HI OK TAIN PETRUL

[Spiny Norman]

Possibly the most pointless post in the whole thread, but I just did a quick bit of sums and discovered that if you travel by Jag XJ6 it's not worth driving 10 miles to save 5p a litre on fuel unless you're putting in more than 10 gallons, and I haven't bought more than 10 gallons in one go since 2008.

[forddeliveryboy]

Don't think I was driving when 2, 3, 4 and 5 star choice vanished, but I still miss it in an absurd way. There was something simple and enjoyable about it, which has never been replaced by the stark choice of 95 or 97. I seem to remember in Germany in the early 90s there was a massive choice of petrol with sub-90 octane freely available. It was almost a holy grail before the advent of ECUs and knock detection, finding what brand and octane suited, possibly even tweaking carb jets and ignition timing to suit.

 

I remember mixing varying quantities of unleaded (95) and 4* (98?) in 2cvs and Dyanes to find the ultimate performance(!) - the lead addition seemed to have a bad effect on the high-revving flat twin but extra octane meant more advanced ignition. On unleaded only, the performance felt thin but throttle response was much better than on 4* - the right mix of the two found a sweet spot.

 

I made the switch to the dark side when diesels were advancing quickly and petrol engines were being throttled with cats (and electronics were spreading like wildfire) - great days of BX turbos which relied on purely mechanical means for their superb performance and ability to equal a 2cv for economy.

 

In turn this led to abandoning my reliance on the forecourt fuel pumps as I was offered a weekly 20 litres of veg oil by a friend who was working in a pub. The thought of relying on someone else for my fuel - and paying thousands of pounds a year for tax upon tax - fills me with horror now. Even the lawnmower runs mainly on misfuel and kero (with a small dash of neat petrol for easy starting), only the chainsaw is treated to fuel which I have to pay through the nose for, since it uses so little.

[dollywobbler]

Don't forget the 244 is a carburettor. Non efi systems can't automatically compensate for fuel quality. Further research has informed me that you should on no account use fuel with an octane rating of less than 97 in the B21 carburettor engine.

 

Of course in later 240 s with efi this is not an issue

 

My Rover P6B was designed for five star 100 octane, but still happily drank 95RON. I still consider 'super' fuels a brilliant sales wheeze and nowt more. "If it's more expensive, it must be good."

[scooters]

My Rover P6B was designed for five star 100 octane, but still happily drank 95RON. I still consider 'super' fuels a brilliant sales wheeze and nowt more. "If it's more expensive, it must be good."

Not the case with the B21 engine.

Having put 95 in it the car would barely run. Now the 97 is in it the car runs sweetly. I was surprised at this because I was of your opinion.

[scooters]

This is from The Economist and really sums up the whole question in detail. It s long bit well worth a read.

 

EVEN at the cheapest petrol station in your correspondent's neighborhood, filling up the family kidmobile with premium (91 octane) fuel now costs over $70. As the meter clocks up dizzying dollar amounts, he looks longingly at the regular (87 octane) pump. Switching from his vehicle’s recommended premium-grade fuel to the cheaper variety would lower his fuel bill by at least 20 cents a gallon (more than five cents a litre). The question is, would it be worth it?

 

On the surface, the decision appears easy. Because the name “premium†implies a souped-up fuel that packs an extra punch, many motorists actually believe it delivers more oomph or miles per gallonâ€â€and may therefore represent good value. The truth, however, is that premium contains no more energy than regular petrolâ€â€around 114,000 British Thermal Units per gallon, depending on the season, the region, the local pollution requirements, and the amount of bio-ethanol that has to be added to petrol in America by law to keep the country’s corn-growers in clover (see “Competition at the pumpâ€Â, August 20th 2012). The difference between premium and regular petrol lies in the blend of hydrocarbons used to make the fuel, and the package of additives mixed into it. 

 

Nowadays, petrol is made up of hydrocarbons (mainly paraffins, naphthenes and olefins) produced in a catalytic cracker or reformer. The refinery process breaks the crude oil’s large hydrocarbon molecules into smaller ones by vaporising them in the presence of a powdered catalyst (an absorbent mineral such as zeolite). The blend varies depending on where the crude came from, the refinery equipment used, and the grade of petrol being produced.

