98 & 95RON Fuel Test

[Splashalot]

UK-based, but still an interesting read:

EVO 087 - January 2006

Petrol Wars; WOULD YOU, SHOULD YOU?

Tesco has just released the highest octane pump fuel you can buy. But is it any good? Does it really matter what petrol you put in your car? Is it worth paying the extra for a premium brand,
high-octane unleaded such as BP Ultimate or Shell Optimax, or will your car perform just as well on regular, considerably cheaper 95-octane unleaded? Newly launched Tesco Super Unleaded 99 Octane could offer the best of both performance and value.

To find out, we rolling-road tested a total of eight petrols in two cars. Four were regular 95-octane unleaded - Shell, BP, Tesco and ASDA. Three were super unleaded - BP Ultimate (97 octane),
Shell Optimax (98 octane) and Tesco Super Unleaded (99 octane), the highest octane pump fuel available in the UK. Our final petrol was Sunoco GT race fuel, a 105-octane brew that can be purchased - at vast expense - in 25-litre cans and which we have included principally to see where the upper limit of each of our test cars' adaptability lies.

Test mule one is a 2500-mile-old Volkswagen Golf GTi - 2-litre, light-pressure turbo, four-cylinder engine, six-speed manual gearbox, front-wheel drive. Test mule two is a 9300-mile-old BMW M5 -
5-litre naturally aspirated V10, seven-speed automated manual, rear-wheel drive. Both manufacturers recommend the cars are run on 98-octane petrol but say that 95 is acceptable if 98 is not available. Both have adaptive engine management systems that should be able to detect between various fuel grades and adjust to make the most of the fuel in the tank.

All UK pump fuel conforms to British Standards, BS EN 228 for 95¬octane unleaded, BS 7800 for higher octanes (including Sunoco GT), so a car configured to use either should have no issues using any petrol purchased at any UK service station. What we wanted to discover was whether there was a measurable difference in power and torque between our eight fuels and also whether there was any perceivable difference in the performance - smoothness, throttle response, low-down urge, top-end zing.

The Dyno Dynamics rolling road at Silverstone-based WRC Technologies produces representative flywheel figures using proven correction factors for transmission losses. The readings are also corrected for ambient temperature, pressure and humidity. All fuel was purchased at filling stations within a 40-mile radius of Silverstone. To ensure minimal contamination between batches, each test car was run until its onboard computer showed a range of 3 miles or less, filled with 10 litres or more of the test fuel, then run down to a range of 3 miles or less again and filled with 20 litres or more of the same fuel. To allow the car to adapt to the fuel, it was then driven hard, allowed to stand overnight, cold-started, driven hard again, and then tested on the rolling road dyno. Any perceived variation in drivability and power and torque delivery was noted.

So, is there a real difference between regular and high-octane unleaded, or between branded and supermarket petrol? Read on ...

ANALYSING THE RESULTS

There were surprises, the first being that on all fuels both cars performed beyond their makers' claims. Against a quoted 197bhp, the Golf gave a best of 212bhp (+8 per cent) and against a quoted 501 bhp the M5 gave a best of 534bhp (+ 7 per cent).

More astoundingly, on pump fuel the Golf's torque peaked at over 250lb ft, 20 per cent up on VW's claim, and gave a minimum of almost 220lb ft. It would seem that German car makers' claims are rather conservative.

The second surprise was that it didn't seem to matter what fuel the MS was running, the spread of power and torque figures being very small. We look into this in greater detail on page 127.

Happily, the Golf showed a clear distinction between the eight fuels - and they delivered tangible differences on the road, too.

THE MANAGEMENT- HOW THE GTi ADAPTS TO THE FUELS

Any engine's output depends in part on ignition timing - the number of crank degrees after the piston has reached the top of its compression stroke (a.k.a top dead centre) that the spark fires. The greater the retardation, the lower the peak combustion pressure and the lower the output. Trouble is, if you combine advanced (near-TDC) timing with a high compression ratio, you hit the knock limit - the point at which the mixture explodes rather than just burning very quickly. The result is 'pinking' which can soon result in a holed piston.

There are two ways of operating close to the knock limit (therefore at the highest possible efficiency and output) without putting the pistons at risk. The first is to run an engine on the bench and determine its knock limit at various speeds and loads, Then you can set the ignition timing of production engines to remain below this (by mapping it into the management electronics).

