Wettin My Shorts

I just to switched to twin outboards, 300's to be exact. The manufacturer's say to use 92 octane, well California has only 91 octane and its also reformulated so it burns hotter. The question of the day is should I use 100LL or mix 91 with 100? Any suggestion would be helpful

tj309

I also run twin 300 ProMax's. It is very important to follow Mercury's recommenced gas of 92 octane. The ECU's are programmed with this in mind, your compression ratio is higher than standard, and at the higher RPMs these motors run at - engine knock will occur with lesser octane gas. The best and cheapest solution is to calculate a ratio of 100LL (available at the general aviation airport) to 91 pump gas and fill your tanks with this mix. Error on the side of caution because those ProMax's are not cheap. It wont hurt to run any motor at a higher octane than it was designed for - just cost more. That is why it it stupid to put premium gas in your truck when the engine and ECU are designed for 87 pump gas.

By the way - what year are your motors? Mine are '99's and I bought an owners manual from Mercury and it shows a fresh water flushing port on the left side. My motors do not have this and I would like to add this - probably will have to contact Mercury.

TJ

Wettin My Shorts

Thanks for the info. Mine are 1999 also. I run a 31 American Offshore and am also wondering what props would make this a little faster. I believe this boat should run 100 but the best I've gotten is 96.

tj309

What pitch props? What type of props? What RPM @96?

mmwalters

I guess we are lucky as we can still buy 93 at the pump. But who knows what you are really getting. I had rapair bump the rev limiters on my pro maxs to 6700rpm. TJ my rev limiters were kicking in at 6200rpm, where do yours hit?
Usually 3 blade cleavers are the fastest but with a heaver boat and twin motors maybe you could try a set of 4 blades. You never know until you bolt them on!

tj309

My rev limiters kick in at 6200.

mmwalters

Wettin my shorts 96 is a very respectable speed with promax motors. I am not sure a 32' skater would run much more than 100mph. Like TJ asked what props and rpm are you running and do you know what your rig weights?

tj309

mmwalters good to hear from such a knowlegable person. I am curious - how fast does your rig go? And how long did it take you to get it to go that fast? It will take me some time to approach the speeds you guys talk about but one step at a time.

As for mr wetting my pants I certainly understand you getting your new motors and wanting to try them out. These are big beasts of huge power just be careful. Feed them what they need (92 at least) and enjoy. Take it slow at 1st.

TJ

offshoredrillin

you're right it wont hurt, but be careful of the long range affects of too much octane, octane is what controls the burn, if your not running hot enough to burn it efficiently that wainds up as carbon deposits on top of your pistons, and in time increases your compressions, there fore making knock. Its a tough call in cali, but mixing it should work fine.

mmwalters

TJ thanks My motion runs just over 110mph..so far. I had the rev limiters moved last season but I did not have much time to test the boat due to a sickness in the family
My motion is a 30' hull with twin full canopies. Let me tell ya the canopies completely remove the sensation of speed. With out wind in your face my passengers have no idea how fast they are running.
I guess it took me a little while to get to speed, not because I didn't push the throttles foward but it takes some time to learn the trim settings.
Also prop rotation can make a huge difference in how the boat carries the bow. It seems the skaters favor turnning the props "in" when I tried that setup on my boat it was scarry, it felt that there was no end to how high the trim would lift the bow and it turns out in back to back test my rig ran about 5mph slower. So now the props are turning out. I like to test my boat durring the week when is is little boat traffic to deal with.