The other day my girlfriend accidentally filled up her truck (my tow vehicle) with Super/91. I guess the station reversed the order highest to lowest and faked her out.

The truck Hauled A** after that. I immediately recognized the truck had more pep and when asking her why she told me that she filled up with 91.

Anyway, I was wondering if this would translate to being on the water and adding Plus or Super gas to the engine (especially since these fuels are now loaded with Ethanol) .... I'd think the more concentrated formula would give you a lot more performance in way of acceleration, getting on plane and top speed.

Anyone care to comment on this?

Nope, it's all in your head. The added octane does not equal more power unless the compression ratio and timing curve calls for it. If it doesn't, you're actually killing power by adding higher octane because it is a slower more controlled burn.

The only way there is an increase in power in an average every day pickup is if you have so much carbon buildup in the combustion chamber that it has by default increased dynamic compression and triggering knock thus timing retard on standard fuel.

only benifits with carb engines.

Even some carb engines it wont help. I have two streetbikes and one is set up from the factory to burn 87 snd the other calls for 91. The difference is how the compression and timing is set up on the different engines

Well how come it made a DRAMATIC difference on her truck??

Maybe a newer trucks computer adjust to make it use the full benefit of higher octane

Is her truck a Dodge Hemi? They call for mid grade fuel so running mid or premium would make a difference over 87 since the computer could use max timing advance. I wouldn't recommend running it in a boat since premium often sits without being sold much so you run a higher risk of getting old fuel.

I am guessing that the wholesale fuel distributor injected more propane into the higher octane than called for. I understand your question because I had the revearse effect last year on my suburban: Stopped at the fuel pumps and got the reg. because we were shopping their. On the way home, went to pass another car, no power and it was clattering almost like a diesel (detonation). Their used to be a fuel additive that I swear if you put a bottle in 20 gallons of fuel, you had a hard time taking off normally without spinning a tire. Nitro 9. We used to run 100 octane aviation fuel in boats at El Novillio. Ran really good because only fuel we had was in the airplane we flew thier in. Forgot to answer your question NO

I was told 91 was not good for my Yamaha HPDI by a professional mechanic. The motor was designed and built to run on 87.

No sir, only if it has the compression and timing curve that can benefit from it. Doesn't matter if it is carb'd, injected, or you have an army of rodents throwing fuel down the intake with buckets.

I used to have to break out dyno sheets for customers to show them the loss in power from too much octane in a motor. It loses power, it is just a fact.

I run a Mercury. My mechanic is VERY picky. Been building and racing boat engines for over thirty years. He told me to always run the mid grade fuel. He said the lower octane did not have enough octane. The highest octane burned a little too hot. I always add quick clean to the tank wnen filling up. He said that helped reduce carbon build up. I've owned many boats. Been doing this over twenty years. No problems yet! (Knock on wood!)

it's amazing how many people just don't understand engines and fuel, i wish i had an army or rodents throweing buckets of fuel down the intake ports lol, that would be awesome.

I like you have had to have these talks with customers and friends over and over again over the years,

My favorite is always the " we put av gas in it and it hauled [censored]!!"

that's just retarded, aviation fuel is made with all kinds of additives to make it burn better at high altitude and low temps, not lake level lolololol...

I run 91 in my efi merc, if I didn't i'd have pieces of pistons and rods scattered all over the place, but my motor needs it.

The guys with the hpdi's yeah 89 is fine, that's what i ran in mine.

older carbed merc's like the little 2.0l XR2's and such had rather high compression for a stock motor , they liked the 93 that was available back then.

Newer stock motors are made to run with more cubes,less compression to get the same power, hense the lower octane requirement of todays stock motors.

I have a 2010 Mercury Optimax. The book said to to use 87 as minimum and said 92 octane was acceptable. I run the midgrade but also add teh Stabil due to the ethanol.

No offense to your mechanical but he's just another shade tree. Higher octane doesn't equal hotter burn, it equals cooler more stable burn unless you're using oxygenated race fuel. This is the only thing you need to remember, you will ALWAYS make the MOST power with the LOWEST octane you can burn without detonation. This isn't internet rumor or something I read in a magazine, I made a living building race cars. This is thousands of hours of dyno time, thousands of track passes, and building more motors than most people will own in a lifetime. Not bragging, just trying to help you guys out because for some reason, fuel requirements and properties are more misunderstood than deep space exploration.


