What causes the popping sound when decelerating?
Discussion
Fimbo said:
I was thinking it might be unburnt fuel being ignited in the hot exhaust but that would surely end up bending valves or damaging the system?
Please enlighten me.
Why do you think it would bend a valve, or damage the system.Please enlighten me.
Most of the newer cars will not pop and bang as they cut the fuel on decelerating.
The older carb, and early flapper injections car will pop and bang the most. you can even get flames on a good day. and overrun from very high rev's
Mmm Over run. Took me a while to perfect it after having the exhaust sleeved but now I can do it on demand. Go for a blast to get everything nice and toasty. Then 2nd gear up to 4k rpm, foot off the loud pedal then gently feather it. Theres a mchine gun hidden in my tail pipes 
edited to add - and if you have a full decat as well (I dont) you will see flames. Recent hoon I was following a certain PH'er with a cerb and could clearly see burst of flame !!
edited to add - and if you have a full decat as well (I dont) you will see flames. Recent hoon I was following a certain PH'er with a cerb and could clearly see burst of flame !!
Edited by bigdods on Sunday 12th August 00:04
Fimbo said:
I was thinking it might be unburnt fuel being ignited in the hot exhaust but that would surely end up bending valves or damaging the system?
Please enlighten me.
That is what i thought but i was told recently that this is not the case.Please enlighten me.
In fact i would go as far as to say that it was a weak fuel mix which causes the pops and bangs.
But what ever the cause it can damage valves
Pasco
Pasco said:
Fimbo said:
I was thinking it might be unburnt fuel being ignited in the hot exhaust but that would surely end up bending valves or damaging the system?
Please enlighten me.
That is what i thought but i was told recently that this is not the case.Please enlighten me.
In fact i would go as far as to say that it was a weak fuel mix which causes the pops and bangs.
But what ever the cause it can damage valves
Pasco[/quote
eh???
its unburnt fuel coming thru the engine on lift off, and igniting in the exhaust system, hence it only happens when hot. haha, i get this on my (somewhat tuned) MGB and have had on several other cars. nothing better than seeing a nice burst of flame in the mirror of a night!
frazer guest said:
TVRleigh said:
you can adjust the overrun valve also to give more or less pop's and bangs.
I think this is only on the flapper EFI, as I think on the later hotwire, that's where the dreaded throtle pot is located.
Any idea what year they changed the system?I think this is only on the flapper EFI, as I think on the later hotwire, that's where the dreaded throtle pot is located.
Even though mine is fully de-catted, it still sounds a bit, well, a bit weak.
I really need more noise and more pops and bangs. Big flames would be nice too.
(no flamer kits though)
well they had both that year, so 1990 on will be hotwire, 1988 with be flapper. and 1989 could be either.
This is on TVR's may have been a year ealier on range rovers, as the system had to be working before TVR started using it. as standard the hotwires had a fuel cut off valve, but this could be disenabled to give pop's and bangs, but will reduced fuel econ.
This is on TVR's may have been a year ealier on range rovers, as the system had to be working before TVR started using it. as standard the hotwires had a fuel cut off valve, but this could be disenabled to give pop's and bangs, but will reduced fuel econ.
bigdods said:
edited to add - and if you have a full decat as well (I dont) you will see flames. Recent hoon I was following a certain PH'er with a cerb and could clearly see burst of flame !!
Ummmm.... you wouldn't be talking about little old innocent me would you?Edited to add:
bigdods said:
could clearly see burst of flame !!
So did the silly bugger in the Peugeot! Caused him to back off and think twice about trying to race TVRs with a french tin-can diseasle.Edited by V8Cerbera on Monday 13th August 14:47
Ripped off directly from: http://www.aauk.biz/Cerbera/manual.htm
Backfiring
Here's an amateur explanation of why cars like the Cerbera (especially the big man's 4.5) pop and crackle on the over-run. Sorry about the length of this, but you did ask for anal detail!
As mentioned before, its because of valve overlap, but what is this, why is it A Good Thing, and why does it cause the impression of a Spanish festival when you let off the gas?
The easiest way to think about this is to imagine what would happen if you didn't have any overlap. Consider the 'obvious' engine design, where the exhaust valve is open for precisely the time that the piston is coming up during its exhaust stroke, closes when the piston is at the top (TDC), at which point the inlet valve opens ready for the mixture to get sucked in during the induction stroke. The problem with this is that when the piston is at the top, you've still got a combustion chamber's worth of spent exhaust in there. Remember that the quoted capacity of an engine is actually the 'swept volume' (the volume swept by the pistons). There is an extra bit at the top of the cylinder which the mixture is compressed into and where the combustion actually takes place. The compression ratio is the ratio between the capacity of the cylinder with the piston at the very bottom compared to the capacity of it with the piston at the very top. On a Cerbera 4.5, with a compression ratio of 10.75 this gives a combustion chamber size of about 57cc - per cylinder. So 57cc's worth of burnt crap would be mixed with the fuel-air mixture each cycle if there was no overlap.
