The Octane Misconception

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#730139 11-Dec-2012 08:45

My Mitsubishi Lancer Cedia has been running on 91 since I ever bought it years ago. However, a couple of months ago I took it in for a service and the new mechanic, who said he used to work for Mitsubishi, said to put 95 in to make the engine last longer as the GDI engines prefer it. I took his advice and I do get more kilometres per tank of fuel.

Whether or not the car has more acceleration performance might be mind over matter - but it is a gutless wonder.

Asmodeus

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#730185 11-Dec-2012 09:31

I have a late 90s Mitsi VR4 and I get quite bad knocking with 91 so 95/98 it is for me

eracode

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#730274 11-Dec-2012 11:10

I guess it's a case of YMMV? (Couldn't resist that).

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dickytim

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#730358 11-Dec-2012 12:28

minigopher17: My Mitsubishi Lancer Cedia has been running on 91 since I ever bought it years ago. However, a couple of months ago I took it in for a service and the new mechanic, who said he used to work for Mitsubishi, said to put 95 in to make the engine last longer as the GDI engines prefer it. I took his advice and I do get more kilometres per tank of fuel.

Whether or not the car has more acceleration performance might be mind over matter - but it is a gutless wonder.

The GDI engines need to run on high octane, unfortunately since it sounds like it has not been there might be quite a bit of coking inside the manifold, this would have a huge effect on the performance, unfortunately the fix is to remove the manifolds and scrape the carbon build ups off.

Unfortunately very few people in NZ know what they have bought and how to run it properly.

Mark

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#730361 11-Dec-2012 12:29

I asked the same questions on a bike forum after I got my DR-Z400SM ... guy who owned it before me said to put in 95/98 RON, I wasn't so sure so did some homework, sure enough people far smarter in engine tuning than me say to stick with the standard petrol.

And the answer is real simple ... if the manual says to put 91 RON in the bike, then put 91 RON in the bike :-)

A good read is this : http://auto.howstuffworks.com/compression-ratio-octane-ratings.htm

In my case the DR-Z does not have a high enough compression rating to justify putting in the "better" petrol, even though it's been quite modified nothing has been done to increase compression beyond stock.

One thing that is important to check is the ethanol content of the fuel, Gull use a lot in their fuel and a lot of vehicles aren't set up to handle it (rots seals and whatnot apprently).. my bike for example should not be run on anything with greater than 10% ethanol ... it's actually worth grabbing those little leaflets at the stations describing their fuels, it tells you the make up of them. Gull even do a 110 RON fuel for racing from some of their stations ... sounds great but it would screw up most vehicles if Kiddie Boy Racer puts it in thinking he'll go fasterer with it :-)

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#730364 11-Dec-2012 12:35

Mark: I asked the same questions on a bike forum after I got my DR-Z400SM ... guy who owned it before me said to put in 95/98 RON, I wasn't so sure so did some homework, sure enough people far smarter in engine tuning than me say to stick with the standard petrol.

And the answer is real simple ... if the manual says to put 91 RON in the bike, then put 91 RON in the bike :-)

A good read is this : http://auto.howstuffworks.com/compression-ratio-octane-ratings.htm

In my case the DR-Z does not have a high enough compression rating to justify putting in the "better" petrol, even though it's been quite modified nothing has been done to increase compression beyond stock.

One thing that is important to check is the ethanol content of the fuel, Gull use a lot in their fuel and a lot of vehicles aren't set up to handle it (rots seals and whatnot apprently).. my bike for example should not be run on anything with greater than 10% ethanol ... it's actually worth grabbing those little leaflets at the stations describing their fuels, it tells you the make up of them. Gull even do a 110 RON fuel for racing from some of their stations ... sounds great but it would screw up most vehicles if Kiddie Boy Racer puts it in thinking he'll go fasterer with it :-)

The ethanol eats rubber, but it also takes more of it to produce the same power so it is not always a good option, i.e. theoretically you'd need to upgrade your fuel pump and regulator to deliver more fuel, then you have the added cost of worse economy!

As far as kiddie racers... let them do it, a few less cars on the road!

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#730371 11-Dec-2012 12:41

Engines produce more power if they have higher compression ratios, the higher (within limits) the compression ratio the higher the efficiency of the explosion. Howver with higher compression comes a sooner knock factor, so a higher octane fuel is needed.

So in an engine that has a compression ratio that will not knock on 91 fuel, putting 95 in it will not make a blind bit of difference.

As has been noted most European cars will not run on 91 because they have higher compression ratios, and typically higher specific power output for a certain CC rating compared to most asian brands.

