Put in mid grade fuel and picked up 1.5 mph.
 

We where out for the day and ran low on fuel.So I stoped at a locall dock and thay where out of 93oct.So I put in 100 gal.of mid grade.When I realy stretched it out it ran faster by 1-1.5 mph.It says on the top of the motor to run mid grade in a hp500s but I run the **** out of it all the time and I thought more octane was beater.Go figure.

Higher octane fuel burns slower

But why did it pick up mph?

Been always told to run the reccomended grade. Anything more is a waste of $ and doesn't always run as well. Just what I've been told.

I have done many experiments and found the same true in many instances, Use what is recomended and 89 is usually good with the exception of blower motors and very high compression ratios. The Ironman.

I think with the higher octane the computer (knock sensor) doesn't work as well....

The lower octane actually burns a little faster but less evenly so with a 8.7 compression motor at 34 deg total timing the burn is completed during the power stroke the higher octane is still burning at the bottom of the stroke so you have to advance the timing or raise the compression with the higher octane to get equal or more power out of the engine. Ironman

higher octane in a lower compression eng. loses hp--the fuel won't ignite as well--the burn rate is slower/less combustable-thats why higher octane won't pre-ignite/detinate

You must have picked up RPMs too. Yes it may be more power but its not good for the motor.

Too high an octane can also be bad for the motor as it will lead to more deposit formation.