Overlap means that the exhaust valve stays open for a while, after the inlet valve has opened. This is in order to lose as much of that burnt mixture as possible. When the piston reaches the top, the mixture has inertia (or momentum if you like, as the term inertia means something different outside of physics). If the exhaust valve is left open for a while, much of it will continue sailing out of the port even though the piston is now going downwards. Also, the mixture coming in through the inlet valve will actually force some of the exhaust gases out - not because its pressurised, as the inlet gas is being sucked in (in a naturally-aspirated engine), but by the inertia of the incoming gas which then pushes against the exhaust gas.
In fact this gets even more complicated with tuned-port induction and tuned exhaust systems. Both of these systems set up standing-waves in the pipes. In the case of induction, a positive pressure node is set up at the inlet port, so that the mixture is actually being forced into the cylinder, even in a naturally aspirated engine. In a tuned exhaust system, the pipes coming from each exhaust port must be of perfectly equal length (hence the spaghetti that you see on highly tuned engines). This length is calculated according to the speed of sound and the revs at which you want the peak effect to take place. The idea is that as an exhaust pulse passes through the collector (the bit where the pipes join), it sends a pressure wave down the other pipes. By getting the length right (the pressure wave always travels at the speed of sound, regardless of revs) you can ensure that the pressure wave that is formed in the pipes is such that there is an anti-node at the exhaust valves of the other cylinders just when they open, causing the exhaust to get actively sucked out of the cylinder, rather than relying on the piston's pushing action alone. Cylinders get paired up together in all sorts of weird and wonderful ways according to the firing order. On a four-cylinder engine (or half of a single-plane crank engine like the AJP8) the arrangement is either 4-into-1 or 4-into-2-into-1. The former gives the most power, the latter a better torque spread.
Anyway, this exhaust tuning is particularly effective when combined with valve overlap, after all, there is no point just leaving the valve open for longer if all that happens is that the burnt mixture gets a chance to wave at the outside World for a while. In an ideal World, the exhaust valve would close just as the new mixture approaches it, with all the burnt stuff having been expelled/sucked out. In a 'normal' engine, the system is set up such that there is no way any of the new (unburnt) mixture could ever escape into the exhaust system - but at the expense of not getting rid of every last bit of the burnt stuff. However in race engines and nasty un-environmentally phallic devices like TVR Cerberas, the balance is more fine, such that under certain circumstances some of this pristine and virginal mixture can go sailing out into the exhaust system to be ignited by the next emission of hot gases from another cylinder, causing an explosion in the exhaust system which makes an audible bang - and in really tuned systems, bloody great flames out of the exhaust!
Now comes an explanation of why this happens when you let your foot off. I must stress that what follows (and some of the above) is stuff that I've worked out for myself from first principles, so please don't take it as gospel.
The effect occurs when you've been on the power, and you suddenly close the throttle. When you're on the power, there's going to be a whole pile of fuel-air mixture in the inlet pipes, all with plenty of kinetic energy. However the inlet valve of any given cylinder is only actually open for about 25% of the time, so this gas is actually bouncing back up the inlet pipe for a while, before being sucked back down it once the inlet valve opens. The periodic nature of this is just perfect to set up standing waves in the gas, as previously mentioned regarding tuned induction above. When you close the throttle, all of a sudden this pipe is effectively capped, causing the pressure wave inside the gases to be strongly reflected back into the cylinder. This means that just momentarily, there is a lump of gas in the system that has much increased velocity which goes rushing into the cylinder when that valve opens. In a 'normal' engine this doesn't matter a bit, but in a highly tuned engine where that exhaust valve is living dangerously on the staying-open-til-the-last-moment stakes, the unusually over-excited mixture goes cascading out of the exhaust valve and into the dark and dangerous world of the exhaust system. Just imagine the look of surprise on its face.
Backfiring
- ********
Here's an amateur explanation of why cars like the Cerbera (especially the big man's 4.5) pop and crackle on the over-run. Sorry about the length of this, but you did ask for anal detail!
As mentioned before, its because of valve overlap, but what is this, why is it A Good Thing, and why does it cause the impression of a Spanish festival when you let off the gas?
