After 4 months with my 200...

[MarkCup]

...I've done some science, and come up with the following...

Firstly, the accountant in me is obsessed with working out the true cost to me of this car, so from day one I have religiously recorded every penny spent using Fuel Log (Android app). After 4 months and almost 6,000 miles I think I've got enough data to support/dispel the myth surrounding higher octane fuel.

Run on 95 from purchase switching to 99 RON fuel early in February, the graph above shows the actual calculated mpg for each fill up over 2,000 miles on each fuel grade.

Mileage covered is mostly minor A and B roads, is consistent from week to week, and my driving style varies from enjoying the car as Renault intended to being on a mission to extract the absolute maximum mpg I can get. Fuel was provided by Tesco.

Headline figures are (95 v 99);

• Fuel price = 135.9 v 140.9
• Average mpg = 33.08 v 35.87
• Cost increase = +3.68%
• Mpg improvement = +8.44%

So higher octane fuel costs more, but you go further on it.

Cost per mile;

• 95 RON = 18.68p
• 99 RON = 17.86p

Saving from running 99 RON = 1.18ppl, -4.369%

Doesn't sound much?

That saving, over a full year covering;

• 6,000 miles p.a. = £81
• 12,000 miles p.a. = £163
• 18,000 miles p.a. = £244
• 24,000 miles p.a. = £326

I for one am sticking with it...not only is the throttle response, flat spot, and general driveability at low revs massively improved, at 18k p.a. I get almost 4 full tanks for free over a year.

 

[alex-avfc]

Interesting stuff. I can't quite work out whether the extra outlay for the expensive stuff will recoup itself for me, pains me when I see 1.44 come up when I reach for the Ultra unleaded pump!

 

[Bren Rogers]

Suppose it could be said that the cooler weather and denser air could contribute to lower MPG during December and January, plus you'd just bought the car and therefore enjoying it a little more, calming down after a couple of months.....

I'm just playing devils advocate really.

 

[Heez]

Informative, thanks.

 

[AK]

Such an Accountant, lol!

 

[LewisTaylor]

Suppose it could be said that the cooler weather and denser air could contribute to lower MPG during December and January, plus you'd just bought the car and therefore enjoying it a little more, calming down after a couple of months.....

I'm just playing devils advocate really.

Fuel becomes denser too. Fill a plastic jerry can up on a cold morning and see how much it expands in the afternoon! Swings and round abouts i guess.

Interesting read though.

 

[Chip-mk1]

Suppose it could be said that the cooler weather and denser air could contribute to lower MPG during December and January, plus you'd just bought the car and therefore enjoying it a little more, calming down after a couple of months.....

I'm just playing devils advocate really.

Agreed, while you might expect a higher RON fuel to give slightly better economy I dont think you can call half a dozen tanks at a different time of year a particularly accurate test.

 

[MarkCup]

Understood that there are other factors here...question to Chip...when switching fuel, how long before the engine properly works its best with it?

I'm going to carry on adding to this data, so will alternate between 95 and 99...but want to make sure I get a good reading from each change of fuel.

 

[warwickstingray]

Nice work! Could it be argued that your car has now loosened up? I didn't need convincing anyway as i was running older hot hatches with timing you had to adjust with a spanner when changing fuels. If the 200's knock sensor is retarding the timing as often as my cars used to pink if i didn't r****d the timing when going back to 95 i'm not suprised in the least.

Cars ECU is trying to give you what you want and the knock sensor is telling it not if you want to fry yourself. Never sounded ideal to me.

My dad couldn't break 35mpg with 95 in his 182 if he got his toe down driving the car to work. On 99 he gets 38mpg and has seen a similar improvement to his eco driving figures as well. He didn't report a major improvement in performance but the 182 isn't as highly strung. He did say it felt and sounded a little more on it but admitted that he may just want that to be the case. He has said he won't go back to 95 as he is also actually saving money!

 

[Chip-mk1]

Its a bit of a myth that ECU's learn very much about different fuel types TBH mate.
There is a degree of learning in some but with only the knock sensor to really go on the ECU will do a far better job of stopping damage on a lower RON fuel than it can hope to do of optimising performance/economy for a higher octane fuel as not all situations are knock limited in the first place so generally the ECU wont just keep advancing timing more and more as it would be fruitless in a lot of cases to do so. So generally the most the ECU will do is realise that yes its ok to go onto its full advance map on the fuel you are running, rather than keep tweaking things further than that.

