Fuel choice

[John]

hi all,
well I've spent all night reading up on the issue of fuel choice and compression ratios;

To sum it up a bike with a compression ratio of 9.3:1 and lower is suited to 91 fuel, and if you use a 96 on it then you risk damage to your bike, and 'knocking' of the engine, am I correct in assuming so?..

while anything higher than 9.3:1 requires a higher octane fuel (96) to utilise its full capability, and if you use 96 in a 9.3:1 compression engine you risk backfiring due to un-burnt gasses exploding in the exhast ?

Well here is the predicament, I've being running 91 since I bought the bike even though it runs a 12.4:1 compression ratio, now if my logic is correct then I'm damaging my engine in doing so? and I should promply move to 96 to gain power and return my bike to a proper running condition?.. also do I need the 'squirt' for the bike also?

anyhelp/closure would be great.. cheers

[loosebruce]

All the tech stuff is a little beyond me John, but i wouldn't imagine running 91 would cause any issues, my mate ran 91 in his ZZR and never had a problem with it, until it ran out of oil (due to a leak by the gearbox) and cooked the engine.
I dont think you would notice a heck of a lot of differce by running 96, i know guys with late model race bikes, 2001 onwards that use 91 in them coz it's cheaper, suppose every cent counts huh.
Like i said i'm no professinal but i wouldn't worry about it, if someone in the know says otherwise for sure listen to them.

[John]

thanks I really think I'm over exaggerating the situation with reading to many random sites, but I do think your correct though because I have not noticed any real problems or otherwise with using 91, and the added bonus of it being cheaper, allows me to buy more beer!

cheers.

[Racey Rider]

To sum it up a bike with a compression ratio of 9.3:1 and lower is suited to 91 fuel, and if you use a 96 on it then you risk damage to your bike, and 'knocking' of the engine, am I correct in assuming so?..

No. 'knocking' - detonation is more likely to occur with 91 than 96 in a high commpresstion engine.
Have you read all the threads on this site about it? We were just talking about it last week. Do a search on here.
Your bike should run fine on 91.
Racey.

[merv]

Generally what you said about fuel choice and compression ratios probably applied back in the old days when engines were typically the same with long stroke etc. Nowadays, go by what your manual tells you for the bike. The manufacturers have got smarter at improving combustion chamber designs and whether the engine will knock or not depends on the length of the flame path from the plug and stuff like that. In this age of multivalves, heads with swirl flow designs, sometimes twin plug heads and all sorts of things you can throw the old rules out the window.

[James Deuce]

Bear in mind also that there are 2 standards (DIN and RON) for measuring octane rating and NZ uses the one (RON) that makes it look better than it is, by about two points.

My FIAT will only run nicely on 98, because it needs a minimum of 96 (DIN). It detonates like buggery on 91. It used to be fine on 96, but when they brought 98 in I am convinced that something changed in the 96 fuel, as the FIAT started pinking on 96.

[Motu]

As Merv says it's more to do with combustion chamber design than compression ratio and the whole octane rating debate is more to do with older designs.Also smaller cyls can usualy run a higher compression ratio - your bike has piss arse thimble pistons and the 12.4:1 means nothing really.You have hardened valve seats so the squirt thing is irrelavent,it is anyway but it's good marketing.

[TonyB]

I have been told by about three or four bike mechanics that 96 is not so good in bikes as it burns dirty and can foul plugs. My FZR has had probs with 96. Years ago I heard that 96 was damaging the o-rings etc in carbs, and Kwaka's were particularly prone. Not sure if this applies any more.

[scumdog]

The old Super (96) WAS hard on O-rings etc but that was 7-8 years ago.

The worst you can do by running 96 when it is not needed is have your wallet empty a bit quicker each time you fill up, however while your bike runs o.k. on 91 it is always possible the computer/ignition system is automaticaly (sp) retarding the ignition spark to prevent detonation in which case the power would also 'retard' but probably not so much that you could tell by 'seat of the pants' testing.

My 2-cents worth.

[James Deuce]

I have been told by about three or four bike mechanics that 96 is not so good in bikes as it burns dirty and can foul plugs. My FZR has had probs with 96. Years ago I heard that 96 was damaging the o-rings etc in carbs, and Kwaka's were particularly prone. Not sure if this applies any more.

That was the over the top amount of benzene and toluene they were chucking in.

[Gasman]

I've tried both in the beemer, and 91 actually seems better. When I ran 96 I got more plug fouling, and gumming. Trouble with this subject is that there are so many myths, general uninformed bullshit, and conspiracy theories! Bottom line is to check out the manufacturers recommendation.

[TonyB]

When I bought the FZR from Red Baron about 2.5 years ago, they said "only use 91- never use 96". The VFR I had before that ran just fine on 96- maybe I should give it a go again...the FZR has a few low rpm grumbles.
Why oh why are there so many myths surrounding virtually everything to do with bikes???

[cowpoos]

okay....octane....91,95,96,98 octane petrols [those are the most comon in NZ] all have the same explosive force...how they differ is in the rate at which they burn...91 burns faster than 96...different octane's are used for different states of tune...which has so many variables that I can't be bothered typing them...but it relates to ignition,compresion ratios,acuraccy of design and or timing...combustion chamber design,average inlet temp,average motor temp,bore and stroke,....blah....blah....blah!!!

Was there a reason you wanted to know this ?

[John]

thanks for all the help on the issue it was solely out of interest of the good health of my motorbike, as it has being playing up alot lately.

[johnny]

To sum it up a bike with a compression ratio of 9.3:1 and lower is suited to 91 fuel, and if you use a 96 on it then you risk damage to your bike, and 'knocking' of the engine, am I correct in assuming so?..

while anything higher than 9.3:1 requires a higher octane fuel (96) to utilise its full capability, and if you use 96 in a 9.3:1 compression engine you risk backfiring due to un-burnt gasses exploding in the exhast ?
anyhelp/closure would be great.. cheers[/QUOTE]

What is compression ratio??
What is al thess 12.4:1,9.3:1 for?? Do i just go and fill up my '98 VTR250 with 91 ?? Do i have to add anything in my tank with the fuel??