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[Briser]

What is the general consensus regarding the standard cheap supermarket UL as opposed to the dearer stuff like Shell Omtimax?  A guy at work swears by Optimax and reckons it gives a bit of a speed increase as well.  Seeing as how I am no expert, I wonder if its worth the extra cost?

[TheBoy]

The high octane petrols - Optimax, Ultimate etc etc - will give a performance boost and better economy.  Whether or not the extra economy pays for itself is for you to try with your driving style.

Also, these tend to have better detergents, keeping the injectors and combustion chamber in good order...

[Kevin Wood]

Higher octane fuels will give you better performance if the engine is going into knock retard with standard fuel. Although the Omega has  knock sensors the engine is probably not in a radical enough state of tune to get into trouble on 95 RON fuel so it probably won't make a noticeable difference. That's my experience at any rate. The knock sensors are there to protect the engine against a really bad dose of fuel or lower octane fuels that are on sale in other markets.

Engines which are turbo charged or higher compression will generally give much more power on high octane fuel because they need high octane fuel to run on full boost or without retarding the ignition.

So, when an Imprezza driver tells you how good a fuel is, don't expect the same difference in an Omega!

I'm sure there are better detergents and additives in premium fuels as against the cheapest supermarket fuel. It's difficult to know what that actually does for you in practice. The marketing hype will tell you it makes a world of difference but I ran my last car solely on the cheapest supermarket fuel locally available and when I rebuilt the engine at 130,000 miles the engine was in decent condition internally and the fuel injectors still had a decent spray pattern.

I changed that policy when, a few weeks after getting my Omega, the supermarket sold me duff fuel and put my emissions light on!

I would say if you're concerned don't run it 100% on cheap fuel, try a tank of Optimax every now and then. If it doesn't improve performance it will, theoretically, at least, clean your fuel system.

One other tip I'd give is, if you end up at a petrol station at the back of beyond where the yokals aren't likely to drive performance cars and they sell super unleaded buy the 95 RON instead. In my other car (which needs super unleaded and has no knock control) I've had tanks of duff fuel in such places because it simply doesn't turn over quickly enough. The additives in a fuel evaporate off while it is stored.

Kevin

[TheBoy]

Higher octane fuels will give you better performance if the engine is going into knock retard with standard fuel. Although the Omega has  knock sensors the engine is probably not in a radical enough state of tune to get into trouble on 95 RON fuel so it probably won't make a noticeable difference. That's my experience at any rate. The knock sensors are there to protect the engine against a really bad dose of fuel or lower octane fuels that are on sale in other markets.

Engines which are turbo charged or higher compression will generally give much more power on high octane fuel because they need high octane fuel to run on full boost or without retarding the ignition.

So, when an Imprezza driver tells you how good a fuel is, don't expect the same difference in an Omega!

I'm sure there are better detergents and additives in premium fuels as against the cheapest supermarket fuel. It's difficult to know what that actually does for you in practice. The marketing hype will tell you it makes a world of difference but I ran my last car solely on the cheapest supermarket fuel locally available and when I rebuilt the engine at 130,000 miles the engine was in decent condition internally and the fuel injectors still had a decent spray pattern.

I changed that policy when, a few weeks after getting my Omega, the supermarket sold me duff fuel and put my emissions light on!

I would say if you're concerned don't run it 100% on cheap fuel, try a tank of Optimax every now and then. If it doesn't improve performance it will, theoretically, at least, clean your fuel system.

One other tip I'd give is, if you end up at a petrol station at the back of beyond where the yokals aren't likely to drive performance cars and they sell super unleaded buy the 95 RON instead. In my other car (which needs super unleaded and has no knock control) I've had tanks of duff fuel in such places because it simply doesn't turn over quickly enough. The additives in a fuel evaporate off while it is stored.

Kevin

As its has knock sensors, it will advance the ignition on higher octane, hence a small, but noticible, performance boost.  Esp that 103 octance stuff I got from some BP station. Shame its so outrageously expensive.

