Talk Morgan Forums Community The Malvern Arms High Octane Fuel

When I was commuting on the M4... 120 mile round trips, I tried Millers additive, claimed to increase octane (cetane) by a couple of digits. It resulted in better mpg (see my earlier post). That was on cruising style driving mainly with everyday urban on top. It was consistant improvement. The extra cleaner in the millers I expected to give a flush out and improvement. On returning to additive free the mpg dropped back. I therefore deduced it had an mpg improvement due to octane adjustment.
I haven't used any since retiring and reduced mileage. I may try the VPiwer as a test.

Personal experiences are subjective at best - too easy to think you are experiencing what you expect to experience. Confirmation bias is what its called.

You might like to watch this 5th gear clip . It shows the high octane petrols producing an extra 2% bhp in return for an extra 6% or so on cost. And thats on a modern car engine which can adjust to the different fuels. On a car with a fixed ignition timing, there can be no improvement unless the engine pinks under low octane fuel.

It will take time to revert back to a lower octane fuel, they sometimes say a tank or two so might knock until then.. not ideal.

Thanks for posting that Howard I had a vague memory of seeing the test but couldn't remember where !
There are also the benefits of less ethanol and the cleaning additives .

I used to go to SanFran monthly and had a good deal with Hertz where I always got a V8 Mustang for the price of a mid-range.

I got an invite to the Sears Point race-way as part of a BMW owners club and took the rental along. They offered me a run. I had some fun but also ran low on petrol. I went to the pumps they had on site and noticed a 107 octane pump. So I filled it up as you would given the opportunity.

I can confirm that it went faster. Lots. I also got evidence, no rear rubber left when I handed it back to the Hertz desk at SFO ! I got a dirty look next time I went to rent and they only gave me a V6.

Interesting. Sorry to get too detailed.

E10 has 2% less energy than a standard gallon of petrol. Petrol is also mixed differently from summer to winter and the difference in energy per gallon is about 3% between them. If you want to get more miles per gallon fill up in the early morning and in cold weather. The fuel is denser so you get a very slightly more energy per gallon. Oh, and there is less ethanol in high octane fuel so more energy.

I still run my cars on higher octane fuel, and octane improver as well, for some!

My experience is varied.
We have 3 cars:

Morgan Plus 8: Natually aspirated. Runs more cleanly/smoothly, especially pickup at low RPM, and slightly better MPG on V Power. So used all the time.
Mercedes E350D: Turbo. Using Shell VPower diesel MPG improves, on a long run, best seen is 48 compared with 35 around town normally. Used V Power every 3rd fill-up or before a long run.
Mercedes CLC 180 Kompressor: Supercharger. No measurable difference, so the cheapest fuel available is used.

When I ran the Skyline GTR many years back the engineer who did the mapping and tuning for them told me the following.

Very modern ECU can adjust on the fly and so putting in high and normal octane is acceptable and it should adapt on the run. This is to assist them in the most efficient lean burn. I gather this may not apply to VW's ?

Modern ECU will need to be run continuously on high octane to really adjust and work well with it. In some cases a "trip" or reset can make them re-learn and optimise. Cheap fuel in the mix will cause them to drop back again to avoid pre-detonation. Many will then stay in the less aggressive pattern so you will not get any benefit of a tank of the good stuff.

Most of the ECU out there older than 4 years will need manual adjustment with the exception of the early eco and turbo models which tended to have a bigger brain to allow adjustment due to their needs.

For the Skyline he said always use high octane and fixed the adjustment. I used to carry a small pot of octane booster for when I got caught short of a decent garage. That was an aggressive engine and you did feel it if you put in the cheap stuff.

His comment on the petrol was that there are three refineries in the UK that serve pretty much every brand of petrol station including the supermarkets. They blend to order. Branded fuels get more additives to improve burn and cleaning but the basics are the same. He said Shell 98 will burn better than Sainsburys 99 in the engines he used to tune.

He used to do passenger runs with a laptop attached to the ECU in customer cars to adjust and map the tuning. He told them to stick to the same brand of fuel or switch to a lower map (three maps in the ECU with a toggle switch) where not available. He would do a couple of runs on Shell 98 and tune it to the edge then run on lower grade and you could see the ECU pull back in real time due to pinking. This made certain the customer understood the difference and protected the investment in the engine. It was the only time I have seen the difference in real time and it could be felt through the drive.

Even on the last Mercedes and Audi I have now I cannot really tell the difference. Given they have more power than you can sensibly use on public roads it would need to be an odd situation for me to feel the difference. Neither of these gines or my driving style will let me see a difference in mpg so I don't try and clock that.

I prefer to put in good quality normal petrol given the price hike they add to high octane. Unless I know I will be driving it hard for some reason.

Things I have seen instantly benefit it far more.
Tyre pressures, this is the instant mpg killer when they are low.
Eco Mode/Performance mode. The mode buttons on modern cars adjust the gear change up points and engine map in most cases. These can have a rapid 2-4 mpg difference.

Out of curiosity I did the sums. Health warning - I am careless according to teachers . Anyway, with today's temperature difference of 5 deg C and assuming a 50 litre fill up, you would get the equivalent of an extra 0.2375 litres of petrol ( in reality 180 grams) in return for getting up at 0600 hours. But thats forgetting that the tanks are buried in the ground where the temperature will barely vary at all.

Changing the subject a bit, the reason that drug trials are done on a double blind system is the human ability to discover the precise effects they expected to discover. I am sure its the same with the effects or non effects of different fuels. The views are subjective

That is an impressive number, 107!. BTW Do you know if it was RON or AKI (which is the usual US unit and shows even lower values than equal RON).

https://en.m.wikipedia.org/wiki/Octane_rating