The second way is actually to sense when the engine is approaching the knock limit. A big advantage of this approach is that it works, and maintains maximum engine efficiency, more or less regardless of fuel RON - it is 'adaptive'. It will even guard against the detonation-provoking effect of combustion chamber deposits in a 'dirty' engine.

In turn, there are (at least) two ways of doing this. Most common is to use one or more knock sensors - highly sensitive accelerometers attached to the cylinder head, which can 'feel' the changes in pressure pattern which mean knocking is close. Signals to the management unit then call for the ignition timing to be retarded. The GTi uses two knock sensors for this purpose. JD

THE M5 - TOO CLEVER?

The MS has arguably the most sophisticated engine management system in production and should, therefore, be able to distinguish between the weakest and strongest performing fuels as suggested by the Golf, but in our hands it didn't. BMW GB tested the car on its own Sun/MAHA rolling road and claimed to see 501.5bhp on a regular 95 and 515.5bhp on Shell Optimax but we didn't see any appreciable difference between any of the fuels. The power and torque traces for all our sample fuels are so close that they are effectively the same. Take the torque curves (below) - at 4500rpm there is just 10lb ft between best and worst, or less than 3 per cent - an amount that would be, and was, undetectable on the road,

BMW GB was as baffled as we were, particularly as the factory recommends that the MS is run on a diet of 98 octane. Like the Golf, the MS performed beyond its maker's claims, even on the 'weakest' fuel as determined by the Golf (our best figure was 534bhp, on BP Ultimate). We wondered if the M5's sophisticated engine management was adhering to an upper torque limit, set by some component either in the engine or the drivetrain, thus pegging all the tested fuels to the same level. If this is the case, our guess would be that the SMG paddle-shift manual gearbox is the weak link. Select maximum power and the fastest shift speed allowed and the first-to-second upshift hits home with a neck-jolting thump that can't do the drivetrain much good.

Perhaps an example of the V10 producing close to, or less than, the quoted 50lbhp and 3831b ft would have shown the anticipated gains on the higher octane fuels. Without finding one, we can't say, but in our hands this well-run-in M5 went just as well with its tank filled with the lowliest 95 as with I05-octane race fuel.


HOW THE M5 ADAPTS

BMW employs a sophisticated alternative to regular knock sensors - it uses the spark plugs themselves as sensors. Even before the spark happens, as the voltage builds up, the air in the electrode gap 'ionises' - its physics change to allow a current to pass through it. With the latest and fastest electronics, the rate at which ionisation takes place can be sensed - the resistance of the air between the electrodes can be measured. As the knock limit is approached, ionisation is faster. By detecting and measuring this, the engine can be kept just below the limit. Because of its accuracy and speed of response, it allows the M5's V10 to be run even closer to the limit than the Golf's turbocharged 'four', and thus at even higher efficiency. Its big drawback, needless to say, is cost. ..

Note, however, that there is a limit to any (naturally aspirated) engine's output. If it can run at its most advanced ignition timing without knocking, then the mean combustion pressure (BMEP) is determined almost entirely by the compression ratio. That's why old, low- compression engines - assuming they can tolerate lead-free petrol - are more than happy to run on 95 octane, and in many cases on 91 octane (known as 'regular grade gasoline' in the US). Using higher-octane in these engines would be a total waste.

TBC.............

[Splashalot]

Part 2:

The fuels tested:

SHELL 95

Price paid 91,9p/litre
Max power 205bhp @ 5600rpm
Max torque 234lb It @ 2650rpm

Rated slightly below BP 95 because although it's power-peak was 1 bhp higher, BP delivered significantly stronger torque throughout the low and mid-range. Driver notes: 'Definitely seemed to be lacking an edge after Ultimate. [Fuels were not tested in a set pattern; Shell 95 was tried after BP Ultimate.] Delivery seems softer, there's less punch from low down and less response in the higher gears too.'

BP 95

Price paid 89.9p/litre
Max power 204bhp @ 5600rpm
Max torque 242lb ft @ 2300-3100rpm

A consistent performance. Lots of low-end torque, which is sustained until beyond 3500rpm, Fades a little further up the rev-range but still stronger than Shell 95 right up to 5000rpm, Driver notes [tried after Tesco 99]: 'Not as punchy and there's much less manic flashing of the traction control light. Foot to the floor more often out of corners - doesn't spin its front tyres up as much. Still felt quick though.'

