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Here is another explanation to clarify. I'll try and make it understandable 
Lets see how this works an a stock, non turbo setup first:
Injector duty cycle= The actual time the injector is "on" or spraying fuel
Our engines are matched to work with the ECU programming, and the stock injector size of 235cc's. Most fuel injectors are rated for max flow around 80- 85% duty cycle. This means that our stock injects will give around 235cc's of fuel flow at 80-85% duty cycle. At WOT, our engines wont go over 0 psi(non turbo remember). AT 0 psi, the map voltage will be around 4.5 volts. With that map voltage, it tells the ecu to give around 80% duty cycle to the injectors, as this will be all the needed fuel at WOT high rpm. Now, we add the turbo to this setup. We have to clamp the Map voltage at 0 psi on up, because if the ecu sees any voltage higher than that 4.5 volts, the ecu will try and give a higher duty cycle than 80%. The ecu is programmed to not allow this, and then we get fuel cut, as injectors can have problems at duty cycles over 80%. So we need to clam the map votage under boost, because the map will increase this voltage when it sees boost pressure. Thats why we need to clamp the map voltage under boost, so it will retain the 80& duty cycle, with no fuel cut. To get more fuel under boost, we up the fuel pressure form the stock injectors, to get more fuel. More pressure= more fuel. Think of a garden hose with a spray nozzle. Turn the water on lightly, it sprays very little. Turn up the water pressure, then we get more water. With me?
Now, if we put in larger than stock fuel injectors, we will get more fuel flow at each percentage of duty cycle over the stock injector. Lets take 440cc injectors for example, as they are around 2 TIMES the flow rate of stockers. This means, they will flow TWICE the amount of fuel at 80% duty cycle over the stock injectors at 80% duty cycle. Yes?
Now to run the fuel injectors on our cars, we need to lower the duty cycle by HALF of what it normally was. We do this by the Map sensor. Normally, stock injects, we need 4.5 volts for 80% duty cycle fuel flow. Now that we have TWICE the amount of fuel, we need to trim the map voltage by HALF to get the same amount of fuel flow as the stock injects. For example, to get the same fuel flow of 235cc at 4.5 map volts, we need to get 235cc of fuel flow out of the 440 injects by Halving the map voltage. Stock injects, at WOT (0psi) the map voltage is at 4.5 volts, and that = 80% duty cycle. In order to get the stock fuel flow out of 440 injects( remember 440cc's of fuel at 80% duty), we need to see a map voltage of 2.25 volts. This makes the duty cycle of 40%, which is half of 80%.
440's @ 40% duty cycle equals the same fuel flow of:
235's @ 80% duty cycle, because we are getting TWICE the fuel flow from an injector that is twice as big as the stockers. (this is just an example for simpicity, actual flow rates will be different)
Now the SMC works this way. You MUST run larger injects for it to work. Lets take the 440cc example again. Stock injects, at 0 psi, we have 235cc of fuel flow. With 440cc injects, we have 440cc of fuel flow at 0 psi, with duty cycles of 80% on both. Now we will get TWICE the fuel we reall need at that flow rate on the 440's, since they flow twice the amount of fuel. We need to tell the ECU to cut the duty cycle in half at 0 psi. We do this by altering the map voltage. Lets say for sake of discussion, at 0 psi, on 440's, that we need a map voltage of 2.25 volts, to get the proper fuel flow. With me? So now we will use the left over 40% of duty cycle for under boost conditions. So now, at over 0psi, the map will increase its voltage accordingly to boost pressure, and the smc will then increase the output voltage goint to the ecu greater than the 2.25 volts. Lets say at 10 psi, the map voltage is 9 volts. The SMC will cut this in half, to read the 4.5 volts to send out to the ecu, and the ecu will run the 80% duty cycle on the 440's. This is enough fuel flow for 10 psi of boost. Again, this is just for the sake of discussion, not acutal usable numbers.. Hopfully this makes sense..
13.5 @108 MPH-2.2 60ft(stupid FWD!)
S16G @ 18 PSI/FMIC/Running on MegaSquirt II (Now with sequential
fuel injection)
My webpage: http://eclipsed4evr.home.comcast.net
-1998 Mitsubishi Eclipse RS-T- "Toy"
-1992 Plymouth Laser Turbo AWD(SOLD)
-2000 Honda CR-V(daily)
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