High Octane Fuel

[kevscrivener]

Originally Posted by Spen

If you're going to use it in your ironhead you might need to use an addative to stop valve seat erosion.

Spen

No need. US technology on unleaded fuel was always years ahead of ours; even my 63 GMC truck is OK for unleaded fuel. When I had my Ironhead top end overhauled about 18 months ago, I asked Matt whether harder seats and valves were necessary, and was told that it wasn't. When he stripped it, there was NO seat erosion at all, and that's on a 34 year old bike that had NEVER been stripped before, and as you know it does get used a bit!

Bear in mind that older Harleys are designed to burn some oil, and this in itself compensates for the additives. Furthermore, US regular fuel is only 92 octane, ours is 95; only found that out a few weeks ago.

[hellon2]

This is an old thread, but i thought I would add to it rather than start a new one.

While in Germany on hols recently touring with my trusty Softail Evo80, I found they sell 100 Octane petrol in many petrol stations. I tried it, seemed to make engine more responsive, even perhaps more power.

Upon my return to UK I was discussing same in the local pub, where I was duly informed what Octane rating is all about (Note: British pubs are source of ALL knowledge, at least useful stuff for blokes).

Basically high octane fuel does not make more power than lower octane. What it does do is provide greater resistance to pre-ignition (aka pinking) due to compression (rather than spark ingnition) thus allowing higher compression engines. High performance engines are usually higher compresion which means they need higher octane fuel to avoid pinking, thus the perceived link between high octane petrol and high performance engines.

See this for (much) more info: Octane rating - Wikipedia, the free encyclopedia

[sproggy]

Originally Posted by hellon2

Basically high octane fuel does not make more power than lower octane. What it does do is provide greater resistance to pre-ignition (aka pinking) due to compression (rather than spark ingnition) thus allowing higher compression engines. High performance engines are usually higher compresion which means they need higher octane fuel to avoid pinking, thus the perceived link between high octane petrol and high performance engines.

That's true, but it's not the whole truth. Other than compression ratio the other thing that affects detonation is ignition timing. Most engines could produce more power than they do but for the need to have an ignition advance curve that prevents pinking/detonation when using a range of fuel grades in a variety of load situations. On an engine with preset ignition timing there is no power benefit in running higher octane fuel although the engine may run smoother (due to more even combustion) and remain cleaner (due to more advanced detergents often added to fuels such as Optimate), however many modern engines use knock sensors to monitor the conditions in the combustion chamber and adjust the ignition timing to give optimal performance within the constraints of the fuel, load etc. These engines can produce more power from higher octane fuel than from regular stuff because they can run their ignition timing closer to 'the edge' without risk of detonation.

Knock sensors aren't very common on bike engines yet (the BMW K1200S and K1200R use them, and different octane fuels do give very different power) but have been used in cars for a while now.

When an ignition system is re-mapped (by whatever means) to give a different advance curve with the intention of improving performance it reduces the engine's tolerance to poor fuel. Push this further and the higher octane fuel is necessary to prevent detonation - it doesn't make more power in itself, but it allows more power to be made without detonation.

All of which is completely irrelevant to a low-compression ironhead engine

[jonr]

Only thing it seems is the north-south divide - not much in the way of availability from the midlands upwards !

[Scotty]

Unless you are running high compression 10/1 and up, you are wasting your money putting high octane fuel in your Harley.

What's the benefit of higher octane?

Higher octane fuel has only one beneficial feature - it allows an engine to run at higher temperatures with more advanced ignition timing under higher levels of compression without detonating / knocking. Higher octane fuel does NOT have more potential energy and will not make an engine perform better unless that engine is knocking. On modern engines with knock sensors, higher octane fuel may make the engine run better if the knock sensors are retarding the ignition timing, which hinders performance. High octane fuel does not burn cleaner, it does not clean your engine, it does not increase horsepower or torque (unless you are experiencing knock), it does not smell better, it does not increase fuel economy (unless you are experiencing knock) and is not better for the environment. If you buy higher octane fuels for any of the above reasons, STOP!

[tardle]

To back up what Scotty said.
I previously ran an oilhead BMW. On hot summer days when loaded up it would ping badly when winding it on in a high gear ,70 upwards on overtakes. I found that if I topped the tank up with Optimax then no more pinging. The bike never gave any bother with pinging during the cooler times of year, loaded or not, & unleaded was used during the rest of the year.
trev