91 vs 95 octane?

[F5 Dave]

much better quailty 98 has had a much better refinery process
91 will go off quicker if left in a tank compared to 98 has happen to me

What? The base stock will be the same. They alter the fuel by adding more arromatics to it to increase the octane.

All i can say is use the same fuel each time that way you know what your bike should perform like when you get to a ride. I always use BP 98 in every thing, ill go out of my way to get it. A friend of mine stopped by Mobil and brought 98 because he had no time to stop by BP, his bike wouldn't even run on it! We drained the carb and the tank put my BP 98 in and it started right up. I am sure my Van gets more Kms on 98, it can't retard the timing enough to run properly on 91.

I love anecdotal stories. Its the same gas. BP & Mobil do swapsies when one of them is running low.


By the way if there is still some doubt on Octane etc- do some google research & look up calorific value. This is the energy of a fuel. It is not octane related. Its like saying your can of coke has more caffeine when its bubbly. Rest has been covered above.

But look up methanol. Low calorific value, but sky high octane. Pour heaps in (because you need more to make power) & crank the compression up silly high & you can make more power. Don't bother even trying it though.

[Jay GTI]

But look up methanol. Low calorific value, but sky high octane. Pour heaps in (because you need more to make power) & crank the compression up silly high & you can make more power. Don't bother even trying it though.

I can't remember the details, but recollect a story about one of the bigger names in MX trying methanol in his factory 2T 250 a decade or so ago. From what little I can remember, it made the power delivery so angry it was barely rideable and he was getting considerably higher lap times.

[F5 Dave]

rules haven't allowed it for some time except in classic roadracing, not even sure if its still there, but you have plenty of time on a big road track compared to dirt.

[Reckless]

I can't remember the details, but recollect a story about one of the bigger names in MX trying methanol in his factory 2T 250 a decade or so ago. From what little I can remember, it made the power delivery so angry it was barely rideable and he was getting considerably higher lap times.

In a 2smoker?
I ran Methanol in my race kart for about 7 years which had various MX based 125 engines. The benefit of methanol is the fact that it runs so much cooler, not that it produces power as a different fuel instantaneously. You use methanol so you can up your compression and timing to gain power not because you use methanol as a fuel par-say. You also use a shit load more of it so in a smoker its a lot harder to get them to run clean down low because you need it to deliver so much more fuel flat out! We had looong sharp tapered needles, We also run manually adjustable power jets feed directly from the fuel bowl that dumped fuel into the inlet which we could adjust on track while racing that meant we could move the main jet tuning more towards the mid range (esp when road racing).

In an inline four stroke?
Mind you in saying that we just gained 20 or more horsepower on the 1982 GPZ1100 goin from petrol (avgas) to meth and not doing a thing to the motor. Bearing in mind this is a race engine already with cams, higher compressions and Non std ignition. Tuned on the dyno of coarse The GPZ did about a litre per lap on Sat at Hampton Downs.

But in saying the above and getting back on topic!
In a crf450r you might find a difference between 91 and 98 because its a already fairly high performance 4 stroke engine??
Try it? It may start a little better when hot and it may feel a bit different! But if your on the way to a meeting and cant find a 98 pump you aint gonna stuff you engine on 91 octane. In my Ktm 200exc, the SV1000 and in the car I use 91 and wouldn't notice the difference if I used avgas or 98 in all those modes of transport without some engine or ignition mods on a dyno.

[F5 Dave]

on my GG 200 enduro I raised the comp somewhat (after optomising the squish it went too high & had to reduce the com to get a suitable ratio), but it was still heaps higher than std (can't remember the figures of hand). I was running 1/2Av & 1/2 91. Lead is wonderful stuff & you get big gains in Octane with just a little but have to add exponentially more to get further gains. Of the available aromatics it is the buss, at least for sensible coin.

Pure Av is supposed to hurt throttle response. All aromatics do I think. Never noticed it in Bucket engines, but ran the dirtbikes 1/2 1/2 just in case & never bothered to experiment running pure.

[caspernz]

All i can say is use the same fuel each time that way you know what your bike should perform like when you get to a ride. I always use BP 98 in every thing, ill go out of my way to get it. A friend of mine stopped by Mobil and brought 98 because he had no time to stop by BP, his bike wouldn't even run on it! We drained the carb and the tank put my BP 98 in and it started right up. I am sure my Van gets more Kms on 98, it can't retard the timing enough to run properly on 91.

Mmmmm, the one about BP and Mobil 98 being that different still has me laughing.....at times it comes from the same tap. How do I know? Aaahh, in the industry.

The Gull thing with ethanol added is a tricky one, for the calorific value of any fuel with ethanol added is lower, thus you need more fuel to make the same energy. In plain english, a tank with an ethanol blend won't get you as far as a tank without ethanol. Try it for yourself, it's quite easy to see the difference.

The basic thing is, if your vehicle can run on 91 you won't get a performance improvement running on 95 or 98, just more miles from the tankful.

[scott411]

the last time I remember methonal being used in NZ motocross was around 1991, when both Darryl Atkins and Shayne King ran 125's on it, i think it was banned from most MNZ sport after that, (i think the exceptions are bears racing and classics)

It was always tricky to use, but i am sure Robert Taylor would know a heap more about it than I would, I think they got a heap more gains back a while ago from air cooled motors where the lower burn temp had a good effect,

as said above, in my 125's i ran 50/50 avgas and 91, lead is wonderfull in two strokes for keeping denotation away,

[F5 Dave]

back in the day (haha, it doesn't seem that long ago cause it wasn't) in my race 50 with silly high compression ratio (got up to 19:1 full stroke) it loved the old 96 Super leaded pump gas ()it was Red remember?. The first batch of piss unleaded came & it barely pulled on the dyno & from that point on we changed to Av gas & the rules changed to allow it. Try as I might with jet changes, every conceivable ignition timing change & 3 different head shapes & altering the com; I still made 5% less power on Av Gas from 96 Super leaded. Pity they got rid of it in some deluded greenie initiative.

[Jay GTI]

the last time I remember methonal being used in NZ motocross was around 1991, when both Darryl Atkins and Shayne King ran 125's on it, i think it was banned from most MNZ sport after that, (i think the exceptions are bears racing and classics)

Yeah dredging the furthest recesses of my addled brain, I'm pretty sure the story I read was something along the lines of it was end of season, one of the mechanics was from a snow mobile tuning background (methanol is still used in drag racing those things as far as I know), so they had a play with this guy's 2 stroke race bike. Tried finding the story last night, but no luck...