So i have a 1991 Chevy Caprice wagon which i put a 350 from a 93 Buick Roadmaster into. I yanked the ecu out of the buick and put it in my car with the engine. I took my car down to the local dragstrip with and ran a 17.2 at 78mph and just for the hell of it i put the old 305 ecu in and ran it. Its first run with the 305 ecu it turned a 16.8 at 83mph with nothing else being changed. I have heard rumors that the 305's had a more agressive spark curve but if anything that make it run more lean. My question is does anyone know why it would be 4 tenths faster with just a ecu swap? The car has headers, functional cowl induction hood with a velocity stack and i ran it with open headers everything else was stock including the 2.73 rear end.

More advance in the ignition timing would give yah a little boost in your 1/4 mile time and may account for the total gain. In terms of fuel mixture, I'm not sure if the 305 ECM has any different programming over the 350. Remember, your launch has a lot to do with overall time. So you may have had a better launch along with the more agressive ignition timing working together to give you that better time. Now change out the rear end and watch your time drop by another second or more.

Your ECM controls the fuel/air mix, but it doesn't control the rate of delivery. That's decided by the injectors themselves. The 350 uses larger injectors than the 305 does, but if you are using the 350 injectors you're OK. 305s do indeed run a more aggressive spark curve and this is likely where you are seeing your gain. I am running a 305 PROM with my 350 for the same reason (even though my car is carbureted). It just flat 'runs better' with the 305 PROM.

The ECMs are the same between 305 and 350. The difference is in the programming - my ECM uses a simple PROM (due to the fact it's a feedback carb setup) where yours runs a PROM/CALPAK or MEMCAL for that reason (the MEMCAL is a PROM/CALPAK combo, the CALPAK runs the injectors in the event of computer failure).

The other related items are the detonation retard attack and recovery rates. Since the 305 used a lot more sensitive knock sensor, the attack rates are a bit slower and recovery rates faster, so detonation isn't going to pull quite as much timing quite as fast. This should further add to the 350's increased timing advance under load.

BTW - You don't really have to change the complete ECM, just the CALPAK. However, for the most thorough comparison, any time the ECM is disconnected from power, it loses the BLM tables and has to rebuild them. While in that mode, the engine is running on default parameters until the integrator has had time to start writing new BLM tables for the engine. In your case, swapping EMCs would mean you were running on defaults, and that the BLM tables in your ECM may not have been optimal for acceleration. If you run that 305 CALPAK long enough, things may change back closer to what you experienced with the old ECM.