fuel grade/ignition timing issue

I have a 135 V6 mariner 1991 model. I prefer to run higher grade fuel 98 octane. Is it recommended to alter the ignition timing to suit this grade of fuel?? Also what compression should this model engine run at?

Re: fuel grade/ignition timing issue

98 octane fuel...I've never even seen that in my neck of the woods. I would recommend using the fuel grade mentioned in the manual.

Re: fuel grade/ignition timing issue

I don't have the manual so am not sure what grade gas to use??

Re: fuel grade/ignition timing issue

I find that most engine mfgrs. recommend a fuel like USA Regular Grade rated at 87 Octane R+M/2. The lower the octane rating, the faster the fuel burns, but with other things to consider there is a lower practical limit to usable octane. <br /><br />In a 2 cycle, you have to get a "pop", a fast burn, out of the fuel. It has to burn off and yield it's power before the piston clears the exhaust port. Otherwise your bang goes out the exhaust.<br /><br />Higher octane fuels burn longer and in a 4 cycle that's great cause you want the fuel to burn the whole time in the power stroke.<br /><br />So I use regular and go about my business. FYI, I have experimented with fuel octanes and really didn't notice any marked difference in performance. But atmosphere, water conditions, and boat loading, vary so much from trip to trip it's really hard to analyze unless you were in a laboratory using a dyno or something like that.<br /><br />My2cents,<br /><br />Mark

Re: fuel grade/ignition timing issue

The octane rating of a fuel is it's "resistance to combustion under pressure"<br /><br />All it does is prevent pre-ignition due to high compression ratio's. Not ratio's actually, but absolute combustion chamber pressure.<br /><br />To explain it in an example, I'll use a modified street racer.<br /><br />Say you have a regular car, that runs on 93 octane as per manufacturer spec, and the engine is stock standard with let's say a 10:1 compression ratio.<br />Now you decide you want more performance, and you have the head skimmed, and now you have a 13:1 compression ratio. But all of a sudden that engine performs poorly, and after you switch off the ignition, the engine continues to run! It runs without any spark! Like a diesel!<br /><br />That's pre-combustion. The fuel is combusting due to the high amount of pressure it is under. I'm not going to go into all the disadvantages and other technical stuff about it. But suffice to say, the fuel air mixture is spontaniously combusting while the piston is still on it's way up. The way to overcome that is to go to higher octane fuel. Let's say 98 octane. The 98 octane fuel will not combust under the pressure, and will "wait" to be ignited by the spark plug.<br /><br />So all in all, it's not worth running your engine with higher octane fuel than required. There's no performance increase and no other benefits.<br /><br />On a side note, if you live at high altitude asl, and your fuel octane rating is marginal, you'll get pre-combustion if you visit the coast. I don't know about the US, but here in South Africa, we get 93 octane at inland, and 97 octane at the coast from all pumps. And your car performs better at the coast due to the increased air density, not because of the octane rating of the fuel.<br /><br />Edit:<br />Sorry, back to the original question. No, you don't need to change your timing if you are using a higher octane fuel.

Re: fuel grade/ignition timing issue

Originally posted by andy6374:<br /> 98 octane fuel...I've never even seen that in my neck of the woods.

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It's an Australian Spec of Gasoline.<br />specs as follows<br /><br />Regular unleaded typically has a Research Octane Number (RON) of 91 to 92.<br /><br />Premuim unleaded petrol 'PULP' usually has a RON of 95 to 96, although a number of products are produced with a RON of 98. PULP products are also marketed as having various detergents that claim to provide cleansing benefits to vehicle engines.

Re: fuel grade/ignition timing issue

From Rovernaut:<br /><br />"It's an Australian Spec of Gasoline.<br />specs as follows<br /><br />Regular unleaded typically has a Research Octane Number (RON) of 91 to 92.<br /><br />Premuim unleaded petrol 'PULP' usually has a RON of 95 to 96, although a number of products are produced with a RON of 98. PULP products are also marketed as having various detergents that claim to provide cleansing benefits to vehicle engines."<br />------------------------<br /><br />In the USA we used to use only the Research Method and had higher octane numbers.<br /><br /><br />We now use Research + the Motor method (whatever that means) /2 and the numbers are lower. It's been so long ago, I forget what octane Regular Grade (leaded at the time) was prior to the changeover but the fuel didn't change at the time of the change as I recall.<br /><br />Course later it went to unleaded regular, where you had regular and unleaded regular at the same filling (petrol) station, and then leaded was dropped, then on to formulated unleaded and then every podunk town in the country had to have a different formula to satisfy the EPA.<br /><br />My2c<br /><br />Mark

Re: fuel grade/ignition timing issue

The U.S use pump posted ratings that are an average of the RON & MON combined. Aus use RON only, even if it is posted on the pump is is not the U.S. 'Pump posting', it is RON.<br /><br />RON is typically 2 to 3 points higher than the U.S. pump posting.<br /><br />The U.S. pump posting of 89 is eqivilent to our regular unleaded which is RON 91.<br /><br />Where it gets confusing sometimes is that most manuals refer to the U.S. system of pump posted ratings, for us aussies we need to convert that to RON<br /><br />Confused yet?<br /><br />Marinerman66, you are wasting your money using 98 (optimax I presume)- IMO. Regular unleaded mixed with good quality oil is all you need.<br /><br />Aldo

Re: fuel grade/ignition timing issue

Speaking of 'good quality oil' as in the post above, I have heard that synthetic oil or semi synthetic oils can react with the additives in our unleaded fuel causing a gell like substance to form, As I am pre-mixing my fuel, what oil would be the best to use (available here in Australia?)

Re: fuel grade/ignition timing issue

Hi Mariner,<br /><br />I havn't heard about unleaded fuel gelling, but...<br />In the days that I still flew ultra-light aircraft, we were told not to add a synthetic premix to a tank that has mineral premix in it. or visa versa.<br /><br />It was said that a synthetic premix mixed with a mineral premix could cause gelling in float bowls or bottom of your fuel tank.

Re: fuel grade/ignition timing issue

Mate, your wasting you money using 98 in your 2 stroke outboard. High octane fuel does not make any more power than low octane fuel, what it will do as mentioned above is allow you to alter compression and spark maps to take advantage of the higher octane fuel, but its the mods that make the power not the fuel!! In late EFI engines that are fitted with knock sensors you can get better performance from 98 but its due to the ECU's ability to adjust the timing and fuel maps to suit the better grade of fuel not the fuel itself. Your outboard is not fitted with anything that will allow such changes to take place and just advancing the timing on your 2 stroke will not make benefit from the higher-grade fuel. If you advance the timing on the 2 stroke you risk damage and will suffer bottom end power to (perhaps) gain a little at the top end. Set the timing as per the factory specs and run clean standard unleaded fuel.<br /><br />Synthetic oil will not mix with mineral oil in a 2 stroke and will cause deposits to form in all the wrong places. Any well know brand of marine grade 2 stroke, mineral-based oil will look after your engine well. They are all guaranteed to conform to the minimum specs required for your oil injected or pre-mix requirements. I use Penrite and buy it in 20lt drums, works out to get around 5lt’s free per 20lt. All the major brands are as good as each other. It is wise to stick to the one brand though than your unlikely to wrong.