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Petrol Grade 95 or 97 ??? - Botchit, Soddem & Leggit
Setting the marketing aside, when is it really beneficial to use the 97 grade of petrol?

I had a discussion about this with a colleague a while back who was wise in the ways of modern engine coltrollers. I believe his explanation was something along these lines...

- - - -

Ignition timing (I think) is adjusted in order to prevent pre-ignition (knock sensor provides this info). When the load on the engine is greater, so too will be this adjustment. When the adjustment is greatest, fuel efficiency is reduced.

By using 97 in place of 95, the onset of knock is delayed so the engine is able to operate closer to its optimum level for longer and you will get more miles to your gallon.

- - - -

The general rule, I understood from this at the time is that for modern engines (where adustments of this type are made automatically as part of normal operation) you may be better to use 97 when doing long periods of high speed motorway driving with a car load of family or luggage. Around town, on your own in stop start driving, you are wasting your money.

Any truth in this???

Any comments??

Anything from perople who really are in the know?
Petrol Grade 95 or 97 ??? - GregSwain
I tend to think it's a waste of money whatever you're using the car for. A constant 70mph on the motorway doesn't exactly strain most engines, so I wouldn't use high octane fuel for motorway trips, even whilst fully laden.

Yes, there's a slight saving in terms of fuel consumption, however at my local BP garage, Ultimate Unleaded is 8% more expensive than normal Unleaded. I've tried it, and certainly didn't see an 8% drop in fuel consumption. The engine is marginally more responsive under heavy acceleration though.
Petrol Grade 95 or 97 ??? - L'escargot
Check you car's "Owner's Guide". Mine (2.0 Focus) specifies 95 octane, and adds that although a higher octane fuel may be used it offers no significsnt advantage.
--
L\'escargot.
Petrol Grade 95 or 97 ??? - Mad Maxy
In my experiments with SUL I've never noticed any difference in fuel consumption or performance. However, SUL does reduce reliance on the knock sensor, which is probably a good thing if the sensor isn't performing 100%, especially for high performance engines used in a high performance way.
Petrol Grade 95 or 97 ??? - jc2
A lot of modern cars do not have knock sensors-they work by picking up high-frequency vibrations thro' the block and are not particularly reliable-being very hard to calibrate-if the sensitivity is set too high-it retards too frequently-if too low-no point in having it.Most modern cars rely on an excellent spark/fuel calibration in the first place.Knock sensors were mainly used in the early days of electronic engine control.All modern EU vehicles are required legally to run on 95 tho' higher octanes may give improved performance on a few vehicles.
Petrol Grade 95 or 97 ??? - MVP
If I put Tesco 99 ron or Shell V Power in our Forester 2.0x, I can go up the A3 Hogs Back in 4th instead of 3rd - seems to give a lot more torque.

I read an article a while ago - in some cars the extra octane improves power & economy, in others, no real change.

MVP
Petrol Grade 95 or 97 ??? - Hamsafar
Most cars won't take advantage of higher than 95 RON, in the EU market, they have a baseline setting of 95 RON and retard the ignition if knocking is detected by one or more microphones. Most cars won't knock on 95RON in normal conditions and so would very rarely get retarded unless they are being abused/laboured in a high gear etc...
Petrol Grade 95 or 97 ??? - OldHand
As a rule of thumb forced induction cars with modern engine management systems will benefit most from higher octane fuel.

V-AG cars that use the FSI technology even more so. I used VAG-COM to monitor the output of my car running an identical performance map- using 97 ron English fuel and 100 RON V-Power I bought in Germany. The 100RON fuel gave 262bhp v 251 for the English stuff.
Petrol Grade 95 or 97 ??? - OldHand
I forgot to say VW and group cars that use FSI recommend high octane fuel only although you can run them on 95 with economy and power being sacrificied.

Likewise SUL was recommended for all the Japanese (but EU market) turbo cars I've owned.
Petrol Grade 95 or 97 ??? - Mad Maxy
JC2: thanks for the technology update. :-)

OH: my recently sold Audi 2.0 FSI was part of my experimentation referred to earlier - insensitive to SUL.
Petrol Grade 95 or 97 ??? - OldHand
In turbo applications the instruction to use high octane fuel is written on the inside of the filler cap. I have no idea about low powered non-turbo.

The manual also instructs me to use it in the RS4 stating that the engine won't run for as long in fuel stratified mode on lower octane fuel.

Petrol Grade 95 or 97 ??? - tilda99
In Mrs T's 98 Polo 1.4 - using Shell V Power it does 48mpg. With normal Shell Unleaded does 38mpg. All this mostly around town. Just about justifies the price gap in the petrol...

