Your classic lotus probably requires leaded fuel, which burns cooler than modern unleaded. It's possible that your friends have experienced preignition and pinking issues - which is really quite serious.

It shouldn't be an issue here.

The head on our lotus's were all designed to run unleaded, the issue was the poor running when they switched from optimax to Tesco [/quote]

Interesting! Used to run a S2000 and I, among many other S2000 owners, switched from Optimax to 99 tesco stuff and while the general consensus was that the ride was maybe slightly rougher, it improved fuel economy. S2000s are designed to run at 98 or above, but can be run on ordinary 95 stuff though I wouldn't recommend it.

Just to clarify, I don't think it's the fuel per se, but rather the fact that the bike was running lean, and perhaps just a little too lean for the 99RON.

I've tried sticking the baffle on the exhaust, but it's made no difference at present.

The battery is only a few months old, and the starter motor is running nice and strong.

Carb icing I doubt, as I haven't ridden it anywhere yet, this is all in the driveway.

I'll try draining the tank and using some optimax again, check the plugs and then get it dyno'd

Anyone recommend someone in London/Essex? Cheap.

if in any doubt you say you've done 60 miles, dump 6L of your favorite optimax in there, shake vigerously, and drain the carb float bowls so you have nice clean fuel in there?

[/quote]

Two things at play here... your new heads are able to take the hotter engine temperatures of the unleaded fuel, and probably have tougher valve seats as well for the same reason.

However, your engine itself may not have high enough compression to push the higher octane fuel to a sufficiently volatile state, to burn properly.

This leads to misfiring and backfires out of the exhaust as the fuel is ignited down the hot pipes.

Shell Optimax is about 98.5 Octane, depending who measured it, and the batch it was. These other fuels are pushing well into the 99 Octane level, some I imagine, are approaching 100 octane.

Find me a factory car engine that is designed to run this, and I'll be very surprised. It may well be fine in many performance engines, once they've warmed up... but up until then, it's wasted fuel.

Your S2000 has a very advanced engine. It is able to adjust when the mixture is ignited, how much is injected into the cylinder, what mixture is injected, and adjust the valve timing, and the amount they are lifted.

In essence your engine can change its compression ratio.

High octane fuels are not very volatile, they take a lot of effort to burn efficiently - this is usually in the form of very high compression, and a hot cylinder. Your S2000 would have had to ramp up it's compression significantly to burn this kind of fuel - which probably lead to a "lumpier" running engine. However, the rewards are obvious - more power.

If using a lower octane fuel... they burn very easily, but that's the problem - they're volatile. They, like diesel, could be ignited just by the compression of the piston in the cylinder, and ignite before the spark plug has fired. This, obviously, is very bad - hence your VVT&L on your S2000 compensating for the fuel.

I think either top or fifth gear did a test on this, they found as billy says that your normal family hatchback cant make use of optimax and the such like but IIRC they did a dyno run with a Subura Imprezza and its engine management could allow the engine to make use of the optimax and it made a fair bit extra BHP, I cant remember how much exactly, but it was certainly into the 10 - 20 bhp area extra on Optimax or maybe it was the BP version I cant remember.

The figure I read, again for the Impreza WRX, was as much as 40bhp difference between 95RON and Optimax.

It can make a difference though... I used to use it in my 306, not a terribly advanced engine, but Optimax still delivered an extra 60 miles per tank, or nearly 20% extra miles.

Now, as expensive as Optimax is, it's not 20% more expensive - therefore, for that car, was more efficient.

Hi Billy,

Thanks for the info, although i'm interested to know how you've come to the conclusion that my engine has a new head, and doesn't run on a high enough compression to run higher octane fuel, without knowing what is!

Paul