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Interested to read the thread in the IS area about fuel quality and it made me wonder what other 'F' owners have found to be best.

I've sort of swung towards Sainsbury 97 or Tesco 99 because it seems such a huge output engine to trust to 95 octane but perhaps I'm just paying an extra few pence a litre for the sake of it if the ECU's up to self-adjusting for the lower grade stuff - I don't suppose any of us would moan about dropping a few horses' worth of power if it saves the odd shilling a tankful!

Any thoughts?

Incidentally - how many 'F' owners do we reckon we have on this forum and is it all the cars registered so far

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what is specified in the owners manual? Toyota/Lexus are normally good at programming the ECUs to work with normal fuel for the country they are sold in, unlike Audi which seems to insist on 97 RON fuel for the most ordinary of engines.

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what is specified in the owners manual? Toyota/Lexus are normally good at programming the ECUs to work with normal fuel for the country they are sold in, unlike Audi which seems to insist on 97 RON fuel for the most ordinary of engines.

Handbook says "minimum 95 octane" so anything (except perhaps from a certain petrol retailer with a three letter name that my solicitor says I shouldn't disparage in print) should be OK....

But - with my BMW twin turbo diesel I got several mpg benefit by using the demon BP super-fuel, certainly enough to justify the extra cost, so I wondered if anyone has had their 'F' long enough (or at least travelled far enough!) to form an opinion.

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no experience of the F but for other Lexus I've owned I see an increase in mpg using higher octane fuel but never enough to compensate for the extra price.

This depends on where you live in the country because the cost difference between 95 and 97/98 seems to vary. It's about 7p more expensive where I live.

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It's about 7p more expensive where I live.

Guess that must be Knightsbridge then!

Not like us lucky Midland yokels who get by on an extra "groat a gallon" (2 to 3 pence a litre that converts to!) for the better stuff....... :winky:

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Interested to read the thread in the IS area about fuel quality and it made me wonder what other 'F' owners have found to be best.

I've sort of swung towards Sainsbury 97 or Tesco 99 because it seems such a huge output engine to trust to 95 octane but perhaps I'm just paying an extra few pence a litre for the sake of it if the ECU's up to self-adjusting for the lower grade stuff - I don't suppose any of us would moan about dropping a few horses' worth of power if it saves the odd shilling a tankful!

RON ratings. The final test of petrol is done using a special standard single cylinder engine to find the "knock" coefficient. With the best equipment available, it is only just possible to measure RON to within about +/-1.5. So the actual figures at the pumps for petrol rated as 95 RON could be 93.5 to 96.5 & for 97 RON 95.5 to 98.5. But these small amounts really make almost no difference to anything. This does not mean that different petrols with different additives will not behave in different ways. RON has nothing at all to do with the "Power" in the petrol, just in the way it burns. So, most engines with reasonable ECU's will attempt the best fit to what the system thinks the fuel is, but it is only an approximation because they are just not that accurate. This in turn means that a car may best match brand X 97 RON today, but the next delivery is 2-3 RON different, so it is no longer the optimimum choice. So in the end - choose what works for you. For me Sainsburys 97 RON seems fine, but to be fair 95 RON is what I get rather than wait for a pump, and it seems just as good :tomato:

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The system doesn't really have knowledge of the fuel that is in there. It will rely on the knock sensors to make sure the timing isn't too advanced. The problem is when you add higher grade fuel in the ECU is reluctant to try and advance the timing because it doesn't want to reintroduce knocking. Therefore the engine will quickly adjust if you go from 98RON down to 95RON but be much slower going the other way. This is why we advise resetting the ECU and it speeds up the learning.

Correct mixture settings will be adjusted very quickly once the engine is warm and in closed loop, this is the job of the O2 sensors. There are more longer term trims also maintained by the ECU and are used when the system goes open loop (cold running, wide open throttle etc). These trims also take a long time for the ECU to adjust in the same way as the timing.

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The system doesn't really have knowledge of the fuel that is in there. It will rely on the knock sensors to make sure the timing isn't too advanced. The problem is when you add higher grade fuel in the ECU is reluctant to try and advance the timing because it doesn't want to reintroduce knocking. Therefore the engine will quickly adjust if you go from 98RON down to 95RON but be much slower going the other way. This is why we advise resetting the ECU and it speeds up the learning.