The easiest way to think about this is to imagine what would happen if you didn't have any overlap. Consider the 'obvious' engine design, where the exhaust valve is open for precisely the time that the piston is coming up during its exhaust stroke, closes when the piston is at the top (TDC), at which point the inlet valve opens ready for the mixture to get sucked in during the induction stroke. The problem with this is that when the piston is at the top, you've still got a combustion chamber's worth of spent exhaust in there. Remember that the quoted capacity of an engine is actually the 'swept volume' (the volume swept by the pistons). There is an extra bit at the top of the cylinder which the mixture is compressed into and where the combustion actually takes place. The compression ratio is the ratio between the capacity of the cylinder with the piston at the very bottom compared to the capacity of it with the piston at the very top. On a Cerbera 4.5, with a compression ratio of 10.75 this gives a combustion chamber size of about 57cc - per cylinder. So 57cc's worth of burnt crap would be mixed with the fuel-air mixture each cycle if there was no overlap.
Overlap means that the exhaust valve stays open for a while, after the inlet valve has opened. This is in order to lose as much of that burnt mixture as possible. When the piston reaches the top, the mixture has inertia (or momentum if you like, as the term inertia means something different outside of physics). If the exhaust valve is left open for a while, much of it will continue sailing out of the port even though the piston is now going downwards. Also, the mixture coming in through the inlet valve will actually force some of the exhaust gases out - not because its pressurised, as the inlet gas is being sucked in (in a naturally-aspirated engine), but by the inertia of the incoming gas which then pushes against the exhaust gas.
In fact this gets even more complicated with tuned-port induction and tuned exhaust systems. Both of these systems set up standing-waves in the pipes. In the case of induction, a positive pressure node is set up at the inlet port, so that the mixture is actually being forced into the cylinder, even in a naturally aspirated engine. In a tuned exhaust system, the pipes coming from each exhaust port must be of perfectly equal length (hence the spaghetti that you see on highly tuned engines). This length is calculated according to the speed of sound and the revs at which you want the peak effect to take place. The idea is that as an exhaust pulse passes through the collector (the bit where the pipes join), it sends a pressure wave down the other pipes. By getting the length right (the pressure wave always travels at the speed of sound, regardless of revs) you can ensure that the pressure wave that is formed in the pipes is such that there is an anti-node at the exhaust valves of the other cylinders just when they open, causing the exhaust to get actively sucked out of the cylinder, rather than relying on the piston's pushing action alone. Cylinders get paired up together in all sorts of weird and wonderful ways according to the firing order. On a four-cylinder engine (or half of a single-plane crank engine like the AJP8) the arrangement is either 4-into-1 or 4-into-2-into-1. The former gives the most power, the latter a better torque spread.
Anyway, this exhaust tuning is particularly effective when combined with valve overlap, after all, there is no point just leaving the valve open for longer if all that happens is that the burnt mixture gets a chance to wave at the outside World for a while. In an ideal World, the exhaust valve would close just as the new mixture approaches it, with all the burnt stuff having been expelled/sucked out. In a 'normal' engine, the system is set up such that there is no way any of the new (unburnt) mixture could ever escape into the exhaust system - but at the expense of not getting rid of every last bit of the burnt stuff. However in race engines and nasty un-environmentally phallic devices like TVR Cerberas, the balance is more fine, such that under certain circumstances some of this pristine and virginal mixture can go sailing out into the exhaust system to be ignited by the next emission of hot gases from another cylinder, causing an explosion in the exhaust system which makes an audible bang - and in really tuned systems, bloody great flames out of the exhaust!
Now comes an explanation of why this happens when you let your foot off. I must stress that what follows (and some of the above) is stuff that I've worked out for myself from first principles, so please don't take it as gospel.
The effect occurs when you've been on the power, and you suddenly close the throttle. When you're on the power, there's going to be a whole pile of fuel-air mixture in the inlet pipes, all with plenty of kinetic energy. However the inlet valve of any given cylinder is only actually open for about 25% of the time, so this gas is actually bouncing back up the inlet pipe for a while, before being sucked back down it once the inlet valve opens. The periodic nature of this is just perfect to set up standing waves in the gas, as previously mentioned regarding tuned induction above. When you close the throttle, all of a sudden this pipe is effectively capped, causing the pressure wave inside the gases to be strongly reflected back into the cylinder. This means that just momentarily, there is a lump of gas in the system that has much increased velocity which goes rushing into the cylinder when that valve opens. In a 'normal' engine this doesn't matter a bit, but in a highly tuned engine where that exhaust valve is living dangerously on the staying-open-til-the-last-moment stakes, the unusually over-excited mixture goes cascading out of the exhaust valve and into the dark and dangerous world of the exhaust system. Just imagine the look of surprise on its face.
my standard 3.6 T350 pops and bangs like big fireworks going off in a drainpipe and after removing the cats it shoots flames too!!!
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