T99
Petrol Grade 95 or 97 ??? - OldHand
I don't believe a 10mpg gain on high octane over 'low' octane is possible.
Petrol Grade 95 or 97 ??? - tilda99
well you might be right. Mrs T filled it up, and I did the maths, so I admit room for (2 lots of) error. MPG definately improved though.
Incidentally, in my V70, it just cost more with no noticeable difference. Hopefully cleaned the engine a bit though....

T99
Petrol Grade 95 or 97 ??? - nortones2
Tests by various mags (EVO recently) show distinct benefits in 4-valve high compression engines. The main benefit from changes quantified appear to be mid-range acceleration in the higher gears - e.g Honda Type R. But, some high octane fuel batches were "off", or stale, so one particular point is to buy only from busy garages.
Petrol Grade 95 or 97 ??? - Brit_in_Germany
Wasn't there a problem of VW FSI engines running on 95 RON suffering from higher failure rates?
Petrol Grade 95 or 97 ??? - GregSwain
Wasn't there a problem of VW FSI engines running on 95 RON suffering from higher
failure rates?


Not sure, but Mitsubishi's GDi system doesn't like 95 RON, apparently the injectors pack up after about 60k unless they're frequently treated to higher octane stuff.
Petrol Grade 95 or 97 ??? - jc2
I thought the GDi problems were caused by running on EU unleaded fuel as opposed to Japanese lead-free.Jap fuel has all the lead removed while the EU fuel has none added and has a maximum content of 0.012% if I remember rightly.
Petrol Grade 95 or 97 ??? - GregSwain
I thought the GDi problems were caused by running on EU unleaded fuel as opposed
to Japanese lead-free...


I remember reading that GDi's were fine if they'd been run on Ultimate or similar.
Petrol Grade 95 or 97 ??? - GregSwain
In fact, a quick journey to CBC section reveals..... "GDI system of 1.8s prone to failure at around 60,000 miles due to carbon build up if cars are run on 95Ron petrol rather than the 97 - 99 Ron Super the engine is designed for."

Must've read that bit before and remembered it!?! Very odd, as it's a year since I've had any cause to visit the CBC section!! :-S
Petrol Grade 95 or 97 ??? - jase1
In fact a quick journey to CBC section reveals..... "GDI system of 1.8s prone to
failure at around 60 000 miles due to carbon build up if cars are run
on 95Ron petrol rather than the 97 - 99 Ron Super the engine is designed
for."


Most Japanese engines are designed with higher-octane fuel in mind.

The fact that they will run OK with the lower-octane stuff is immaterial -- most Japanese (and some Korean by extension) engines do run better with higher-octane fuel and as such I always put in a tank of Ultimate/Optimax every so often. The engines do seem better afterwards; whether this is down to better detergents or better fuel I'm not sure.

European engines won't benefit to the same degree as these are designed for the inferior Euro fuel in the first place.
Petrol Grade 95 or 97 ??? - OldHand
I hadn't heard of failure from lower octane fuel being used in V-AG group FSI engines. I have heard many reports of poor running, lower power and much worse fuel economy however.
Petrol Grade 95 or 97 ??? - Brit_in_Germany
I would appear to have mixed up the FSI and GDI engines. When I was looking at an Octavia, the dealer said that although the FSI would run on 95RON, this wasn't recommended.
Petrol Grade 95 or 97 ??? - Saltrampen
I read that V-power (98-99 RON?) has Bioethanol in it (not sure how much).
Now I know many cars have to be modified to run on E85 (85% ethanol) and that too much ethanol can confuse the oxgen sensor in the exhaust system on an unmodified car (possibly other effects as well, do they need different valve seats?)
Are there any issues with Shell V-power confusing the oxygen sensors?
(I expect there is a previous discussion on this somewhere as well on this forum)

Petrol Grade 95 or 97 ??? - Aprilia
There's a variety of things mixed up this thread - all sorts of technical inaccuracies.

First off - Mitsubishi GDi engines. HJ's CBCB is not totally accurate. The carbon problems are not due so much to the Octane of the fuel but to two other factors:
1. The GDi engine was designed to run on Japanese low/no sulphur petrol. Using higher-sulphur EU and US petrol you get clogging of the injector tips (which are in the combustion chamber) with sulphur deposits which disrupts the spray pattern and leads to localised carbonisation in the cylinder head. Mitsubishi sell an sulphur solvent additive to help prevent this. 97RON Ultimate is lower sulphur and so there is less of problem if you used this. Its not the higher octane which is really the issue - direct-injection tend to be much more fuel-tolerant than premix injection engines because the injected fuel directly cools the cylinder - so less tendency to knock.
2. The main problem with the GDi engine is the EGR. In a conventional 'premix' injection engine the fuel/air mixture is inside the inlet manifold. EGR gasses are added into the manifold and the fuel/air mixture acts a solvent and prevents EGR deposits from sticking to the manifold walls. In a direct-injection engine there is no fuel at all in the inlet manifold and carbonised oil particles from the EGR stick to the walls. Over a period of time this builds into a thick crust which affects the running of the engine. This happens irrespective of the type of petrol used. It has to be cleaned off with a crushed walnut shell blast.

