I filled my car up with midgrade instead of premium the other day just to see what kind of difference it would make. Does it make ANY sense that I am getting about 6 mpg better than I did with premium?? Nothing else is different, just the grade of fuel. It idles just a little rough and if there is a drop in power it isnt much. Also, what should my fuel pressure gauge read? I dont know what pressure the regulator should be set at.

 

There can be subtle differences, but 6 mpg??? You're not going off what the trip computer is telling you after only a couple of miles are you???

The difference in MPG between different grades of gasoline depends on your engine and it's requirements... You'd be surprised that even some of today's economy cars run a little better on midgrade than regular... I've never seen it work the other way around like you're observing, where midgrade gave better economy than premium...

My ex-gf's base 2.2L Cavalier did a little better (1-2mpg) on midgrade than regular, but premium did nothing over midgrade...

My Vette, only got regular a couple of times in it's life... Both times I noticed about 2mpg decrease with regular... So I didn't really see any savings since I'd need about 2 more gallons of regular to go the same distance as premium...

My 94 Northstar Eldorado is the same way... The dealer gave me a full tank of regular gasoline when I bought it... The vehicle is marked "Premium Required" and that first tank of gas didn't last very long, nor did it accellerate correctly at WOT (pinging)... I ran the tank almost dry and filled up with Premium and the power and performance returned...

Every modern car I've ever owned ran better with either midgrade or premium...

 

It sounds very possible, just not 6 mpg. With premium, you might not be burning all the gas if combustion pressures aren't high enough. So, in theory, you could get a more complete burn with mid-grade, but not 6 mpg better, and probably not in a high compression engine like your Corvette has.

All that being said, I'm going to fill up with midgrade next time. I've been running premium since I bought the car, and it might not be giving me any gains. :crazy:


[Modified by CentralCoaster, 2:04 AM 3/17/2003]

 

You are correct... I was taking the Corvette as the model since he mentioned it as the subject of the MPG observation...

A long time ago I had a 78 Buick LeSabre with a whimpy 301V8... I believe the CR on that car was 8.5:1... That car never had premium or midgrade ran through it and never had any pinging issues... I think the lowest compression 350 TPI motor was 9.5:1, which would benefit from midgrade and premium...

From what I remember about gasoline and octane rating was that you run the lowest octance you can without detonation... The higher the octane, the slower the burn, but you do get the resistance to pre-detonation which is worse than the slower burning fuel...

 

I found out it also depends on the brand of gas. We did a test of about five fillups and noticed that gas from staitions like AMOCO or SUNOCO got more miles per tankful. We tested it against ones like SHEETZ and generic brands. Pinging was also more noticeable in the cheaper gas. Even one of the cheaper gas had a higher octane rating! We did one car with midgrade and one with premium. It ranged from 1-3 gal per tankful better with name brands. Same type of cars and same runs everyday. I also found out that cheaper gas comes from the lower part of the tank when tankers get filled. Most generics use name brand but from the bottom of the barrel. It can be in different in your area but this was in MD. Lower compression can use lower octane like said but it depends on mods also.

Race fuel is expensive so I'll just add octane booster to pump gas:- BIG MISTAKE, first pump fuel is notoriously inconsistent in its blend, often changing weekly if not daily. Different manufacturer's mix fuel in different ways. If you travel across the country to ride then fuel can and does change between area's. therefore you have no baseline to work from. How much octane booster will you need? You will never know. Add to this the fact that octane booster's will only increase the octane by small amounts, no matter how much you add, you will get problems.

The first test to determine the (research octane rating)RON rating of the fuel is done at low rpm's and cool conditions, this is not realistic for use in a high revving engines.
The second test to determine the (motor octane number)MON is more realistic as it is carried out under higher rpm's, higher temperature's and grater load's.
Check out the trype of ratings at your pumps.

:auto:
This was on an L88 vette with 12.5:1 comp

[Modified by sumoboy, 1:51 PM 3/17/2003]


[Modified by sumoboy, 1:53 PM 3/17/2003]