As for how long this takes on the 200, I dont actually know I am afraid mate, lets face it though if its not within a few litres there wouldnt be much point in trying to do so at all.

So I wouldnt expect it to keep getting better and better over time if you ran it on 99 all the time for example, im sure that within a tank you would be at the best point you're going to be anyway.

 

[Dan519]

Its a bit of a myth that ECU's learn very much about different fuel types TBH mate.
There is a degree of learning in some but with only the knock sensor to really go on the ECU will do a far better job of stopping damage on a lower RON fuel than it can hope to do of optimising performance/economy for a higher octane fuel as not all situations are knock limited in the first place so generally the ECU wont just keep advancing timing more and more as it would be fruitless in a lot of cases to do so. So generally the most the ECU will do is realise that yes its ok to go onto its full advance map on the fuel you are running, rather than keep tweaking things further than that.

As for how long this takes on the 200, I dont actually know I am afraid mate, lets face it though if its not within a few litres there wouldnt be much point in trying to do so at all.

So I wouldnt expect it to keep getting better and better over time if you ran it on 99 all the time for example, im sure that within a tank you would be at the best point you're going to be anyway.

it makes a lot of difference to modern ecu's like the 197's. There are a series of target tables that the ecu uses to learn long term trims from, this keeps learning and relearning as conditions change to make the job of the lambda sensor quicker (ie get the target fueling closer to perfect so smaller changes are needed)

Knock feedback is instant, and has no reset period or special procedure.

 

[Advikaz]

I've always found through my racing the setup of the engines are vital! reeds, carbs, chains, sprockets, fuel pumps,exhaust valves, fuel pipes, exhausts ect... we ran on pretty much standard out the box motors that a friend of mine used to setup for me! all that we were concerned about at that time was squish and freeing up the crank (seals).. we where caught champs putting machined seals in the engines that a certain engine tuner had sent me !!! i removed them before time practice as the organisers had suszzed us !! but it is not illegal as you could have a worn set of crank seals that would be the same measurement .......see the problem ?? i would suggest that if you have the best engine in the world and you do not set up correctly the "fuel in" and the "fuel out" !! ie airbox all the way through the system to the exhaust then you will find that engine 3 tenths off.. and trust me the words "this engine is s**te as the others are pulling away from me down the straight" !!! could be that it has the best barrel measurement in the world... the lowest squish or the highest !! see already a conflict ! some tracks like i high squish some a lower ! so how do you sort this ??? 2 different engines ???? or an average squish that works everywhere !! better to finish 2 x 3rd places than a win and a 10th place
the best advise is the mechanic needs to be totally into engines and understand every part of the motor and its needs !!! dont rely on a chassis mechanic who can screw it together to go around corners ! he needs to work very hard on the engine to get the speed.... it will be cheaper in the long run also !!

We did tinker with different fuels & to be honest it does make a difference theirs a huge amount of fuel doping that goes on, people wouldn't do it other wise! In cars the reality is if you're running a 1.2 corsa & run it on super you'll notice nothing, but that is because the vehicle isn't designed to run on such a fuel. Sh*te fuels cause pre ignition & really you should run as per book as that is what your engine was designed for!! & yes most ECU's are learners and they do reset to run on certain mixtures, but if the book says 98 then run 98 or above especially on performance engines, anyone thats owned an import will tell you that.

Sorry for the long winded explanation.​

 

[Chip-mk1]

it makes a lot of difference to modern ecu's like the 197's. There are a series of target tables that the ecu uses to learn long term trims from, this keeps learning and relearning as conditions change to make the job of the lambda sensor quicker (ie get the target fueling closer to perfect so smaller changes are needed)

Knock feedback is instant, and has no reset period or special procedure.

The RON of the fuel doesnt effect its lamba readings mate, so while what you say is correct about target fuelling tables being constantly updated with trims based on lambda values, its nothing at all to do with learning different RON fuels, the lambda reading for 95 or 99 RON fuel wont be measurably different anyway.

