After reading time and time again that engines using 9:1 compression cylinders should run on super unleaded, I decided to fill up with it the other day. I've used standard for years with no ill effects, but figured it was worth a squirt.

After using the van for a whole tank worth of super, I've gotta say that I've noticed some significant differences - mainly that my fuel consumption has increased, the engine noticeably stinks of petrol when it first starts, and a whopping great cloud of smoke billows out of the exhaust every third start-up or so.

Why exactly do people say you should use super unleaded...?

That ain't down to the difference between the two fuels. Can't be. It's gotta be coincidence, possibly a leaking carb float.

The only difference between the two is that one ignites slightly easier than the other - 95, strangely, is the easier, which is why it's more prone to "pinking" - it pre-ignites, through the stress of compression (like a diesel), rather than waiting for the spark to say "Boo!".

If your engine's timed to run on 98, it's liable to pink on 95. If it's timed to run on 95 without pinking, there's no point in using 98. I've found that French 95 pinks in cars that don't pink on UK 95...
Changing the timing from a 98-friendly setting to a 95-friendly one will reduce power. Reduced power MIGHT increase fuel usage, because you might find yourself using a bit more throttle to keep up with traffic.

Adrian,
I'd agree that a change from 95 octane to 98 cannot have been the cause of the problems described, however I'm not so sure about your reference to 'pre-ignition'.

Detonation: Detonation is the spontaneous combustion of the end-gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug.
The initial combustion at the spark plug is followed by a normal combustion burn. For some reason, likely heat and pressure, the end gas in the chamber spontaneously combusts.
The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug.

Pre-ignition: Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition.
The two are completely different and abnormal phenomena.

ken

p.s. An interesting dissertation here, from which I've learnt why the term 'detonation' is used.
All to do with subsonic vs. supersonic deflagration, it would appear...

http://home.earthlink.net/~tszekely/id60.html

I find it takes about a tanks worth of super to get working, on the second tank full I can feel a difference in power and the engine seems to run better. I find if I drive slowly the fuel consumption greatly increases but that doesn't happen very often. Also I've tried all the different fuel stations, the best is BP ultimate and the Total stuff is good too. Tesco super is shit my car doesn't like it at all but I think that was/is 99. Adrian said about French fuel and I agree. I only use super in France.

Standard gas for me boys

Im to ignorant to notice any difference

ive tryed the expensive fuel and the cheapest no difference afaik
mind, i only potter along on the first choke hey, its a 2cv!

How would pre-ignition happen in a 2cv engine. What can possible ignite the fuel/air mixture before the plug? The compression wouldn't. If it did, timing at 8 dg plays no significance.

If there's a lot of carbon build up this can glow and cause pre-ignition and/or run on.It does not seem to be common practice these days to decoke an engine. A dose or two of redex used to shift a lot ,also there used to be a liquid decoke you squirted down the plug holes and left over night.The start up in the morning would produce clouds of smoke.I used both these method many years ago with out any detrimental effect!!

A quick Google found this ----
http://www.ecotek.eu/PowerBoost.htm

There are several possibilities;
A spark plug which has a heat range that's much too hot.
( Folk can get confused by the fact some manufacturers use a lower number for a colder plug whereas others use the opposite arrangement.)

Very heavy carbon buildup, to the extent that a part of it can act as a glowplug, although that's very rare now that MOT tests will take smokey engines off the road.

Badly fitted helicoil/plug insert which protrudes into the combustion chamber.
Not very common, but I've found a couple over the years, one of which wrecked both the cylinder head and piston and barrel on that side.
If the timing's way out, it's usually both sides which suffer...

I use super unleaded in all the cars when in France as the standard unleaded is E10 and it's usually only a cent dearer.

Over here the 2CV only has 95 but the fiat always has 98 (until I had it six years ago it had only ever run on leaded 4* (half a tank full a year!). The merc. usually has 95 but occasionally (if I have an extra shell points voucher) put in 98.