what octane?

Just wondering what octane gas you all run? The owners manual says LEADED gas!... min 91 octane. I was running the highest I could find but someone mentioned that it may make the bike run to hot and that carb. bikes don't need high octane. Any opinions?
Thanks

 

Higher octane gas actually burns colder, ormore specifically has a higher flash point.
I don't believe these bikes should require leaded fuel as they were produced after 1973, and would have to conform to US emission standards.
I run mid-grade in all my bikes, just because I don't trust the bottom of the barrel stuff here in NY.
My VF750c is a higher compression engine, and runs fine on 89 octane.
I will be interested to hear what other XJ'ers are running, as this will be my first year on this bike.

Nachoman

 

I am running 87 octane in mine. I believe that putting higher octane in your vehicle than what the service manual is NOT a good thing. Gas mileage should go DOWN. This is because your bike is designed to compress a certain amount and then have a spark, and a certain amount of the gas is going to burn - both from the spark (a tiny amount) - and then the rest of it will burn from the heat AND the PRESSURE SPIKE that the initial burn causes.

However, when you use a higher octane gas, less of it is going to burn, because it is harder to burn by compression. In other words, you have to use MORE fuel for the same amount of power. This might be why higher octane burns cooler. By the way - the hotter the combustion gases, the more power - because hotter gases expand more - putting more pressure on the piston, therefore, more power.

Try it - see what your gas mileage is using different octanes. Let me know if I am wrong.

But, if you can hear pinging in the engine - By all means, use higher octane because predetonation is a very bad thing.

 

Right - and about pinging.

The spark sparks before top dead center. This allows the flame front to travel properly before TDC so that you get maximum (time) power out of the (power stroke) explosion.

Retarding the timing moves the spark earlier - if it is too early you get a pinging ( the cylander being pushed downwards by the expansion of the gases BEFORE it gets to TDC. the piston doesn't travel backwards but it is hindered in reaching TDC prior to the power stroke)

If you retard the timing until you get pinging, then, advance it a bit you get where you should be.

Now - where this matters is that timing is variable - at high revs the piston moves faster but the flame does not. You need more retardation then to get the same explosive downstroke. Using a higher octane will help retard the flame front - or in otherwords - resist ignition until the right time.

These bikes were designed to run on lower octane fuels - using a higher octane fuel might delay the explosion (advance the timing) more than nessisary - this means you get less power out of the power stroke and potentially more unburnt fuel out the exhaust.

Certainly ride with both fuels and see what you get the best milage from. I'd say that what ever gave the best millage was also the best for the bike.

On my car I have the ability to adjust the timing easily - and where it is set right now seems a bit too retarded... high loads cause it to ping a bit. I'll be notching it up this weekend to see if I can get it sorted.

Woot.

 

As Woot said - these engines are designed for unleaded 87 octane fuel - so don't know what manual you're reading there.....

Shouldn't need 91 octane unless you're running a really high compression engine.

I occasionally use 89 octane during hot weather to help a little - usually has better performance, and not as susceptable to pre-ignition on our older engines.

But as a rule, fill up with what you can get....

 

I've run 91 octane (in my Maxim 650) since purchase ('88 ) and have never ever had any problems. The higher the RON, the more resistant to predetonation (this is a good thing in my book), and the cooler your engine runs (for an aircooled machine, this is priceless). I also have a much cleaner engine as a result and that is priceless too. Richard, your logic is sound and I agree with your prespective. In practice I can tell you I never noticed any improvement (or degradation) in fuel economy using high test, nor did I notice anything amiss with my spark plugs when checked for heat range. I've torn down the motor twice (long, stupid story but thats for another time) and have not noticed any coking or other heat issues within. I'm going to sit on the fence on this one. I can't remember what my owners manual said, I'll look for it at some point.

 

Mine does specify leaded 91, but the leaded tended to run cooler and I "assumed" that I would maintain the temperature range by sticking with the higher RON fuel. I'm going to have to experiment now. Darn you, Rich, I'm in a quandry now!

 

Robert - I agree you want to avoid early detonation... but I wonder if you increased the octane too much wether you'd essentially advance your timing so far as to basically waste the expansion of the gas... ie by the time you start getting power from the explosion it is late in the powerstroke?

