Octane differences

[CHANCE1234]

So with fuel so cheap now a days ($1.69 here just off cape). And with me just hanging a new 2015 Suzuki 140 on the back of the 20, I feel I can justify medium or even high test for the boat. My question is what fifference will I feel or realize in performance? Economy, speed, a lighter wallet, nothing or something else? Thanks

[Bushwacker]

If it's rated for 87 Octane, you won't gain anything but a lighter wallet by running higher octane. Higher octane fuel burns slower however, so that may raise the temps on the exhaust valves which is not a good thing, since that's the weak link on most 4-stroke motors anyway!

The only scenario I can envision where higher octane might help is IF the engine has a knock sensor, and IF you're over-propped or one day happen to get lucky and load up with 1000 lbs of fish so you're really overloaded! A knock sensor will retard ignition timing if it hears detonation or pre-ignition, which is less likely to happen with better fuel, or if it does, the ECM won't have to retard the timing as much to stop it, so you get slightly more power.

The non-ethanol fuel we can buy here (Rec 90) is at least 90 octane and much more expensive than even 92-93 octane E-10 gas, but I think the benefits of it are worth the extra expense. Don't really need the higher octane, but avoiding the corrosion problems you can get with phase-separated E-10 gas, which can easily wipe out any cost savings from the cheaper gas is the best reason to run Rec 90 fuel if you can get it!

[CHANCE1234]

Good info. Thanks Bushwacker

[htillman]

So I've heard that octane degrades over time and higher octane boost older gas that's Been sitting for a while. Any truth to this...

[kmoose]

For 11 years I pumped the cheapest gas I could by thru my DF250 with little issue but at one time I had coking on the exhaust valves from enough to cause compression variances in a couple of cylinders. After talking with a very good Suzuki mechanic the fix was to just run Lucas fuel treatment and it would clear up... and it did. The other thing he recommended was to run at or near WOT for 3 or 4 full minutes in the channel on the way in. Again, this was very good advice as it actually improved overall performance and a slight oil making problem I saw occasionally.

As Denny said, propping is a big deal with 4 strokes and you need to make sure you are not over propped. At anytime under any load I could at least hit mid operating rpm limits and could hit max on a light load.

Now that I have repowered I have made the move to running rec 90 in the new motor. The manual calls for 89 and prefers no eth so I'm going to follow the rules with this one but I wouldn't be too scared to run less if I ran an additive.

[CHANCE1234]

I still have a bunch of ring free from the Yamaha. Do you think I should continue to run this through the new Suzuki? Is it similar to the Lucas treatment?

[pelican]

soft carbon deposits on intake valves is nothing new - BMW had this problem with their v12's,that was YEARS ago...

it's never a bad idea to do a little decarb,once yearly...merc power tune,squirt a can down through the throttle body...that will clean the deposits away.

propping is the most important thing you can do ! take the time required to get your rig dialed in !

an often overlooked point about propping - summer months,the humidity is the highest,the motor can actually lose 10% RPM,in these conditions...always keep that in mind
duplicate weight conditions,full tanks,and enclosure down - run it wide open throttle - you want to see max rpm,with a full load...every inch in pitch,it yields about 150RPM
get the motor height correct,get the correct prop - the results are worth the time !


ethanol fuel / non ethanol fuel - it's the same !

phase separated fuel - it will shut the motor down...fuel seperates out,when water is in the fuel.works like this: water,the alcohol mess,then fuel.the pickup tube,it draws fuel from the bottom of the tank - water is drawn up first,choking the filters and killing the motor - isn't any motor,that I know of,burning/running on water...NOTHING will cure this,except drawing the water/alcohol mess out - then finding the source of the water intrusion...sea foam,startron,whatever - it's a waste of money! and it's useless !

phase separation is caused by water entering the fuel system - fuel will not phase separate without water.if it did,every boat,gas powered in a northern climate,would have a serious problem,after sitting for the winter months...

my pressure washer - it hadn't been used since last January - it was sitting with fuel in the tank since then...it was loaded into my boat,for my move to florida...the boat sat at a friend of mine's place,in south Carolina,for the torrential rains they had - recall the flooding and the closure of I95,due to flood damage ?
I finally needed to use the pressure washer a few days ago...3rd pull it fired and ran,untill I was done what I needed to do...
my chain saw - it has sat in it's case,with fuel in it,for over a year - few pulls,and it was screaming !

water in fuel - it causes problems....



4 strokes:
these will make oil,due to condensation - a few other problems can cause it as well...let's focus on condensation...

these motors need to be ran at temp,to "burn" off any moisture...a good full throttle run,will accomplish this...

[76Red18]

Definitely have to keep the water out but I have also had lots of problems with deterioration of plastic and rubber parts in small engine carbs; 2 stroke and 4 stroke. That could be cheap Chinese rubber though. I think alcohol burns a little cooler also.

[Capt Terry]

I used Amoco high test in my high compression Corvettes in the 60s - 80s because that was how they ran the best. It was also recommended in the Merc owner's manual, so I used it in my Merc 2 cycles from 1976 until maybe 2003, when I started having issues and discovered even the Amoco high test had ethanol. My issues- very hard starting & initial running following a lunch break of trouble free running in the morning, almost like vapor locked. The fuel bulbs would not stay firm and I tried several bulbs, replaced fuel lines, and on & on. Sometimes had to use starting fluid to get it started. My problems disappeared when I started using real gas as recommended by a Merc mechanic, who also suggested I mount the bulb vertical. A friend who burned up two 2 cycle engines on weedwackers was told by his mechanic to only use real gas in 2 cycles. I've had an ETEC for two years which is supposed to be ok on ethanol- but I have stayed with real gas.

[Capt Terry]

I should have added that Frank Brown (Carl Moesly's Merc race mechanic) recommended Amoco high test on my two engine purchases from him. He chewed out a friend of mine who was running cheap fuel in his Merc 2 cycle on a SeaBird.
But all that preceded the ethanol daze (intentional spelling)!