Octane vs. Gas Mileage question

[Fanta]

A friend of mine posed a question that I couldn't answer, and I'm curious to as well.

 

We all know about the price variations between 87, 89, and 93 octane. I also more or less understand that the higher the octane, the better it burns and such. My question is though for an N/A say, Honda Civic, which is better for your money? Is it better to go cheap with 87 octane, or does the 93 pull off better mileage ( more efficient? ) so that paying more in the end might actually be cheaper than the 87 in the long run?

[carguygibby]

If the recommended octane for your car is 87, stick with it. There is no advantage in using a higher octane. My 300M runs on 87 no problem, my flatty did too.

Colin

[GoFastBeater]

Higher octane does not mean it burns better, it means it just has a higher flash point (more heat/compression needed for ignition). Lower octane increases the chances of pre-ignition. If your car does not have a hot enough ignition or enough compression, then running higher than the recommended octane will most likely not get a complete burn, and mileage may even suffer slightly---not to mention spark knock is annoying.

[mstieg]

Good question. I'd like to hear the G54b answer myself, not a NA Honduh. =)

 

I've got a 1400 mile trip coming up for the Starion, and may just program a seperate tune for some 87 highway cruisin' if it'll save enough $$ to be worth it. I won't need full boost/octane to scare my friends I'll be staying with. A more aggressive low boost 87oct tune should be plenty.

 

Mine usually got 19-23mpg on 93 pre-MPI, never really used the 87.

[Fanta]

Good question. I'd like to hear the G54b answer myself, not a NA Honduh. =)

 

I've got a 1400 mile trip coming up for the Starion, and may just program a seperate tune for some 87 highway cruisin' if it'll save enough $$ to be worth it. I won't need full boost/octane to scare my friends I'll be staying with. A more aggressive low boost 87oct tune should be plenty.

 

Mine usually got 19-23mpg on 93 pre-MPI, never really used the 87.

 

Ehhhh, you can run 87 octane on our cars, but its generally not recommended ( at least try to keep out of boost as much as possible to avoid pre-detonation )

 

So there isn't any advantage in gas mileage using 93 over 87?

[Truth]

my Maxima recommends premium, but i've been running middle grade on it with no spark knock as of yet.

[RWDAddict]

With my 02' Lancer I always put 93 in it and the one time I ran 87 the throttle wasn't as responsive and since i've only ran it once so this may not be too accurate I did notice a slight drop in gas mileage. By only 1 MPG I believe. If your tank is roughly 10 gallons it's only $2 more per tank figuring premium is only .20 cents more.

[vbrad511]

I only run the 92-93 in my vehicles. When I used to run the mid grade I'd get worse mileage, and seemed to run a bit worse.

[mikec]

Cars with more modern ECUs - those that look at the knock sensor and constantly push the ignition timing to the edge of knock - will tend to get better MPG with higher octane fuel. They'll advance the timing to take advantage of the higher octane fuel's resistance to pre-ignition.

 

Something like a StarQuest though, with an ECU that doesn't learn much of anything and fixed ignition & fuel maps, won't get any different MPG with higher or lower octane fuel unless there are other engine/fuel system problems that make the engine ping/detonate on low octane fuel... when the engine is borderline you tend to have to give it extra throttle to drive so MPG suffers. If you want to experiment, get the highest octane stuff you can buy and then manually tweak the distributor timing a degree or two and you'll see some tiny MPG improvement. Just modifying your driving habits will have a greater effect on MPG - like lifting off the throttle but leaving the engine in gear (and the clutch pedal not depressed) to decelerate will put the engine in full fuel-cut mode until the RPMs drop to near idle. If you lift and put the tranny in neutral, or press the pedal, the engine RPMs drop quickly and the idle fuel mode kicks on. Coasting in gear will leave you in fuel cutoff mode longer.

 

Higher octane allows more compression and/or more timing advance. For most engines, more compression = more torque, more timing advance=more torque & power. Turbos and superchargers create more "effective" compression ratio which is why they generally need higher octane fuel. Folks that run higher PSI than stock, or run larger flowing turbos, are more at risk when running low octane fuel on StarQuests. The factory setup is fairly conservative - a tank of 87 octane probably won't lead to engine knock/detonation/meltdown on a stock car with a healthy fuel injection setup. Especially on the 87-earlier cars with the lower boost levels; on the 88-later cars the increased factory boost limit pushes the limits on 87 octane. Personally I'd never run 87 octane in my 88 unless there was no alternative; then I'd limit boost to no more than 5psi to be safe.

 

mike c.