Best octane gas?

[Machi]

That's a neat tip, Pete.
I wonder what we would say about octane if we were all running at over 5000 ft above sea level - like I am.
Any alternate advice? I use 86 Chevron and the bike seems to run better at these altitudes.

[Burns]

With ignition timing coming off the end of a camshaft that is chain driven, with a worn chain, and the advance curve contolled by a couple of springs with god knows how much variance in their rates due to god only knows how many thermal cycles, and the whole thing depending on timing marks that can be 2 or 3 degrees off anyway I'd say "spot on" timing is a figment of imagination.

Also, the octane number on the pump is just a guess. The pumps mix two octane blends together with some variance in the resulting blend.

So, and please feel free to correct my thinking here, I ain't sensitive, it seems to me you ought to pick the fuel that you are going to run (and I vote that you keep the 20 cents a gallon in your pocket unless you are racing) advance your timing to where it starts to ping and then back it off a little at a time until it doesn't.

That's pretty much what the computers/knock sensors do in modern engines only of course with the XS your setting will be static and not dynamic.

Of course you don't have that option with the TCI's so I guess high octane is not bad insurance there, though it is a waste of money to buy more octane than you need. Run the cheapest stuff that doesn't rattle.

Of course you could pull yout TCI parts, sell them on Ebay and put Pamcotes Iggy in and go with my original suggestion.

Lastly, it seems to me that most holed pistons are melted through, not hammered through. Melt throughs are usually caused by too much air, not by not enough octane.

I'd like a piston-engined airplane kind of set up with the spark advancer on the handle bar. Then you could advance it way over and ask a Harley Guy if he'd like to give it a kick.

[tomterrific]

You're giving bad advise here. It is detonation that leads to a holed piston. I've studied this for years and it was Smokey Yunick that first described it in an artical in Circle Track. I'll say again that every summer I get emails about holed pistons in old bikes and they all run cheap gas. The XS is as sturdy as a rock but it can still be sidelined with low octane. It is possible to lower octane requirements by running the mixture rich and the ignition retarded. Both will kill gas mileage so that's a bad idea compaired to just using the proper octane.

The thin air at altitude is like running a part throttle even when wide open. The octane at the pump is lowered at high altitude because it is cheaper to manufacture.

Tom Graham

[pamcopete]

Well, somewhere in this thread someone asked what octane the manufacturer recommended. The emissions placard on my '81/H says minimum 91 Research which equates to 87 at the pump.

[Six-Five-O]

So, if you have a TCI equipped model or an aftermarket dual output coil setup, then you should swap the plugs at each oil change to equalize these differences.

How about swapping the plug wires every time before you leave home? ;D Seriously, whatdaya think?

[grizld1]

The "deposit line" I referred to is the line between the clean-burned area of the tip and the area covered by fuel deposits. That has nothing to do with migration of metal, type of ignition system or coil, etc.

Most of the XS650 piston failures I've seen have been associated with ATU failures where the stepped nut had loosened, allowing the aligning pin to hammer the ATU slot wide, advancing the timing drastically. The pistons showed melt-through.

Cam chain stretch will slightly alter valve timing over the life of the chain; ignition timing won't be impacted by this as much as by wear on points, cam followers, etc., and adjustment at regular intervals will return it to spec. It's a good idea to locate true TDC and correct the timing marks.

Tom, I entirely agree with you on fuel selection, but most tuners will take issue with you on adhering too strictly to factory advance specs. After a high-speed run ending in a throttle chop, the clean-burn band on the center electrode should be approximately 1 mm. wide, as measured from the tip down. That's not only where the engine will run safely, it's where it will develop the best power and run most efficiently. Retard timing to narrow the band, advance timing to widen it.

[pamcopete]

May 20, 2008 20:44:46 GMT -5 Six-Five-O said:

So, if you have a TCI equipped model or an aftermarket dual output coil setup, then you should swap the plugs at each oil change to equalize these differences.