Supermarket 95 - What you’re buying.

Supermarket 95-octane couldn't be included in the final results for the reason that supply is not consistent, so testing a batch would merely be a snap-shot. A spokesperson for ASDA, which sells
unleaded at just 85p per litre, told us that it draws fuel from no fewer than seven suppliers, including Esso, Texaco and BP. Tesco is similar, although 200 of its 380 filling stations stock a part bio-ethanol unleaded 95 octane which, like Tesco 99, is supplied exclusively by Greenergy. These are in the south east and a small area in the north west. For the rest, as with ASDA, it's a bit of a lottery as to what you get. Out of curiosity, we did test 95 from our local ASDA [Corby] and Tesco stores in Northampton and Wellingborough which, according to a spokesperson, might have been bio-ethanol as these branches do get it 'but not 100 per cent of the time', The ASDA fuel [which we were told was drawn from an Esso depot] was storming stuff and a match for BP 95, Tesco 95 was on the pace power-wise but up to 30lb ft down on torque.

SHELL OPTIMAX

Price paid 95.9p/litre
Max power 209bhp @ 5500rpm
Max torque 242lb It @ 2500-2900rpm

Shows useful gains compared with Shell 95. Compared with other higher octane fuels, it's on a par with Tesco 99 low down but lacks its mid¬range [and that of BP Ultimate) with torque fading slightly earlier. Driver notes [tried after 1st batch of BP Ultimate): 'The Golf suddenly discovers a top end - you want to rev it right to the red line. Of all the fuels, the GTi felt the crispest and most fun to drive on this. It would be my choice.'

Tesco 99

Price paid 93.9p/litre
Max power 212bhp @ 5500rpm
Max torque 242lb ft @ 2400-3500rpm

Highest octane rating of the pump fuels didn't quite give the top results. Massive gains over our sample of Tesco 95. Doesn't deliver like BP Ultimate low down but then gets ever closer before matching its power and staying stronger for longer, A fine result, and it's part bio-ethanol. Driver notes [after Shell Optimax]: 'Mega mid-range but feels a little laboured to the redline. Somehow less clean feeling, sound was less positive, too.

BP ULTIMATE

Price paid 100.9p/litre
Max power 212bhp @ 5300rpm
Max torque 252lb ft @ 2400-2500rpm

Level-pegged with Tesco 99 on peak bhp but significantly out-performed it and every other pump fuel in terms of torque [and even matched the race fuel into the mid-range!. Very strong right from the off and stays ahead of the others right up to 5000rpm, Driver notes [tried after a 95]: 'Explosive low down - really quick on boost and sharp response, too, Woolly top end, though - the last 1000rpm is a struggle. Strange'

SUNOCO RACE FUEL

Price paid 352,5p/litre
Max power 218bhp @ 5200rpm
Max torque 248lb It @ 2500-2800rpm

Mixes it with BP Ultimate initially, but beyond 3500rpm it's well clear of the pump fuels, as expected. Storms to highest peak power, though the flat topped curve suggests it's capped. So costly, only justifiable in a race car (with a high compression ratio], where a second a lap is a lifetime. Driver notes [tried after 2nd batch of BP Ultimate): 'Felt like it kicked hard low down and then kicked again at about 4000rpm, when it just took off.'



FUEL Q & A - TECH ED JEFF DANIELS ANSWERS THE BURNING QUESTIONS

Q What is petrol?

A Pure petrol is a blend of hydrocarbons, each one combining hydrogen and carbon in a particular ratio. Very small quantities of various other chemicals are added as a 'package' to prevent stored petrol from deteriorating, to keep the fuel system and combustion chamber clean, and to discourage throttle valve icing, 'Green' petrols include a proportion of alcohol [usually vegetable-derived ethanol].

Q If all forecourt petrol has to conform to British Standards (BS EN 228 for 95 regular unleaded and BS 7800 for super unleaded) how can different fuel suppliers claim their fuel to be better than others?

A All standards are minimums, So long as the product conforms to the standard, manufacturers can seek land advertise superior quality, for instance by tweaking their additive packages or adjusting their hydrocarbon blends.

