Sorry if this is a newb question but I've searched, I know the B16a2 comes with a P30 ECU, but will a non-chipped virgin P28 ecu run the b16a2 just the same? Any cons to it?

The reason I'm asking is I got a b16 swap from my buddy and it came with a p28 ecu, I don't think its chipped but its running in my car right now. I noticed I've been getting some real shitty gas mileage, probably in the low 20s average, I was thinking maybe it has something to do with the ECU?

 

It definitly has A LOT to do with the ECU. It can run on a P28 but its not good for too long. Look for a P30 or chipped P28 made for your setup, but P30 if you want the best gas mileage for your engine.