[loudnlow7484]

Gas mileage shouldn't drop, but I, personally, would use 90 octane or better with an advancer on. Advancing ignition timing makes your engine more likely to pre-ignite, or detonate. Using a higher octane gas would lessen that possibility. In my experience with cars, advancing the ignition has never caused worse gas mileage. Actually, come to think of it, it should give you marginally BETTER gas mileage. It essentially makes the engine more efficient in the midrange, where you do most of your driving. The engine isn't sucking any more air, or injecting any more fuel, it's just igniting it earlier. Hmmm. If my logic is flawed, once again, somebody please slap me and point me in the right direction.

[SweetLou]

I volunteer to slap you, but I think your right.

[AkBigN]

That makes sense I just had an interesting experience. I put the advancer on and ran for about 15 miles with no problems. A girl got on the back for a ride around the city, still no problems. I stopped to fill up the tank which was pretty much empty and when I went to leave my engine was coughing hard. It wasn't just in low RPM's, it was the whole range. After riding a mile or so I drove her back home because it felt like my engine was going to destroy itself. As soon as she got off the problem went away. I don't know what the hell happened but I didn't dare tell her it was the extra weight!!!

Anyway I'm still on the same tank and my mileage seems like it went way down. But I could have just forgot what my odometer was at and am just looking to hard because of what happened. Hopefully it doesn't happen again.

[SoloScott]

it should give you marginally BETTER gas mileage. It essentially makes the engine more efficient in the midrange, where you do most of your driving.

No slap. You are correct again. Having your ignition advanced allows the engine to make full use of fuel with higher octane. The added midrange power (efficiency) means that you need to roll on less throttle to get the desired results.

[The CyberPoet]

Gas mileage shouldn't drop, but I, personally, would use 90 octane or better with an advancer on. Advancing ignition timing makes your engine more likely to pre-ignite, or detonate....[snip]

No slap. You are correct again. Having your ignition advanced allows the engine to make full use of fuel with higher octane. The added midrange power (efficiency) means that you need to roll on less throttle to get the desired results.

I think not...
And here's why (and where you fell into the pitfall):

Unlike a typical car, the cylinder in a Kat is 149cc's for a Kat 600 (187.5cc's for a 750). 149cc is slightly less than 1/2 can of coke (or about 1/4 of a 20 ounce bottle). This is a tiny space and should not be compared to the dual-coke-can sized bores of a Chevy 350 (717cc each) or even the 1.4-times-a-coke-can sized bores of a typical 2-liter 4-cylinder car engine, when talking about predetonation -- the behaviors are totally different.

Compression on the 600 is 11.2:1, for the 750 it's 10.5:1. The compression ratio doesn't change with an ignition advancer, so you're no more likely to get compression-related preignition than you were previously. You are running slightly hotter (in theory), but with such a small cylinder, cycling so fast, the heat is moving out of the engine quite rapidly and unless your oil jets are defective (blocked), you shouldn't be able to raise them temp high enough to force pre-detonation from temp.
Thus, if you're not causing predetonation, there is no benefit in higher octane fuels, since they:
(A) contain less power per volume (since they are less explosive as a result of their higher octane ratings), AND
(B) you haven't changed the predetonation tendency in any real sense that would make it more apt to occur (slight temp increase at the cylinder)... AND
(C) The further you advance the spark, the less likely predetonation is to occur (because the spark happens sooner -- thus less time in the cycling events for the fuel-air mix to pre-ignite for other reasons).

Now, if we compare burn speeds of heptane/octane mixtures with higher octane counts, the flame front speed decreases slightly as octane quantity increases. Since we're talking about a very very small area, the flame front moves through it at so high a speed that the octane-related reduction in flame speed is irrelevant (or of negative benefit at particularly high RPM's, where it can in theory result in incomplete burns).

Thus, the only benefit you are getting from high octane fuels is *possibly* the bundled detergent chemistry helping clean your engine -- or keeping your engine clean -- and that's only an issue if it's not clean to begin with.

If this sounds poorly written compared to my normal posts, it's because I was just on my way to bed and my eyelids are drooping... but the underlying logic is still 100% valid.

Cheers
=-= The CyberPoet

[loudnlow7484]

I understand your logic, and it all makes sense, but....... in my experience it has always been good practice to move to a higher octane fuel when advancing ignition timing. If nothing else, then because the slower burning of the higher octane fuel would lessen the ill effects of over-advancing the timing. Not a big concern on our bikes, where the advance is set, and unchangable, but a big deal in cars where you could set nearly infinite advance.

[The CyberPoet]

Oh, and the final bit of the proofing puzzle that I couldn't remember in my sleep-addled state last night:

The European market GSXF's already have more advance built-in (11 degrees) than the US bikes end up with when running the 5 degree advancer (which brings them up to 9 degrees after installing the 5 degree advancer) -- and the Euro-spec still spec regular fuel.

Cheers
=-= The CyberPoet

[loudnlow7484]

Really? So what would you have to swap in to get up to the 11 degree advance of the euro ones? Is it in the magical black box, or do they have a different factory advancer?

[The CyberPoet]

Really? So what would you have to swap in to get up to the 11 degree advance of the euro ones? Is it in the magical black box, or do they have a different factory advancer?

Different advancer, different CDI. The factory service manual has the specs for this in the very back, with the per-country break-outs.

Cheers
=-= The CyberPoet

[loudnlow7484]

hmmm...... sounds like an ebay search to me. did they, by chance, have more power overseas than here? this might just help me with my 10 second goal....

[The CyberPoet]

hmmm...... sounds like an ebay search to me. did they, by chance, have more power overseas than here? this might just help me with my 10 second goal....

I don't know if there's any difference in top-end HP, but there should be a difference in the torque band shapes. You can find the CDI and advancer part numbers at www.motofiches.com if their site is up.

Meanwhile, if you want to knock the times down drastically, move to a different engine that can bolt into the same space (such as a Bandit 1100 or 1200 engine). You'll find a whole thread on that in the Mechanics forum

Cheers
=-= The CyberPoet

[loudnlow7484]

mmmmm, bandit 1200 engine........ perhaps I should re-word that. My goal is to get a 10 second pass with less than $1500 total invested. I'm at $900 so far, I have $600 to play with and about .8 seconds to lose. Matter of fact, I'm going to start a thread on that.......

[The CyberPoet]

mmmmm, bandit 1200 engine........ perhaps I should re-word that. My goal is to get a 10 second pass with less than $1500 total invested. I'm at $900 so far, I have $600 to play with and about .8 seconds to lose. Matter of fact, I'm going to start a thread on that.......

Oh, you could have gotten there for under $1500 if you had gotten the engine at the outset.
Hmmmm...
I'm going to guess that you've already regeared the sprockets to give you faster launches and more torque through the power band, right? Thinking about it, if you're trapping at 115 or so, there's no need to have gearing that will carry you to 130...

Start a new thread.
Cheers
=-= The CyberPoet

[loudnlow7484]

nope, no gearing done yet. my thread is in the mechanics101 section now, so I don't pirate this thread.

[Lagmo]

'I have run race gas in this bike, and ALL pinging was quashed, not to mention noticably improved low end response... besides, i love the way it smells when its burning ... of course, at $4.50+/gal, it isnt very economical...
a

ROFL at that price you'd get like 3/4 Gallon regular 92(87 pump grade) gasoline here...
I just go with the regular 92 Octane(87 pump grade US) stuff, havn't had any trouble so far on my '90 Kat 600
Then again i don't use engine mods of any kind either