Talk Morgan Forums Morgan Sports Cars Morgan Three Wheeler & Super 3 99 Octane Fuel (RON) - worth it?

My local Esso garage has just started selling 99 Octane petrol. This has made me think about how much of a “high performance” engine the X Wedge really is. S&S say it’s a high performance engine and it should be fuelled with minimum 96 RON but in the grand scheme of things 80ish HP from a 2 litre isn’t huge although the torque is impressive. I wonder how much benefit better pre ignition control through higher octane fuel will be for this engine given it has a knock sensor and presumably the programming to exploit it? Anyway assuming that the ECU would be doing less retarding I filled up with it anyway but discounted what I perceived to be a crisper performance as probably placebo effect.

My engine hasn’t been upgraded apart from a better intake filter, has anyone run a M3W on higher octane fuel say 99 vs 96 RON on a dynamometer and shared results? I imagine those that have done engine upgrades will know more about this than me.

To my knowledge, the higher compression rate the engine has, the higher octane number is needed, and that's it. So if your S&S engine has a compression rate that demands 96, that is the minimum number you should you should use. If you fill up with anything higher, you are buying fuel with a property you do not need, so it is a waste og money IMO. I did try 98 octane fuel on my previous Lotus Elise S2, which is more of performance car/engine than a MW3, I'd say. I noted no difference at all.

"The higher compression rate the engine has, the higher octane number is needed" is definitely part of the story, the broader part of the story is that the more prone to detonation an engine is, the higher the octane of fuel you want to put in it. A relatively low performance, low compression engine with a poor combustion chamber design might well need higher octane fuel than a much higher performance engine with better chamber design and cooling.
In some engines higher octane fuel does give more power and in some engines it might just reduce the chance of it going boom!

Higher octane will help with predetonation, “pinging” if thats what you call it. Carbon buildup is another thing. Why go cheap on fuel and go expensive in engine?

Notice how petrol stations never advertise the price of the higher octane fuel, not even on the pump itself until you lift the holster. The premium can sometimes be considerable in my locality, particularly with V-power. A licence to print money!

Since the early days of ownership with everything very standard, including the engine pinking at about 4000rpm that the restrictive standard exhaust did, I have run mostly with the highest octane fuel I could get. It's a little more expensive but in the shape of things for the general mileage, not much in an effort to reduce the harmful pinking. Since going Stage 1 G56 and an unlocked, rolling road optimised ECU, the pinking has gone at all rpm's and doesn't even happen on those occasions when I have to use 95 octane fuel, so it's not the fuel that causes detonation. Your Brooklands system should be similar to a Stage 1 exhaust, so I think it will run OK on UK standard 95 RON fuel but maybe better of 99 but that may be just a perceived performance improvement, as you say. It's not a turbo charged 150bhp per litre engine, so that little bit extra with the fuel shouldn't make the difference.....but I still use the 99 RON stuff when I can.

My Uncle very many years ago always insisted on using the most expensive, highest octane petrol he could find. I explained to him the higher lead content made it burn slower to suit higher compression engines only and reduce pre ignition but my warnings fell on deaf ears despite the modest cars he drove.

Having said this I did need to use 5 star (101 octane) when it existed in the 70/80's in my Jensen CV8 as that had a 10.5:1 c/r and would pink on anything less. I currently also use premium in my 1960 Velocette for same reason.

The X wedge in the M3W has knock sensors though and will run fine on regular. I never used anything else in my Stage 1 car. It has been suggested you might get slightly better fuel mileage with premium but not enough to compensate for extra cost.

Thanks for your thoughts everyone.

The higher octane numbers are nice to have. My other consideration is that the higher the octane is the less chance of having ethanol in the fuel mix. The fuel pump has issues with ethanol and ethanol also attracts water. If you're burning it up by the tankful and traveling a lot this is no worry but try to make sure that no ethanol fuel mixture is in the tank for long term storage.

Under other circumstances I would let this slide but more and more governments are using ethanol to 'improve' what comes out of the exhaust pipe. They may have their points lined up but it also causes other issues such as mpg degradation and more instances of water in the fuel.

IMHO

Spot on Michael👍