Correct mixture settings will be adjusted very quickly once the engine is warm and in closed loop, this is the job of the O2 sensors. There are more longer term trims also maintained by the ECU and are used when the system goes open loop (cold running, wide open throttle etc). These trims also take a long time for the ECU to adjust in the same way as the timing.

Sorry, oversimplified the issue. Was trying to say that over this range RON really doesn't make much difference anyway, and what you get at the pumps is in any case variable. Add to this that car sensors are not laboratory standard, and that the majority of ECU action is profiled - "mapped" rather than closed-loop; the ECU can't improve much with relatively inaccurate inputs. The electronics involved are very accurate because it is easy and cheap to achieve this, but deal with input from sensors made to a price. An analogy is electronic thermocouple thermometers which are profiled to suit the inherently predictable but non-linear output from thermocouples, achieving about 0.2 degree accuracy, but the manufacturing tolerances of sensors reduce the accuracy of the whole thing to about 2 degrees.

ECU closed loop control tends to be slow to maintain stability whilst having enough open-loop gain to be effective. Generally ECUs are designed to ignore small, fast changes to avoid reacting to limited behaviour to make things generally worse - with some exceptions. They are usually best at compensating for big, gradual changes.

What is said about resetting the ECU is doubtless true. But when nothing has changed, is unlikely to restore to exactly the same as before reset due to differences creeping in because of inconsistencies in the input from sensors etc. Sorry if this all a bit techie! :tomato:

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Some interesting dyno tests here:

Thorney are sponsored by Tesco 99 so the argument that Tesco 99 is better than Shell V Power is not the conclusion that Which came to in their test...

http://www.thorneymotorsport.co.uk/tuning/...ts_Update.shtml

I suspect running the ISF on 99 would give it a 3 to 5% performance improvement.

Paul

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no experience of the F but for other Lexus I've owned I see an increase in mpg using higher octane fuel but never enough to compensate for the extra price.

This depends on where you live in the country because the cost difference between 95 and 97/98 seems to vary. It's about 7p more expensive where I live.

If you want to work out out your exact mpg, it is harder than you think.

First: Forget the mpg display on the car - its just a guide, but will show you if it gets better or worse. The petrol flow rate side of it is significantly affected by how fast the fuel flows (how hard you drive) and by temperature though - usually seasonally. So is your mpg anyway.

Second: When you buy from the pumps, the allowable error is -0.5% to +1% which for 50 L is a range of 3/4 L - only if the pumps have been recalibrated within living memory. Otherwise it could be worse. Petrol with a bit better mpg could be just that there is more of it.

Third: How far did you travel? Again, the car Odometer is a guide, but not very accurate - it is hard to find information on Odometer accuracy, but 5 miles in 100 seems to be about there. It varies with temperature and speed. A satellite system can do better, but as with speed it depends on update rate and a number of other factors.

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Nice to see that updated Thorney test - I'd only ever seen the one before V-Power came out before!

Quite interesting actually - both V-Power and the 102 stuff look better really low down than Tesco 99, whereas Tesco 99 seems better at the top end than all three - even the 102!

Shame they didn't try it on some full on turbo cars as that would be quite interesting.

Pete at Thor did a similar test ages ago - that can be seen here:

http://www.peterbetts.co.uk/Fifth_Gear-Thor_Racing.avi

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Big error in the maths here, If the allowable error is -0.5% to +1%, tell you what lets round that up.

Lets make it -1% to +1%. Thats a 2% swing range correct ? What is 2% of 50 litres ?

Please don't tell me it's 4 litres as it is actually 1 litre and that was adding 0.5% to the swing..........

Lets put it this way, 2% of 100 litres is 2 litres, hence 2% of 50 litres is half that.

Then lets add the 2% swing to fuel comsumption figures.

20mpg = 19.6mpg-20.4mpg

30mpg = 29.4mpg-30.6mpg

40mpg = 39.2mpg-40.8mpg

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Big error in the maths here, If the allowable error is -0.5% to +1%, tell you what lets round that up.