 

Additives are included to reduce carbon build-up inside the engine, improve combustion, inhibit corrosion and allow easier starting in cold climates. Fuels that meet the requirements for “Top Tier Detergent Gasolines†(a voluntary standard endorsed by BMW, General Motors, Honda, Toyota and Volkswagen) contain more detergent in their additive packages than the minimum required by the authorities.

 

Another key additive that blenders stir into their brew is ethanol. That is done these days primarily to boost the fuel’s octane rating. A higher octane rating allows an engine to use a compression ratio of, say, 12-to-one instead of a more usual ten-to-one. The greater the compression, the higher the temperature within the combustion chamber. And the higher the temperature, the greater the thermal efficiency and power produced. In a nutshell, high-compression engines designed for performance need high-octane petrol.

 

Though ethanol has less energy per gallon than petrol, it has a considerably higher research octane number (RON)â€â€around 108 to premium’s 97. It should be noted that this is not the octane rating seen on the pump in America. The RON figure results from a laboratory test done using a special engine with a variable compression ratio.

 

In the fuel test, the compression is raised until the engine begins to “knockâ€Ââ€â€ie, the fuel in the cylinder ceases to burn smoothly and instead detonates before it can be ignited by the spark plug. The cylinder pressure at which this occurs is then compared with that achieved while the engine is running on a reference fuel (a mixture of iso-octane and n-heptane). The ratio of the two pressures provides the RON of the fuel in question.

 

A better way of measuring a fuel’s ability to resist knocking under load is the so-called motor octane number (MON) test. This uses a similar test engine, but with a preheated fuel mixture, a higher engine speed and variable ignition timing. Because it uses more real-world conditions, the MON rating is typically eight to ten points lower than the equivalent RON figure.

 

In Europe, the octane rating on the pump is simply the RON figure. America, by contrast, uses the average of the RON and the MON figures, called the AKI (anti-knock index). Thus, 97 octane “super unleaded†in Britain is roughly equivalent to 91 octane premium in the United States.

 

Whatever the test, the point is that knocking needs to be avoided at all cost. If allowed to continue, it can quickly cause an engine to disintegrate. That is because when the air-fuel mixture in the cylinder detonates spontaneously before reaching the top of its compression stroke, the rising piston confronts a wall of rapidly expanding gases from the explosion, which attempt to force the piston back down the cylinder. The stresses caused by suddenly trying to reverse the rotation of the engine can become high enough to shatter the pistons, connecting rods and parts of the crankshaft.

 

To prevent that happening, a high-compression engine uses a blend of hydrocarbons that is somewhat less combustible than normal. Ethanol has an auto-ignition temperature of 362ºC, while petrol bursts into flames without a spark between 246ºC and 280ºC, depending on the blend. Therefore, adding a little ethanol to petrol can raise the auto-ignition temperature enough to prevent the blend from igniting purely from the heat generated during compression.

 

On the face of it, then, a motorist would seem ill-advised to use regular petrol in a car with a high-compression engine. That was certainly the case in the past. But cars today have sensors that listen carefully for the knocking sound, and instantly retard the ignition system when they detect that detonation is about to happen.

 

The delay in delivering the retarded spark allows the piston to start moving downward on its expansion stroke before the ignition actually occurs. That provides additional room in the cylinder head for the gases to expand and thereby reduce their damaging peak pressureâ€â€and so burn in a more controlled manner.

 

To sum up, if the car’s handbook says that premium petrol is “recommended†(rather than insisting it is “requiredâ€Â), then the engine will automatically adjust itself to run smoothly on a lower octane fuel. Because of the retarded ignition, the engine will, of course, produce less power, and have slightly higher fuel consumption. But the poorer fuel economy is likely to be outweighed by the savings at the pump.

 

Even so, your correspondent remains reluctant to make the switch. One reason is that no one has been able to tell him what damage is done, if any, by running the engine permanently in a retarded state, and forcing the anti-knock system to remain active all the time.

 

Another reason is because all the vehicle’s emissions testing was done using the recommended grade of fuel. Despite the fact that modern fuel-injection systems adjust the air-fuel mixture for changing conditions, your correspondent still has no idea how much more pollution the car might dump into the atmosphere if he switched to regular. Premium certainly has a better additive package, which helps keep the tailpipe clean as well as the inside of the engine.