Also, I have been told by someone in the know that supermarkets now tend to buy Russian fuels, and these are very low octane (around 80), and have additives to increase it to 95

[Kevin Wood]

Out of interest - can you watch the ignition timing (specifically the knock retard) with a Tech2?

Would be an interesting experiment to see what it does on different fuels.

In theory (at least) once you've got high enough octane for an engine to run to maximum power with the optimum ignition timing without knock it won't make any difference if you increase the octane. Higher octane will just give you a bigger margin against knock occuring.

It would be interesting to watch the timing with different fuels as this would tell us which are good / bad and what level of fuel quality it makes sense to run on an Omega.

Kevin

[TheBoy]

Out of interest - can you watch the ignition timing (specifically the knock retard) with a Tech2?

Would be an interesting experiment to see what it does on different fuels.

In theory (at least) once you've got high enough octane for an engine to run to maximum power with the optimum ignition timing without knock it won't make any difference if you increase the octane. Higher octane will just give you a bigger margin against knock occuring.

It would be interesting to watch the timing with different fuels as this would tell us which are good / bad and what level of fuel quality it makes sense to run on an Omega.

Kevin

Yes it can, and on v6 with DIS packs, it adjusts per cylinder pair (presumably on coil per plug, it does per cylinder).

[Kevin Wood]

Yes it can, and on v6 with DIS packs, it adjusts per cylinder pair (presumably on coil per plug, it does per cylinder).

Quite a clever ECU if it can trim individual cylinders.

Kevin

[Martin_1962]

Test was towing on M5 with 2.0 kept jumping between 3rd and 4th - stopped for petrol at Brent Knoll - Shell - stayed in top on the flat no hunting up and down along with better consumption

[TheBoy]

Yes it can, and on v6 with DIS packs, it adjusts per cylinder pair (presumably on coil per plug, it does per cylinder).

Quite a clever ECU if it can trim individual cylinders.

Kevin

I'll check next time I plug into a CPP engine (if I remember )

[Martin_1962]

Yes it can, and on v6 with DIS packs, it adjusts per cylinder pair (presumably on coil per plug, it does per cylinder).

Quite a clever ECU if it can trim individual cylinders.

Kevin

I'll check next time I plug into a CPP engine (if I remember )

I'll volunteer

[Chopsdad]

I use BP Ultimate when towing and can feel the extra torque and responsivemenss pretty much immediately.

But as it's so expensive I used Shell Optimax from the busiest and cheapest place in town, which only cost 2p per litre more than normal BP.

Didn't feel the same gain but enough.  BP gave extra MPG but Optimax...mmmm...not really.

But recently engine felt "odd" when starting from cold and I switched back to BP normal - runs better and not really noticed and power loss after a few days.

So yes they are worth a punt, and no I wouldn't use supermarket petrol.  Switch from BP to Texaco every now and again helps too so try different brands for a change and see which suits your car.

[NaughtyNigel]

I tried running Optimax and BP Ultimate in my 2.5 V6 CDX Auto. I actually found that it used MORE fuel than regualr BP, and did not run as smoothly.

A lot of these fuels work well in small, high reving engines, but do not provide any real benefit in bigger, slow revving engines.

I think if I tried keeping the 'Mega engine running above 4,500 or so in top in the UK I would probably be offered B&B by Her Majesty!

For reference, I usually get about 34 MPG on a longis run with the speed set at 77 MPH.

----------

Nigel.

[MickAP]

I always use ASDA which is close to me for my diesel. I filled up at a TOTAL garage last week and regret it. The car had been running really well until I put that TOTAL diesel in, now it's sluggish and sounds noisier too.
They did have another diesel on offer which was 3p litre extra, do you think they sell the cheaper and in my opinion poor stuff in the hope you will buy the more expensive one.
Back to ASDA I think with a dose of REDEX injector cleaner.

Mick