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I run a 1995 R100R and where possible fill with E5 but I also run E10 if forced.
I'm convinced the engine runs smoother with E5. Is this all in my head or has anyone else found this.

I didn't notice any difference in running moving from E5 to E10 although it must have weakened the mixture by a few percent.

I'll find out what it's done to fuel hoses and O rings in the carbs at the next overhaul. I've already had the bowls off and seen nothing untoward.

I've run on E10 apart from a few 'test' fill ups with E5 since it first came in. Can't say that I've noticed any real performance difference apart from a slight reduction in fuel consumption (less than 10%) which is what you would expect. Also no issues with degradation of plastics so far.

I do wonder if the engine were tuned specifically to run on E5, whether there would be more difference but since it (E5) isn't universaly available, especially in rural areas, and since I would expect a comensurate degrading of performance on E10, it's not somewhere I'm going to go.

Rob

Are there different burn rates of the flame front with E5 vs E10? Or volatility of the fuel? Could something like that contribute to better running?

Kurt

Kurt in S.A. wrote: ↑Sun Sep 28, 2025 9:47 am Are there different burn rates of the flame front with E5 vs E10? Or volatility of the fuel? Could something like that contribute to better running?

Kurt

There must be. I ran a Chinese 125 Yamaha copy some years ago. These are set to run very lean. Going from E10 to E5 and the idle speed would increase and require a small adjustment to the carb slide stop.

Kurt in S.A. wrote: ↑Sun Sep 28, 2025 9:47 am Are there different burn rates of the flame front with E5 vs E10? Or volatility of the fuel? Could something like that contribute to better running?

Kurt

I doubt there are significant burn rate or volatility differences for E5 and E10 but it might depend where you are located. In the UK E5 is 98 Ron and E10 is 95 Ron but where E5 and E10 are the same octane rating I would expect both fuels would be formulated to perform the same.

The one thing we know for sure is that E10 will have less calorific value than E5 due to the increased oxygen content so old open loop engines like ours will run a slightly weaker mixture as a result. If you run worse with E10 then it may be the weaker mixture that makes the difference. If you are in the UK then there is also the lower octane rating of E10 that might make a difference but it won't be burn rate. It's a myth that a slower burn rate is the primary effect produced by higher octane fuel.

My bikes run just fine on E10 but they don't like sitting with it. When I lived in California and could only get E10, if the bike was going to sit for a while I'd park it with a full tank and Startron fuel stabilizer. I always run the float bowls dry when I park the bikes.

I had good luck with this plan--the bikes would start in the springtime and I'd run through a tank quickly to get fresh fuel in them. Except for the time I didn't, which was the first spring after my divorce was final. I had to ditch the bike in the next town and come back with the truck and my son the next day to rescue it. All pretty normal, until my ex decided to follow us to my new home.

As we were setting up the ramp, she popped up, "Hey, can I do anything to help?" "Yes, if the bike starts to fall off the ramp, please throw yourself under it!" Then she invited herself in to the house for a tour. I think she was disappointed that there was only bike stuff (gear, Krausers) in the whole house. I was a little disappointed that the three bikes I'd bought that she didn't know about weren't already in the garage.

Anyway, avoid E10 so you can avoid uncomfortable situations with former spouses.

I mostly run on E5 which I buy at my local Tesco where it is only 8p per litre more than E10, in other places it can be as much as 20p a litre more, in which case I switch to E10. There is a difference in fuel consumption of around 10 % so price matters. However be wary at remote rural country filling stations as their E5 could be stale.

I've not seen anything like a 10% difference in fuel consumption between E5 and E10. Based on the caloric value of the two fuels it should only be approx.1.6% although there are some other subtle things going on besides the caloric value change which may alter that figure slightly.

Based on
Calorific value of gasoline without any ethanol of 34.2 MJ/L (this might vary slightly in different markets e.g. 33.7 MJ/L)
and
Calorific value of ethanol 23.4 MJ/L

E5 should be 1.6% lower caloric value than straight gasoline
E10 should be another 1.6% lower than E5

So we should be 3.26% worse off since the introduction of ethanol.

I think it's fair to say that, to get the best results from either type of fuel, the engine needs to be set up to run on that type. This is possible why switching from one to the other without re-tuning will result in larger than expected changes in economy and performance.

Rob