An interesting story about Octane and Altitude....

[Toe Dragger]

I was reading in another thread about using lower octane because of our higher altitude and I found this article about the subject.

Do motorcycles compensate for altitude the same way cars do? I'm curious to try but don't want to cause engine damage over time by riding hard with lower octane fuel. :?


Colorado's Octane Secret: Part 1 (http://www.koaa.com/features/investigates/view.asp?ID=251)

Colorado's Octane Secret: Part 2 (http://www.koaa.com/news/view.asp?ID=250)



In fact, Industry experts say the only research ever performed was done roughly 20 years ago, with carburetor engines.

We even contacted Ford and General Motors in Detroit and experts there say cars with carburetors can run on 85 octane at higher altitudes. But virtually all cars sold since the mid-1980's don't have carburetors and use fuel injectors, which GM and Ford say you should never operate on anything less than 87 octane.

The engines are built to compensate for changes in altitude and they're built to handle a minimum octane of 87 at all altitudes. If you use an octane lower than 87 your engine must compensate even more, which adds more stress to it and could cause damage over time.

[Anonymous]

There is also a vast difference between car FI systems and bike FI systems - also, a lot of bikes do still come with carbs.

Car systems are closed loop - where the FI system adjusts what it does using data from both before the charge is burned, and after. Bike FI systems are open loop, really only processing the pre-combustion effects. Some (read: very few) are starting to add on post-combustion sensors, but the computers are still waaay behind the curve compared to car FI systems.

[Quick636]

I was told when I bought my bike to put nothing but the premium gas in it due to the high compression that bikes run at these days. If this is indeed true, why would the octane issue even pertain to bikes? Shouldn't everyone be putting 91 in their tanks anyway?

I'd be curious to find out if anyone on here runs the lower octanes.

[Mr. Turbo]

Only person i know who runs regular gas in their bike is my older bro, but his bike is an 82 kawi KZ750. Compression ratio is 9 to 1, and kawasaki specs 87 octane gas for it. The bike runs fine, no knocking/pinging, gets good mileage too. But on a bike with 11 to 1 or higher compression i don't think the dip in air pressure would allow you to run lower octane gas. The only thing i can think of that would cause an engine to run "normally" on lower octane gas when at altitude would be a reduced overall compression pressure due to the air being at lower pressure. Not sure if that's right, but its the only halfway sensible explanation i can think of.

[Anonymous]

Your ZX6R is pretty high at 12.8:1 (stock) compression ratio. Generally you can drop one grade of gas up here, sometimes more, on a stock motor. I run 85 in my superhawk - no pinging, no knocking, but I also have lower compression than you. You can always try to go one grade down at your next fillup. If you start to notice pinging or knocking from the motor, just mix in some premium to bump the octane back up.

[Filo]

My car manual says I should only put 87 or higher in, regardless of altitude. I guess that supports the argument that we are getting screwed. I always put the highest octane gas in my bikes because that way I am faster :D . I tried increasing my talent, lowering my weight by wearing flip flops and shorts and nothing else, putting race takeoffs on my street bike, watching Biker Boyz, everything. The only thing that really worked was 91 octane gas :lol: . Now if I can only find higher octane, I will be even faster :) . Which reminds me, <hijack>I got my Faster video last night :D, very nice footage, needed more umbrella girls. </hijack>

[Boy Scout]

2002 CBR600F4i manual says pump octane number 86. I have never seen a pump with 86 (right between low-grade 85 and mid-grade 87).

I used to use the high-grade stuff until I read a discussion (might have been on CSC), the end result of which was that from a power/performance perspective, you should run the lowest octane you can that doesn't cause pinging/knocking. Based on that I started dropping grades. I've been running 85 since then without any issues. Bike was in for 16k service last week and received a clean bill of health. Valves didn't even need adjusting.

[yakuza]

Yup, I race with 85 octane in my F4i. :D

[Quick636]

Curious, what is the compression on the f4is?

My manual says to use nothing lower than 90.

[Boy Scout]

F4i=12.0:1

[Quick636]

Hmmm...

While you guys are probably right and I could get by on 87 or 85 octane, I think I'll stick to my plan of using 91.

Sorry for the thread hijack, but its an interesting discussion.

[Lurch]

Lower the octane the more power you will make. I run 85 in my 03 GSXR 1000 during practice and everything is just fine. At this alt. you are wasting your money on 91 octane fuel. You are compressing a hell of a lot less air so you don't need the octane.

Dyno runs at TK have proved that 85 makes more power then 91.

Lurch

[Quick636]

Well THAT is an interesting note. If you run 85 in your 03 g1k then there is no reason why I shouldn't be able to in my 636, right?

[Lurch]

It will run just fine...

Lurch

[MattTLS]

Another thing to consider is the actual versus stated compression ratio. As with hp and torque numbers (and just about everything else) the manufacturer claims for its product, the actual compression ratio is OFTEN lower than stated. The only way to really know is to measure it.

To find it you need to know the bore and stroke, be able to accurately place a piston at top dead center, and be able to accurately measure the volume between the top of the piston and the combustion chamber (using a light oil or similar introduced through a buret). The rest is simple calculations.

I haven't checked my TL (stock internals for now), but they typically are closer to 10:1 than its reported 11.3:1 cr.

Matthew

[Anonymous]

Curious, what is the compression on the f4is?

My manual says to use nothing lower than 90.

Your manual was also written for sea level. We have about 66% of the oxygen here that you'd normally get down there. Adjust accordingly.

Your call, but it'll run fine at least a grade down at this elevation, and a shitload better when you go for trips into the mountains...