stevesxm

yup lots of friends. and they are all smarter then you are.

pullmytrigger

The reason I ask about the knock sensor is many yrs ago I had a Donzi with a pair of 502mpi's.....when I went on the water test at the dealer before I bought it it ran 75 on the speedo in flat water and had great acceleration....I asked about what fuel I should run in it and everybody said that 87 would be fine....the boat did run fine, but I only ever saw maybe 70 out of it and it didnt accelerate like that water test/first tank of fuel.....and if you call gassing up at the local busy Sunoco being "stupid or careless or negligent" with fuel selection then I guess I was.

unfourtunatly later on that summer the boat was stolen and I never saw it again but Ive ofter wondered if the dealer had premium in it when I water tested it and thats why it ran so much better.

My buddy also has a pair of Procharged 502 mpi's in his Active Thunder and if he doesnt run premium hes down like 7 - 8mph

stevesxm DONT respond.....I dont want to hear anything from you.

jeffswav

What time of the year did you hit 75? If the weather is cooler your boat will run faster, hot humid weather will scrub about 3 or more MPH off the boat. Also were you going by the "Dream O Meter"? Because they can be all over the place.
Steve is brutally honest, I am also when people do not listen. You just are not getting it.
All the knock sensor does is tell the computer to pull out a little timing. There is no feedback for AFR and no system is fullproof.
Stock engines run fine on 87, higher octain fuel burns slower, actually having less HP if the engine is not set up for it. Higher compresion engines require higher octain fuel so they do not blow up. If you run old gas it will blow up, either the tops of the rod berrings or the pistons. This topic has been beat to death here, do some searches here and you will find many hours of reading.

28Eliminator

Pullmytrigger, not stupid or careless. We are at the mercy of the fuel distirbutors. Our gas stations have recently dropped down to 85 octain and if you want 87-88 it has 10% ethenol which is not good for most boats, and I don't think would not be a good choice for the 502 and knock. Your buddy with the prochargers should never even consider anything less than 91, but sometimes you are stuck on a return trip, but I sure wouldn't kick it into boost using a less octain fuel. I can still get 91 octain, but none of the stations carry 93 anymore around here. I think it's a horsepower conspiracy!

BADS335

Lots of good points here.

I just wanted to be sure I am not losing anything or risking hurting anything on 87...from a land based auto fuel pump station.

I agree with the supercharged statements or higher compression...Premium is a must. Maybe one day...Probably not though. Boat is too small for big water to take advantage of the power and I barely have enough lake to top out as stock. But a 27'+ boat for bigger water is in my future...when kids are older.

Thanks again guys and I am open to any other feedback or suggestions on anything related to this boat or motor.

onesickpantera

My engine was built to run on 87 and that is what I used to run. I now run 89 as I have seen a decline is fuel quality the last couple of years. It is really apparent in the winter as the two-stroke sleds are much more finicky to fuels. I know winter fuel is different but the point is fuel quality is down and ethanol content is up!

For me running 89 is just a little insurance.

Dennis Moore

Mercruiser/GM has designed the marine engine to be used in any part of the world. We think our gas in the USA is crap...no, sorry, our gas is pretty good compared to what Chevy marine engines see in Saudi Arabia, Brazil, China, Mexico and other parts of the world. Our 87 octane gasoline is racing fuel compared to some of the stuff out there!
Mercruiser and GM cover warranties on engines used outside the USA too. They see it all and are aware of this.
Marine engines can run on much lower octane gas than what we feed them in the good old USA!
More engines are damaged by water in the fuel than low octane and the solution to that problem is keeping a clean water/fuel filter on the engine.

Detonation isn't just about compression ratios.
Combustion chamber design and ignition advance are the most crucial aspect of detonation control. Advancing the ignition advance to quickly (below 3000 RPM) will kill an engine quicker than excess (total) ignition advance at WOT.

Thunderbolt IV V6 ignition modules installed on a V8 engine with 24 degrees initial advance at the balancer is a real detonation inducer! Scary!

Most Mercruiser marine engines have a very modest amount of initial advance at idle, usually 8 degrees. The ignition advances very slowly until aproximately 3000 rpm(full advance only after the boat gets on plane). This slow advance curve works great for low octane fuel!

Remember: If you are installing a modifyed ignition, don't bring in full/total ignition advance to quickly and you will be alright!


Dennis Moore/Moore Performance

jeffswav

Are you the same Dennis Moore that had the performance book everyone talked about?

Dennis Moore

Yep, That would be me...
Dennis Moore/Moore Performance

jeffswav

You have some cool parts on your web site. You should update your book and re-publish as version II, never saw the old one but heard it had some great info.