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[niterydr]

Originally Posted by MustGoFaster

Spot on! The ECU will always think the injectors are 360cc. Piggy backs just lie to the ECU (tell it there is less air than there really is) to get the result of the right amount of fuel with injectors that are bigger. With the "less air" the ECU then runs in a different part of the timing map, and that can sometimes be a substantial difference.

There are other piggybacks that can adjust after the ECU (Emanage Ult.) but most common is to adjust the air flow signal.

But, if you make enough extra power, you get the modified air flow signal into a good range on the ECU map and timing numbers will be ok. That's why it's best to run the smallest injectors that will support your power level. If you put in injectors that are 50% larger than stock and only make 10% more power, you will have an issue with timing. If you put in injectors that are 20% larger than stock and make 20% more power, the ECU is going to give you a timing that should be close enough to run with out issue.

Example: (fictional made up #'s)
Lets say a Hz number of 2600Hz is enough to max the stock injectors. We'll say 20 degrees of timing is correct here. ~16 PSI from 9b's.
Now we put in injectors that are 20% bigger than stock, and an SAFC, and change NOTHING else. To get the right amount of fuel you will need to tell the ECU it's only getting 2080Hz of air. The ECU gives the right timing for that HZ count, say 24 degrees. But your still running 16PSI from your 9b's... too much timing for these conditions.

Now we add 13T's and better IC's and turn the boost up more. The HZ reading from the MAS goes up to 3300. Take 20% away and we end up with 2640Hz to the ECU. ECU feeds 20 degrees of timing, which will be ok since we have nice cool charge temps from our bigger IC's and efficient turbo's. Now we are making 20% more power.


All those #'s are numbers that I picked that may or may not be in the ballpark. Just trying to give an example of what happens.

Well said Ryan, great use of example!