There are two reasons why bikes can get away with higher compression with lower octane ratings. (There may be more, but these are the two i know of.)

There's a difference between "static compression" and "dynamic compression". Those numbers you see on the spec sheet are in static compression. It's the math of the piston stroke and bore and before and after cylinder volumes. Dynamic compression, on the other hand, is the pressure actually realized within the cylinder chamber when in use. It's really quite complex and i'm not sure how to explain it without sitting here typing for another 20 minutes. Since we're somewhat off topic already i'd rather not. Quick version. What it doesn't take into account is the valve timing (opening and closing). You can effectively lower cylinder pressures of an engine by putting in a more radical camshaft with longer intake and exhaust durations (or lobe separations), even to the point of overlapping their actuation. This allows the engine to breathe better at high rpm, but sacrifices cylinder pressures (usually effecting torque) at the lower part of the rev range by reducing the dynamic compression of the engine. Our bikes (and most sportbikes) have a pretty radical cam profile that does this, where you car does not. Makes sense as our bikes are made to run up to 14k rpm while cars generally don't see anything over 6k, don't you think? This difference is also a primary reason why your bike has to idle at ~1300 rpm while your car is probably between 600-800 rpm (unless it's a rotary engine). I know that's not a great explination...but i could go on and on and on...

The second difference is that smaller cylinders are less likely to detonate than larger ones. I'm not quite as up to snuff as to why this is, but my mechanical engineer friend knows all about it...and i'm sure there someone else around here who does, too.

So there you go. I used to use premium all the time, too, for all my vehicles as i thought it just had to be better in some way. But since then i've actually gotten educated and basically, unless there's a specific reason to do otherwise, it's useless to use anything other than what the manual specifies.

 

You guys do know that 93 is "cleaner" right? it has more additives in it for cars that require premium gas with less detonation and less carbonization.

Me personally my car requires it and its all ive used, ive had the motor apart (just for the hell of it, i was bored one day haha) and checked all my cylinders and well there was minimal carbon.

The sticker on the back of the bike inside the rear fender under the back seat says to use 91

 

what does the manual say?
use 91+ if you like...use whatever you like...whatever you can afford...
but...'just because i want to' arguments does not float against scientific facts.

you can have a super high compression motor and retard timing and use 87, or a low compression and advance timing so much and use premium. it's in the tune...the TUNE...the TUNE

 

my car stock uses 91. The manual says 89 i believe then the bike itself says 91. You should always run 91+ with high compression regardless of tune.


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Quote: You should always run 91+ with high compression regardless of tune.

What? Regardless of tune? WTF, mate. Didn't you hear what Tahoe said? He's right...compression doesn't mean squat by itself. It's all in the tune of the engine (ie ignition timing, camshaft profile, etc). If you change one of these other things, timing for instance, you change the octane requirement for that engine even if compression is the same. You could take your stock engine that requires 91, adjust the timing and run it on 87 without any trouble. Now it would lose some power, but not from the octane, but because you retarded the timing. You could also advance the timing on that same stock engine with the exact same compression and all of a sudden it would start detonating because now it's gonna need a higher octane gas.

And yes, generally higher octane gas has more detergents in it. But running a higher octane than is necessary for your engine will burn less clean than the octane it was designed for...so you're not really cleaning your engine if it only requires 87 and you're running 91. Now i know this isn't the case in your specific car since it was designed for 91...but in a lot of other cars this is the case. Like if you decided to run 110 in your car...it wouldn't do any good or run any cleaner or more powerfully as 110 is more stable and will not burn as completely in a car designed to run 91.

In other words your stock engine doesn't need 91 because it's high compression...it needs 91 because that's the way it's tuned from the factory. What you're saying about compression isn't wrong...it's just part of the whole equation that goes into why your engine requires a specific octane gas.

 

Makes sense, so what do i follow for the bike, the manual or the sticker in the rear fender?

 

they all say the same thing...believe it or not...just one is saying RON (research octane number), the other MON (Motor Octane Number) and the last one is PON (Pump Octane Number), which is (RON + MON)/2

i can't remember the exact numbers i think it came out to at least 86 PON.

 

There was a discussion a few weeks ago that roughly concluded it wasn't even necessary to use 91 pump gas, let alone premium with octane boosters.

Let me guess, you use it in that Eclipse in your avatar? GS-T or GSX?

 

I was riding with some bros recently and they kind of thumbed their nose at me when I filled with a mid-grade fuel. I didn't care...I just count my extra change every time I fill up!

 

you shouldn't be hanging out with people like that...i would stop calling them bros too...

you should come ride with me, we'll hit up all the great gas stations carrying 87 octane gas...