correct octane

[fordman]

I have a 1998 Cheetah 23" tunnel with a 350 Mag and Bravo 1. It has factory fuel injection and 4" through transom exaust. I have been getting a large amount of black residue on the back of the boat. We boat at lake Havasu and there are areas of extended no wake zones. I have been using 92 octane fuel but was told by someone that this engine should use 87 to 89 octane. I was also told that by running the higher octane fuel my computer can't adjust the timming correctly thus the black residue. Any other ideas?

 

[ODDD1]

Re: correct octane

well, they yold you half right....you should be running 87 or 89 octane or you wont get thr perfofmance designed into the motor....see, the puter will advance the timing till it senses detonation..then pull it back to a safe level. With 92 octane the puter cant advance timing far enough to make engine detonate, so it thinks the detonation sensors arent working and pulls timing back to a 'safe' [and less powerful] level.<br />All this has nuthin to do with sooting the transom....these engines run rich at idle to hold a stable idle speed you gonna live with it.

 

[uflanke]

Re: correct octane

Okay, let me jump in here...<br /><br />Is this the case for all engines? Is there a general rule for fuel? I run a '91 Evenrude 3-cyl 70hp, should I also be using the lower octaine?<br /><br /><br />------------------<br />Thanks,<br />KF

 

[Dhadley]

Re: correct octane

Absolutely! And as for the 91 Evinrude, use 87 that's fresh and good oil only.<br /><br />Good luck!

 

[JB]

Re: correct octane

Howdy Uflanke. There may be exceptions, but I don't know of an unmodified 2 stroke of any kind that doesn't run better on the 87 octane or a manufacturer that doesn't say so in the owner's manuals. The ignition timing is optimized for the lower octane (burns a little bit faster than the mid- or high octane)so that you may actually be getting less power and paying more for it.<br /><br />The very popular notion that higher octane burns cleaner and makes more power is MYTH. The only value in high octane is that it allows higher compression ratios and more power from high compression engines. Unless the higher compression ratio is there, the higher octane is useless.<br /><br />Good luck.<br />JB

 

[ODDD1]

Re: correct octane

OOOOOHHH, JB We gotta argue about this....I have always told people who own OUTBOARDS to buy the highest octane they can get their hands on..here is my logic behind this..2 cycle engines like octane, they are a relatively 'cold burn' engine , they are not designed to tolerate high combustion temps, and high temps[for whatever reason] are the major reason outboards destroy powerheads.<br />Now in my foggy mind, i have 2 fuels i can burn....87 octane or 92 ...one burns faster than the other[the 87] and in a BORDERLINE SITUATION, like a weak water pump, partially clogged cooling system, clogged jets in the carbs, a fuel restriction,some rummy screwwing with the timing, a higher octane fuel will help suppress UNCONTROLLED COMBUSTION and resulting engine damage..also remember..most call for 87 minimum....take 87 octane fuel..add oil mix..the more you put in the lower you push the effective octane...and toss it in a damp, hot fuel tank for two weeks.....IT AINT ANYWHERE NEAR 87 NO MORE! high octane fuel is cheap insureance every outboarder needs.

 

[12Footer]

Re: correct octane

Actually, (I agree with you otherwise), additives like oil INCREASE the octane.<br />The higher the octane number,the LESS explosive it is when mixed with air.<br />To a just-purchased engine, I allways sugest<br />87plus, (as does most owners' manuals). But<br />if they hear the pre-ignition ping, add oil<br />or another additive (sta-Bil,seafoam,ect) to<br />knock up the octane. So I guess we all basiclly agree. And these new-fangled F/I motors are finicky eaters.<br /><br /><br /><br />------------------<br />the Early bird get's the worm...So, he don't have to stop for bait.

 

[Dhadley]

Re: correct octane

If you want to see something interesting run a motor on a dyno that has the ability to read exhaust gas temp and switch the fuel from 87 to 93. Or hook up a pyrometer to your outboard and switch between the two while youre cruising. Todays fuels do not act like the old leaded fuels. You'll be surprised. <br /><br />Good luck!

