Have both options local to me

997.2 C2S

 

Great question, I'll be curious to hear from experts. My opinion is this:

If your 997 is factory tuned for 93oct, use that. Is it possible cars tuned for 93oct simply rely on their sensors when running 91oct (i.e. California) to retard timing, etc? Based on that 91oct will have a bit less power.

Also, all modern cars are designed for 10% ethanol in case that is a concern.

I would be curious if mpg and hp are similar for both.

 

I m going to say the price is too high on the eth. free gas so I would go with the 93.

 

I believe you can run either version. The car's ecu will compensate for the 91. As to milage the 10% ethanol blend would most likely return the worst milage.

Yes, your car was designed to run with 10% ethanol.

 

Alan, so will the ecu retard the timing and lower the engines hp if 91 is used?

 

Compensating to run the 91octane implies power would be reduced. Also, if that is the case, then the higher octane with 10% ethanol could possibly give better mileage.

 

Ethanol has less energy (calories per unit volume) than gasoline, so it's likely you'll get worse mileage with the 10% ethanol blend.

In Brazil where E85 and gas are both sold, and all cars can use either, there is a significant discount for the E85 because of the lower energy/mileage with ethanol.

I think ethanol has something like 67% of the calorific density of gasoline, so you could probably calculate what the break-even price difference is.

Ethanol, of course, is naturally 110 octane so is an easy octane booster for the fuel companies to blend up with. So, the ethanol will allow the engine to produce higher peak power.

 

Like many thing, it depends.

What is the price per gallon of each?
Generally, you will get better performance from the 997 with higher octane.. but if you won't really benefit from that slight upward push in the performance curve because you are not driving it at that limit, then the retarded timing with lower octane isn't a problem and you are just spending extra money needlessly.

 

Yes, very true. However, would timing advance to take advantage of the slightly higher octane rating offset that lower energy density in terms of daily driving rpm vs. peak power? Therefore increase mpg AND peak power?