I'd guess there is a large window of correctness built into our bikes...

You would think that the optimal timing of the explosion would be obtained by the right mixture of sparkplug tempatures, octane and advance... and this optimal setup should give the most bang for the buck - or in other words the best fuel milage.

also - isn't there a number of ways of measuring octane - like one that is RON + ??? / 2 and one that is just RON...

I'm not up on my gasoline specs... if you could help fill in the gaps here I'd be thankful.

Woot.

I do keep detailed fuel milage logs but I find them a bit too variable to give good feedback on milage. Maybe when I get out I'll run 5 tanks of supreme, 5 tanks of regular and 5 tanks of the middle grade and report my average fuel milages for each grade.

 

Sorry I was a bit fanatic earlier. :lol:

I just talked to a guy who works on quite a few engines at work, and he said that he wouldn't expect any problems or even that much lower gas mileage from higher octane, just much higher prices! 8O

I think I will be taking logs too! It should be easy with such a small tank, right? My ride to work is pretty constant, and it won't be too expensive an experiment...

I defenitely don't think that 87 is any "dirtier" than 89, that I am pretty sure of. In fact, it could be cleaner depending on the fuel station, since 87 is used so much more, and so the tank at the station is "flushed" out often. Maybe this isn't true. I don't know. But I do think that most gas companies don't put any additives in 89 or 91 that they don't put in 87. I think they just seperate the different hydrocarbons at the refinery, and the longer (?) ones go to 89 and 91. And then they all get the same additives?

I would just much rather ride on 87 if I can, because the reason I told my wife we were getting a motorcycle was to save money! I don't think she would appreciate me using premium.

 

A thought - since our bikes are carbureted, is the gas mileage going to be too varied based on air tempreture/humidity to get any meaningful results?

 

On the back of my right hand side Maxim cover there is a sticker, that was put on the bike at the factory, that lists a few different specs. Here's what it says Vehicle Emission Control Information and the second to last thing on the list says “Gasoline Grade (leaded) Research Octane : 91 MIN. Also states what kind of spark plugs, valve clearance and a few other items. I always put at least 89 in my bike and 93 the other times. They don’t seem to sell 91 Octane gas around here so this averages out to around 91 if I fill the tank half way. I would stick to using 91 octane at least especial without leaded gas.

 

Just wondering CT - what version of octane is listed on the bike? Is it Europe only that gets into the (RON + ??? / 2 ) = listed octane at the pumps?

Richard - I've always ran 87 - didn't feel that it needed more... I'll try more and see. As for the variance... I'm not sure we could tell in just 5 tanks unless it was a significant change... however - it would need to be a significant change in order to get your money back on the more expensive gas.

My average mpg over the last 34 tanks of gas is 34.005 mpg with a standard deviation of 3.12mpg. That is over a period of just shy of 3000 miles.

 

You crack me up Woot - standard deviation? :wink: Is statistical analysis a hobby?

 

Woot,

With your analogy higher octane fuel would retard the the fuel air explosion, as it would move it to after TDC.
Personally I don't think it would make that much of a difference in the timing.
The main advantage is resistance to detonation, but once the spark plug fires, the burn rate will not make enough of an impact to change the timing.
Around here they do all kinds of things to the fuels to create the octane rating, including adding ethanol, and oxygenating.
The octane rating is a ratio of RON and MON.
I will try to find the link that explains all this....

Nachoman

 

I didn't mean to say that it would move the firing timing after TDC, rather that it would make it slightly advanced. In an extreme advanced situation it would be bad... in a slightly advanced situation I would have thought it would have been less efficient.

I guess what I'm worrying about is that a microsecond change at low RPM is different that a microsecond change at high RPM. The flame front moves at a steady (incredibly fast) speed but the pistons move at a much faster rate... ie twice as fast at 9000 rpm as opposed to 4500 rpm.


Richard - Maritime Risk analsys... I write computer software and write reports for a few agencies... alot for the Canadian Coast Guard. I play with numbers alot. All I meant to get at is that each tank varies quite a bit, but that in order to get your money back on a tank you'd need it to vary quite a bit. Hang on and I'll do the math on that.

 

HRM - surpring math.

I'm working in both Canadian and American units... so I should recheck this.

My cost for a litre of fuel is 113.9. Say supreme costs 116.9.

