Pop and bangs.......the video
Discussion
Mark, just for you fella
When the 420 was on the rollers you mentioned the old girl didn't pop and bang
Was out today for a taz and took this video......hope it proves once and for all on the popping and banging front lol
The town of Henley in Arden were taking cover, I'm sure they thought it was a rifle attack lol
Ps apologies for the shaky video......watch to the end :-)
https://www.youtube.com/watch?v=KI6Qo0yklJE
When the 420 was on the rollers you mentioned the old girl didn't pop and bang
Was out today for a taz and took this video......hope it proves once and for all on the popping and banging front lol
The town of Henley in Arden were taking cover, I'm sure they thought it was a rifle attack lol
Ps apologies for the shaky video......watch to the end :-)
https://www.youtube.com/watch?v=KI6Qo0yklJE
With either deliberate or consequential maladjustment of the 'Overrun Valve', during cranking, fuel may be injected into the plenum from the cold start injector directly opposing the incoming stream of air. This produces a rich mixture, not all of which is ingested if, for some reason, the engine does not fire and run first poke. This leaves variable amounts of residual fuel in and around the plenum, trumpets and tunnels which, for example, can be strongly smelled when investigating a non-starting engine.
In addition to fresh air rushing into the plenum a stream of foul air is sucked up through the crank-case ventilation system containing unburned hydrocarbons mixed in with other noxious gasses.
The different conditions and sources of fuel combine to leave varying quantities of juice which, at average pressures experienced in the inlet area, condenses out into liquid form on the walls and surfaces of the plenum chamber and other parts of the inlet system, a normal phenomenon! These fuel residues do no harm, they just innocently exist thereabouts, until conditions change..
Inevitably, when the engine next encounters rapid deceleration with the throttle plate(s) shut, otherwise known as Overrun, a very high vacuum occurs in all the inlet areas which encourages any liquid fuel lurking there to vaporise. Messer's Boyle and Charles wrote some laws about this!
With throttle shut, a small volume of air enters the engine via the Air Flow Meter (AFM) through the idle air gallery plus whatever sneaks past the closed throttle disc. The ECU responds accordingly by trying to inject correctly matched amounts of fuel at a level similar to the quantity at idle. Unfortunately the Overrun depression felt in the inlet manifold is much larger than that normally corrected for, by the Fuel Pressure Regulator, so the overall pressure differential across the injectors is much larger than the usual maximum of 36 psi.
This has the effect of sucking even more fuel from the injectors than intended by the ECU, exacerbating the overall richness making it impossible for any resultant mixture to ignite inside the combustion chambers from where it passes unburned into the exhaust manifold.
Here, the rich unburned mixture encounters variable quantities of air, very hot gasses and piping hot metal surfaces enabling random combustion right there in the exhaust system. The result is loads of popping and banging, typically a very loud crackle, capable of generating spectacular flames from the tail pipe depending upon the state and nature of the silencers.
Apart from the anti-social nature of the associated decibels these random detonations outside the combustion chambers create excess stress and eventual damage to any local manifold gaskets.
Rover and Lucas' solution to this phenomenon was to introduce a valve that responds to the high vacuum generated in the inlet manifold to allow in a more generous quantity of metered air from the air rail which in turn operates the AFM flap resulting in the signal to the ECU to inject an appropriate amount of fuel. A secondary effect of the opening valve is to reduce the pressure differential across the main injectors nullifying the excess sucking forces on them,
This larger combustible volume now combines successfully with the vaporising residual unburned fuel to ignite and burn properly inside the combustion chambers. Thus, the unwanted noise levels and it's associated potential damage generated on Overrun are suppressed.
As mentioned, either deliberate or consequential maladjustment of the 'Overrun Valve' creates the right conditions for all the popping and banging.
Probably deliberate, methinks! lol.
In addition to fresh air rushing into the plenum a stream of foul air is sucked up through the crank-case ventilation system containing unburned hydrocarbons mixed in with other noxious gasses.
The different conditions and sources of fuel combine to leave varying quantities of juice which, at average pressures experienced in the inlet area, condenses out into liquid form on the walls and surfaces of the plenum chamber and other parts of the inlet system, a normal phenomenon! These fuel residues do no harm, they just innocently exist thereabouts, until conditions change..
Inevitably, when the engine next encounters rapid deceleration with the throttle plate(s) shut, otherwise known as Overrun, a very high vacuum occurs in all the inlet areas which encourages any liquid fuel lurking there to vaporise. Messer's Boyle and Charles wrote some laws about this!
