RX-7 AFM has a physically bigger air passage, but is electrically
the same as the smaller 1.6 AFM, including (and this is the problem),
the same angle of flapper door rotation gives the same voltage signal
to the ECU... Why is this a problem? Because for any given amount of
air-flow (power) at a given RPM/Load level... the volume of air is the
same... and if you pass that same volume of air through two different
sized passages, they will flow at different speeds, and open the
flapper doors more or less... (less in the bigger passage, because
the air-speed is slower). So for a given load situation, the bigger
AFM will be open less, and will therefore be giving readings of less
air-flow than is actually present to the ECU, so it wont fire the
injectors for long enough, and you'll be running lean.
The solution to this problem, is that if you use an RX-7 AFM in a
normally aspirated 1.6 with stock 1.6 injectors (205cc), you need to
loosen the spring on the flapper door, so that it DOES open to the
same angle for the same amount of air-flow. (there are other problems
with this)...
But...I've seen it suggested that if you dont loosen the spring, and
the bigger AFM tells the ECU there is less air flowing... that this is
balanced out by having the 1.8 injectors (which are trying to flow
more than the 1.6 ones for the same pre-programmed injector on times).
In theory this is certainly possible, but I dont know how close the
match is?
I tried the RX-7 AFM in my NA 1.6 (with stock injectors) and although
I got it running pretty well, and definitely had more power at the
top-end, I never got some idle issues sorted, and removed it. I'm not
the only one to have suffered these issues, but it is rare according
to all the US miata-forum users... I may have got a faulty AFM from
the wrecker?
Also, too much fuelling can wash the oil off cylinder walls, leading
to bore-glazing, ring wear, poor sealing and oil-burning... It will
also kill a catalytic converter in the unlikely event that there is
still one under your car. And high fuel bills!
If I haven't explained anything clearly, or you'd like more details,
please ask...
Fletch.
Your basic fuel injector is just a nozzle like your garden hose
nozzle... that can be switched on and off by the engine computer
(ECU). And just like your garden hose, the amount of liquid that can
get through it is proportional to; a) How big the nozzle is (cc
rating of the injector), b) how long it is held open for, and c) the
pressure of the liquid behind the nozzle.
Your basic MX-5 (or most any-other fuel injected car), already has a
fuel pressure regulator that changes the fuel pressure depending on
the vacuum of the intake plenum (so you get less fuel through the
injector at idle, with the throttle closed making high vacuum, than
you would get for the same injector on-time when the car is under load
(less, or no vacuum)). They need to do this because injectors have a
minimum on-time (because of the mechanical opening/closing of the
valve), and a maximum on-time specified as a duty-cycle. (depending on
who you believe, injectors should not be on more than 80-90% of the
time). Most any car requires more fuel at full-power, compared to
minimum requirements (idle) than can easily be covered by injectors
running at a fixed pressure. ie. for a given flow rate, you couldn't
open them for a small enough time at idle, or a long enough time at
max-power to give the proper air/fuel ratio.
A super/turbo charger or other power adder puts more air in the
combustion chamber, so you need to have more fuel to get the correct
fuel/air ratio...
You can a) make the nozzle bigger, b) increase the injector on-time,
c) increase the fuel pressure, or d) a combination of a,b, and c.
The Sebring supercharger and Greddy turbo use a fuel-pressure
regulator that leaves the fuel pressure where it was set by Mazda when
in vacuum, but increases it during boost. This is elegant because you
dont need to change the injector on-times (so you can use the stock
ECU).
A aftermarket ECU like a Link, Tec, Haltech, Motronic or others,
leaves the fuel pressure alone, and changes the injector on-times...
But, the stock injectors are already running near their duty-cycle
limit in the normally aspirated set-up, so you need bigger injectors
as well...
Why not just put in bigger injectors?
Lets say you put in Injectors double the size of the factory ones...
The factory ECU has no idea what size injectors its running, do it
just keeps firing the injectors for the same time it always did... At
full power, this may be just right... but at idle, you're now getting
twice as much fuel as you wanted... This is why you need a
programmable ECU with the bigger injectors, to get the idle and
partial power settings correct.
The adjustable fuel-pressure regulator method is a simple and elegant
but slightly imprecise method of achieving your aims. There are also
practical limits to how high you can raise the pressure. An ECU is
fully adjustable for injector on-times over a "table" of RPM and load
(vacuum/boost) levels, and is a more expensive, but more complete
solution. You adjust the settings to suit your car, so if its
over-fuelling, its because its programmed wrong.
Other topic...
The RX-7 AFM has a physically bigger air passage, but is electrically
the same as the smaller 1.6 AFM, including (and this is the problem),
the same angle of flapper door rotation gives the same voltage signal
to the ECU... Why is this a problem? Because for any given amount of
air-flow (power) at a given RPM/Load level... the volume of air is the
same... and if you pass that same volume of air through two different
sized passages, they will flow at different speeds, and open the
flapper doors more or less... (less in the bigger passage, because
the air-speed is slower). So for a given load situation, the bigger
AFM will be open less, and will therefore be giving readings of less
air-flow than is actually present to the ECU, so it wont fire the
injectors for long enough, and you'll be running lean.
The solution to this problem, is that if you use an RX-7 AFM in a
normally aspirated 1.6 with stock 1.6 injectors (205cc), you need to
loosen the spring on the flapper door, so that it DOES open to the
same angle for the same amount of air-flow. (there are other problems
with this)...
But...I've seen it suggested that if you dont loosen the spring, and
the bigger AFM tells the ECU there is less air flowing... that this is
balanced out by having the 1.8 injectors (which are trying to flow
more than the 1.6 ones for the same pre-programmed injector on times).
In theory this is certainly possible, but I dont know how close the
match is?
I tried the RX-7 AFM in my NA 1.6 (with stock injectors) and although
I got it running pretty well, and definitely had more power at the
top-end, I never got some idle issues sorted, and removed it. I'm not
the only one to have suffered these issues, but it is rare according
to all the US miata-forum users... I may have got a faulty AFM from
the wrecker?
Also, too much fuelling can wash the oil off cylinder walls, leading
to bore-glazing, ring wear, poor sealing and oil-burning... It will
also kill a catalytic converter in the unlikely event that there is
still one under your car. And high fuel bills!
If I haven't explained anything clearly, or you'd like more details,
please ask...
Fletch.
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