If I get 34 mph on regular, on 10 litres of fuel to get the same cost per distance then I need to raise the fuel milage for supreme to 34.9 mpg.

With my 3mpg std dev. it would be really hard to tell... not without a huge sample size.

 

Altus,
I was reading the factory owners manual from my 82 xj650 maxim. That is where I got the 91 min leaded gas info.
Woot,
How do you figure your gas mileage, you mentioned liters and then MPG. My bike gets at least 48mpg...I believe I have even gotten 50-52. 34 seems pretty low. Even my Night Train gets 45+, and the engine is 2.5 times bigger..
I think I am going to try 89 octane in my Maxim for the next tank and see how it runs. I have to run 93 in my Night Train because of the EFI, and that is getting expensive! Thanks for the replies everyone.

 

I'm in Canada so I have to convert everything to mpg for the web

My milage is really low - that's what happens when you don't have a windshield of anysort and have wide shoulders before putting on gear. I'm a sail!

If I put a windshield on I think I'd get a lot better milage.

How do I measure?

Every tank I fill up to the lip, while the bike is on the sidestand. On each receipt I right the kilometers travelled and the odometer reading. At home I put the receipts into my vehicle database (which has maintance, parts, gas receipts, etc)...

I usually get about 145km for 10 litres of gas (rough numbers)
OR
90.1 miles per 2.64 Gallons or 34.x mpg.

Most of my miles are highway, on backroads the mpg gets better even if I flog it - and if I take it slow I might sqeeze a 44mpg out of it... I did that once when I was limping home with a tire that could delaminate.

My old ninja got way better milage... 350km per tank at 15 litres... or about 55 mpg (and sometimes better). That bike I got huge milage on the highways - tucked in on the tank I could get 400km out of it. Shame I sold it. :-/

 

Woot is just says Gasoline grade regular (leaded) Then below that "Reseach octane : 91 MIN.

It’s been a few years since I have been to Canada and I forgot that the gasoline is quite a bit more expensive there then in the States. But well worth the drive to Mont Tremblant Quebec for some great skiing. So I guess your paying close to 4 US dollars a US gallon.

1 US gallon = 3.7854118 liters
1 liters = 0.264172051 US gallon

 

It is about 3.60 a gallon now - last summer it was 4.60

when it was 4.60 yours was around 3 in NY? There are rumors of $5 gas in the states this summer - so I suspect we'll be north of that :-/

I really need to improve my milage. I could install a shield, new carb boots and ride slower... or I could just buy a sport touring bike and keep the maxim to restore.

(EDIT) with the change in exchange rate - it is now actually 3.72 :-/

 

My Maxim gets that 45-50 with no windshield, cracked carb boots(hope to fix that) and I weigh over 200...
I don't think it has anything to do with having a windshield!! LOL
How many miles on your XJ, Woot? Mine has 30,000. My friend just bought another 82 Maxim with only 19,000 and I bought my old one back from her to drive to work. I will try to post a pic of her bike after I detailed it yesterday. I don't have one handy of mine, maybe a another day.

 

I'm north of 70,000km... I average 10,000km per year. Less in the past two years as I've brought a house and am tied to the cage more.

:-/

Very nice looking bike

 

You all have such beautiful bikes. Sigh, makes me think of when my baby was new (to me anyway). I've been such a bad owner, she deserves better. I'm shamed by you all. She has been shuttled around through all the PCS moves, left in the rain, ignored, but always got the best in mechanical maintenance. Hope that will stave off the rot for a little while. I've got to get moving on putting her back on the road. If only I could give her say, two weeks of my time, uninterupted, I think I could bring her back to her former glory. Woot, don't worry about converting, your going the extra mile. Very nice of you. I got one of your VW Rabbits once, loved going 110 KPH! Impressed my then girlfriend to no end. Great car, sadly sold to several owners and dissappeared into obscurity. VW of Canada did really good work, I'll give them that. The Mexican cars were trouble (electrically speaking). Enough of that. I'm motivated by your picture Nightraingirl, gourgous beast. Hope your parking her indoors.