Lets make it -1% to +1%. Thats a 2% swing range correct ? What is 2% of 50 litres ?

Please don't tell me it's 4 litres as it is actually 1 litre and that was adding 0.5% to the swing..........

Lets put it this way, 2% of 100 litres is 2 litres, hence 2% of 50 litres is half that.

Then lets add the 2% swing to fuel comsumption figures.

20mpg = 19.6mpg-20.4mpg

30mpg = 29.4mpg-30.6mpg

40mpg = 39.2mpg-40.8mpg

PMSL :lol:

3 / 4L = 0.75L

Think he meant that mate :P

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I got a B at O Level (last year before the GCSE's came in). No jokes about O Levels, I have the ban button !!

I know lots of girls who have O levels and A levels too :P

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  • 7 months later...

When i got my ISF the other day the guy in the dealership said the car only requires 95 octane and they had all been advised to stick to 95 as the car runs better on it and that anything greater would just be a waste of money. I dont know who told him this (Lexus maybe?) but i cant see why this would be the case.

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  • 2 weeks later...

In my ISF i once managed an entire tank of 95 Ron in 95 miles !!!

When my wife borrowed the car for a week she managed 231 miles from a tank.

I guess she found somewhere selling RON230 ??

No, but seriously, I do see a general increase in MPG using the higher grade motion potion but can't notice any performance increase. In my opinion ...and of course this depends on road conditions, traffic, how i drive, temperature and what colour pants I am wearing and in addition factors xy and z ...i would guess I get 10-15 miles more on tesco 99 but i have no scientific evidence...it's just a feeling.

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  • 3 years later...

Interested to read the thread in the IS area about fuel quality and it made me wonder what other 'F' owners have found to be best.

I've sort of swung towards Sainsbury 97 or Tesco 99 because it seems such a huge output engine to trust to 95 octane but perhaps I'm just paying an extra few pence a litre for the sake of it if the ECU's up to self-adjusting for the lower grade stuff - I don't suppose any of us would moan about dropping a few horses' worth of power if it saves the odd shilling a tankful!

RON ratings. The final test of petrol is done using a special standard single cylinder engine to find the "knock" coefficient. With the best equipment available, it is only just possible to measure RON to within about +/-1.5. So the actual figures at the pumps for petrol rated as 95 RON could be 93.5 to 96.5 & for 97 RON 95.5 to 98.5. But these small amounts really make almost no difference to anything. This does not mean that different petrols with different additives will not behave in different ways. RON has nothing at all to do with the "Power" in the petrol, just in the way it burns. So, most engines with reasonable ECU's will attempt the best fit to what the system thinks the fuel is, but it is only an approximation because they are just not that accurate. This in turn means that a car may best match brand X 97 RON today, but the next delivery is 2-3 RON different, so it is no longer the optimimum choice. So in the end - choose what works for you. For me Sainsburys 97 RON seems fine, but to be fair 95 RON is what I get rather than wait for a pump, and it seems just as good tomato.gif

Errm, to be precise it's not "the way it burns". The RON is how much you can compress the fuel before it self-ignites, like diesel. Diesel engines have a compression ratio typically about 16:1. This is why they're more economical and also why (if well maintained) the engines are stronger.

Modern cars adapt to lower RON with the anti-knock sensor which retards the timing. But this means you're compressing the fuel, even as it's trying to expand, for longer in the cycle. NOT Optimum.

95 RON ordinary unleaded is good for 10:1. My 10.5:1 engine is best on at least 97 RON.

Watch out for ethanol though which is often used as an octane booster. It has 102 RON but has less energy (it expands less when burnt). Another dirty trick by those oil companies.

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talk of 10:1 or 10.5:1 compression ratios isn't too relevant on this IS-F forum. The use of direct injection stops the fuel from being heating as much, heat which causes the detonation, and as such the IS-F engine with its compression ratio of 11.8:1 is happy to run on 95 RON fuel.

Diesel engine compression ratios would typically be in the 18:1 to 19.5:1, and is one reason why they are so noisy. Very rare to get standard diesel engines these days on current vehicles, turbo-diesels are around the 16:1 as the turbo itself is providing some of the compression.

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