Anyone done the maths?
I’ve seen the high octane stuff like V Power for sale for a few pence more per litre. With the current trend for small capacity turbos, does it work out cheaper to use that, assuming more “energy” per litre and a lighter throttle?
Lots of assumptions and I’m no engineer. Just saw it at the pumps today and wondered if it’s just for power in fast cars or whether justified on economy grounds if you’re after high mpg?

IANAPetroChemicalEngineer, but I’m fairly sure that higher octane petrol doesn’t actually have any more energy in it. It’s to do with its resistance to pre-ignition, so it’s good for very high compression petrol engines. Won’t make a jot of difference in an engine that is designed to run on ‘normal’ petrol, I don’t think.

as above, higher octane only relates to an ability to resist pre-combustion. As high compression ratios (aided by turbos) and low revs, this allows more air/fuel to be added before pinking, generating more power.

In theory, the marginal extra power (maybe ~5% on something highly tuned) could result in marginally more economy if you were in a higher gear for longer, but it is highly unlikely that it would be enough to counter-act the cost difference.

if it was cheaper to use, shell (etc) would advertise it as such. They don’t, because the can’t without being sued. That should tell you all you need to know (same as chipping an engine for more economy)

A modern engine, especially a turbo, will adjust ignition timing, boost, etc to the fuel, so you probably will get better economy, but it will depend enormously on the particular car, route, and your driving technique. I personally doubt it will be cheaper, but the only way to know for sure is to try it. I would run a few tank fulls of each fuel on your regular commuting and calculate the cost per mile over the same route, driving in the same manner.

On my most highly tuned motor, I got slightly better mpg out of the expensive stuff. £ per mile worked out the same so I used the good stuff.

http://www.whatcouldbegreener.com/142/fuel-octane-choosing-the-wrong-octane-will-cost-you/

[video]https://www.youtube.com/watch?v=WYlk9C1o0nk[/video]

Not petrol but on an older diesel I found that I generally got a better MOG using shell v power diesel than normal shell diesel. However this was offset by the higher cost of the diesel.

No idea about the petrol version.

Totally different concept with diesel, so the story is not relevant.

Just give it a try. You’re only a fiver out of pocket.

If you are going to notice a difference you will only notice a difference at wide open throttle and high partial throttle loads – on a petrol engine at least. At wide open throttle the engine is sucking in more air and filling the cylinders more, so compression pressures are the highest and knock is a risk/threat. So under these conditions any improvement in knock resistance will be exploited and you will get more power and economy.

However most of us don’t drive everywhere at wide open throttle, in fact some of us probably never get to fully wide open throttle, and we drive on partial throttle for 99% of the time, therefore we’re ‘throttling’ the engine, restricting the amount of air that is sucked into the cylinders, so they are not filled as much. So the compression pressures are much lower, ignition is retarded anyway so knock is not an issue, so the higher octane of the fuel is of no benefit as you’re getting nowhere near the knock threshold. Anyone who grew up driving cars that did knock, they only knocked under very high load at partial throttle where ignition is advanced (usually due to the driver holding too high a gear for too long), or wide open throttle, where you’re filling the cylinders and advancing ignition, and you stopped knocking by pulling back on the throttle.

With diesel engines it is less clear as the engines are not throttled so under all conditions you are getting lean combustion, rich combustion and stoichiometric combustion happening simultaneously within the cylinder under all conditions. So you should be getting some benefit of higher octane fuels all the time, but it is probably not noticeable or measurable and certainly wont square the business case to use the more expensive premium fuels.

Despite this I confess to using the premium fuels in both our cars (a petrol car and diesel van), not because of any potential fuel economy or performance benefit, but because (or so the marketing bumf says) the fuels include various additives that clean engines, so my thinking is that if you keep the engine clean and free from carbon build up then it is going to run more efficiently for longer and the business case MIGHT stack up over a long period of time. I have no idea if this is true or not, it’s a leap of faith – and potentially falling for the marketing bumf, but I know of a few people, especially with diesel cars, who have had issues with injectors and high pressure fuel pumps due to carbon build up with expensive fixes, so I guess I’m trying to avoid anything of this nature.

Compression changes according to throttle opening? Are you kidding me?

Diesel doesn’t have an octane rating it has a cetane rating. This indicates how well it ignites under compression. So higher cetane basically just burns better. The effect does not depend on the tuning of the engine.

Wobbli

I’m another use the “good stuff” diesel user for same reasons.

For the extra £10-15 a month it seems a worthwhile chance.

On the petrol I used to use the higher RON fuel in my 306gti for track days. I used to use it on the road sometimes too but I never really noticed a real world difference on road. Haven’t bothered with any other car as the throttle is rarely truly wide open.

Tescos 99 used to mske my mini go like the clappers.

Odd, I heard the Tesco high octane stuff was both cheep and really good too.. there’s a car nut just down the road from me (Ferrari something, Porsche somethingorother speedster, massive Merc 4×4 with ginormous wheels and rummbly engine) who I bumped into at our local Tesco petrol station (I normally use Shell) and he was filling his Ferrari up with that Tesco high octane stuff..