With throttle shut, a small volume of air enters the engine via the Air Flow Meter (AFM) through the idle air gallery plus whatever sneaks past the closed throttle disc. The ECU responds accordingly by trying to inject correctly matched amounts of fuel at a level similar to the quantity at idle. Unfortunately the Overrun depression felt in the inlet manifold is much larger than that normally corrected for, by the Fuel Pressure Regulator, so the overall pressure differential across the injectors is much larger than the usual maximum of 36 psi.
This has the effect of sucking even more fuel from the injectors than intended by the ECU, exacerbating the overall richness making it impossible for any resultant mixture to ignite inside the combustion chambers from where it passes unburned into the exhaust manifold.
Here, the rich unburned mixture encounters variable quantities of air, very hot gasses and piping hot metal surfaces enabling random combustion right there in the exhaust system. The result is loads of popping and banging, typically a very loud crackle, capable of generating spectacular flames from the tail pipe depending upon the state and nature of the silencers.
Apart from the anti-social nature of the associated decibels these random detonations outside the combustion chambers create excess stress and eventual damage to any local manifold gaskets.
Rover and Lucas' solution to this phenomenon was to introduce a valve that responds to the high vacuum generated in the inlet manifold to allow in a more generous quantity of metered air from the air rail which in turn operates the AFM flap resulting in the signal to the ECU to inject an appropriate amount of fuel. A secondary effect of the opening valve is to reduce the pressure differential across the main injectors nullifying the excess sucking forces on them,
This larger combustible volume now combines successfully with the vaporising residual unburned fuel to ignite and burn properly inside the combustion chambers. Thus, the unwanted noise levels and it's associated potential damage generated on Overrun are suppressed.
As mentioned, either deliberate or consequential maladjustment of the 'Overrun Valve' creates the right conditions for all the popping and banging.
Probably deliberate, methinks! lol.
gmw9666 said:
Errr my head hurts
I'm sorry about that, but I don't see why?Let's take ALL the points in sequence!
- With either deliberate or consequential maladjustment of the 'Overrun Valve', during cranking, fuel may be injected into the plenum from the cold start injector directly opposing the incoming stream of air. This produces a rich mixture, not all of which is ingested if, for some reason, the engine does not fire and run first poke. This leaves variable amounts of residual fuel in and around the plenum, trumpets and tunnels which, for example, can be strongly smelled when investigating a non-starting engine.
- In addition to fresh air rushing into the plenum a stream of foul air is sucked up through the crank-case ventilation system containing unburned hydrocarbons mixed in with other noxious gasses.
- The different conditions and sources of fuel combine to leave varying quantities of juice which, at average pressures experienced in the inlet area, condenses out into liquid form on the walls and surfaces of the plenum chamber and other parts of the inlet system, a normal phenomenon! These fuel residues do no harm, they just innocently exist thereabouts, until conditions change.
- Inevitably, when the engine next encounters rapid deceleration with the throttle plate(s) shut, otherwise known as Overrun, a very high vacuum occurs in all the inlet areas which encourages any liquid fuel lurking there to vaporise. Messer's Boyle and Charles wrote some laws about this
- With throttle shut, a small volume of air enters the engine via the Air Flow Meter (AFM) through the idle air gallery plus whatever sneaks past the closed throttle disc. The ECU responds accordingly by trying to inject correctly matched amounts of fuel at a level similar to the quantity at idle. Unfortunately the Overrun depression felt in the inlet manifold is much larger than that normally corrected for, by the Fuel Pressure Regulator, so the overall pressure differential across the injectors is much larger than the usual maximum of 36 psi.
- This has the effect of sucking even more fuel from the injectors than intended by the ECU, exacerbating the overall richness making it impossible for any resultant mixture to ignite inside the combustion chambers from where it passes unburned into the exhaust manifold.
- Here, the rich unburned mixture encounters variable quantities of air, very hot gasses and piping hot metal surfaces enabling random combustion right there in the exhaust system. The result is loads of popping and banging, typically a very loud crackle, capable of generating spectacular flames from the tail pipe depending upon the state and nature of the silencers.
- Apart from the anti-social nature of the associated decibels these random detonations outside the combustion chambers create excess stress and eventual damage to any local manifold gaskets.
- Rover and Lucas' solution to this phenomenon was to introduce a valve that responds to the high vacuum generated in the inlet manifold to allow in a more generous quantity of metered air from the air rail which in turn operates the AFM flap resulting in the signal to the ECU to inject an appropriate amount of fuel. A secondary effect of the opening valve is to reduce the pressure differential across the main injectors nullifying the excess sucking forces on them
- This larger combustible volume now combines successfully with the vaporising residual unburned fuel to ignite and burn properly inside the combustion chambers. Thus, the unwanted noise levels and it's associated potential damage generated on Overrun are suppressed.
- As mentioned, either deliberate or consequential maladjustment of the 'Overrun Valve' creates the right conditions for all the popping and banging. Probably deliberate, methinks! lol.
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