Splashalot
10-07-2007, 08:58 PM
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.............
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.............