I know this is a rather ragged topic but I have asked a few people and have been reading about it for some time now (no expert but have a pretty strong warm and fuzzy on the topic). I am also looking for more technical advice not snappy one liners please.

I am getting my bike rejetted here soon with the factory pro race kit, I know under most circumstances 87 is still preferred, engine has to work less and the fuel will burn more efficiently. However, where it started to get rather gray for me was not compression but ignition advancement. I have advanced my ignition 6* for a total of 4* BTDC; The rest is all stock except for a shift star. Would I see any benefit of running 91 or higher because of that? Can I tune it to run more efficiently on higher octane without loosing any power? I have Aviation quality fuel readily available too but I want to get the most power out of what I have and if that mean 87 is what the bike wants than that's what it gets. I subscribe to neither leaner nor richer is better when it comes to A/F and I don't want to up the compression right now as not enough $$. Bike has the BMC Race Air Filter if it matters. Thanks and I'm eagerly awaiting the responses.

 

As long as there is no 'pinging' sound coming from the engine at any rev then 87 is your best bet. But as long as the bike is tuned for whatever octane you are using you'll be happy.
Just switching octane wont do much of anything for performance. so pick one, jet your bike for it and stick with it.
Ive got my bike jetted for 87, had to get 89 one day due to the station being out of 87, and there wasnt a huge difference, but she just didnt run as good as she would have with 87.

 

Well see that's my conundrum, I'm rejetting my bike right; normally octane and jetting are - for the most part - independent of eachother, except when compression is changed or pre-det is present. Advancing the ignition can also have an affect on those factors. I'm trying to find out if I can increase performance by increasing the octane and benefiting; jetting a bike controls the A/F ratio.

 

Higher octane ratings are for preventing ping or knock as you know. That ping or knock is the result of the fuel igniting before the piston has reached the proper position in it's stroke. That happens for the most part because of cylinder temperatures. The compression ratio has the biggest impact on those temperatures. A/F ratios also affect it. Advancing the ignition timing isn't going to change whether or not knock or ping is going to happen. If you think about it, the further you advance your ignition timing, the closer you are to generating the same type of issue that happens with knocking. The fuel ignites and tries to make the engine run backwards.

 

I'm not sure I fully understand it fully, I'm getting hung up on the part that's underlined. So What I'm gathering from you is that I need to see if advancing my ignition has induced a knock? If so then I would need to increase my octane until it is eliminated; if no knock stick to 87?

 

Yes and no.. lol. Yes it's possible that your advance timing is causing it to have a knock. If the knock goes away after retarding the ignition, then yes it's being caused by it. I don't think increasing the octane will make that knock go away. One thing to keep in mind is that I'm not there and you are. It's certainly worth a try. I'd be interested it how it turns out.

 

I"m researching this topic now.. but obviously differently because obviously you're dealing w/ a carb while I'm dealing with FI. I'm unsure yet if the F4i uses both the MAP and IAT to determine advancement. Plus then you have the whole mess of monitoring say the crank sensor using a VR. So I'm trying to figure out what measurement will work best to determine how far to advance at higher TPS positions.

 

Guess Ill just empty the tank and bring a gal of each with me to the shop hand it over too them and see if they will let me sit and watch.... Prob not, I'd be asking too many questions

So what your saying is increasing the octane will not offset the possibility of the knock. hmmm very interesting, still waiting on some engine builders to email me back

 

If the knock is being generated by the advanced timing, then no, I don't believe that going higher in octane will eliminate it.