What exactly makes the OHV 2.2 MPFI ECM capable of firing 4 injectors through two wires accurately, yet the TBI ECM cannot? Is it a low and high impedance thing with the injectors?
I'm tired of my 90 MPFI car blowing raw fuel out the tailpipe and idling/running funny. Dont get me wrong... it runs good, just not *right*.
JLP
90 Cav<--- "1LE" MPFI car
79 Monte<--- 11.82 @ 112.72
MPFI is batch fire... it fires 1,3 and 2,4 at the same time., just like the spark.
Chris
'02 Z-24 Supercharged
13.7 @102.45 MPH Third Place, 2007 GMSC Bash SOLD AS OF 01MAR08
Sooo..
If I pull the loom off an MPFI injector harness, I'll find one wire paired to 1,3 and the other on 2,4? I never pulled the loom off, but thats the only way I can see that working. One wire pulses one pair, and the other wire does the opposite pair.
Am I correct or off my rocker?
JLP
90 Cav<--- "1LE" MPFI car
79 Monte<--- 11.82 @ 112.72
Hold on!!!!
First things first.
The 92-93 2.2 mpfi ecm fires all 4 injectors at once. There are only 2 wires, power and ground, connected to the injector harness. Check a wiring diagram or a harness if you don't believe me.
Each injector is about 12 ohms of resistance. With all 4 wired together resistance is calculated as parallel resistance. The total resistance of (4) 12 ohm injectors wired in parallel is 1/ (1/12 + 1/12 + 1/12 + 1/12) or 3 Ohms. The total peak current passed through this system is 4 amps (12v / 3 Ohms). Because each injector is high resistance, the ecm fires all of them in "saturated mode," meaning that once the current reaches 4 amps, it stays at 4 amps until the injector is shut off. The reason this doesn't burn out the injectors is that each one shares roughly 1/4 of the current.
A single TBI injector is about 2 Ohms resistance. Like the 92-93 ecm, the tbi ecm fires the injector with every ignition pulse received from the ignition module. And the total peak current passed through the system is 12V / 2 Ohms or 6 amps. This is a bunch of current! If you try to hold the injector open for any length of time with this current it will not live. So the tbi ecm uses a special circuit which senses when the current peaks and the injector is fully open, then drops it to 1/4 of the peak to hold it open. This circuit is calibrated for the maximum current of the injectors it's connected to at the factory, in your case a single TBI injector. This is called a "peak and hold" circuit and it's what is missing from many ecms/ pcms with high resistance injectors.
Now in your case you've replaced the single 2 Ohm injector with a roughly 3 Ohm set of 4 injectors. This replacement set uses 4 amps constant current when they're open. Let's see how this works out.
The peak and hold injector driver opens the injectors. The current rises from zero amps, but because the total resistance of the replacement injectors is higher than the originaltbi injector, the current stops rising at 4 amps. The peak and hold circuit is looking for 6 amps peak, but it doesn't see it. So it never goes to "hold" mode. Instead it keeps the current to the injectors at the 4amp maximum, saturating them with current like the stock mpfi ecm would do. From the injectors point of view, absolutely nothing has changed.
The TBI ecm is completely capable of accurately controlling (4) mpfi injectors. But your car is running rich, so something has to be going on. Let's check flow rates:
The stock tbi injector for the 90 Cavalier is rated at 56 lbs/ hr fuel @ 10.5 psi. If you were to open the injector for one hour it would deliver 56 lbs of fuel. The stock injector for a 93 Cavalier is rated at 16 lbs/hr @ 43.5 psi. So four of these injectors opened at the same time would deliver 16 * 4 or 64 lbs/hr of fuel.
By switching to the mpfi system you have gone from a 56 lb/ hr injector to a 64 lb/hr injector. This is a 14% increase in injector size. No wonder you're running rich. But how to cure the problem?
The easiest way is to lower fuel pressure. Since the port injectors are rated at 43.5 psi, we can use this formula to calculate the pressure needed:
New pressure = ( new flow / old flow * square root of old pressure ) squared
We want the mpfi injectors to flow the same as the tbi injector, 56lbs/hr.
(56 / 64 * square root (43.5)) ^ 2 = 33 psi
The new injectors should be run at about 33 psi fuel pressure. I'd set 'em for 34 psi and let it run a tiny bit rich. You can find adjustable fuel pressure regulators on Ebay or on the web.
FYI some of the characteristics of the TBI injector are different than the mpfi injector. The tbi ecm will not know how to correctly adjust the injector on time if battery voltage changes (like at idle on a cold day with the heater on) or if the injector is only open for a tiny time. This may not be noticeable, or it may cause issues. The only way to know for sure is to drive it and see. The only way the change these issues in the tbi ecm are to reprogram it or switch to a mpfi ecm.
LMK if you have any more questions.
BTW, nice Monte. Some day I'll get my 455 powered '79 back on the road.