 

[JB]

Re: correct octane

Hey, ODDD1. Your reasoning seems very logical and I have gained great respect for your knowledge and judgement over the past couple of months. . . . .but I would expect the higher octane, slower burning fuel to burn hotter because of uncovering of the exhaust port when combustion is farther from complete. We agree that excess heat is a major enemy of heads, but so far, we disagree on which fuel is more likely to produce the excess heat. DHadley obviously knows something that one of us needs to learn and can bring us into full agreement. How about it, DHadley??<br /><br />JB

 

[ODDD1]

Re: correct octane

Agreement? But, JB, I REALLLLY like a good arguement!

 

[JB]

Re: correct octane

I'll bet you remember, ODDD1, back in the 50's when we all urged any outboard user to seek out "AMOCO white" gas for fuel. White cost more than regular leaded and had higher octane rating. But that's not why we recommended it. We recommended it because it had no lead or other deposit-creating additives. Nevertheless, the myth that "HI-test" made more power and made engines last longer was either started or reinforced by the recommendations to use "white" gas. It also gave some the impression that hi-test was "purer" and burned cleaner. Amoco's marketing didn't do anything to correct that myth, they pushed white gas as though it were rocket fuel. Now we have lead free regular with minimum additives and lead free premium with octane boosting additives and we have ethanol enhanced gasoline that dissolves everything in sight. The fewest additives are in the regular . .87, and it burns at a speed that allows peak power from most 2 strokes without burning up a lot of surplus fuel in the exhaust system and cooking the engine. Slower burning fuels peak combustion pressure later in the stroke and can push a hot flame front right out the exhaust ports into the exhaust manifolding. Now. Granted, no 2 stroke except maybe a direct-injected engine is likely to have really complete combustion at cruising rpm, but my point is that the lower octane fuel comes closer and thus converts more of the chemical energy into piston kinetic energy, and less into surplus heat in the exhaust system. That, according to your very good logic about protecting the engines from heat damage, argues for using the lower octane, unleaded, unenhanced fuel to do so. And it lowers the rate that fuel-based deposits foul plugs, clog rings and combustion chambers.<br /><br />Your move, Sir.<br /><br />Respectfully,<br />JB

 

[MASTERTECH]

Re: correct octane

Should you use premium gas in your 2 stroke outboard?? Or 87 octane??<br />NO! here's why. The octane ratings are composed of 2 components, research octane and motor octane (RXM/2) divided by 2. The RESEARCH octane is the quality of the base stock, the MOTOR octane is derived from additives. The oil company will never tell you the ratios. Problem is the highest octanes are achieved by MOTOR octane additives, which will just gum up the pistons in a 2 cycle engine. Base stock is generally the same. The MID-GRADE gas has the detergent additives needed to clean your motor and sufficient octane for MOST motors. Using 87 octane is certain death for older model motors with higher compression as the compression ratios and timing advances will not tolerate this stuff. If you have a MERCURY, FORCE or OMC motor from the early 70's thru mid-80's refer to technical bulletins from the manufacturers for timing changes, richer jets and replacement head gaskets to lower compression for use with todays gasolines. <br /><br /><br />------------------<br />Mastertech in Paradise

 

[12Footer]

Re: correct octane

My Force is eating too much fuel. I'm gonna ask you guys about that .<br />Idle speeds produce high concentrations of unburned fuel in the test drum. I've tried cutting back on idle mixture, but the beast runs terible like that, even if I cut back on the octane.<br />I can't use additives (like seafoam or sta-bill) due too it bumping up the octane.<br />While we are on this subject of octane,is there any way to determine what additives exist in the fuel from the pump?<br />How can we determine if it contains alcohol or other crapola?<br />I was once told that,due to federal laws,they must post "contains Alcohol" at the pump.So far, I haven't seen any such posting.<br />What gives?<br />BTW, I burn anything in the 87 range...<br />Shouldn't I cut-back to a lower grade?<br /> <br /><br /><br />------------------<br />the Early bird get's the worm...So, he don't have to stop for bait.