 

But his biggest reason for sticking with premium, though, is that he was well aware that the car needed 91 octane to work properly when he bought it. And having paid upfront for the higher performance, he is reluctant now to throw that benefit away.

 

As for those who earnestly believe (and quite a few do) that filling the family Toyota with premium will somehow make it go faster or deliver more miles to the gallon, all one can say is don’t bother. As one wit noted, the only thing it will make run faster is money from your pocket.

[scooters]

So essentially the issue for the chod operator is that if your car has a high compression engine and is pre ecu you might find that it causes bad knocking.

 

Interesting about the amount of additives in modern fuel. The old volvo was built at a time when petrol was less buggered about with. Bring back leaded 4x!

[scooters]

This also explains why the Federation of Classic Car Shitters is up an arms about the amount of bio ethanol being put into petrol in the EU.

[dollywobbler]

This also explains why the Federation of Classic Car Shitters is up an arms about the amount of bio ethanol being put into petrol in the EU.

 

That's a different argument. Ethanol is very good at eating brass and older types of rubber. That's why the Feds are kicking off about it. It also seems to stop petrol and oil mixing, which is bothering the two stroke crowd. If you're engine is knocking/pinking, just retard the ignition slightly.

[NorfolkNWeigh]

When I was a kid ,my dad only put 5 star in the Viscount when we going on holiday with the caravan. But in those days it was the difference between £4.80 and £5.00 for a tankful.

[Pete-M]

I've ran my Focus on poverty unleaded for the last 8 months, until last week when there was a humongous queue at the petrol station which resulted in me shoving £20 of Super Unleaded in as that pump is shunned by the majority of folk.

 

It has made a big difference. Engine is more responsive, smoother and it appears to go a fair bit better too.

[MikeKnight]

I've timed the Triumph GT6 to run on nothing but BP Ultimate (97/98 RON).

 

This was after a head conversion to unleaded. It just runs a lot happier with it. It's also a very clear low ethanol petrol which makes the rubbers happy.

 

As long as you set the timing right you can use anything you want.

[Asimo]

As long as you set the timing right you can use anything you want.

I ran out out of lawn mower petrol and, anxious to finish the job without a 12 mile round-trip to get some more, put B&Q white spirit in the tank (just before the engine stopped). It pinked horribly and would only run flat-out, but I got the job done. Smelled vile!

[PhilA]

Comparing the smell of regular 100LL aviation petrol and the nasty mix of whatever crap they can get away with providing the public to run their cars on is surprising. I've had an accidental mouthful of both in the past. Avgas tastes like cheap whiskey and the taste is gone after washing your mouth out a few times. Can't say that for pump fuels.

 

Either way it's interesting to watch the knock figure on the diagnostics computer on my car, which will change in "percent" (of what I don't know) as the engine is revved up.

 

It's also interesting to put a strobe light on and watch the timing at idle whilst tapping the cylinder head next to the knock sensor- the amount of timing retard and the hysteresis is really quite large. It dials out a lot of degrees and takes a few seconds to return back to normal, so if you have poor fuel I could see the timing easily being ten to twenty degrees further back than it should for optimum performance and economy.

 

Phil

[forddeliveryboy]

My '85 900 turbo 16, one of the first, gave lousy economy on standard unleaded with so-so performance. I seem to remember soemthing like 23 instead of 28 on Super Unleaded. That engine was electronically controlled, with Saab's own APC system.

 

A few years ago in Norway, I met a Traction Avant owner who ran a 50/50 unleaded/kero mix. With a long piston travel and low compression, it was designed to run on 82 ish RON. He said the kero/unleaded gave superb running characteristics - more power, same economy and a much smoother engine. You just had to watch for the sump filling with unburnt fuel if you spent ages in slow traffic, apparently.

 

Any Traction I've ever driven has returned around 30mpg-ish when driven hard or at 70-80mph, 35 on more gentle running. Given the engine is from 1934, a very low compression and the body's hardly slippery by today's standards, you do wonder why today's stuff isn't much better.

[STUNO]

Way back in the 1973's I bought a Morris 1800s, previously owned by a "specialist" truck dealer on North

London. The s bit meant it had a high compression engine and twin carbs, I understand it was MGB spec.

