Flames from exhaust on Rover V8 during shift up

[scottieb]

Hello all,

Not really a problem, but i was wondering if anyone with an in depth knowladge of engine calibration (mentioning no names!) could tell me why I get a flash out of the exhaust during shifting up the gears. You can only really see it at night, when you change up, as you dip the clutch and your shifting gears there is a rather nice sounding pop and a blue flash from the exhaust pipes.

I'm just curious as to the chemistry behind this, I know that flames on overrun are cause by to week a mixture, but not sure about this one.

Thanks in advance,

Regards,

Scott

[rob@vems.co.uk]

When you take your foot off the throttle the vacuum shoots up and I bet that its settling on a rather rich setting low on the KPA scale but high up on the RPM and possibly a retarded ignition value.  If the combustion is incomplete and the exhaust is hot enough, when the unburnt fuel meets free oxygen it will auto ignite. 

[cliffb75]

When you take your foot off the throttle the vacuum shoots up and I bet that its settling on a rather rich setting low on the KPA scale but high up on the RPM and possibly a retarded ignition value.  If the combustion is incomplete and the exhaust is hot enough, when the unburnt fuel meets free oxygen it will auto ignite. 

Yep, that pretty much covers it

Running over run fuel cut will remove this 'problem' (no fuel=no fire), whereas keeping the fuel on and just running very retarded spark at low load (lots of fuel and heat in the exhaust), or alternatively doing cylinder cut (i.e. rev limiting) will make it bigger.

Generally requires a reasonable amount of initial flow though i.e. high load and/or high revs, in order to have a gas velocity high enough that there is still a flame/enough heat when it reaches the tailpipe

Looks cool if you're into that sort of thing, but just makes me think that whoever mapped it doesn't understand what over run fuel cut is for......

[scottieb]

Cheers Cliff!

I'm not the sort of guy that purposely goes out of the way to turn my exhaust pipe into a flame thrower, but it did look quite interesting in the dark!

Its just nice to understand whats going on. Electronics I understand, Combustion in engines is not my stong piont but does fascinate me.

Scott