-->Slow
so when did thay go to batch fire?
Chris
'02 Z-24 Supercharged
13.7 @102.45 MPH Third Place, 2007 GMSC Bash SOLD AS OF 01MAR08
The 92 and 93 are batch fire. Batch fire just means "more than one injector at a time."
In 94 they changed to the style you described, 2 banks of 2 injectors. 95 used the same setup. If ya want the official name, it's called "alternating batch fire."
-->Slow
Slow, Thanks for the words on the Monte and also the tips for the Cav.
The fuel pressure thing always confused me.. I never knew if lowering pressure actually decreases the fuel output from the injectors or not. Looks like I'll need to snag an AFPR off Ebay and give it a shot. Even though the car is quick right now, I'm hoping it perks up a little when I lean it out.
JLP
90 Cav<--- "1LE" MPFI car
79 Monte<--- 11.82 @ 112.72
slowolej wrote:Hold on!!!!
First things first.
The 92-93 2.2 mpfi ecm fires all 4 injectors at once. There are only 2 wires, power and ground, connected to the injector harness. Check a wiring diagram or a harness if you don't believe me.
Each injector is about 12 ohms of resistance. With all 4 wired together resistance is calculated as parallel resistance. The total resistance of (4) 12 ohm injectors wired in parallel is 1/ (1/12 + 1/12 + 1/12 + 1/12) or 3 Ohms. The total peak current passed through this system is 4 amps (12v / 3 Ohms). Because each injector is high resistance, the ecm fires all of them in "saturated mode," meaning that once the current reaches 4 amps, it stays at 4 amps until the injector is shut off. The reason this doesn't burn out the injectors is that each one shares roughly 1/4 of the current.
A single TBI injector is about 2 Ohms resistance. Like the 92-93 ecm, the tbi ecm fires the injector with every ignition pulse received from the ignition module. And the total peak current passed through the system is 12V / 2 Ohms or 6 amps. This is a bunch of current! If you try to hold the injector open for any length of time with this current it will not live. So the tbi ecm uses a special circuit which senses when the current peaks and the injector is fully open, then drops it to 1/4 of the peak to hold it open. This circuit is calibrated for the maximum current of the injectors it's connected to at the factory, in your case a single TBI injector. This is called a "peak and hold" circuit and it's what is missing from many ecms/ pcms with high resistance injectors.
Now in your case you've replaced the single 2 Ohm injector with a roughly 3 Ohm set of 4 injectors. This replacement set uses 4 amps constant current when they're open. Let's see how this works out.
The peak and hold injector driver opens the injectors. The current rises from zero amps, but because the total resistance of the replacement injectors is higher than the originaltbi injector, the current stops rising at 4 amps. The peak and hold circuit is looking for 6 amps peak, but it doesn't see it. So it never goes to "hold" mode. Instead it keeps the current to the injectors at the 4amp maximum, saturating them with current like the stock mpfi ecm would do. From the injectors point of view, absolutely nothing has changed.
The TBI ecm is completely capable of accurately controlling (4) mpfi injectors. But your car is running rich, so something has to be going on. Let's check flow rates:
The stock tbi injector for the 90 Cavalier is rated at 56 lbs/ hr fuel @ 10.5 psi. If you were to open the injector for one hour it would deliver 56 lbs of fuel. The stock injector for a 93 Cavalier is rated at 16 lbs/hr @ 43.5 psi. So four of these injectors opened at the same time would deliver 16 * 4 or 64 lbs/hr of fuel.
By switching to the mpfi system you have gone from a 56 lb/ hr injector to a 64 lb/hr injector. This is a 14% increase in injector size. No wonder you're running rich. But how to cure the problem?
The easiest way is to lower fuel pressure. Since the port injectors are rated at 43.5 psi, we can use this formula to calculate the pressure needed:
New pressure = ( new flow / old flow * square root of old pressure ) squared
We want the mpfi injectors to flow the same as the tbi injector, 56lbs/hr.
(56 / 64 * square root (43.5)) ^ 2 = 33 psi
The new injectors should be run at about 33 psi fuel pressure. I'd set 'em for 34 psi and let it run a tiny bit rich. You can find adjustable fuel pressure regulators on Ebay or on the web.
FYI some of the characteristics of the TBI injector are different than the mpfi injector. The tbi ecm will not know how to correctly adjust the injector on time if battery voltage changes (like at idle on a cold day with the heater on) or if the injector is only open for a tiny time. This may not be noticeable, or it may cause issues. The only way to know for sure is to drive it and see. The only way the change these issues in the tbi ecm are to reprogram it or switch to a mpfi ecm.
LMK if you have any more questions.
BTW, nice Monte. Some day I'll get my 455 powered '79 back on the road.
-->Slow
someone has done there homework.... but you are exactly right.... I had to do research on this before I put a turbo in my 2.2. where as my injector actually puts out higher impetence than the mpi.. just like you said 52lbs.
**changes are here**