That did not explain why it would only run properly on 5 star and needed an Italian tune up frequently as it hated London crawling. I still believe it had been heavily breathed on as it was a very fast car. The

tune up involved 70 mph+ in 3rd and clouds of black s**t from the exhaust for a minute or two, then it

cleared and the car started to perform and settled down. 3 star induced bad running and stalling with hard starting. Fortunately it was sold before 5 star disappeared.

I also still rate that as the best car I have ever had. And I have had a lot!

[DS20]

The only one of mine that is noticeably happier on superunleaded is the Citroen. It starts more happily, pulls more strongly, and is a bit more efficient (less inefficient) on super than on standard unleaded. The Saab works well on either, and the Range Rover couldn't care less what's in the tank as long as there's lots of it.

[PiperCub]

My experience and I'm old enough to recall 2,3 &4 star leaded is that is really varies from car to car as I think all the above indicates.

My 205GTi 1.6 is sharper and does a couple more mpg on Shell V-whatever-its-called, whereas in my early MX5 doesn't make any noticable difference so I use ordinary Shell 95-type stuff in that. The same goes for the Eunos which is slightly odd as I'm lead to believe that JDM-spec cars are tuned for the 100 octane (so 5star) unleaded that's available in Japan.

 

As a matter of interest, about seven or so years ago, had a XR2i as a smoker, I used to run this on waste Avgas 100LL (again 5 star but heavily leaded despite the LL - Low Lead title) from my local flying club mixed with some 95 unleaded. To be honest, it made no detectable difference to the running/performance of the thing and it was in reasonable condition. The only thing apart from the fact the 100LL was free (it was the stuff that was kept for a week after daily testing) was that it made the exhaust smell nice!!

[e287yba]

Comparing the smell of regular 100LL aviation petrol and the nasty mix of whatever crap they can get away with providing the public to run their cars on is surprising. I've had an accidental mouthful of both in the past. Avgas tastes like cheap whiskey and the taste is gone after washing your mouth out a few times. Can't say that for pump fuels.

 

Either way it's interesting to watch the knock figure on the diagnostics computer on my car, which will change in "percent" (of what I don't know) as the engine is revved up.

 

It's also interesting to put a strobe light on and watch the timing at idle whilst tapping the cylinder head next to the knock sensor- the amount of timing retard and the hysteresis is really quite large. It dials out a lot of degrees and takes a few seconds to return back to normal, so if you have poor fuel I could see the timing easily being ten to twenty degrees further back than it should for optimum performance and economy.

 

Phil

 

The "change in timing" will be 'percentage of dwell', which (from memory, I could be VERY wrong!!) is 100% for 90degrees of timing as far as the distributor is concerned (so 180degrees of crankshaft rotation or 1/4 of the Otto Cycle).

 

When I was a kid (in the later '70s), 4* fuel in the UK had a nice, sweet (even fragrant?) smell - unleaded petrol never ever had this, it's very nasty in comparison (and tastes awful - don't ask!!).

 

Your experiment with tapping the block adjacent the knock sensor is revealing - Maestros and Montegos with the 1.6 (S-series not R-series) and 2.0 engines have such a knock sensor, and they (pre-'89 when the Ignition was re-mapped to be based on the use of 95RON unleaded) would cope with unleaded (with a 'leaded fuel' additive to protect the valve seats) or 4* leaded fuel with no problems except a little bit of occasional pinking and went slightly less well on the unleaded fuel.

 

I believe SAABs also had a similar set up, coping with anything from American 87RON (equivalent to 91RON in the UK, that was the equivalent of ye olde 2*) to Super+ which is 99RON (equivalent to nigh on 5*) but without needing 'leaded fuel' additive to protect the exhaust valve seats...

 

I run the Jazz on Morrisons Unleaded, and have set the timing on the 309SR for the same fuel (it's still fast enough to surprise other road users)... I have currently got the MG Maestro 1600's timing set to suit Morrisons' product as well (it goes well enough, the Jazz - a 1.2 - seems to be as quick if booted!), but it runs better on Sainsburys' Super+ (97RON) with the timing advanced to suit... I am assured that V-Power is the best but haven't tried it (with the further advance to the ignition timing which I am told will be needed for the best results) yet.

 

The 309 runs fine on Tesco Ultimate (99RON) if I am feeling extravagant, advancing the timing to suit makes for a great drive ; the MG hates it and runs like a sulky dog with soiled trousers ...