 

You must remember that RON is higher than the octane numbers posted on the pump. The pump octance number is RON+MON (motor octance number) /2, so it is an average of the two. RON and MON are calculated a bit differently, so one is actually higher than the other for the same fuel sample. RON is the higher one, which leads many owners to believe that they need a higher octance fuel than is actually the case. It is also why many Europeans think thier gas is better than ours, since they use RON numbers exclusively on thier pumps and in their engine specs. I did quite a bit of research on this awhile ago, since my turbo is listed as requiring 95 RON. I thought, where the hell am I gonna find 95 octane gas, since 94 octane is the highest I can find, at Sunoco. Turns out, the 91 octane does just fine in my bike, although when the 87 is at $1.00 l, I'm tanking up at nearer $1.15 or so.

 

Great replies... I have a windshield for mine, but I took it off. Maybe now I have to do a two-way experiment. One variable is the windshield, and the other is the octane! Bummer...

 

This also makes me wonder how a quarterfairing would do for wind resistance? Hopefully those of you with those add-on fairings could post up!

I guess we kind of thread-jacked this one, right! :roll:

 

We jacked it to hell... sorry to the original poster... we did talk a little bit about octane though

that and we're community building.

 

The quarter shield would still get the majority of the windblast off of your chest and, consequently, lower wind resistance, which equates to fuel savings. Comfort, lower fuel consumption, what's not to like?

 

Thanks for the compliments, Robert, and everyone else. Remember, that is not my bike, that is my friends bike. I had just gotten done detailing it. I will post mine another day. And yes, she lives indoors as do my bikes.
As far as thread jacking, I was the OP and then I posted the pic, which has nothing to do with octane so I don't mind, I thread jacked myself!! As long as everyone is having fun! Anyway I found out what I wanted to know, apparently everyone uses whatever gas they prefer. LOL

 

What fuel octane do you find works best in your bikes??

 

Unless modified, our engines are designed for 87 Octane, regular unleaded.
If you live in really hot areas where the engine stays warm, increasing to 89 or 90 helps the engine some. Other than that, mid-grade or premium is just a waste of money.

 

Our motors (at least the 650-750 from 81 to 84) are at 9.2 to 1 compression ratio. They should be find with 87 octane. I agree with Altus, unless you encounter predetonation, stick with the cheap stuff.

 

My manual called for 'Regular' gasoline. Correct me if I'm wrong, but in 1981, wasn't 'Regular' actually leaded (with 'irregular' being specified as "Unleaded")? Not that you can buy leaded gas at the pump these days (though I still use it on a regular basis ) but isn't that what they're actually designed for?

 

With gas at about $3 a gallon, 5 gallons is $15.
93 here will cost you 20 cents a gallon more, or with 5 gallons a buck extra.
$15 for cheap gas, $16 for good gas?
I like my bike, and "waste" the buck for good gas.
Plus, while I doubt it is true, I feel that premium is made better, and makes the bike run better.

 

Regular (87) vs Premium (91 or 93) is NOT the same as Cheap vs Good.
Regular does NOT = bad.
Premium (high octane) fuel is not always "good gas" -- unless your engine needs it, it's a waste, and could in fact be doing harm to your internals. Most of the price premium you pay is for the higher ocatane, marketing hype and profit, and in some cases a slightly different additive package - and I do mean slight.
Octane is an ignition inhibitor - meaning it slows down the flame wave - in esscence causing the fuel to burn 'cooler'. This also means it reduces the total amount of energy that you'll get from the same amount of fuel. In high-compression engines, or if you're experiencing pre-detonation ("pinging" or "knock"), the higher octane levels suppress the ignition slightly to prevent or reduce these effects.
Unless you've modified the engine internals to give higher compression, or live in an extremely hot area, or getting pre-det -- you're not getting any benefit from higher octane fuel - just paying more.

 

Fuel price should not be an issue for those of you who live in Canada/US!
Here in Denmark the price of premium is $6.70/us-gallon and $6.25/us-gallon for regular. So no matter what you put in the tank, the price is crazy and any increase in mileage is welcome.
Windshields and fairings will pay them selfs in a matter of days.

 

I lived in Europe and Japan for about 3 years each, and can attest to the outragious fuel prices.

Japan back in 2003 gas was around $4.00 a gallon. I couldnt imagine how much it is now.