 

[sony2001]

Re: correct octane

Dhadley: what are the differences in temps?<br /> I say that if you don't have variable leaning of the high speed jet then the motor<br />won't reach max exhaust temp of 12to1400 F.<br />The range of octane is only 6 points to play with in an effort to ELIMINATE detonation.<br />Outboards are always a bit rich and thus<br />can't reach max. exh. temp.<br /><br />------------------

 

[ODDD1]

Re: correct octane

JB, you flatter me...I was born in '62! I gotta give you the credit due...I had your post stuck in the back of my head all day...an heres my rebutall..we seem to be agruing about which fuels burn hotter cooler /fasster/slower, with their respective changes in combustion temps....i argue from the side of engine longevity....the fuel question is not which burns hotter or colder, but which is more tolerant of heat,not producing excess heat, but more tolerant of it....the ability to not detonate or preignite under high heat high load conditions, conditions the motors are maybe not designed or specified for but subjected to regardless.<br />The 2 biggest piston eaters are detonation and preignition...both caused by excess heat/pressure in the combustion chamber...what fuel is more tolerant/can be pushed harder before those 2 ugly twins show up? I still think premium. Reformulated/ethanol doped fuels compound the problem..they will inherently lean out any given mix..[a given that all 3 octane levels of fuel doped at the same ratio] .what octane rating will tolerate a lean overtemp burn and be less likely to detonate? still say premium. Peak power is not my goal, making the motor last is...fuel is still way cheaper than pistons. You said yourself slower burn fuel [premium] reaches peak pressure later in the combustion stroke, your concern being elevated exhaust temps...to me, the same event means peak pressure at a larger cylinder volume, and a better rod to crank angle, better suited to handle the push on the piston....less stress on rings, ring lands, rod/crank bearings.<br />Again I think premium fuel gives you a cushion against the adverse, insureance if you will.<br /> by the way i never recommend leaded fuel for 2 cycle either.<br />Thanks, JB I hope i hear from you.

 

[JB]

Re: correct octane

You're d****d smart for a youngster, ODDD1. You clearly have wisdom beyond your tender years. It appears that we didn't disagree except on priorities, and I will concede that an engine in trouble for any of a variety of reasons might well survive longer on (unleaded, un-enhanced, non-reformulated) higher octane fuel. I think we may continue to disagree on what fuel to recommend for an engine in proper tune and state of maintenance, but I am sure our discussion, with comments by DHadley, 12Footer, Jersey, et al has improved many people's underestanding of the issue. That's what this forum is for. Peace.<br />JB

 

[ODDD1]

Re: correct octane

Well, JB you a right ,too. we all know what happens with fuel in 2 cycle motors.....its a matter of the degree of importance we place on the different factors.....in a properly tuned, setup motor 87 is fine....but as a boat wrench I couldn't tell you how many times I have seen motors fried by a ignition/carb/cooling prob that, if they would have ran better fuel I would be repairing that prob instead of replacing pistons....My veiw on this is a little skewed....you go to a hospital to see sick people, and to a boat shop to see busted motors......an I see alot of em.<br />Once again, Thanks, JB, we are all glad you spend time here....

 

[sony2001]

Re: correct octane

My neighbor just blew his head gasket on a<br />Toyota 4X4. I bet you that truck rarely saw<br />4000 rpm. But me I'm on the lake, it's 80 F,<br />I'm turning over 5 grand for 20 minutes, I<br />stop, then throttle to the horizontal, boosting 30" of manifold pressure to get me<br />rite back up there!!! knowing that all the Force engineers and EXXON engineers are right<br />on the MINIMUMS. Balderdash! GIVE ME PREMIUM.<br /><br />------------------

 

[krosemond]

Re: correct octane

That was a very informative exchange of ideas, although it got a little more technical than most of us weekend boaters can (or want) to understand. So is this thread going to end without a conclusion?<br />Bottom line is, what fuel do I put in when I get to the marina? I am leaning towards 89, for no other reason than it is in the middle, thus decreasing the variance of the mistake of going to either extreme.

 

[ODDD1]

Re: correct octane

krosemond, you gotta make that choice for yourself, you gotta balance what you drive, how you drive it, how you maintain it, and how long it sits between uses. oh and dont forget, if you live in a "non attainment area" and have reformulated fuel, this has to color your decision, too. we have been burning gasoline in combustion engines for over 100 years..and there is no black and white 'blanket answer'.