OutdoorKing Repair Forum Forums Restoring, Customising and Modifying Tech Talk Octane Fuel

Hi guys,the other day i mowed a paddock with the ride-on and the old victa 160 two stroke.the victa coughed and spluttered (unusual)so i was wandering if i should be using 98 rated octane fuel to mix my 25-1 fuel,caltex vortex or bp ultima,remembering that when i was a kid the only fuel was (super or standard)as well my two strokes and four stroke are of the 70s/80s era...ken

Ken, the old standard-grade petrol in the leaded fuel days was 87 octane, and the Victas of that era ran perfectly well on it. Current standard unleaded is 91 octane, and that level is more than adequate for old Victas. Old B&S 4-strokes are in the same situation; their compression ratios are very, very low. However be careful with Honda and modern OHV B&S engines, which have much higher compression ratios. Follow the manufacturer's recommendations for octane rating.

Simple 2-stroke engines such as Victa made have rather low effective compression ratios due to leakage through the ports. They also have very poor scavenging, which means they retain a lot of exhaust gas - the same effect as exhaust gas recirculation on a car engine. It dilutes the charge, reducing burn rates and tending to suppress detonation. Engines of that type are seldom very demanding of fuel octane level - but be very careful with high speed, high output two strokes such as high performance 2-stroke motorcycle engines, and some chainsaws - follow the manufacturer's fuel recommendations closely with all types of engines.

Thanks grumpy,as usual most help-full...ken

Not to high-jack your thread, but I was wondering if anyone had heard of late 90's utes running better on the e10 petrol's than standard unleaded?

When I bought my '96 toyota hilux I made the mistake of automatically putting premium fuel in and she coughed and farted and shat herself something shocking until it was empty. I didn't know *WHAT* the hell was wrong at first. After a while it dawned on my and I've used straight unleaded ever since, but I had a client yesterday ask me how it handles e10 fuels as his sons old ute seems to run better on it.

Any idea's?

There are several issues with E10 fuel (10% ethanol). First, it is a bit unfriendly to some polymers in the vehicle's fuel system, though only 10% of ethanol does not do much more than speed up normal deterioration. Second, being an oxygenate E10 effectively makes carburetor vehicles run a bit leaner (modern vehicles with oxygen sensors are not affected in this way: the engine management system just injects more fuel to compensate). Third, you use a few percent more fuel: E10 requires more fuel to provide the same amount of fuel energy, and it requires a richer air/fuel ratio. Fourth, ethanol has a higher octane level than 91 octane normal fuel, so unless the oil company changes the mix to compensate for this, you might end up with a fuel that is a fraction of one octane number higher than the conventional fuel. That is a slight advantage if the vehicle is prone to detonation.

I do not know of a reason why a 1996 Hilux would run better on E10. I am guessing that the model is new enough to have fuel injection and an oxygen sensor, so it shouldn't make any difference to the way the vehicle runs, except for a barely measurable difference in the fuel's octane level.

Premium fuel has a different mix of ingredients: a higher proportion of high octane components and a lower proportion of low octane ones. Since your vehicle only required 91 octane fuel, I don't see why it would run noticeably differently on 95 octane unless the fuel company was careless and allowed the Reid Vapour Pressure to rise compared with the normal level suited to the time of year. My guess is that for one reason or another, you either had contaminated fuel, or unseasonable fuel (if the gas station's supply of premium unleaded was old stock from the winter, and they sold it to you in summer, you might have had hot fuel handling problems due to excessive vaporisation of fuel in the fuel lines).

Hmm okay. I'll take your word for it! I'm an ex-baker by trade and business admin before that, so I'm not as experienced in mechanics as I'd like. I still manage to sort out problems with my gear and parts though hehe.

It maybe worth trying her on higher octane fuels again come warmer weather, but for now I guess I'll be better of sticking with what works I suppose.

Cheers Grumpy