Octane.

So I noticed the other night that my bike is supposed to take 91 octane leaded gas. Obviously can't get leaded anymore, but I've been using 87 unleaded. Haven't noticed any knocking or anything, so am I hurting anything other than power output by using 87? And I doubt I'm affecting power all that much either.

 

87 is perfectly fine.

 

Marketing lie printed in American owners manuals - all XJ's are designed for 87 ocatane unleaded

 

x2, what they both said.
Mid grade does have some additives though...but heck, reg unleaded is already too darn high.

 

The lowest octane that doesn't result in pinging (and you need to listen for it carefully on a motorcycle engine!) is the one to use.

 

While we're on the subject... Since I live in Iowa and the cheapest here is the ethanol blend, (not E85), does it have any adverse effects? This is the 89 octane stuff... No way I'm going to try running E85 through it, since I doubt the thing's designed to run what is essentially alcohol.

 

I have had a bunch of questions on octane. What I found in a nutshell is the higher compression the higher the octane. The goes for all engines. I own a coupe of personal watercraft and find the same with those. You bump up the compression, you must bump up the octane. Since no one here that I know of other the the Seca Turbo group runs high compression, 87 octane actually works best. Octane is used in high compression to get full compression before igniting the mixture. Thats why low octane in a high compression situation cause premature detonation or pinging. High octane in a low compression situation makes for an incomplete burn, and can actually rob some of your mileage and power. So, running 91 in a regular auto or motorcycle, you are just wasting money. I owned a Subaru WRX turbo that anything but 91 could be heard loud and clear. Hopefully that makes sense.

 

Never thought about that, but very good point Salty

Here's a laymans video on octane, check out his other motorcycle videos too!
http://www.youtube.com/watch?v=ZZx5w1jVUDA

 

Well, just for shits and giggles, I put 91 yesterday when I filled up. It seems to run smoother, idle easier and pull a little bit harder than it did with 87. I used to be able to hear it quite clearly above about 4500RPM with the faceplate on my helmet closed. I could barely hear it at 6000 now.

 

Buy what you want, but octane ratings simply reflect the fuels resistance to detonation. The higher the octane rating, the more resistance to detonation they are and the less 'knock'. Any perceived boost in power or smoothness is likely a subconscious response and not an actual one.

JMHO

 

I usually agree. I've accidentally pushed the wrong button at the pump a couple times and put 91 in my car as well. No noticeable difference there. I'm usually of the opinion that you should run what the manual says to. My car's designed to run on 87. The only difference I noticed between 87 and 91 in the car is that I get better mileage with the 87. It's what the car's designed to run. Following that line of reasoning, the bike is supposedly designed to run 91 and it does seem to make a difference.

 

Well wraith, BUY that 50 cent more expensive gas and have a great time.
only time the mid grade is in the tank...when I am looking to spend more money, or use the additives, or when it goes on sale
I have used 87 and cant tell a diffrence good or bad, and the compression ratio does not warrant using performance octane fuel.
my .02
WAIT!!! with inflation and high gas prices....heres my .05


Food for thought

 

I'm not so sure The Old Engine should be on 87. My old Beetle (bough New in "71 was designed for 87 well Doh it's a Low comp Aircooler. (aircooled motors need higher Octanes than their more Modern brothers .. all things being equal)
BUT in the intervening 40 years.. Octane ratings have Changed! 87 is more akin to Urine than gasoline.
Pore old VW engine ((yes it's healthy and running as dr Porsche intended) is currently needing at least 91 to run as it should...ie neasy starting passable idle smooth acceleration no missing etc.
Same Fuel, just the "modern' ratings have been changed
I would think even the lowest power Yama 4 cyl Aircooler here is well above 9:1 compression .. a wee bit higher than the VW's 7 :1
I won't be trying to save a $1.50 per tankfull... thanx.

 

Your call Danilo, but I'm not bitting! ;-)

 

Danillo - Where are you coming up with this? I agree that perhaps octane ratings have changed, but we have come a long long way in gasoline since the 70'. Through refining and other advancements we actually have cleaner, more efficient burning gasoline then we did back then (leaded and unleaded for starters). "An aircooled engine needs higher octane?" I'd love to know where you get that from! Your decision of fuel is your choice of course, just for the rest of the people reading this thread most likely want facts and not preferences. I'm open to learn from you if you have a place I can find these facts. I'd like to say that higher octane fuels are on average cleaner than their lower octane counterparts, but that isn't as true anymore because the lower octane fuel tanks at most gas stations are emptied and refilled more often bringing in fresher fuels.

 

if our gas is so great now, why do we all have to add inline fuel filters to our bikes? i have no rust in my tank, but my carbs filled up with junk last summer. that's the first time i've had to touch the carbs since 2000 when i got the bike. why has my gas mileage gone down each of the last 2yrs? why do the shops tell you to add a stabilizer if you have a can of gas for the lawn mower that might not be used up in a month?

 

We end up using those filters because there is crud in the tanks at the gas stations we go to. Have very little to do with the gas it's self, and certainly has nothing to do with octane. It was just as dirty in the 70's. As for fuel stabilizers, that is to prevent the gas from turning into varnish. I'm not excatly sure of the process, but I can ask my neighbor who is a chemical engineer for Chevron if you like. Again, nothing to do with octane. Gas in the 70's turned to varnish too. I wish I would have taken pictures of my bikes carbs when I got it. 18 year old gas! I'll show you varnish!!

 

So anybody know offhand what the compression ratio is in my 400cc 2 cylinder?

 

I pitched this question out to VERY sucessful racing engine builder and tuner who also works on and rebuilds old bikes. He says and I quote " If you are putting anything but 87 octane in your XJ except for the turbos, you are putting hard earned money down the drain. It is impossible for these machines to burn all the fuel being introduced into the combustion chamber. any improvement in performance is a pipe dream." This is not MY opinion because I know so little about it.

So we put my bike on an exhaust analizer with the 91 I usually use and took the measurement (what ever they measure). I then ran the bike to well below the reserve and then filled with 87 octane with 10% ethonol. We put it on the analizer and the differance was amazing. Far less unburned fuel was present and carbon emmisions were also greatly reduced. Infact if I lived in a state that required an emissions tes my bike would have failed with the 91 octane fuel.

He also stated that he runs 87 octane with 10% ethonol in ALL of his stock vehicles. He does not however have any high performance stock vehicles like the subaru mentioned in a previous post.

I thought this was interesting. I now am going to use the "low grade" from now on.

The Buff

 

Interesting. Now to add fuel to the fire. (no pun intended.)

I just finished up that tank of 91 this afternoon. I've been driving more and more spiritedly as I get used to riding a bike. My gas mileage went up from 52 to 56. I'm not accusing anyone of being full of sh!t, but I am saying that the changes I noticed were not in my head.

Granted this one only one tank of gas, so your mileage may vary. (pun intended this time. )