Ruminations on the Effects of Octane on Motorcycle Engines

Ruminations on the Effects of Octane on Motorcycle Engines

In early April 2004, a member of the Ducati ST Owners group asked about octane and carbon buildup. I wrote the following. Note that Phil rides an ST2; I ride an ST4s; and the previous bike i mention owning is a 2001 Ducati 750Sport.

On Sat, 3 Apr 2004, Phil Roach wrote:

I was on the phone to BCM Ducati to purchase a FIM chip for my ST2. Bruce was telling me about the 966 kit they do (wish I had the dough but I don't) but said "you'd have to run high test". WHen I mentioned I frequently do anyway he said it wasnt a good idea as high test will horribly carbon up the stock bike as it burns too slowly. I saw a post not long ago about someone seeing lots of carbon on their valves. Did he run high test a lot? Just got me thinking. I thought I was doing good things buying the extra cleaners, etc. that the high test usually adds but it sounds like I need to buy cheaper in future....thoughts?

first, keep in mind that higher octane fuels burn more slowly. in certain engines, this is desirable to prevent the fuel from burning unevenly or too quickly for the engine's design. in others, this is a hinderance to proper operation. running 110 race gas in your unmodified street car is not going to help it any; if anything, it may lower your fuel economy and destroy your catalytic converter as it spews unburned fuel into the exhaust system, which then may become an afterburner... similarly, running really low octane (fast burning) could result in the fuel burning too quickly for conditions, burning before the piston has come to top dead center, and working *against* the engine, and possibly damaging it.

i run 87 octane in my bike most of the time (out of a weird sense of caution i use 89 in the heat of summer). the valves were clean at the 16k checkup -- hardly any traces of build-up. my 750S had far, far more build-up at 6k -- from using 91 octane. they were black and visibly built-up.

so, one school of thought says use the lowest octane you can without pinging. bruce says this; he uses low octane in some (though probably not all of) his race bikes, and they seem to do okay. (there are anecdotes, too. my favorite is about a race bike that was brought to them. they drained the 110 octane race gas, put in 87, and gained over 10hp. why? the low-octane fuel burned faster, and thus the engine got maximum power out of it.)

Wonderful Mike takes a by-the-book approach, pointing out that ducati intended the engines to use 91 octane, and you should use it because predetonation can destroy a cylinder. and he has a real and valid point. fuel octane is pretty closely monitored by the refineries; the more consistent their gasoline, the more consistent their profit margins (too much additives to increase octane and the cost of refining goes up). but there is some variation, and you could get old, bad gas from a small town gas station that doesn't do much business, and that really could be bad for you.

further, automobile engines have really smart engine computers that can detect predetonation (pinging) and adjust the ignition timing to prevent it from recurring. they're smart and robust, and if they get bad fuel, they can deal. they won't perform _well_, but they'll go. our motorcycle engines, on the other hand, are not nearly so sophisticated, and are more vulnerable to such vaguaries of fuel.

so, i believe that engine damage from low-octane gas is possible, if you're using unknown octane horrible gas in mexico, but i've never heard of it happening to any modern bike, ducati or otherwise, in north america or europe.

i think mike's being too cautious; he swears he'd never buy a bike from me. :) that should tell you about two extremes of opinion and experience.

otoh, i sure as heck wouldn't run 85 octane in a turbo charged 1.8L Audi engine, for example -- i wouldn't want to screw with the turbo-charged fuel-air mixture.

in practice, it appears ducatis seem to deal with 87 octane US pump gas just fine, and even thrive on it. some prefer to play it safe, by the book; others believe their engines do well on quality but low octane gasoline.

make your own decision.

disclaimer: I am not any sort of spokesperson for Ducati, BCM Ducati, Wonderful Mike Mullen, or anyone else. I'm speaking from my own experience and what I've learned from other. Consider all the facts you can gather, then make your own informed decision. :-)

Original message archived at YahooGroups