If you adapt your driving to use less throttle then you benefit from the improved MPG. In regards to economy you will probably spend more to cover the same mileage as the cost of the premium fuel is usually more than the miles you gain. If you can go through the inconvenience of adding you own fuel additives to regular fuel you will benefit from increased MPG and miles per £

Compression changes according to throttle opening? Are you kidding me?

No. Compression ratio doesn’t change, but compression PRESSURE varies to throttle opening in a petrol engine. If you have more stuff in the cylinder at BTC then the actual pressure at TDC will be higher.

http://www.carbibles.com/fuel_engine_bible_pg3.html

use the fuel that the car handbook says to use.
I expect that to be either 95 or 98 but can use 95 too.

Not sure what these new fangled Euro6 small engine turbos prefer, btu the manual should say.
Mine is an oldschool turbo and says 98 is preferred.

On a statistically significant sample of 1 return journey to the Alps, I got 7% more economy on the autobahn with 98 one way than i did with 95 going the other (edit: with temperature below 3C so no aircon either way to mess up the figures). So if it’s all about pence/cents per litre, then is 98/V-Power etc. 7% more expensive than the normal cheap stuff?

I wouldn’t waste time or money on the fancy 98 stuff unless the manual says so.

Compression changes according to throttle opening? Are you kidding me?

Yep – have a look at volumetric efficiency. The static CR doesn’t change but the compression in the cylinder does.

<righteous brothers>You’ve lost that cammy feeling, now its gone gone gone oh oh oh dawoop dawoop</righteous brothers>

Engines come on the cam because the gas flow is starting to be consistent and therefore the effective compression is getting close to the static.

FWIW its worth my 60’s min, running about 10.5:1 CR and a light fly won’t shut off on regular. It just runs on with the heat in the cylinders igniting the fuel. Super is much more controlled in the burn so the key works and you can switch off.

I found that I used to get slightly more economy, but this was offset by the extra cost.

One thing I really noticed was a reduction in engine noise.

Does reduced noise mean less engine wear? I am not sure.

My car gets regular diesel until it starts to feel lumpy at slow speed from cold, then it gets fed a tank of vpower which clears it up for a few months.

My motorbike gets fed the good stuff always, but then it is a small two stroke that makes somewhere between 200-240hp per litre, and has been lightly tuned, so will most likely see the benefit of the high RON. It also used to be the case that VPower didn’t have ethanol in it, as you could balance the ethanol content across your supply, which made it better again for old bikes with carbs and metal fuel tanks (ethanol is the devil for absorbing water), I don’t think that’s still the case, but old habits…

The increased compression thing – is it because of the momentum the air has in the intake?

Despite this I confess to using the premium fuels in both our cars (a petrol car and diesel van), not because of any potential fuel economy or performance benefit, but because (or so the marketing bumf says) the fuels include various additives that clean engines, so my thinking is that if you keep the engine clean and free from carbon build up then it is going to run more efficiently for longer and the business case MIGHT stack up over a long period of time. I have no idea if this is true or not, it’s a leap of faith – and potentially falling for the marketing bumf, but I know of a few people, especially with diesel cars, who have had issues with injectors and high pressure fuel pumps due to carbon build up with expensive fixes, so I guess I’m trying to avoid anything of this nature.

Me too,

I don’t know the science behind it but what I do know is that my car is getting on a bit, only gets driven short city journeys and sometimes has a lot of issues with juddering/loss of power and general sluggishness. This gets a lot worse when I use generic supermarket fuel, and a lot better when I use Shell/BP etc, and even better when I use Shell V power or equivalent.

Are we talking about premium fuels, or octane ratings?

[Quote]At wide open throttle the engine is sucking in more air and filling the cylinders more, so compression pressures are the highest and knock is a risk/threat[/quote]

Surely that only applies to a turbo, or supercharged engine? Otherwise there’s nothing to force more air into the cylinder?

Back to the OP, it’ll likely work out much the same in terms of cost. You certainly won’t end up saving loads of cash.

edit

This gets a lot worse when I use generic supermarket fuel, and a lot better when I use Shell/BP etc, and even better when I use Shell V power or equivalent.

I know that this has been done to death before but as far as the supply is concerned all the fuel in the UK comes from a limited number of places. E.G. if you are in Scotland it probably came from the grangemouth refinery no matter what the label says.

This is interesting, but the website’s awful-

http://mototuneusa.com/think_fast_intake_porting.htm

Most normal cars have knock sensors which retard the ignition away from a default safe setting if they detect pre-ignition. A very few cars are able to advance the ignition beyond the safe setting until knock is detected to take advantage of higher octane petrol (Subaru WRX etc i think).
Thats my understanding anyhow 🙂

You’ll be able to run a more efficient ignition map, and your engine may adjust for this on the fly based on sensors.

There may be more cleaning additives etc. which could make it worthwhile to put in every now and again to keep things working better, and the engine working more efficiently, although the marketing suggests that the normal Shell stuff does this too.

I would be very surprised if the efficiency benefits made it worthwhile at 5p extra a litre (even if your engine will adjust the map for the better fuel), and my fuel economy is far too affected by weather, mood, type of driving and how much of a rush I’m in, to get a good comparison.