Q 'Better' petrols have been launched in the past - Formula Shell, BP Advanced - but didn't catch on. Why have they come good now?

A Some of the 'better' petrols of the past caused problems in particular engines, and word got round. It was probably time someone had another go, and the fuel companies seem to have learned, in marketing and technical terms, from earlier mistakes, fortunately, most of the 'problem' engines are now obsolete.

Q What are 'detergents' in fuel?

A Just that. Like the detergents in your washing machine, they ease dirt out of crevices in the fuel system and engine, and keep them in suspension to be flushed down the exhaust.

Q What sort of engines stand to benefit most from higher octane fuel- small or big, forced or natural induction?

A Any engine will benefit from high octane fuel if its compression ratio is high enough to take it to the knock limit on 95. Many now use knock sensing and retard the ignition to stop it happening - at a cost in performance and efficiency. This enables them to run happily on 95, but they will perform better on 98. The management systems of turbo engines take the knock limit into account via the wastegate or blow-off valve setting [or by controlling the variable geometry of the turbo]. The golden rule: if in doubt, consult the handbook. The rule of thumb: almost any four-valve engine will do better on 98.

Q Are there any engines that won't benefit from higher octane fuels?

A Yes, plenty, but mostly old designs with low compression ratios. There's no point in feeding a Morris Minor 98, for example, But remember some 1970s cars were designed to use high-lead fuel, up to 101, which was then available, The worry with a really old car is whether it will knock out its valve seats when using unleaded fuel ...

Q Why do some car makers strongly suggest 98 octane is used in their performance models - will a constant diet of 95 harm the engine in any way?

A So long as the management system can adapt, there should be no harm in using 95, but you will suffer a performance and efficiency penalty. The handbook is the best guide, Car manufacturers have no vested interest in persuading you to buy needlessly expensive higher-octane petrol. If they recommend it, use it.

Q Tesco 99 octane unleaded is 'up to five per cent' bio-ethanol. Is this what helps it to its higher octane rating? How is it made and are there any drawbacks to bio-ethanol?

A Pure ethanol has a very high octane rating - well over 100 - because it contains oxygen as well as hydrogen and carbon, and this helps to cool the charge entering the cylinder, Even with only 5 per cent ethanol, this is enough to give a lift to the overall fuel octane rating. Ethanol can be made from a variety of plants, mashed, strained and distilled - in effect, the same process that's used to make whisky, brandy, vodka, calvados, etc, etc, Ethanol in high concentrations can attack rubber and plastics in the fuel system, unless the car has been properly adapted, but five per cent appears to pose no problems.

Q Why do some high-octane fuels have a reputation for 'going off', if stored for too long?

A Any petrol will 'go off' if stored for too long. Some of it will oxidise, and some will react with metals that may be around, even in small quantities. High-octane fuels tend to contain particular 'cracked' hydrocarbons which oxidise more readily. Good fuels contain additives to slow the process, but they can't stop it altogether, Thus it's probably best to buy fuel at a big, busy forecourt where the throughput is high and the petrol is unlikely to be older than the last delivery but one.

Q Would you expect to get more mpg from a higher octane fuel (ie. can you offset the higher purchase price with better range)?

A So long as your engine's able to benefit from high-octane fuel [see earlier answer], and has an 'adapting' management system, then you should. The ability to run more advanced ignition improves efficiency as well as maximum power, and not just at the top end. Remember the knock-limit benefit isn't at the top end so much as around maximum torque, My day-to-day Honda Jazz typically does 46mpg on 98, and 44mpg on 'regular', That's nearly a five per cent improvement, and some high performance cars will likely benefit more.

[rayray086]

Decent read, though the driver comments would be biased and largely affected by what the car was running beforehand.

Oh, and why do they use adjusted power figures at the flywheel rather than raw power values at the wheels?

[brackie]

Thanks... As Ray said, "A decent read".

Interesting that they chose 2 German cars for their tests .

I guess I would draw 3 main conclusions from the article:

1. BP (for example) fuel is a better bet than "supermarket fuels" because of it's consistency
2. ethanol blending gives more octane
3. you get what you pay for!

The description of knock sensing and especially the way that BMW is doing it, just goes to show how modern electronics can even out many of the variables that affect fuel performance.

[syncro]

Lots of errors.
Remember it has been written by a journalist, not an engineer.
After top dead centre?????