I recently started burning regular and have had no issues.

Since the car is not supercharged, if the engine does not ping, it'* a waste of money. I would never attempt to burn regular on my supercharged car (2004 SVT Cobra).

You may notice a decrease in mpg. I had a cadillac once that was not supercharged, but was required to burn premium. I did the same thing you did...tried it and found no obvious problems. BUT, when I took a look at the mileage, it actually cost me MORE money to burn the cheap gas because the mileage was lower.

There also may be hidden issues that can occur (increased gunk building up on sensors, fuel injectors, etc)...someone else should verify that thought since I'm not a gearhead person...

The Owner'* Manual states "If your vehicle has the 4.6L V8 engine, use regular unleaded gasoline with a posted octane of 87 or higher. However, for best performance, you may wish to use middle grade or premium unleaded gasoline."

I run with mid-grade that has a posted octane of 89. I seem to get adequate performance and have had no problems.

Without a scantool to determine timing advance under all driving conditions as a comparison between octanes, you can't be sure.

I would strongly suspect you'll get better mileage and better performance with higher octane.

I've done the testing over fifteen full tanks of gas (3 separate tests @ 5 tanks/test) calculating tank average fuel economy. From a fuel economy stand-point only, you save money in fuel economy by running premium if and only if the difference in price between regular and premium is $0.40/gallon or LESS. If the difference goes above about $0.40 you begin breaking even.

Test Disclaimers on 04 GXP:
1) Mileage = 26-29k
2) Driving conditions - town and highway combined average
3) topography - hilly, few flat spots, except test 3 which was done on a trip to Michigan, i.e. mostly flat driving after Pittsburgh.
4) "normal" regular and premium gas from same station(not "special" gas from Exxon/Mobil or BP) except test 3 which was also "normal" gas but not from same station as tests 1 and 2.
5) all tested in spring, summer, fall conditions with no A/C and no extra load on the electrical system beyond OEM equipment, windows down.
6) car driven only moderately hard, only a few hard starts
7) new tires beginning test 1
8. test 1 and 3 had oil change within 500miles of beginning of test.
9) no fuel economy "improvement" devices
10) color = crimson tintcoat

Worst average fuel economy from one tank = 17mpg nearly exclusively town driving in WV and Pittsburgh (a little interstate between my house and Jerry'* in Squirrel Hill)

Best average fuel economy from one tank = 29mpg - fill up on OH turnpike at Eastgate measured at end of tank just south of Lansing, MI around Jackson.

I was surprised at the variation (2 standard deviations) between the best and worst mileage (although an n=15 doesn't really allow for a reliable measure of SD). From a non-statistical point of view this was unexpected. I would guess it would narrow as the car gets older. If I were to choose the single outlier it would be the 29mpg tank. Most measures came in between 21.5 and 24.5 mpg.

Again, it pays to run premium from a fuel economy/cost consideration IF the difference in price between reg and prem. is LESS than $0.40/gallon.

I haven't seen testing that shows premium is better for the engine mechanically so can't comment on that aspect.

That testing is skewed though...everyone knows red cars get better gas mileage, plus theyre faster.


Anyone know the compression ratio for the GXP right off hand? Given the relatively small displacement, yet good power output, its possible it might have a pretty high compression ratio requiring a little higher octane.

The first couple years the Aurora stated to use premium gas because it didnt come with knock sensors, I believe they later went to the "for best performance yada yada".

I also think several of the Cadillac engines fall into the same category.

I briefly contemplated using higher octane for the 3.5, but after I determined the compression ratio for it is actually slightly lower than the L36, I didnt see the need for it.

Unless your compression ratio is higher then 11.1 then you don't need to run higher octane fuels. The 2001+ version of the Northstar has lower compression, 87 is all that is required to obtain factory performance spec, but we all know that doesn't apply long term in real world situations.

Frankly, use whatever suits you best, but 91 is not necessary.

I took apart the throttle body in my 95 SSEi and it was clean as a whistle. After 158,000 miles it looked almost brand new. No black gunk or anything, and I had never touched it before. I attribute that to nothing but AMOCO ULTIMATE 93 gasoline for 12 years.
Oh, by the way, did I tell you I own two Amoco (BP) gas stations.

In my GXP I'm still investigating mileage and performance.

I've seen Amoco'* research and development campus in Naperville by Chicago and looked at engines using premium and those that didn't. It was pretty impressive. The premium left the valves and intakes nice and clean. The only problem is... in my opinion, (other than knock) these things are not usually what kills an engine. So I can't say it'* really worth the money.

Of course the GXP will run fine on 87, but mine gets nothing but 93. The way I look at it, the extra 7% expense over 87 octane is spent for increased performance. And that 7% is likely to be at least partially offset by an increase in gas mileage.

Without extensive testing, it’* not possible to know what the performance increase is. But there is every reason to believe there would be a significant performance improvement with higher octane fuel in this vehicle. Keep in mind that the increase may not happen right away; the PCM is going to have to learn that it just received some good fuel and timing can be increased.

The message I’m sending to my PCM is that I only run the good stuff, so knock is not a problem. Therefore I should get the maximum timing advance available in the program.

I wouldn't mind trying 93 octane, but I have yet to see it in Alberta. Our "Premium" is 91. I've heard of it in BC and other places. As it is, 91 octane is about $1.09/litre which translates to about $4.12/US Gallon. I don't do near the highway driving I used to so great performance seems less important. I tried running 91 octane for a while but honestly didn't notice a positive change in fuel economy. In fact, it seemed a little worse.