Why does my bike go better on 98?

[lb99]

83 GPz1100 Unitrack, fuelled up with bp98 as an experiment, I was avey surprised to notice a significant difference in smoothness and response, I am very impressed. But I am confused. The motor is hardly specced to run such a high octane, 2v fuel injected air cooled 8.7-1 compression, so hardly a high state of tune, but the difference between 91 and 98 is instantly obvious. Why?

[AllanB]

Good question. With that low compression I'd expect a lower octane to suit it.

Tried 95?

[BMWST?]

my old bmw car felt ncer using 96 at the time even though it said 91 was ok.fuel economy improved so that the cost of running was the same.It wasnt clever enough to know to adjust ignition timing,but it def felt better

[lb99]

Good question. With that low compression I'd expect a lower octane to suit it.

Tried 95?

Yup, is ok on that too. I have only pootled around town at this stage, but puddling along at 50 in top for example is much less snatchy, Although it is obviously still happier in 4th at that speed.
Certainly crisper response when you give it a handful in low gear from low rpm, maybe the ignition map has a really savage advance curve. It was the pinnacle of tech for its day

[FJRider]

Yup, is ok on that too. I have only pootled around town at this stage, but puddling along at 50 in top for example is much less snatchy, Although it is obviously still happier in 4th at that speed.
Certainly crisper response when you give it a handful in low gear from low rpm, maybe the ignition map has a really savage advance curve. It was the pinnacle of tech for its day

The bigger bangs in the cylinders do make the difference ... but how long will the motor last with those bigger bangs ... ???

[lb99]

The bigger bangs in the cylinders do make the difference ... but how long will the motor last with those bigger bangs ... ???

Thats the thing, higher octane has LESS bang than 91, so far as I understand, the higher the octane, the less volitile it is, to resist pre ignition (knock) and as far as i can understand it has less BTUs too. Higher compressions, and full on turbo engines need high octane to reduce the likelyhood of knock (or ping) so the ignition can be advanced to the most optimal position, and therefore take advantage of the available BTUs as efficiently as possible.

So WHY does my bike like it? There should be no difference

[FJRider]

... So WHY does my bike like it? There should be no difference

I had a read of few stories on higher octane fuels ... and found this ..

The benefits for fuel injected vehicles depend on the sophistication of the car's engine management system. Systems that are capable of adapting fuel and spark maps to the fuel being used are most likely to show improved performance, while vehicles with less smart systems typically won't benefit.

[HenryDorsetCase]

How do you know that it does? had it on a dyno?

because confirmation bias is a thing you know.

[caspernz]

This topic has many angles. What is the recommended octane rating for your bike? Might be 95, so using 91 makes the engine go a bit limp, whereas the 98 allows it run in full noise trim. Beyond that, the symptoms you describe may well be largely imaginary, like those swedish twins in my bed

For the purposes of your research, run a tank of Mobil synergy or whatever it's called, with the 8000 number. Basically same stuff as BP 98.

[veldthui]

My bike you can't tell the difference between 91 or 98. It works the same. Recommended is 95.

[Akzle]

calories. that's why.

[Laava]

So WHY does my bike like it? There should be no difference

Have you had your butt dyno correctly calibrated?

[robajs]

I did an experiment using 103 octane in my old ossa trials bike to improve pulling power which it did!
after riding thee trials there wasn't enough left in the tin to do another mix so poured it into my 89 r100gs
as there was only half a tank of 95 in it ....not enough to get where I wanted to go.
the bmw would pink in top at 50 kph but with around two litres of 103 octane it just pulled like a school boy .
the vp racing fuel was pricey but fun as an experiment. pre quake pioneer auto parts were in chch so it was easy to
get but now ...... who knows.

[rastuscat]

because confirmation bias is a thing you know.

Cheers. I always knew it was a thing, just didn't know what to call it.

[lb99]

Have you had your butt dyno correctly calibrated?

Yup, I do it by measuring the clench marks on the passenger seat after ridin 2 up