I know that this has been done to death before but as far as the supply is concerned all the fuel in the UK comes from a limited number of places. E.G. if you are in Scotland it probably came from the grangemouth refinery no matter what the label says.

Well what’s going on then? I’m not imagining this. Could the garages themselves be diluting it or something? If all the fuel in the UK is the same, how is it possible that I can fill up from one place, and be running rough until the next lot starts coming through where I filled up at a different place?

how is it possible that I can fill up from one place, and be running rough until the next lot starts coming through where I filled up at a different place?

Honestly I’ve no idea. It might just be coincidence it might be related to better or worse storage at those specific locations rather than branding or it might be something else entirely.

On a slightly more flippant note (and I’m paraphrasing someone from on here) it is worth remembering that neither Shell or Tesco operate oil refineries in the UK.

It doesn’t need to be a clever engine- my motorbike had carbs and a crank position sensor and that was pretty much it but still got marginally better mpg on premium pump fuel. I don’t think that was octane, more likely it was the washing up liquid or whatever it was that they put in it. It had pretty wild cams and a bunch of ignition advance mind but it ran fine on standard uk pump.

But, it wasn’t as big an increase as the price increase

It’s mixed to spec at the storage place, so even if it came from the same place it can still be different. Don’t they also mix additives when they put it in the ground at the filling station?

Surely that only applies to a turbo, or supercharged engine? Otherwise there’s nothing to force more air into the cylinder?

Cylinders are accessed via Valve in ports and from manifolds – all of these are of a constant size and therefore will have an optimum flow.

Below this flow you will get a less than perfect cylinder fill (imagine the air in the manifold stopping and starting, then as you get going it becomes a continuous stream or flow) thats what changes the volumetric efficiency. — you can do stuff to change this variable valve timing, additonal plenums / or intake runners.

Cylinders are accessed via Valve in ports and from manifolds – all of these are of a constant size and therefore will have an optimum flow.

Below this flow you will get a less than perfect cylinder fill (imagine the air in the manifold stopping and starting, then as you get going it becomes a continuous stream or flow) thats what changes the volumetric efficiency. — you can do stuff to change this variable valve timing, additonal plenums / or intake runners.

Or more simply, if the throttle valve is only part open, the intake manifold is under a vacuum, so most of the time the engine is seeing considerably less than atmospheric pressure.

how is it possible that I can fill up from one place, and be running rough until the next lot starts coming through where I filled up at a different place?

confirmation bias

Did the maths a few years back so would depend on the price difference now but over 3 cars, 106gti, e46 330i and 20v 1.8t a3 is saw Super ( whatever the BNP one was) as break even in cost but cars went better on it and saw 5-10% Difference in economy most notable on the 330i

Another upside on the 330i was it used less oil on super, went from 1/2 litre every few 1000 miles to 1/2 litre in 5000 miles so was obviously making the engine a happier place

My car’s manual strongly recommends using 98ron, but can use 95 if necessary. It does seem happier on Tesco 99 ( but this may be confirmation bias I will admit) so I am happy to spend what is less than a cost of a beer every third week extra on it. I only do about 7000 miles a year so the difference isn’t too significant

AFAIK regular petrol in the UK is all the same and can be manufactured pretty much anywhere by any refinery for any brand to use. I seem to remember though that someone on here was a fuel engineer and claimed that vpower was synthesised from an entirely different base gasoline.

I rarely drive anywhere with much focus on economy, so I can’t really answer the original question though I do notice significant differences in behaviour in my cars and motorbike. My cinq is normally aspirated with a simple batch injection system. It has no knock-sensing capability. It makes no difference what petrol goes in it. My 500 abarth is quite significantly turbocharged (~1.8bar at full chat). It behaves better with 99 ron in, but only from a responsiveness point of view, so lag is reduced and boost appears to be held for longer.

Where I see significant differences is on the motorbike. Like northwind’s its carb fed with simple electronic ignition. Fed on higher octane (vpower, or when touring aral’s 102) it has much more low-end torque, is far less likely to bog down and during the winter its annoying carb icing issues simply disappear. The downside is that if I can’t get v-power and fill up with someone else’s fuel, it leaches out of the plastic tank like a leather canteen in the desert. I found bubbles and condensed fuel trapped beneath the tank stickers last weekend!

how is it possible that I can fill up from one place, and be running rough until the next lot starts coming through where I filled up at a different place?
confirmation bias

Although it’s true that all the petrol will come from 1 or 2 refineries, the additive packs are different and can significantly affect things (detergents, octane boosters etc.).
I would guess confirmation bias is the answer for this though, the engine should run fine on any fuel of the appropriate spec, and if it’s not running right some of the time, something’s wrong with it.
Even if you were getting sediment in your fuel or something from one garage, this would clog up your fuel filter and you would still have probs on the next tank.

Mine gets worse MPG on posh petrol. No idea why. Its a 2009 Focus.

I seem to remember reading somewhere that Tesco 99 achieve the high octane by adding bioethanol into it.

I noticed in france this summer that everywhere sells SP-E10 which is 10% bioethanol. Much cheaper. But again worse MPG