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Fuel effects on exhaust sound

Discussion in 'Technical and Troubleshooting Torque' started by Barnz, Mar 31, 2014.

  1. Recently tried to see if there was any benefit to running 98 octane fuel in my bike as opposed to 95, to see if there were any discernible performance changes.

    From the ass dyno I felt no change what so ever, but I did unexpectedly sense that the engine was producing a slightly louder exhaust note and overall sounded more aggressive. The change was very minimal but still interesting.

    Am I hearing things?

    (to clarify, tank was down to reserve level when tank was then fully filled with 98, shell v power. bike has now traveled 65km since fill. bike is cbr500r)
  2. On a cbr500r you'll find no benefit at all with 98.
  3. I ride a 06 hornet and have noticed that 91 sounds rough and feels like there is more vibration, 95+ sounds and feels smoother. So there is a difference. not sure how much it would show on a Cb500
  4. Yes.
    • Like Like x 1
  5. April Fools
  6. It might be psychosomatic, but I like the 98 octane from 7/11, which is Mobil fuel.

    I'm forced to use 98 in my cage, but it seems to run smoother in the bikes as well. I also get more mileage out of it.
  7. higher octane fuels do burn at a different rate. Also, your model may be borderline on the 91.

    so it is possible you noticed a change in sound.

    Mine has a rortier sound on 98.
  8. I mostly run 98 in my bike but cant say I've notice any difference in the exshaust note between 95 and 98. My preference is Shell, then Mobil-7/11. I try to stay away from BP Ultimate as it consistantly returns the lowest numbers in tests some of my mates do that run race cars. The octane number is the Minimum that the fuel must contain and there are variations in the brands. Also I reccomend staying away from United's 100 octane as it contains up to 10% Ethonol. Most bike manufacturers say not to use Ethonol blend fuels in Motorbikes.
  9. I don't think it would change the note, but it may run smoother.

    Generally, higher octane won't give more power, but it will allow you to tune for higher power more safely.
  10. Depends on the engine. On any modern engine with compression ratios above about 11:1 or 12:1, it will give better power and fuel efficiency from advancing the ignition timing.
    • Agree Agree x 2
  11. No.
  12. They are more complex hydrocarbons, so explain why I'm wrong.
  13. Because fuel will only burn as fast as it's being injected into the engine, which is controlled by the throttle and carbs (or EFI). With modern EFI systems and knock sensors there can potentially be a more complicated feedback mechanism here but that's not directly related to their chemical structure.
  14. Always run the lowest octane you can without encountering detonation or pre-ignition. Higher octane simply means slower burning....not more power.

    Racing fuel is not the same as pump gas for on-road use. Using a higher octane fuel than needed only results in combustion chamber deposits and unburned hydrocarbons out the exhaust (your soot).

    Not to mention a waste of your money.
  15. Yep, and this seems to make the difference, especially in milage and smoothness in power delivery.
    As far as I'm concerned, the relatively small price difference on a full tank is definitely worth it to me. Cage or bike.
  16. Octane rating has nothing to do with flame propagation velocity (AKA: burning speed). There are high octane substances with high flame velocity and high octane substances with low flame velocity. However, an easy way to increase the octane rating is to put some crap in that burns slowly, delaying peak combustion pressure. There are other methods, but I won't bore you all to tears by explaining them.

    I don't know whether it's true or not, but it's rumoured that Aussie PULP is ULP with crap in it to reduce the flame velocity. For some engines that don't have knock sensors, this might (or might not) mean that the cylinder pressure could be increased late in the power stroke (compared to operation on ULP), leading to a louder, sharper sound when the exhaust valve opens.

    Other factors include the compression ratio, the exhaust,(which could simply muffle the crap out of the sound), the exhaust valve opening timing, inlet charge swirl and tumble velocities in the combustion chamber, the velocity of jets of combustion gases coming from "squish" areas in the combustion chamber (if any are present), and the rate of heat transfer through the combustion chamber/cylinder walls - just to name a few.

    So yes, it's possible/plausable for a change of octane rating to change the sound of some engines.
    • Agree Agree x 1
    • Informative Informative x 1
  17. Ahhh, I get where your coming from.

    That's not what I meant. I should have been clearer. I meant the flame propegation rate. It's different for higher octane fuels.

    i.e. the explosion is different, hence it could explain a different exhaust note.

    [edit] Just saw Danny's post. Bit confusing there mate. It has nothing to do with it but it does?

    I think I know what you mean. Octane rating alone doesn't influence flame propegation rate, but the additives they use to achieve it may?
  18. Yes. Aren't you glad I didn't go into the other possible methods of increasing the octane rating! ;)

    BTW, regarding the idea that the timing/duration of fuel injection will be the thing that controls the combustion time, this isn't much of a factor for petrol engines. For diesel engines (which inject high pressure fuel near Top Dead Centre - TDC), yes it's certainly a contributor. For petrol engines (excluding some experimental engines and maybe some ultra-high-end production cars), the fuel is injected at relatively low pressure into the inlet manifold (most common, especially in bikes), or into the combustion chamber near Bottom Dead Centre (BDC). Both of these types of injection are over long before combustion starts, let alone stops. Because of this, it only has an indirect effect - much less than combustion chamber swirl and tumble.
  19. How much can you tell about the new Mazda petrol engine, Danny? I'm fairly sure they run well lean, which to me says stratified and high pressure injection.
  20. #20 danny_tb, Apr 2, 2014
    Last edited: Apr 2, 2014
    Stratified, yes. High pressure, not necessarily. Same thing for the current Holden V6, except I know it's low pressure.

    Just found more info. The latest generation of SKYAKTIV-G gasoline engines use 2,900PSI, which is very high for a petrol engine, but still much lower than needed for injecting fuel into a burning mixture near TDC (diesels use 15,000+PSI). However, the next generation SKYACTIV-G engines seem to be effectively common rail DI diesel engines injecting petrol, and with a spark plug.