The ECU learns a lot about the way the engine is altering over time, and its sensors etc, but not about the fuel itself.

The mapping differences between fuel types vary the ignition requirements greatly, but dont alter the fuelling requirements when aiming for lambda 1 which is what the ecu is attempting to do.


So I think you might have slightly misunderstood the question.

 

[Chip-mk1]

We did tinker with different fuels & to be honest it does make a difference theirs a huge amount of fuel doping that goes on, people wouldn't do it other wise! In cars the reality is if you're running a 1.2 corsa & run it on super you'll notice nothing, but that is because the vehicle isn't designed to run on such a fuel. Sh*te fuels cause pre ignition & really you should run as per book as that is what your engine was designed for!! & yes most ECU's are learners and they do reset to run on certain mixtures, but if the book says 98 then run 98 or above especially on performance engines, anyone thats owned an import will tell you that.
​

​

Yes this is my point, the ECU will pull timing out in certain circumstances due to it encountering knock, and that will hinder economy, but what it wont do is add more.
So if you run on a worse fuel than your car is mapped for you will lose economy, but if you run on a better one you wont gain anything.

 

[Dan519]

The RON of the fuel doesnt effect its lamba readings mate, so while what you say is correct about target fuelling tables being constantly updated with trims based on lambda values, its nothing at all to do with learning different RON fuels, the lambda reading for 95 or 99 RON fuel wont be measurably different anyway.

The ECU learns a lot about the way the engine is altering over time, and its sensors etc, but not about the fuel itself.

The mapping differences between fuel types vary the ignition requirements greatly, but dont alter the fuelling requirements when aiming for lambda 1 which is what the ecu is attempting to do.


So I think you might have slightly misunderstood the question.

RON does effect lambda reading actually, although it's only to a small degree.
Ignition timing will also effect the fuel required as it changes efficiency.
So no, i understood perfectly

 

[Chip-mk1]

RON is only one part of fuel, a higher RON fuel might have a lower calorific value which means it will require more fuel even though the fuel can take more timing.
E85 being the most obvious example, 25% or so worse economy, despite its massive (well over 110) RON rating.


So the results of his tests if he does them scientifically might be reflecting an underlying difference between the fuel other than the RON value, such as the fact that I believe the cheaper supermarket fuels are relying more on this ethanol than say a premium quality fuel like shell Vpower does.


So no its NOT the RON value specifically effecting the lamda value of the fuel Danny, a different fuel with more RON could mean either more OR less fuel required, depending on how the extra RON is acheived when the fuel blend is created



But I suspect that going into details about fuel blends is probably a little beyond the level of any test likely to be done on this forum.

 

[Chip-mk1]

PS

To the OP:

One other thing to consider, is that you may find at full throttle, where the dynamic compression is highest you see improvements in fuel economy, but that at part throttle where the engine has much lower dynamic CR that increased octane makes no difference at all as the engine is so far from knock limited anyway.

So you might find that on trackdays you get better economy with higher RON fuel (assuming other aspects of the fuel are equal) but that on motorway cruising it makes no difference at all.

 

[MarkCup]

OK, plenty to think about then. If I'm honest, most of the previous 5 posts are simply noise to me LOL

Going to keep the data coming in, think I might alternate each month so 4 tanks or so on each grade.

 

[Chip-mk1]

Bear in mind while doing these tests that at full throttle on 95 RON each time the knock sensor triggers its because your engine has been subjected to a potentially damaging detonation event which has just sent huge shockwaves through the piston and rod etc and that over time you may well increase the chance of the rubbish quality cast pistons breaking up as a result.

So while its quite interesting to do, I wouldnt really be up for doing the same testing myself if its a car you care about and you are likely to drive hard.



This is the sort of end that the pistons are likely to end up at prematurely if they experience a lot of det.

 

[MarkCup]

Bear in mind while doing these tests that at full throttle on 95 RON each time the knock sensor triggers its because your engine has been subjected to a potentially damaging detonation event which has just sent huge shockwaves through the piston and rod etc and that over time you may well increase the chance of the rubbish quality cast pistons breaking up as a result.

So while its quite interesting to do, I wouldnt really be up for doing the same testing myself if its a car you care about and you are likely to drive hard.



