well, you would put premium in a GXP unless you WANT it to be doggy and slow. You don't spend all that extra money for a northstar engine over an N/A 3800 and not want to get the best performance and efficiency out of it as possible..

Or unless you want it to be doggy.....Some vehicles are actually SLOWER with the higher octane because the compression isn't there to utilize the higher octane principles.

I would like to see someone'* 1/4 mile times on regular and Premium. Even better would be dyno slips. If it'* not knocking then premium is useless.

It'* not knock that causes anything with the higher octane running slower, it'* the fact that the higher octane itself burns slower (if I remember correctly)

My high compression Premium V8 takes 91 octane. Noticed a difference in power. $3.00 a gallon though, but do I care? No.

Higher octane fuel requires more energy to ignite, but realeases less per unit mass. That'* why engines not tuned for it run worse vs. regular.

I highly doubt that you notice a difference.

I don't notice a difference between 87 and 93 in my medium mod'd SSEi.....

That'* for clearing that up wikoff

I notice a difference between regular and premium. If I step on it with regular, it seems to dog down the engine and I have a problem getting about 26 mpg on the highway. With 91-93 octane, I can manage 30 to 32 on the highway based on the miles I put on everyday[105 miles]

yes. however, the 4.6 is not one of these engines. With the premium fuel the PCM is able to advance timing to increase performance. With the regular octane the timing has to be retarded to prevent engine knock. My engine isn't a 4.6 but it'* still a decendant from a northstar and the owner'* manual says 87 octane is fine to run, but 89 or higher is recommended for best performance.

IIRC the northstar engines have a higher compression ratio than the supercharged 3800 yet the supercharged 3800 does need premium. Are you saying that the 3800 also does not need premium fuel? If you are, we've ran 87 in our 00 SSEi and it'* a dog with it. Fill it back up with 93 octane and it drives like an entirely different vehicle. Octane does matter.

You probably take off with an egg under the accelerator. I, on the other hand have a tendency to hammer the gas from a start, where the high octane is needed most.