Kawasaki recently launched the H2R motorcible. The world’s only supercharged production hypersport streetbike. This bike has 326 bhp. I saw few videos of this incredible bike.

My question is about the flames that come out of the exhaust system. Because I saw at most of the videos.

How dangerous is this flames for the care of the motorbike? Where this flames came from?



The phenomenon of flames coming out of the exhaust of any vehicle is called a Backfire

Basically when you either run rich or lean in the power stroke of the combustion which is not completely burnt , it is sent through the exhaust which obviously being hot reignites with the outside air on contact creating the Backfire effect.

This can be of two types:

  • Unintended

Usually super cars and in this case the Kawasaki H2R which have extreme degrees of free flow exhausts and when the diver lets go of the throttle the ECU sometime leave an extra spark to keep the vehicle in power range while shifting gears. So the when the exhaust valves open not all of the fuel is burnt and is sent out of the exhaust which since being a free flow rapidly ignites with the outside air causing flames to shoot out with a pop sound.

  • Intended

Some sports cars(almost all rally cars) have something called as a ALS or Anti lag system this system , the system purposely ignites the exhaust air laden with fuel to create a secondary exposing causing the turbocharger which works on the exhaust to spool up prematurely thus reducing the turbo lag.

People often try to achieve backfire for thrill seeking and fun proposes.

Now to Answer your question:

Where this flames came from?

If you dig deep into the whole concept of backfire you must know that the backfire happens outside the combustion chamber either in the induction system or the exhaust system. The flames you see are the ones from the exhaust system as explained above.

How dangerous is this flames for the care of the motorbike?

  • Backfire in the Induction system

If your Valve timing is not proper ie, if the intake valves open even before the combustion is complete then there are chances that the flames might propagate up the intake and explode there which is destructive to the engine and is seriously harmful. This can destroy intake manifolds.

  • Backfire in the Exhaust system

This is the more common form and the causes are the above mainly free flow exhaust and delay in exhaust valve timing.This type of backfire is not harmful to the engine and does not cause a serious issue.

  • I believe you mean "flatshifting" when you mention extra spark. For which, the throttle has to be kept open. Bike's do not use this. They rely on shifting as quickly as possible and exploit the dog clutch by quickshifting. – chilljeet Jul 13 '15 at 6:36
  • Agreed, i was talking about the concept as a whole. – Shobin P Jul 13 '15 at 7:18
  • @ShobinP New MC chat :-) chat.stackexchange.com/rooms/46288/motorcycle-diaries – DucatiKiller Oct 5 '16 at 5:43

Anarach has provided a good survey of the Backfire (or afterfire in this case, if you're being pedantic) phenomenon.
I will attempt to add to that keeping it specific to the H2R. Watch this video, the flames are a lot more exaggerated in this case.
The H2R, is a track-only hyper-sport bike with every consideration given to make the engine produce most amount of power and so emissions and noise take the non-existing back seat.
Now that that's out of the way.
There are

  • Afterfire between shifts - The H2R comes stock with a quick-shifter. After the initial 2 shifts (in the video you provided), the rider begins to use the quickshifter for clutchless full throttle upshifts. The quick-shifter works by cutting the ignition to help unload the gear and then progressively re-applying power (Using either the electronic throttle body and/or solely controlling the ignition). Cutting the ignition will have the effect of unburnt fuel going out the exhaust where it ignites due to high exhaust temperature.
  • Afterfire as a consequence of the tune - As anarach explained - rich mixture. In performance applications, this could have multiple causes -
    • Mixture is sometimes intentionally kept rich to cool the flame-front and prevent detonation in high compression ratio/ forced induction engines (H2R is supercharged)
    • Quickly closing the throttle - As seen in the video, causes the mixture to become rich.
    • Opening the throttle quickly while the engine is being overrun - Engine is tuned for quick throttle response, and hence slightly overcompensates when encountering little rotational inertia.
    • Engine Braking control setting - Not relevant in this case, but in the case of formula 1, some cars intentionally run hot exhaust on closing throttle to aid to aid downforce (or rather to keep it consistent and prevent suddenly losing downforce) as the exhaust, by design, flows over the rear wing.
  • Afterfire on hitting the rev-limitter - As seen in the video (this link takes you to the exact time) added in this answer. Ignition is cut to keep the engine within RPM limit. This, like in the case of the quickshifter, causes unburnt fuel to burn anywhere within or outside the exhaust.

And as anarach explained - having a free flow exhausts just brings everything out. There's no sound muffling, and flames can be openly seen.
As an addition - Afterfires can also occur (quite easily) on an extremely lean mix as it burns much slower and as a consequence continues to burn through the exhaust.


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