This is the sort of end that the pistons are likely to end up at prematurely if they experience a lot of det.

Really?

Even though on the filler cap it lists both fuels?

On my 172 Cup the flap says 98 recommended, but there's no recommendation on my 200.

 

[Chip-mk1]

Really?

Even though Renault on the filler cap it lists both fuels?

On my 172 Cup the flap says 98 recommended, but there's no recommendation on my 200.

If Renault list it as 95 RON safe then it shouldnt be an issue mate, apologies, im too used to the earlier cars and wasnt aware they had made that change on the 200.

Although personally even still, I would prefer the added safety of a more det resistant fuel.

 

[TimR26]

I use 99ron VPower all the time but I don't think it gives any mpg benefit.

In December and January were you having the engine running while defrosting the car? Also shouldn't you exclude the lowest MPG tank of 95ron which I'm guessing was your trackday at Goodwood.

 

[Will.]

I use what ever fuel I feel like. If I'm feeling baller than I go for super.

 

[Thrust-Rated]

So if I was to fill my car up next with the pricey stuff. What am I expected to notice? Anything?

 

[warwickstingray]

I find that sort of damage hard to believe from the standard cars map. Maybe from an aggressive remap that has a smaller safety margin by being on the brink all the time.

Should have got my dad to do one of these tables as he only uses his car too and from work. Same journey 5 days a week.

Who knows depends on how you use the car but if you think you'd feel cheated if there wasn't a huge change then don't bother. It says 98 recommended not 98 only.

 

[bmit]

I found similar results in my 197 . Think I figured anything over 7ppl or so more expensive for premium makes the extra mpg more expensive. . Still run on 99 for the improved running at lower revs

 

[KMB]

My 200 Cup has from new and will only be run on 98RON fuel. On all my past cars the increase in economy pays for the difference.

 

[Christopher.]

Nice bit of info there Mark.

 

[uk300matt]

always used vpower in all my cars from my subaru days.
as said,it may be in my head but im sure it runs better & pops n bangs more.

 

[Jordi17]

Thing is you have to remeber it costs more to fill using vpower etc.

 

[Nick172Cup]

Thing is you have to remeber it costs more to fill using vpower etc.

Lol, you are some sort of special breed aren't you!

He's already calculated that the vpower is more expensive and it still ends up being cheaper to run on that than it is normal 95Ron!

 

[Steven103]

Very interesting thread (though all of your posts are particularly interesting!) Hmm, this has ended up a lot of quoting.

it could be said that the cooler weather and denser air could contribute to lower MPG during December and January

No-one seems to have pointed out that it's the other way around? (or am I about to look silly :rasp

Colder air = denser air = higher volumetric efficiency = more power per gram of fuel (as even though the AFR will always be kept at an optimum value therefore fuel used increases, it is outweighed by the fact that engine efficiency is proportional to power, which is proportional to torque, which is proportional to volumetric efficiency)????????????

So you might find that on trackdays you get better economy with higher RON fuel (assuming other aspects of the fuel are equal) but that on motorway cruising it makes no difference at all.

IMO this is the problem. We measure efficiency of a vehicle in MPG because it's relavent to real life. To measure efficiency of a fuel you need Brake Specific Fuel Consumption (how much fuel needed to create X amount of energy) which, although related, is the exact opposite!

So if I was to fill my car up next with the pricey stuff. What am I expected to notice? Anything?

I think you raise another very good point, lots of things absolutely 100% make some difference for better or worse: induction kits, grade of engine oil etc but so many of these things are unnoticable even if theory tells us it's better (or worse).

Thing is you have to remeber it costs more to fill using vpower etc.

You. Are. Incredible.

 

[Steven103]

Apologies, only just realised how patronising "how much fuel needed to..." came across. It was sort of there for those that didn't know, it wasn't aimed at you Chip.

 

[Advikaz]

Naturally the higher the octane the more concentrated the fuel is if you like, therefor the ecu in theory at least should equate for this thus changing the fuel to air ratios. Higher octane = bigger bang.

 

[Dave 200]

Really?

Even though on the filler cap it lists both fuels?

On my 172 Cup the flap says 98 recommended, but there's no recommendation on my 200.