90 5.0, I hear you man, my favorite was always when a customer would explain how his buddies mustang was built to run jet fuel. I would always ask why they put a diesel in a stand lol. Jp5 and jp8 are just really clean aviation grade diesel.

My manual calls for 87 octane, period. That's what I burn. I recently began adding an occasional can of seafoam. It seems to have helped overall smoothness of operation.

My truck manual on the other hand says flex fuel is approved. I have run some of those blends and the engine mileage and performance was below that of straight gas or 10% ethanol.

I run the 93 octane. It has more detergents than the lower grades,burns smoother,and burns cooler,than the lower grades. Octane deteriorates over time,so the longer your fuel is in your boat tank the less,actual,octane you have,say a month or two,from when you put it in. I start high on Octane so,two months later,I still have enough octane to avoid detonation.

Usually manuals say minimum octane 87. As far as comparing 4 cycle engines to 2 cycle engines they are 2 different animals. As far as the ethonol issues manufactures have been building engines that run on it since the summer blends have been used that is over 30 years. Race cars have been using alcohol for as long as I can remember I'm 52 and cars have been running it for at least 30 years. As far as octain I run 89 in mine been running fine for 16 years Compression is within spec and leak down is less than 2% I also only burn synthetic 2 stroke oil.
My 4.3 V6 gets better milage towing with 89 than 87 and manual says min. octane 87. Last I checked a Dyno is a controlled enviornment in other words no real world friction to fight like wind,water,or altitude changes which is why they are considered BENCH TESTS.

No offense man, but he is a factory trained and certified Mercury mechanic. He has owned and operated his own authorized Mercury repair shop for over twenty years. He only works on Mercs. Nothing else in his shop. He has always been regarded as the best Mercury guy in this area. Not a "shadetree"!!!

He may very well be, but if he told you that he does not understand octane. slodsm is correct in what he is saying.

Slodsm is correct and knows what he is talking about. I used to work for Ford then had my own shop and raced and built race engines for many years. Octane is widly misunderstood. It is not an additive but is a rating system to determine the ability of a fuel to resist detonation (exploding). The air fuel mixture in an internal combustion engine is supposed to burn in a controlled way, not detonate. When you hear an engine that knocks under acceleration or heavy load the knocking you are hearing is the air fuel charge detonating rather that burning. That is why knocking is so hard on pistons, etc.

Dude! I only came on here to answer his question. Didn't attack you or anyone else. Do what you want! I have followed his advice for decades. I met him a long time ago when I had trouble with an old engine. He rebuilt it for me and I ran it for years with not one problem. Still running as far as I know. He is the only person I will let work on my engines and I have never had problems with his work. Thanks for the input. I am getting damn tired of posting on here and getting these kind of responses. Makes you want to just stay off the site! One final note. Boat engines are not the same animal as a car engine. Very technical with very close tolerances. I would never follow a car mechanics advice for my boat engine!

Lots of octane discussions on BassBoat Central and under each motor manufacturer. Do a search on "octane" limited in your motor's forum.

http://www.bbcboards.net/

My 2005 Mercury XR6(carbed) 150hp general opinion is to run 87 or 89 octane, but not 91. My Mercury forum is moderated by a Mercury Master Tech and I trust him.

This article will explain all you want to know about gasoline ratings and more.

http://auto.howstuffworks.com/fuel-efficiency/fuel-consumption/question901.htm

SLODSM seems partly incorrect and partly correct to me. Your mechanic sounds to me like a competent and experienced mechanic and is not really a "shadetree" as he was called. He surely understands fuel basics and a cause and effect on performance. He the term "hotter" to describe to you premium's ability to under higher compression engine designs to create more (higher)engine power which is wasted if not used in the right engines. He is though mistaken if he thinks 'Premium' burns at a physically higher (hotter) temperature, but he isn't a physicist so who cares.... he just repairs engines.

We all get peer review on this forum which helps us both help and be helped ... and that is how we learn.