87 Octane timing question???

[3-Liter]

I live in the middle of nowhere. During the last "gas crisis' we could only get 87 octane fuel. I understand the correct timing procedure and specs for a stock 2.5 Merc fishing motor is 25 BTDC. When we can only get 87 octane do I need to retard the timing a couple of degrees? Motor is a stock 2.5, all timing boxes and modules removed. Timing is set with dial indicator at .150 before TDC. In a perfect world I would not need to ask this question, but sometimes you do what you have to do.

[mach351]

Pretty sure 25* isn’t stock. That is what we set it to for performance, but the rule has always been 25* and 145psi is about the limit of 93 octane. In my head I want to think 19* is your stock timing. So when you put a 2.5 back to 38 or 42cc heads and 19* it should run on 87 octane. Gotta figure Mercury built it that way because they knew some people would be putting crap fuel in their new 2.5’s and they didn’t want to have warranty issues.

If youve raised your compression id think you’d have to dial back the timing even more. Of course boat weight, load, and rpm have a lot to do with how much the motor before detonation.

[3-Liter]

I may have asked this question incorrectly. The Merc service manual states max timing 16 degrees while cranking, and 22 degrees at 5800 rpm. With all boxes removed should I be able to run 22 degrees with crap fuel?

[Merc 2.5]

22 or 23 max

[billybob]

Yes, I just looked up in service manual, 0D82000 and up. It says 86 octane rating. Just as long as you haven,t shaved heads, or done any other Perf. mods., you will be OK. If in doubt use an octane booster. The only thing about octane boosters, I feel you can only rely on them, for about 2 weeks, before they are undependable. Also don,t run to the jagged edge. That,s where you tend to pop a motor when least expected.

[Capt.Insane-o]

I run 23 degrees with 125 psi heads on 87. The extra 2 degrees isn't worth it on stockish motors.

[3-Liter]

Thanks much guys. Hope we do not have another 'gas crisis' where we only get crap fuel, but this helps. I may just set timing at 22 degrees all the time and go with that.