G-Body

iTrader: (4)

So I was running my El Camino on 87 octane most of the winter. I was also playing around with the PCM and tuning it during the early spring. I had a LOT of time spent logging and abusing the car and there was only once that I saw it pull 1 degree of timing out of 2 cells at WOT.

Now I switched to 93 octane like I used to run and on the first tank I picked up 2 MPG. Was there knock before and it was retarding the timing to the low octane table before it showed up on the spark knock retard table?

I thought that if there was knock it would show up on the knock retard table and then the advance would swing down towards the low octane table?

I don`t understand how I am getting better fuel mileage with the same amount of advance.

cast iron

two things:

93 octane has slightly higher energy content (lower heating value) but that usually isn't a first order effect. The knock limitation of running a fuel with +6 octane points would probably be more apart of the answer, along with better location of peak cylinder pressure and when its applied to the piston.

You may be running a tad to much timing for the given rpm/load cell you spend the most time in on 87, requiring you to push the accelerator more, to obtain more power to overcome road load (everything that wants to slow you down - areo, friction, dead bodies in the back ). 93 octane may be providing more torque output per throttle position, hence no need for more air (and fuel) to overcome losses.

In engineering, that's known as BSFC, brake specific fuel consumption. Fuel Mass / HP output (aka torque x rpm / 5252)

ringram

I agree with most of what you just said Mr Iron, except that its lower octane fuels that have the higher calorific value not the higher octane ones.

cast iron

I'm going off of what's reported by a collection of SAE papers (lower net heating value) and information found in Greg Banish's Book. It's about a 1kJ/kg difference, so its probably contributes to very little of the change.