Ignition kill while shifting mod cause backfire?

[Guest]

So I recently heard a highly tuned sport-bike of unknown make. racing down a road near my home. Listening closely as I always do in the presence of "internal-combustion-speed-metal" I noticed that each shift was followed by a backfire. I am trying to understand how a bike must be tuned to cause this? here is the only idea i could come up with; the engine is revving at speed and as the rider shifts up the ignition is momentarily killed, filling the header/exhaust with unspent fuel/air. then as the gear is engaged the ignition returns burning up the unspent fuel in the header causing the backfire?

[BlackStar]

If it was a fuel injected bike, it was probably the fuel Cut function. Basically what you explained, but with fuel. If he is running an air or electric shifter, those do momentarily cut the ignition.

[ashdricky]

what if the bike used carbs instead of injected? would an ignition cut cause the same shifting backfire?

[Seana23]

running too lean (too small a main jet? Too "Breathy" an exhaust?
Hole in exhaust/ manifold?
A Triumph???
only joking!

xxx

[TheCafeKid]

Pretty common. Even the GP bikes do it. SBK. The lot. It's a simple spark cut when the lever is toed. Allows for wide open throttle shifts. Doesn't (or didn't last I read, theyre changing the rules all the time..) cut fuel delivery as they want that wide open to get from one gear to the next as quick as possible. (that new Honda tranny theyre usin in the GP is even faster. Slick stuff) So, you have a momentary spark cut but fuel is still being delivered. Shift. Spark comes back, and BOOM. Backfire out the tail pipe. You can replicate this with your GS to a degree. Cruise down the road, pull the clutch in, hit the kill switch, coast, at te same time let the clutch out and hit the killswitch back to "on". BOOM.

Same difference roughly. Motor us still spinning, still sucking fuel but has no spark to ignite it. It builds up, then ignites.

[ashdricky]

thank you cafekid this is the information i needed, I am building a very special GS and want to replicate this, My ignition module has an input for a shift ignition kill switch, now I just need to figure out how to rig a switch to my shift lever.

[earlfor]

Pretty common. Even the GP bikes do it. SBK. The lot. It's a simple spark cut when the lever is toed. Allows for wide open throttle shifts. Doesn't (or didn't last I read, theyre changing the rules all the time..) cut fuel delivery as they want that wide open to get from one gear to the next as quick as possible. (that new Honda tranny theyre usin in the GP is even faster. Slick stuff) So, you have a momentary spark cut but fuel is still being delivered. Shift. Spark comes back, and BOOM. Backfire out the tail pipe. You can replicate this with your GS to a degree. Cruise down the road, pull the clutch in, hit the kill switch, coast, at te same time let the clutch out and hit the killswitch back to "on". BOOM.

Same difference roughly. Motor us still spinning, still sucking fuel but has no spark to ignite it. It builds up, then ignites.

I bet muffler life is pretty short due to ignition cut/overpressure cycles. Not a problem on a race bike that you can put a new exhaust on for every race, but a bit expensive for a daily rider.

[TheCafeKid]

thank you cafekid this is the information i needed, I am building a very special GS and want to replicate this, My ignition module has an input for a shift ignition kill switch, now I just need to figure out how to rig a switch to my shift lever.

Ask the drag guys. They use air shifters and spark cutout a lot. Of course all you're really replicating is a "hot shift". Toe the shift lever as ready to shift, unload the trans with a quick flick of the throttle toward closed, while at nearly the same time shifting.
Not easy on the one-two shift as the gap between gears is pretty steep, bu cake from 2 on up. Down shifting takes more practice and some say it's not really good on the dogs. But if you've built the engine for racing, you've built it for racing and have spares on the shelf

[TheCafeKid]

I bet muffler life is pretty short due to ignition cut/overpressure cycles. Not a problem on a race bike that you can put a new exhaust on for every race, but a bit expensive for a daily rider.

To those street guys that can afford those tricks, baffle packing is cheap.
And on a race bike...what mufflers?

[ashdricky]

so it is not the actual metal of the muffler/header that is damaged by this but the packing in the silencer of the exhaust? so in theory if a baffle-less or "straight-pipe" design was used damaging the exhaust would not be an issue?

[TheCafeKid]

so it is not the actual metal of the muffler/header that is damaged by this but the packing in the silencer of the exhaust? so in theory if a baffle-less or "straight-pipe" design was used damaging the exhaust would not be an issue?

I don't see how the metal could be damaged unless it was made out of Alu foil.

The Aprillia RSV4R and Factory, and the BMW S1000RR both have quick shifters from either the factory or an aftermarket company designed for plug and play. Granted they're both on the extreme end of "sport" bike but one would assume they'd done some homework. Then again, if you can afford one of those...changing a baffle is pocket change...