Cyril

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#730417 11-Dec-2012 13:15

Cheers - N

Please note all comments are from my own brain and don't necessarily represent the position or opinions of my employer, previous employers, colleagues, friends or pets.

bender84

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#731415 12-Dec-2012 14:34

FWIW (may not be much admittedly)

I acquired a JDM 2006 Toyota Avensis (at the suggestion of my mechanic) to replace my old 1996 Honda Accord after it was written off in Otahuhu by some driver who didn't seem to understand how to use a roundabout (Failed to give way to his right, Luckily the Police attended and put him straight and ruled that he was at fault)

1. Out of ignorance, the first fill was with BP regular 91 and got 9.34 litres per 100km. Did seem slightly sluggish and asthmatic though.
2. Later on read about the potential issues (mostly carbon build up on the air intake valves) surround D4 engines and that I should be running the engine off of the highest octane possible, so for the next 3 fills, I put BP Ultimate 98 RON in the tank and got 9.89, 9.91 and 9.90 liters per 100km. Although I noticed more "torque", the engine wasn't running as smoothly and seemed to be consuming MORE petrol over the single fill of 91. Seems like the VVTI system occasionally had a fit and the RPM's for what ever reason would often go down to 1200rpm on acceleration and ran slightly rough (which really surprised me considering the glowing talk about BP 98, but may be my specific engine just simply doesn't like it).
3. Decided to try Techron 95 RON from the local Caltex and it seems the Engine is most happy with that running a lot more smoothly and felt a lot more "normal", also returning around 9.4 - 9.5 litres per 100km. From the documentation I've been able to find on the net (UK and Poland. Could not decipher the Japanese Car manual), it seems the 1AZ-FSE engine is tuned for 95 RON

Regarding these Gasoline Direct Injection engines (in my case, 1AZ-FSE)... Noticed that on the Caltex NZ site, that had an animation which effectively claimed that Techron will help you clean the intake values of the Direct injection engine and keep them clean? Would have thought that valves would be completely closed when the fuel is injected in to the chamber and the backsides of the valves will still get gummed up??
I wonder how difficult it would be to spend a day's DIY every several thousand km to give the valves a clean ourselves?

ubergeeknz

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#731419 12-Dec-2012 14:41

Petrol injectors very rarely require cleaning in a healthy engine. I had some pulled out of an MR2 which had done 140kms and flow-tested, they were as clean as a whistle. Of course I only ever ran 98 it being a turbo vehicle.

mrphil

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#731427 12-Dec-2012 14:52

most petrol flap will state what ron petrol is best used in that vehicle but i personally found 98 to be a cut above the rest and yes dont put 91 into a euro....during the earthquake period we could only get 91 and man our cars did not like it one bit

here is a cool site i use to track my fuel mileage
www.fuelly.com

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#1638597 22-Sep-2016 09:53

The American AA says in 2016 that using premium fuel doesn't give you better performance or quality, but costs more, and to only use premium if your car requires it.

MikeAqua

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#1638627 22-Sep-2016 10:29

ajobbins:
insane: I found that throwing 98 into my Primera designed to run on 91 did slightly increase the milage I got per tank, however the extra KM/L did not justify the added cost of 98.

I guess it's another case of RTFM

I just can't see how that is possible. Unless your car was 'knocking' on 91, there is no way you could get better mileage.

It's like saying a car painted with 'racing stripes' goes faster.

My Primera (P11 SES) was able to run on 90+ octane fuel (owner's manual) but was noticeably better to drive at higher Octane levels. My understanding (from dealer mechanic and the owners manual) is they are optimised for higher octane and compensate electronically for lower octane.

After researching the subject on the Primera car club site (where people had done dyno testing with different fuels), the day I bought it I filled it with 95 set it to remap and drove like I stole it. On the few occasions after that when someone else put 91 in the tank, it it ran like a bag of wees for a while.

Mike

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#1638638 22-Sep-2016 10:43

Timely that someone should bring this thread back to life. My father has recently bought a car with a high compression engine (2005 Toyota Mark X). I have the same car so had done a little research when I got mine, and the general consensus online was that it should be run on 98, but could likely get away with 95 (but with reduced power as computer would adjust timing to prevent knock). Everyone seemed to be in agreement that you shouldn't run it on 91.

I explained this to my father when he got the car, but just discovered last night that he has since filled it up with 91 "because the dealer never mentioned needing 95 or 98". I, of course, had words with him about why he would just ignore my advice!

But my question is, just how much can the anti-knock sensors compensate for? Surely it can only adjust the timing so much? Could he be damaging his engine?

At this point, with half a tank of 91 left, should he top up with 98? Will this bring the overall octane up as the fuel mixes together? Or is he better to wait until the tank is near empty and fill it completely with 98?

timmmay

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#1638652 22-Sep-2016 10:47

Paul1977:

Timely that someone should bring this thread back to life. My father has recently bought a car with a high compression engine (2005 Toyota Mark X). I have the same car so had done a little research when I got mine, and the general consensus online was that it should be run on 98, but could likely get away with 95 (but with reduced power as computer would adjust timing to prevent knock). Everyone seemed to be in agreement that you shouldn't run it on 91.

I explained this to my father when he got the car, but just discovered last night that he has since filled it up with 91 "because the dealer never mentioned needing 95 or 98". I, of course, had words with him about why he would just ignore my advice!

But my question is, just how much can the anti-knock sensors compensate for? Surely it can only adjust the timing so much? Could he be damaging his engine?

At this point, with half a tank of 91 left, should he top up with 98? Will this bring the overall octane up as the fuel mixes together? Or is he better to wait until the tank is near empty and fill it completely with 98?

If the manual says use 98, use 98. If the manual says use 91, use 91. If you need to switch then I'd probably let it get pretty low first.

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