Octane for Sustained high RPMs.

[Banshee's Veil]

Hello,

So quick question. I finally got my XS1100SH performing beautifully throughout all speeds and rev range. I've been using it as commuter.

I'm using a 16" rear wheel off an XJ1100, I don't know how accurate my intermittently working tach is, or how fast I'm going beyond the limits of the speedometer, but sometimes I find myself cruising at 6500 to 7500 rpm in top gear.

I know these are low compression engines, but should I put in a higher octane rating for sustained engine speeds, or is 87 fine? I don't have access to ethanol free in my area so I treat my gas with either star-tron or marine stabil.

I have other more modern bikes with flashed ECUs and the programmers state that severe engine damage will occur using anything less than 91 octane above 5000rpm. Those bikes have compression ratios of 10.7 :1 or higher.

Compression increases slightly with RPM, so I wondering if I should start using premium or migrade, since a lot of my miles are now slab riding with a 16" rear wheel.

[jkorn]

Does the engine knock or ping?

if not, I’m guessing your good.

[DiverRay]

These bike were built in the day of "low octane" engines. Using a high octane gas when low octane is called for WILL result in carbon build-up inside the engine. As Jeff said, if it doesn't ping or knock, you're good.

[Bonz]

How/where did you hear compression ratio increases at high rpm with respect to changing an engine's octane requirement? MAYBE if the bike is running constantly at near redline with potential rod stretch, then I guess it's a potential very very marginal amount of compression increase at best.

Compression ratio the manufacturers rate the engine is static compression ratio. Dynamic compression ratio is when the engine is running. Every manufacturer takes that into account when they rate a bike for octane. You'll get better performance and potentially a bit more power running the lowest grade recommended as long as it does not cause knock or ping, which it shouldn't in a well tuned bike such as yours.

Run the lowest octane you can find, you'll keep the combustion process happening when it should. High octane is slower to burn relatively speaking and continues to burn as it leaves the engine which builds up carbon on the back of the exhaust valves.

[XJkid]

Bonz, that was my experience. I ran 89 and 91 for a summer, took off the headers and plugs and could scrape the carbon gunk off the piston crowns and valves.

[Bonz]

Bonz, that was my experience. I ran 89 and 91 for a summer, took off the headers and plugs and could scrape the carbon gunk off the piston crowns and valves.

Good to hear of real life stories, it's not just something uncle albatross made up under the shade tree one afternoon, lol. I am curious where sustained high rpm running was published or read or heard necessitating premium.

[Banshee's Veil]

Good to hear of real life stories, it's not just something uncle albatross made up under the shade tree one afternoon, lol. I am curious where sustained high rpm running was published or read or heard necessitating premium.

I got one of my modern bikes flashed, and the flasher told me this: "Please remember that this flash is running high cylinder pressures at relatively low rpms in order to get such high torque values. This REQUIRES you to run premium octane (91+ octane). This flash has the best of both worlds... throttle control and unmatched power. Very refined in general riding, crazy power with larger throttle application. If you're on a trip and in a pinch on finding premium fuel, you can use 89 octane if you stay below 50% throttle."

I was just wondering with that same logic is premium was necessary since I would mostly running at 75% throttle.

[Radioguylogs]

I got one of my modern bikes flashed, and the flasher told me this: "Please remember that this flash is running high cylinder pressures at relatively low rpms in order to get such high torque values. This REQUIRES you to run premium octane (91+ octane). This flash has the best of both worlds... throttle control and unmatched power. Very refined in general riding, crazy power with larger throttle application. If you're on a trip and in a pinch on finding premium fuel, you can use 89 octane if you stay below 50% throttle."

I was just wondering with that same logic is premium was necessary since I would mostly running at 75% throttle.

So this flasher was able to flash a higher compression ratio? That's quite a trick.

"...you can use 89 octane if you stay below 50% throttle." I think I need to learn something to believe this.

[Banshee's Veil]

So this flasher was able to flash a higher compression ratio? That's quite a trick.

"...you can use 89 octane if you stay below 50% throttle." I think I need to learn something to believe this.

I don't know the specifics, but on that bike, the C-14, 91 octane is required per Kawasaki.

[DiverRay]

Yeah, I owned a connie for a while. They DID require premium! The XS was DESIGNED to run on regular, with its lower compression ratio and lower peak RPM range. It really isn't a good idea to try and compare engine specs/fuel requirements between a bike from the late '70's, early '80's to a modern machine. With all the electronics on your Kawasaki, you are looking at apples and oranges.

[Bonz]

So this flasher was able to flash a higher compression ratio? That's quite a trick.

"...you can use 89 octane if you stay below 50% throttle." I think I need to learn something to believe this.

Here you go, maybe this will help teach you, lol.

Yes. With the Kawasaki C14 sport touring bike, it has variable valve timing. There is an electronic solenoid that controls oil flow to the actuator. That electronic solenoid can be programmed to do different things via a flash. Change the valve timing and you change the dynamic compression ratio, not the static compression ratio.

Based on the values flashed into the ecu, below 50% throttle dynamic compression ratio isn't increased significantly to require higher octane fuel in a pinch. However if you whack the throttle open past 50% or run roll ons against your buddy and use more than 50% throttle, cam timing is changed and dynamic compression ratio goes up to where premium is required.

[Radioguylogs]

I had no idea any motorcycle might have variable valve timing. If the intake and exhaust cams have independent timing, it's easy to imagine changes in compression (and therefore octane requirement).

Thank you for the education.

[Bonz]

You are a scholar on VVT with the C14 now!

[Banshee's Veil]

Here you go, maybe this will help teach you, lol.

Yes. With the Kawasaki C14 sport touring bike, it has variable valve timing. There is an electronic solenoid that controls oil flow to the actuator. That electronic solenoid can be programmed to do different things via a flash. Change the valve timing and you change the dynamic compression ratio, not the static compression ratio.

Based on the values flashed into the ecu, below 50% throttle dynamic compression ratio isn't increased significantly to require higher octane fuel in a pinch. However if you whack the throttle open past 50% or run roll ons against your buddy and use more than 50% throttle, cam timing is changed and dynamic compression ratio goes up to where premium is required.

Well that makes sense, I thought it was the sustained period of higher RPM requiring the premium for a reason beyond my understanding. On the freeway with the 16" rear I'm usually above 6k, hence my question lol.