Better MPG with lower octane gas?

[Cellos86SVO]

So after having been able to drive our SVO for the past several weeks I've noticed that the car gets very bad gas mileage with 91 octane gas. Roughly 20mpg on the hwy and 15 or so around town which is just as bad as my '88 GT and thats all I've been running just to be safe. However, on my long way home the other weekend my curiosity got the best of me, I flipped the fuel grade switch (even though its broken and only boosts at 10psi no matter where the switch is at), ran 89 octane, and I noticed I was getting 25 mpg on the freeway which is a pretty significant improvement. My question is why? Shouldn't the car see the same MPG no matter what fuel grade is being used?

sidenote: I found out that the car had K&N filter already in it which I thought was pretty awesome.

[fastmonkeywheels]

I haven't done the correct research on this, but a friend of mine I trust to give me accurate advice said that a car tuned for 87 will get better MPG running 87 than they will running 92, the same applies for 92, if you're turned for 92 you'll get better MPG running that than 87.

Now, of course you have to take into account what Mike said about the oxgenated fuels, but all else equal the above should apply. If your fuel premium switch is broken, thus you only have 10psi in whatever mode you're probably running in 87 mode all the time, thus you'd get better MPG in 87.

It's possible your switch isn't broke and the BCS is, that puts my whole theory out the window. You could try and see what's wrong with your fuel switch.

[blitzkraig]

I've seen timing maps for the PE premium tables, LA3 for regular and premium, but not for PE regular. Is there a difference in the PE part throttle timing maps between premium and regular? I'm wondering if there's any MPG benefit from running the switch in premium mode on regular fuel just cruising on the highway, providing it's not pinging and pulling spark advance from the knock sensor.

[boostedbruiser]

I've seen timing maps for the PE premium tables, LA3 for regular and premium, but not for PE regular. Is there a difference in the PE part throttle timing maps between premium and regular? I'm wondering if there's any MPG benefit from running the switch in premium mode on regular fuel just cruising on the highway, providing it's not pinging and pulling spark advance from the knock sensor.

According to what i can see, the Regular mode takes 6* of timing out ..... makes the computer think its only at 4* base timing, essentially taking that amount across the board.

[SVOeric]

which is why I run on premimum mode, with the adjustable boost controller set at 10-12. havn't had any pinging yet, and keeps me out of trouble on the roads (well mostly)

[Cellos86SVO]

That's a very complicated question to answer but power and mileage don't come from octane. The heat energy in the fuel dictates how far a gallon (or pound) will move your car. And the heat energy isn't posted on the pump.

Most likely the higher octane fuel you get has more alcohol - oxygenated fuels - in it and that seriously lowers the heat energy in the fuel. Thus fewer miles per pound/gallon/tank.

My dd car looses about 10-15% between summer fuels (not oxygenated) and winter fuels (oxygenated). There are other factors like lower tire pressures, more time spent warming up the engine versus summer driving, etc. but 10%+ is way more than those factors alone.

Well this all happened only a few weeks ago and I live in cali so would the fuel still be oxygenated?

[Cellos86SVO]

right so the huge loss in MPG is due to the premium fuel. What kind of mileage should I see on this car anyways?

[Jcamper]

I thought all pump fuel in Commifornia was oxygenated? I know Oregon is moving there. I would actually suspect that large of a mileage discrepancy would probably be a difference in pump handle shutoff/tank fill height. Could have had an empty hose or a slight difference in metering on top of that. Jcamper

[gbeaird]

The lower the octane, generally, the faster the fuel burns, so the more energy you can get out of it per unit of time. There are a lot of variables involved, but that is a basic generality. So yes, the lower-grade fuel will probably burn more efficiently (quicker) allowing you to apply more of the energy conversion to actually doing work (moving pistons). As long as you don't need the octane to prevent spark knock, you can probably get away with running the regular in that normally low-compression engine. Just remember you are running regular before you hammer the throttle on a hot day in 4th gear, or you could be walking home.

Gene Beaird,
86 2R SVO, G Stock,
Pearland, Texas

[93-331-29psi]

I have been running 87 in my 84 for about 2 months now with no negative results. Then again I have the boost turned down real low also (around 8psi) and I drive like an old lady. When I retire it from DD Duties I will put the 4.10s back in and turn the boost back up and go with 93

[bob tweedy]

mpg

hey mike. put me in the know. as i live in backward montana where we have no emission controls/testing what is oxygenated fuel. while i was in mississippi we used to run white lightning. i do know for a fact that the octane boosters aint worth the plastic they bottle them in. are we talking something like 'torco' additive?
bob

[65ShelbyClone]

Octane also has little to do with the fuel's flame speed. Methanol is a good example of a fuel with a high octane and a flame speed faster than gasoline.

I suspect there was/is either an inconsistency in the fill up or something awry with the engine. I get 24-26mpg in my SVO with 91 regardless of what brand of fuel I use and I don't baby it either. 20mpg hwy/15 city is worse than a 5.0.

Maybe it's all psychological. Maybe you're driving it easier because you put 87 in it.

[luvtolean]

right so the huge loss in MPG is due to the premium fuel. What kind of mileage should I see on this car anyways?

Doubt it. Probably more a statistical outlier, either different driving conditions, pump settings or a "strange" batch. Do 8 tanks of each and average them. I bet you see that difference go away.

Even with oxygenated fuel, most people I know get better milage with premium, enough so that it pays for it.

FYI, premium is typically a better fuel than 87 with different "premium" handling all along the chain. Octane is not the only difference. Also, 89 is usually a mix of 87 and 91.

Oh yeah, and oxygenated gas rocks. Mo' powa.

[luvtolean]

Mike I wholly agree with your follow up post. Oxygenated fuel is not solving any problems today. It probably actually has an environmental cost associated with it due to increased consumption. Neither MTBE or alcohol are good for our water either.

But here are my comments to your first post (I wasn't going to post, but maybe I'll learn something here if I'm picked apart):

There's more power in a gallon or pound of real petrol than any blend of gasohol - in fact there's significantly less heat energy - the stuff that makes power - in ethanol/methanol-laced fuels than petrol.

Yes.

The ONLY way to make more power using alcohol-laced petrol fuel is

1) Burn lots more of it

I dunno about "lots", but depending on the engine, in some cases more...

2) Tune specifically for that fuel.

This I have dyno tested and know to not be always true. Ignoring closed loop feedback controls which of course compensate on the fly (and thus can add more fuel to the extra oxygen without manual fettling), even on motorcycles without Lambda sensors we've done back to back dyno testing and the oxygenated stuff wins with the same fuel map. Unfortunately I don't know all of the changes involved with oxygenated stuff, so if it was something other than the oxygenate, I won't know it.

If you tweak the mapping to your specific fuel/engine of course you can do better.

Interestingly, many race orgs had to change fuel rules allowing oxygenated fuels (as pump was failing), creating a bonanza for companies like VP, and leading to some pretty big gains just by changing fuel. The downside has been a higher cost of racing...

[bob tweedy]

mph

hey mike. i need to clarify country & western. to this ole mississippi boy 'country' is akin' to 'blue grass'!
western is purly 'horse s___ and gunsmoke'!!!
lester flatt and earl scruggs forever.
bob