Merc specifies 91 octane. I have a customer who will be keeping his boat at a marina that only sells 89 octane. Will Torco octane booster get him by? How critical is the requirement. What would 89 octane do to a 575?

I have been running 89 octane... backed timming to 28 degrees...tried the octane boost including "Home Brewing".. kinda of a pain.....

I know a certian customer that tried that in a 42' fountain and spanked one motor after the other. So what does he do after both motors are replaced. Still put crap gas with octane boost in them. However this is 87 octane, plus octane boost.
Thank you, Cam-shaft.

Not sure if this works or not, but if he used a gas caddy or maybe a couple of them I think they hold like 30 gal each, filled them with 110 and filled the rest of tank with 89 would'nt it bring the octane up.

Maybe there is a formula out there to figure out how many gallons of each you would need.

if you have ever had a can of octane booster open and spill in your storage locker you will never bring another one on your boat again. i know.

I don't trust octane booster. I tried years ago in another boat I had and still burned pistons on both motors.

I would boost the 89 octane with some 110 or 114 octane race fuel.

10 gallons of 110 mixed with 90 gallons of the 89 will put it at 91 octane.

I read somewhere that the "points" octane boosters talk about raising are actually .something so if they are saying it raises your octane 4 points they are actually raising it .4.

Ive also heard using Toluene does a better job than anything else but you still need a bunch of it.

No idea if any of this is true or just internet chatter

Yes,
With the stock blower pulley (4"s) and an intercooler, consistent 89 octane should work. It will provide knock protection and horsepower. Expect to lose 1.5 to 2 PSI of boost, which is exactly what you should be doing to run 89.
I'm running 7PSI of intercooled boost with pump gas (91/93 octane) and it is working well. My ECU HAS been reprogrammed by AZSM, as well as some other mods.
The best way to make sure it isn't knocking is to run WOT with the Rinda/Merc scan tool on and watch for the knock sensor to kick in and send a code. If it doesn't, you're probably good to go. Another FYI, like all warranted Mercs, this motor is way fat from the factory, and that should provide some knock protection too.
Here's picture of mine with a Whipple intercooler, which I highly recommend.
Regards,
Steve