 

Right now it is about $3.49/Gallon US here. It's at it's low, and it is expected to be $4.13/Gallon US this summer again. Last year it hit about $4.50/Gallon US.

Europe certainly does have higher gas prices - although - there are some distinctions to be made. In more populous places mass transit is a possibility instead of a theory. Only our major cities have a transit system worth talking about - east of Quebec we have almost unuseable systems. The other factor is annual milage - here I log about 20,000 miles on my car and ~6000miles on my motorcycle. When I was in England I might have logged (extropolated) 6000 miles total and likely less as the transit system was phenomonal.

Gas prices for the bike - well - as you say I don't really worry as much - it is a toy that I occassional commute on.

While we in Canada have higher gas prices than the 'States, we do have lower gas prices than Asia or Europe.

I put at least 50 litres of gas in my car a week... add in the bike and I'd be surprised if I used less than 75 litres a week.

I guess the flip-side question is how many miles do you drive in Europe?

Woot.

 

Now you know why cars in europe are mostly 4 cylinder with 1.3-2.5 litre

I drive 19-20,000 miles/year(diesel app. 40miles/gal)in my car and maybe 4000mile/year on the Seca. Then again I don't live near the major cities.

Here in Denmark the fuel price is not the only hurdle.
Both cars and motorcycles has this registration tax that adds min.180% to the actual price!
This means that as an ex. the 2006 Mustang @ $25,000 in the US is round $125,000 in DK
We are used to it and it pays for a big part of our Welfare State.

Returning to the subject.
Yesterday I filled my Seca with regular octane fuel for the first time.
That was a big succes. The engine has been coughing and cancellig when it came from normal driving revs to idle and again in starting to acc.
So after reading the forum here, I adjusted the idle +50rpm and tightened the clutch cable. Both things helped and gave much more smooth driving.
The last step to succes was the change of octane level.
The engine runs perfect now.

 

i get about 37mpg on my xj650, no windshield, and fill up roughly 140miles/tank. 87 octane

 

You know, i had kinda thought the guys saying higher octane runs better in these bikes were full of bull.... but i put 89 in the bike today and i must admit I thought it did run a lot smoother....

Now im not sayin that everytime i fill up ill put it in but... i dunno. Maybe if im feelin special.

 

Avoid the mid-grades.
I worked for a company that designed and built gas pumps. In 99% of the stations out there, there is NO real middle grade. The pump simply mixes Super with Regular at a percentage determined by the gas station owner (via computer interface software). Although there are laws and regulation forbidding blend and pulser modifiction, I have seen 80/20 blending used in a LOT of privately owned stations. Also, since stations use much less Premium gas, there is a greater chance of H20 contamination or old gas.
I would use regular unless I had knocking, and I would only buy super from a newer station.

 

damn good info right there....

 

Let's think about this. Is not premium a blended version of regular (added anti-knock suppliments)? I had always assumed that gasoline was blended to make the various RON offered. Makes sense to blend at the pump, saves money and additional storage problems. Kinda like chocolate milk, can't pull that out of brown cows now can you?

 

The problem isn't that the gas is blended at the pump.
The problem is that certain station owners are crooks.
Lets say 89 Octane is calculated to be 60/40%.
A dishonest owner can bypass the safegaurds and change the gas to 80/20%, giving you about 87.5 Octane and saving him money. (It is a software change, the owners are not supposed to be able to change it) They just have to change it back before they get calibrated by the state.

 

Nay, say it isn't so! Sneaky twits. I've been purchasing fuel at the base here for years, guess I'm fortunate. A little computer savy can be a very useful tool. Hope I don't frequent any crooks in my travels.

 

So I got curious.....Wondered if 93 would make a difference.

Filled up with 93, ran to about 140 miles, filled with 87, calculated mileage, got 40mpg this last tank. Gas was the only difference. I don't know if it was because the carb got a "cleaning" effect from the higher octane as or what. I didn't notice any difference in the way the bike runs tho. Idle was the same and the bike didn't seem "smoother".

I'll check mileage again after this tank and see if it drops back to 37.

 

I doubt the higher octane would have any significant cleaning effect. I do know that my statistical variation from my mean fuel milage is 4 mpg. Meaning the tanks swing back and forth quite a wide range between really bad and not too bad. Worst milage I get is on the highway and the best is slow speed back roads.