FSI engines suffer both of the above effects.


FSI engines can run stratified-charge at part-throttle cruise (this enables 'lean burn' operation). The snag is that there is the old problem that NOX emissions rise strongly with lean-burn mixtures and so the exhaust system is fitted with a NOX catalyst to get it to meet the legislation. To avoid too much NOX over the course of a journey (and bunging up the cat) the ECU advances the ignition as far as possible without knock setting in (the engine has two knock sensors). Obviously it can advance the engine further if a higher RON fuel is used. So to maximise the amount of time spent in stratified mode you need to use 98RON+ fuel. It WILL run stratified on lower RON fuel, but just for a lower proportion of the journey since the knock limit will be lower and there will be fewer opportunities to run stratified.

For a conventional engine with a knock sensor, using a higher RON fuel WILL make a small increase to the power output since you have shifted the knock-limit for the engine and the ECU will increase ignition advance until knock occurs - so the engine will become slightly more efficient. But going from 95-97RON would be a small change and hardly noticable.

Knock sensitivity of an engine is largely related to compression ratio (knock is really detonation of the uncombusted 'end gas' beyond the advancing the flame front in the cylinder). Turbocharging is really just increasing the compression ratio (the turbo is just 'pre-induction compression'). So a turbo engine with 8:1 compression and 14psi boost is like a naturally aspirated engine running 16:1 compression. The only difference is the preheating of the intake charge by the turbo - which is why we need the intercooler (and why turbo's don't run so well in very hot weather). On a turbo engine you can do things like use a bowl-in-piston or bowl-in-head combustion chamber to reduce knock sensitivity.
Turbo's don't inherently 'need' higher octane fuel - it all depends on how they're built and mapped. A lot of Jap-import turbo cars run higher compression and more advanced timing to suit Jap 100-RON fuel, rather than lower octane UK fuel, so a lot of people think turbo's have to run with SUL.

Petrol Grade 95 or 97 ??? - OldHand
It WILL run stratified on lower
RON fuel but just for a lower proportion of the journey since the knock limit
will be lower and there will be fewer opportunities to run stratified.

Turbo's don't inherently 'need' higher octane fuel - it all depends on how they're built
and mapped. A lot of Jap-import turbo cars run higher compression and more advanced timing
to suit Jap 100-RON fuel rather than lower octane UK fuel so a lot of
people think turbo's have to run with SUL.


I think you'll find I'd already said exactly that re stratified mode but thanks for repeating it..

Re: Japanese imports- it isn't safe to run them on 98 Ron fuel alone without octane booster unless remapped for it. I don't think anyone claimed turbos inherently need higher octane fuel either.
Petrol Grade 95 or 97 ??? - Aprilia
I think you'll find I'd already said exactly that re stratified mode but thanks for
repeating it..


No problem. Just trying to explain the reasons behind it in my own humble way. I don't doubt you know a lot more about it than I do.

Re: Japanese imports- it isn't safe to run them on 98 Ron fuel alone without
octane booster unless remapped for it.


Depends entirely on the particular car model. Japanese filling stations provide fuel from 91 - 101 RON-equivalent. The vast majority of Japanese cars run on 91 or 95RON, you really need to check via the engine code.

>>I don't think anyone claimed turbos inherently need higher
octane fuel either.

A lot of people think they do.
Petrol Grade 95 or 97 ??? - OldHand
Interesting point about them using lower grade fuel in Japan. I had no idea they used such low octane fuel.
Petrol Grade 95 or 97 ??? - nick
Nice to see you two getting on..... ;-)
Petrol Grade 95 or 97 ??? - aaflyer
I use BP Ultimate Unleaded (106.7 p / litre at yesterday's fill - ouch!)

Anyway, to adopt a rather conspiratorial position, how easy would it be for a garage to:

(a.) Put a few 'out of order' stickers on normal pumps, thereby forcing one's hand (save for driving away to another station) to use the higher-priced fuel?;

(b.) Channel/filter all the 'normal' fuel to the higher-priced pumps thereby charging customers over the odds...?

AA
Petrol Grade 95 or 97 ??? - GregSwain
(a.) Put a few 'out of order' stickers on normal pumps thereby forcing one's hand

Easy enough, but very stupid. Not illegal though - if they haven't got the product you want, you're free to go elsewhere!
(b.) Channel/filter all the 'normal' fuel to the higher-priced pumps thereby charging customers over the
odds...?

Illegal, and a bit difficult as different pumps draw fuel from different underground tanks. When the tanker makes a delivery, you'd have to deliberately put 95-RON into the 97-RON tank. Don't think trading standards would be impressed when they come to take samples (which they do on a regular basis)!!
 

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