Fuel-high octane vs. low octane

i have a 90 merc 175 and was told that lower grade is good enough too run in it ,but i should buy it from the better gas stations and not just go with whoever is cheaper

I have never seen an Owners manual that tells you to use 87 octane fuel. Every motorized vehicle I have ever owned has said use a MINIMUM of 87 octane, or whatever the number may be. My vote is for the highest octane. I know for a fact that I get better gas mileage running high verses low and much better performance. I would also have to argue the High oct burns cooler than low. Back in my po boy days when I drove a 78 trans am, I lost the heater core. Didn't have the money to replace it and for some reason was driving around with about a 6"x 8" hole in the fire wall where the old core was mounted. It was winter time and I used the heat off the motor to get by, seaping through the hole. Long story short, when I burnt 92 oct compared to 87 that engine put out more heat burning 92 octane. IMHO, I say high octane burns cleaner hotter and you use less per mile.

the running attitude in different grades all depends on the timing and compresiion of your engine higher the compression the higher grade fuel you should run,ignition timing can be adjusted a few degrees to allow for low grade fuel.my dad has an 89 150xr4 and it will hardly start on anything less than 89 it runs 125psi compresion my 200 johnson runs fine on 87 but it only has 105 psi it may turn 150 rpm more with 89 or higher.

My 2003 225 Opti calls for 87 or higher. I talked to my dealer and he recommended the 87 because it will burn cleaner than higher grades of fuel. I've run 89 and higher occaisionally but all I can tell is that it costs more.

I also use Mercury Quickleen about every other tank.

Charlie

kc is right. the old feul we had had lead in it. Which contained tetra ethyl lead which is a compound to create a free radical reaction during the burn to slow the combustion process in high compression engines. Now octane is tetra ethyl ether which does the same thing without the lead. The old stories about the lead being in the gas to cushion the valves of four strokes is not true. The higher the octane the slower the gas will burn, which is what you want in a high compression engine. Use what is recommended for your motor by the manual. Higher grades of gas,depending on the source, do have more detergents which will delay varnish and there fore ring wear for the long haul but will not improve the performance of the engine. Also better quality 2 cycle oil have better detergents to prevent carbon build up over the years. I have rebuilt my 84 johnson 75 3 times thus far, and its longest life to date has been while using the manufatures oil and top grade gas. Break down in the winter way up the south fork with no boats around and you will become a beliver.