fresh rebuilds on 575sci with 7lbs boost,650 hp, can get 90 oct non eth at marina or put on trailer for 93 eth at gas station [pain ina ass] what would you do???

You will need the 93 octane E10.
Stock 575SCI's require 91 octane with the stock 4.5# of boost and with 7#of boost, you are going need the 93..

I run 93 octane (who knows how much ethonal they water it down with!) from the gas station on my 9psi procharged 500efi engines never had a problem.

My friend swears by non ethonal 90 in his 6psi carburated 540 bulldog blower motor never had a problem.

The best would be to treat your 90 octane with race fuel you might have to buy in a 55 gallon drum.

Good luck

Ethanol adds a lot of octane. So it's not all bad. E85 is actually a pretty good fuel for power. Part of the issue is as the ethanol evaporates off it leaves any water it absorbed behind and lowers the octane of the fuel. A mercury rep told me pump fuel can be as low as 80 octane without the ethanol (e10 fuel that is minus the ethanol). I'd rather have fresh pump 93 than the non ethanol 90. But if the boat sits for a few weeks at a time maybe the 90 with octane boost or race fuel mixed in would be better.

I have tested a lot of pump gas in recent years, all E10. I have never found pump gas to be less that what is posted on the pump, that includes fuel at Marinas, mom & pop gas stations and the local Exxon. 99% of the time the octane is a full point higher than what is posted on the pump. If the pump says MINIMUM 91 octane, the tested octane will be 92. I have test samples of gasoline that are now 3 years old, the gas is yellow, smells bad but still test with the original octane.

Really? That's different from what the mercury rep had for information. Were your samples stored in sealed containers in a controlled environment or a vented varying atmosphere like the fuel tank of a boat?
The fuel in my boat sat for 5 months last year, it would NOT FIRE!!!! Drained the tank put fresh fuel in and it fired right up.

Been wondering about this same thing for my pair of 300 optimax XS. I can get 90 non-ethanol delivered to the house or take out and head to the gas station for 93 E-10.

I was only speaking to the issue of Octane. Many addatives are put into gasoline to help start and control emissions. If the engine won't start it may be "bad gas" but the octane componet of the fuel is still ok. My test are in 1 gallon containers, 1/2 the containers stored indoors sealed containers the others outdoors open spout. Octane & ethanol % remain the same after 2 + years. E-10. The original question on this post was about octane, octane is octane with very little change after several years.

Ethanol doesn't ADD octane when you buy pump premium, it REPLACES what USED to give you the octane rating, like MTBE, ETBE, iso-octane and toluene . The lowest base stock of most pump gas I have ever heard of before being blended is 84 octane, fwiw, Smitty

How are you testing the octane, I test ethanol content OFTEN at all the pumps in my area as I have found it varys GREATLY compared to what its SUPPOSED to be but haven't seen a effective way to cheaply test octane, Smitty