Octane revisited

Alright, I have to admit that I am stupid.

I took off the sidecover of my bike, and it told me I needed 91 octane leaded.

Hmmm... I have done a little research on this now, instead of shooting from the hip. Lead does not increase the octane of gas, but prevents knock in some other way (I don't know how).

So - logically - my bike would require MORE than 91 octane if the gas is unleaded.

I did put 93 in yesterday, and it did seem to run better! The compression ratio on my bike is 11.2:1, which according to a chart I just looked at, would require about 105 octane theoretically!

The other kicker is that because our engines are older, they have a layer of carbon and other crap on the cylinder walls, which INCREASES the compression ratio, and prevents heat from escaping. This would also tend to increase the octane requirements!

Some things to think about: As mentioned earlier, richer mixtures tend to prevent knock by running cooler. Retarded timing helps prevent knock. The closer you are to knocking, the better gas mileage you will get! (More power with a slightly richer mixture, better mileage with a slightly lean mixture.)

From what I read though, the variables are many, including engine temperature, mileage, driving style (duh), small details like porting, etc...

So the question that I have is: It sounds like I need to figure out what octane my engine will knock at. How do I do that? ECUs use accelerometers (sp?). I don't have one! I am not quite sure what knocking would sound like. I also believe it is most likely to occur under acceleration - like when I have my helmet on - and can't hear well.

So - what do we do? I don't believe anymore that taking gas mileage logs will help! :? Maybe I will start running lead in my bike. My brother put lead in his older car.

 

It is mathmatically possible to figure the Dymanic Compression.

That involves RPMS (speed of piston), chamber size, valve overlap (some compression loss normal) (i think a couple more parameters..

but you still just end up with a compression # even if more accurate than static.

It becomes harder when you try to figure in heat loss (ie cast iron vs Alum. heads/block), timing, etc..

I'd say if you feel like you notice a difference (dyno handy ?) go for it

 

I am not concerned about power - I am more concerned about knocking. Sorry about the confusion.

Power that we can dial in is not a factor of octane unless you are running way too low of an octane so that it is knocking the crap out of it (it is a tiny bit - but nothing compared to air/fuel ratio).

 

hum aside from hurting my wallet lol could using a higher octain gas in my bike cause any poroblems? and would it be worth buying if it wont cause problems?

 

Using the octane that the bike calls for (91), or a little higher won't cause problems.

I am surprised more people haven't responded! Has noone ever heard their XJ knocking? Has anyone had it run a little rougher with a lower octane? Maybe nobody knows the difference. I hope not.

 

I don't think people are responding because this is 'one of those topics'. This topic despite apparent victories on either sides is still not won - just like tires, politics, religion and what oil one should use - there never will be a common 100% set in stone answer.

I've never heard my bike knock.

I've not noticed a difference in fuel economy between different grades of gas, certainly nothing statistically significant.

My cost per mile math through the past 2 years shows no savings by using more expensive gas.

If you have a properly running bike the carbon buildup will be negligable - certain cleaning agents might reduce carbon buildup...

I run the regular grade gas and it works perfectly.

My vote? Well - living in Halifax Nova Scotia, riding in a temporate somewhat humid climate, from cool to somewhat warm tempatures, with no windscreen on a bike with potentially questionable carb boots and somewhat dull chrome - regular gas is better. If you have any differences your milage may vary.

Woot.

 

I've run 91 in my 81 650 since day one and haven't had a single problem. I am aware however, that one should not need it since my bike runs 9.2 to 1 compression ratio. With anything over 10 to 1 I would run high test. My 10.5 to 1 VW needs it but my 8.6 to 1 doesn't.

 

What do you mean by high test? Do you think I should start running leaded 91 or 93? My compression is over 11:1! I am leaning that direction.

 

why is your compression so high? Shaved your head or new pistons and rods? I read that my compression should be around 9.2 stock for a 85 750 maxim midnight

 

I've had my bike for 4 yrs and I have never heard it knock or ping with cheap gas. I usually opt for the 93 ocatane though because it is only maybe an extra buck a tank. Detonation is affected by the shape of the combustion chamber, load conditions, rpm, timing, effective compression ratio, efficiency, engine temperature you can go on and on. Maybe the head design guards against it, maybe the rpm range of a bike's engine and the timing curve allow you to run a lower grade. I never felt it in my bike , however in my Dodge truck the thing knocks so bad you think it was missing a pistion 93 octane helps but under certain conditions it gets the shakes.

 

Thanks easygo, that is good to know. So I can asume that if it does knock, that I could feel it. Thanks again.

I have the Maxim X, so I guess it has higher compression... I need to confirm this though. I got it off a spec sheet on the internet, but I didn't confirm in the manual.

 

My 25,000 mile XJ runs like crap with 87, and runs great with 93.

In the 6 months I've had the bike (Florida always biking weather), I always used 93 since it cost less than $1.00 extra per tank. Then, as a test, I tried the cheap stuff. I could tell the difference 5 miles from the gas station. Still, kept testing that tank, then to confirm, filled up again from another station with cheap gas. Again, ran tank out to confirm, and same bad results. Next, filled her up with 93, and within 5 miles, my old great running bike was back. Just added the 93 back yesterday, so these results are all current.

Do a couple tank test yourself, and you will spend the 80 cents extra.

I don't know if it is the octane or the additives, but the extra money is well spent. Also, have a Mac 4x1 which might feel the fuel better than 4x2 stock exhaust would.

 

By high test I'm referring to the 91 octane we Californians are stuck with. All apologies for the missunderstanding.

 

Richard:

A few thoughts:

1) I think you'll know if your bike is knocking. Even inside an enclosed car the "rattle" is obvious. Think rocks in a coffee can.

2) There is no harm or risk in running higher octane than you need to (other than to your wallet). Higher octane prevents detonation. It is not hotter, faster burning, more explosive, etc.

3) If your compression ratio is truly that high, I would think that you would need high octane fuel. But if you are satisfied with your bike's performance on regular, and you don't hear any detonation under load or acceleration, then there's really no reason to switch.

Take care and have fun-
Herb

 

sidenote: Octane DOES infact affect the burn time of the fuel, but it is minimal and for our purposes can be ignored. The only true diffrence is flash point.