My 200 states both 95 and 98 on the inside of the fuel filler lid.

 

[Oldtimer49er]

Really?

Even though on the filler cap it lists both fuels?

On my 172 Cup the flap says 98 recommended, but there's no recommendation on my 200.

quoted off Renaultsport

What fuel should I use in the petrol versions?
The recommended petrol for all Renaultsport cars (all Clio, Mégane and Spider) is 98 RON. Lower octane fuel (either 95 or 97 RON) may be used although the performance will be slightly affected.

 

[.Kev.]

Lol, you are some sort of special breed aren't you!

He's already calculated that the vpower is more expensive and it still ends up being cheaper to run on that than it is normal 95Ron!

You forget, he's no doubt one of the mongs paying £4k to insure his 182, so every penny counts.

Honestly in all my warm or hot hatches i've put super in, gotta love all the tight asses who want to save a few pence and put the cheap stuff in.

 

[Chip-mk1]

No-one seems to have pointed out that it's the other way around? (or am I about to look silly :rasp

Colder air = denser air = higher volumetric efficiency = more power per gram of fuel (as even though the AFR will always be kept at an optimum value therefore fuel used increases, it is outweighed by the fact that engine efficiency is proportional to power, which is proportional to torque, which is proportional to volumetric efficiency)????????????

No, that doesnt really follow, denser air just means that to stay at the same speed cruising along the motorway for example you now need slightly smaller throttle angles to flow the same MASS of air, and the efficiency of the engine is unlikely to change, in fact in certain circumstances a hotter charge will actually result in better economy.

IMO this is the problem. We measure efficiency of a vehicle in MPG because it's relavent to real life. To measure efficiency of a fuel you need Brake Specific Fuel Consumption (how much fuel needed to create X amount of energy) which, although related, is the exact opposite!

Absolutely true, for endurance racing where you want to minimise pitstops, or racing where fuel weight is particuarly important (like F1) its the BSFC that matters far more than most people realise.
In the context of mpg though, weight of car and aerodynamics etc are obviously a factor, its possible to have a fuel efficient engine that is still not a fuel efficient car.

I think you raise another very good point, lots of things absolutely 100% make some difference for better or worse: induction kits, grade of engine oil etc but so many of these things are unnoticable even if theory tells us it's better (or worse).

Gearbox oil temp can make quite a marked difference to transmission losses, same for engine oil temp for pumping losses, so on short journeys its not just the obvious things like still being "on choke" (or rather out of closed loop for modern cars) there are many other factors that mean you are never going to get good economy on short trips.

 

[Chip-mk1]

Naturally the higher the octane the more concentrated the fuel is if you like, therefor the ecu in theory at least should equate for this thus changing the fuel to air ratios. Higher octane = bigger bang.

Thats simply not true at all.
E85 for example is normally quoted at 113 RON, and yet you require approx 25% more of it to get the same energy released as its got a lower calorific value.

Its very common for people to misunderstand what the RON value means, its just how much pressure the fuel can take before it explodes basically mate, its NOTHING directly to do with the amount of energy the fuel contains.
In fact you can raise the RON of fuel just by adding ethanol to it yourself if you want, but you'll then find that you need to inject MORE of the high RON mixture afterwards, so economy will go down not up.

 

[Steven103]

No, that doesnt really follow, denser air just means that to stay at the same speed cruising along the motorway for example you now need slightly smaller throttle angles to flow the same MASS of air, and the efficiency of the engine is unlikely to change, in fact in certain circumstances a hotter charge will actually result in better economy.

Hmm I suppose the volumetric efficiency argument only really applies to W.O.T where you actually want to ingest as much air as possible. Even then, fair enough, there isn't a rho or an m in that equation...

What sort of scenarios is a hot charge more efficient? Presumably only in the sense that hotter charge requires a wider throttle & therefore less head loss over the TB or are there other reasons?

Finally, and I know I can google this, but whilst you're here ( ) what even is the point in whatever "RON" or "octane" is? From energy principles it's all about Qlhv & from a stoichiometric AFR point of view it's all about the correct mass of air for a certain chemical composition of fuel. Neither 113 nor these E numbers seem to feature joules/gram or the number of oxygen atoms?