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Thread ID: 139508	2015-05-14 00:02:00	91 octane versus 95 - advice please	BBCmicro (15761)	PC World Chat

Post ID	Timestamp	Content	User
1400640	2015-05-14 00:02:00	email from daughter below. She lives in Gisborne and accidentally filled her car with 91. It's a '98 Corolla sedan, NZ-new, 1.8L. Quite high specs for the time (airbag, anti-lock brakes). Uses about half a litre of oil Wellington-to-Auckland. Though it's done over 300,000 km it has significantly more power than our son's 2001 Corolla 1.6L (which uses 91). She's been using 95 because I told her. The car used to be ours (Mrs Micro and me) The handbook in the glovebox says 95 but I have to wonder. Any advice? >>>> yo, so now three people, men of course, have all scoffed at me for putting 95 in my car in the first place. One is an engineer, one a mechanic and one just a capable man who owns waders and knows how to live off the land. So I'm feeling a little lonely in my defence of why I use 95 in the first place - can you please provide me with better evidence than "dad told me to" coz it turns out that is only convincing for me and not anyone else. The car is alive and happy at well over 300K km so I feel like it's working out pretty well but I dislike being scoffed at and seek more robust reasoning to combat the scientific knowitalls. cheers	BBCmicro (15761)
1400641	2015-05-14 00:32:00	91 will be fine. Offers far better value than 95. 95 gives slightly better fuel economy, but not enough to offset the highest cost. Your daughter would be better off using 91 and saving herself some money.	Nick G (16709)
1400642	2015-05-14 01:02:00	If the owners manual says use 95, then use 95. Does Joe Average really know more than the manufacturers engineering staff ? " 95 premium can give around 4% lower fuel consumption than 91, assuming the engine computer adjusts to take advantage of the octane difference. " www.mynrma.com.au Not sure why that would be true....after all 95 is just 91 with some anti-knock additives (nasty carcinogenic additives) perhaps this is why.. ? " Modern road cars fitted with advanced engine management systems can detect knocking and make adjustments so that fuel with lower octane can be used safely but with some loss of power as a downside. For these vehicles, it could be said that high octane fuel found in premium unleaded petrol will improve performance compared with using regular unleaded petrol. This is why the man with the deep voice on the ad who claims improved performance by switching to high octane premium fuel is not completely wrong. " www.frugalfoo.com	1101 (13337)
1400643	2015-05-14 01:22:00	For a car that old with that high mileage I'd just use 91..	paulw (1826)
1400644	2015-05-14 01:44:00	She accidentally filled her car with 91. It's a '98 Corolla yo, so now three people, men of course, have all scoffed at me for putting 95 in my car in the first place. very few cars these days need 95. Old classic cars...but then they did better on 98 actually. Anyway, changing from one to the other usually require a bit of adjustment to the timing is all. Personally I'd stick with 91 - way cheaper.	pctek (84)
1400645	2015-05-14 01:49:00	Generally 91 has a higher calorific content than 95 not the other way round as many people think. Wherever possible you should use what the manufacturer recommends as someone else already stated. If you have to guess 95 is actually the safer choice but more expensive. I'd expect a corolla to run fine on 91, generally not considered performance cars but toyota would know better than me. If an engine is designed for 91 and you use 95 in it the only effect it'll have is costing you more, a lot of people convince themselves of a difference that isn't there. However if an engine is designed for hi octane fuel you can potentially damage it by running it on a lower octane. 95/98 whatever is designed for high compression engines and prevents pre-ignition (the anti-knock 1101 mentions). They don't run well on 91. Some engines (in fact a lot of them these days) are designed to run on either and will adjust themselves and in that case use whatever you prefer or which works better. For my 2002 V6 Camry I ran it on 91 for a couple of years then tried 95, you can't feel the difference but it gets ~ 50km more from a tank of gas on 95 making it actually marginally cheaper than 91 per km.	dugimodo (138)
1400646	2015-05-14 04:07:00	Well getting all technical: The higher the octane number, the more compression the fuel can withstand before detonating (igniting). In broad terms, fuels with a higher octane rating are used in high performance gasoline engines that require higher compression ratios. Gasoline engines rely on ignition of air and fuel compressed together as a mixture without ignition, which is then ignited at the end of the compression stroke using spark plugs. Therefore, high compressibility of the fuel matters mainly for gasoline engines. Use of gasoline with lower octane numbers may lead to the problem of engine knocking. In short, there’s none; you should use what the manufacturer has specified for your vehicle. Despite what some people claim, there is nothing to be gained from running your vehicle on a higher octane petrol than that specified by the manufacturer. Unless your engine is knocking , buying higher octane petrol is a waste of money. Of course, if your car is designed to run on 95 or 98 octane petrol, you must use the octane rating that has been specified by the manufacturer, otherwise you risk causing damage to the engine. In modern computer controlled engines, the ignition timing will be automatically altered by the engine management system to reduce the knock to an acceptable level. Octane ratings are not indicators of the energy content of fuels. They are only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner. Where the octane number is raised by blending in ethanol, energy content per volume is reduced. Even more earth-shattering stuff about this can be found at: en.wikipedia.org Actually, I have found quite often, never mind the car manual, the petrol recommended is often found on a sticker inside the petrol cap.	pctek (84)
1400647	2015-05-14 04:55:00	I would have thought a 1998 Corolla 1.8 would be using the 7A-FE or 1ZZ-FE engine, for which Toyota generally say RON91 is recommended. Is it some special Corolla variant?	inphinity (7274)
1400648	2015-05-14 05:34:00	Thanks guys I will ask her to check the manual for engine number and compression ratio . If it is the 7A-FE engine that's probably 9 . 5 CR according to my skim-reading of the Internet . Apparently 10:1 is probably OK for 91 I hadn't heard of engine-computers adjusting for lower-octane fuels . . . As to why it's specified at 95 octane, it might be to give extra margin in some countries . I mean, if it were specified at 91, Russians might try to run it on 86 . Just my guessing . . . . Putting it all together (particularly inphinity's comment on the engine) I'm inclined to go with 91 . . . I wonder what I would have saved if I had done that 200,000 km ago	BBCmicro (15761)
1400649	2015-05-14 05:49:00	I use 91 in my 2000 Corolla 1ZZ-FE it's fine. Do a couple of tanks on it then run a couple of tanks on 95 and see what you get better mileage out of. You may find that 91 doesn't work out any cheaper at all. If it was a 4A-GE or 2ZZ-FE then 95 at a minimum, but sounds like she has the econobox version (don't think the 2ZZ hit our shores until a few years later and 4A-GE is 1.6). "Of course, if your car is designed to run on 95 or 98 octane petrol, you must use the octane rating that has been specified by the manufacturer, otherwise you risk causing damage to the engine. In modern computer controlled engines, the ignition timing will be automatically altered by the engine management system to reduce the knock to an acceptable level." Yeah but timing can only be retarded so much. If you run 91 in something high CI or turbo you're asking for a melted piston or nice big window in the side of the engine block.	Alex B (15479)
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