How about swapping the plug wires every time before you leave home? ;D Seriously, whatdaya think?

Well, 6-5-0 good point. You could also do what Ford does with the platinum plugs in there new vehicles. One cylinder bank has the platinum on the tip, the other has the platinum on the ground electrode. They used to supply double platinum plugs but some whiz kid MBA figured they would save about $1 per vehicle if they went with two different plugs due to the high cost of platinum.

I'll do a little research to see if their is a matched set of plugs for the XS, or you could just use double platinum plugs.

Alternatively, you could install a double pole double throw switch to swap the polarity of the primary which would have the effect of swapping the secondary polarity.

[pamcopete]

An Autolite APP3923 double platinum should work @ $3.98 ea. I "plugged" a couple of them in my '81/H and I'll report back with results in a week or so. These plugs are warranted for 5 years against electrode erosion.

[Burns]

true or false:
If it doesn't ping you do not need any more octane.

[Burns]

Ok I did a little home work. For a very thorough discussion of this topic go to www.faqs.org/faqs/autos/gasoline-faq/part3/section-1.html

That article has a table indicating that 91 is probably the best octane for the 650.

But I stand by my earlier suggestion to pick the fuel that you are going to use and time the bike to the fuel. Set the timing just short of where you get ping because that's the "sweet spot" for engine effeciency.

Your choice of fuel should be based on what you intend to do with the motorcycle. If you want all the power that you can get, run "high test" and advance the timing accordingly.

If you want as many miles to the dollar as you can get (like me) run the 87, time accordingly, and don't treat this thing like a road racer - which it ain't any way.

Since I've put probably 100 thousand miles or more on XS's with timing set at factory spec and NEVER put high test in any of them and NEVER holed a piston (or had any other engine failure for that matter), I feel pretty sure that if folks out there are getting holes in their pistons it ain't the gasolene.

That's just my opinion, based on my personal experiences and riding style.

[yamdibo]

Just to throw a little gas on this fire, the service manual for my 78E states Regular Gas (90 Octane)!

[pops]

and a little more gas on the fire.....
My '79 states 91 (leaded)

pops

[Burns]

Here's another little squirt (wikipedia)

"the octane in the United States will be about 4 to 5 points lower than the same fuel elsewhere: 87 octane fuel, the "regular" gasoline in the US and Canada, would be 91-92 in Europe."

[Six-Five-O]

May 23, 2008 11:49:44 GMT -5 Burns said:

Here's another little squirt (wikipedia)

"the octane in the United States will be about 4 to 5 points lower than the same fuel elsewhere: 87 octane fuel, the "regular" gasoline in the US and Canada, would be 91-92 in Europe."

I'm not sure that all the info posted at Wikipedia is 100%...

How can 87 octane in the U.S.A be considered lower in octane when the article states that regular gas here would be considered 91-92 octane in Europe?

Following that line of reason?


More gas on the fire...Does anyone believe Yamaha produced XS-650s intended for various countries without consideration of overall octane, and with no regard to the evident issue on how octane ratings are calculated from one country to the next?

[pamcopete]

This just in from Petro Canada:

To determine your vehicle's octane requirement, look at the manufacturer's recommendation in your owner's manual. Most auto manufacturers recommend 87 octane gasoline, as measured by the (R + M) / 2 method on a test engine under defined operating conditions. If the vehicle knocks on the recommended grade, a higher octane grade should be selected. Some foreign vehicle manuals recommend a Research Octane Number (RON) instead of the more common octane rating that appears on most gasoline pumps. As a rule, the recommended octane rating can be determined by subtracting four (4) from the recommended RON number. A vehicle that calls for "91 RON" could use 87 octane gasoline (as measured by the (R + M) / 2 method).

The placard on my '81/H says "Reasearch Octane 91 min", which translates to 87 octane minimum at the pump.

The (R+M)/2 is the standard displayed on the pumps.