V8 BACK PRESSURE?
Discussion
No car needs back pressure...There is a big difference between flow and so called back pressure..I did a fair bit of research when i was supercharging the 280i...Back pressure is something you do not want and especially with any forced induction...Most race cars have a baffle for noise but overall the exhaust systems are either tuned or straight through...You want the gases to escape at a specified rate...Blow air through a toilet roll tube and then through a straw, Although there is equal amounts of air going through both its the rate of flow that changes considerably...Back pressure is a myth..block your exhaust and see what happens to your engine...But do this only if you want to see it towed off to a scrappy..
The only back pressure i want in a car is the G force pushing me into the seat...
The only back pressure i want in a car is the G force pushing me into the seat...
mrzigazaga said:
No car needs back pressure...There is a big difference between flow and so called back pressure..I did a fair bit of research when i was supercharging the 280i...Back pressure is something you do not want and especially with any forced induction...Most race cars have a baffle for noise but overall the exhaust systems are either tuned or straight through...You want the gases to escape at a specified rate...Blow air through a toilet roll tube and then through a straw, Although there is equal amounts of air going through both its the rate of flow that changes considerably...Back pressure is a myth..block your exhaust and see what happens to your engine...But do this only if you want to see it towed off to a scrappy..
The only back pressure i want in a car is the G force pushing me into the seat...
get on them beans zigThe only back pressure i want in a car is the G force pushing me into the seat...
I have changed mine to a single straight through box and had the two downpipes put together through a combiner before the box so its a single pipe box.
Result...its quite loud but not crazy when driving carefully (you still get the rv8 rumble) but when you open the taps, its quite anti social
Here are two vids, one of a drive off (a Ferrari on there too!) And another just idling then revving:
http://youtu.be/5z1WETrLap8
http://youtu.be/TzGeEP92mG4
Here is my box configuration
(Old PIC so excuse the wet chassis! I had just cleaned it down!)
Result...its quite loud but not crazy when driving carefully (you still get the rv8 rumble) but when you open the taps, its quite anti social
Here are two vids, one of a drive off (a Ferrari on there too!) And another just idling then revving:
http://youtu.be/5z1WETrLap8
http://youtu.be/TzGeEP92mG4
Here is my box configuration
(Old PIC so excuse the wet chassis! I had just cleaned it down!)
superwedge said:
have you got youtube video on this?
Turn it up LOUD scroll down for the vid http://www.pistonheads.com/gassing/topic.asp?h=0&a...
.....New year..New noise....on the original MK1 wedge it's a single pipe into a side silencer and then another at the rear, later went onto one larger silencer midway down the system. I have often considered the need for my middle box, which is what I call a dead end type with in and outlets on the same end of the box. So would be very easy to cut it out and put a straight pipe in. So if it's not required and back pressure is a myth, why is it there?
Back to science stuff again (apologies in advance !!)
To get the best power out of your engine, you need to get the air in, and the exhaust gas out as fast as possible, with no flow restrictions. Forget fuel, it's very easy to inject more fuel than the engine needs. At high rpm it's actually HARD to get the gases (air) moving quick enough, both in and out, because an engine both sucks and blows in big PULSES.
No back pressure is ideal for power, but it means you get the full BOOM-BOOM of the exhaust gas escaping from each cylinder, which makes it very noisy. A silencer smooths out the pulses, but this means by its very function, that it has to restrict the flow, causing some back pressure. So like lots of things in real life, a silencer is a trade off between good flow (and back pressure) and reduced noise out the pipe.
Striaght through boxes have better flow, but also are noisier, and if you stack them one after the other, eventually they add up the back pressure too, as the system gets quieter. Big diameter pipes have more flow, but by these same rules, will be noisier with straight through boxes (because less of the gas gets captured by the box packaging)
I hope that all makes sense......
To get the best power out of your engine, you need to get the air in, and the exhaust gas out as fast as possible, with no flow restrictions. Forget fuel, it's very easy to inject more fuel than the engine needs. At high rpm it's actually HARD to get the gases (air) moving quick enough, both in and out, because an engine both sucks and blows in big PULSES.
No back pressure is ideal for power, but it means you get the full BOOM-BOOM of the exhaust gas escaping from each cylinder, which makes it very noisy. A silencer smooths out the pulses, but this means by its very function, that it has to restrict the flow, causing some back pressure. So like lots of things in real life, a silencer is a trade off between good flow (and back pressure) and reduced noise out the pipe.
Striaght through boxes have better flow, but also are noisier, and if you stack them one after the other, eventually they add up the back pressure too, as the system gets quieter. Big diameter pipes have more flow, but by these same rules, will be noisier with straight through boxes (because less of the gas gets captured by the box packaging)
I hope that all makes sense......
bradderztvr said:
So if it's not required and back pressure is a myth, why is it there?
Well mainly to quieten it down..I should imagine that a 4 into 2 into 1 would be bloody loud and it would certainly foul an MOT...You will be constantly pulled over by the police and slapped with a fine and ordered to have a type approved exhaust fitted...The legal limit for exhausts is 82 db...In general classic cars and classic sportscars are okay even if they are over the new limit as long as they are no louder than when manufactured or leaving the production line...Vehicles have received warnings under the Police Reform Act 2000, which enables officers to warn individuals that they are likely to cause alarm, distress or annoyance to the public and that if they continue to do so their vehicles will be seized, resulting in a fine.
DVLA
Modified exhaust systems
(b). "It’s illegal to modify the exhaust system to make a vehicle noisier after it has been ‘type approved’ (checked it meets environmental and safety standards)
The police can also take action if your vehicle’s silencer doesn’t work in the way it was designed or if you’re driving in a way that creates too much noise."
Tuned exhausts are for highly tuned engines and they are not cheap as most of the time cylinders are reconnected in a different group for higher performance and i doubt very much if it would fit under a Wedge bonnet.
Some boxes are to create tone ..I have played the sax for 30 years and so know about air flow through pipe and variations in pitch in response to pipe diameter.
If you are ultra fussy and want a particular sound then you can create an exhaust system that gives you exactly what you want...A large rectangle mid muffler will produce a flat heavy deep tone whereas an oval type will produce a curved somewhat smooth deep tone...Then there is the tail pipe diameter which will level off the tone...Too small and it will rasp like a fart...TVR definitely created an individual exhaust note..Even my 280i sounded nothing like any Capri/Granada or Sierra that shared the same engine..Even my friend who has worked with Fords for over 30 years has never heard one sound the same as a TVR...And thats with custom systems.
Effects like reverb and gate can be applied...St Paul's cathedral has a domed roof which allows the sound to flow/Roll round instead of slamming into flat edges...This reduces reverb and controls the gate...
I guess in theory you could have key pads along the exhaust and then these could be synced to a control box so that you can play a pre programmed tune while you are driving...For some flight of the bumble bee might apply but for me it would be "Mars"...The bringer of war....
matt-man said:
I have changed mine to a single straight through box and had the two downpipes put together through a combiner before the box so its a single pipe box.
Result...its quite loud but not crazy when driving carefully (you still get the rv8 rumble) but when you open the taps, its quite anti social
Here are two vids, one of a drive off (a Ferrari on there too!) And another just idling then revving:
http://youtu.be/5z1WETrLap8
http://youtu.be/TzGeEP92mG4
That sounds really nice. Do you reckon the combiner pipe makes a difference?Result...its quite loud but not crazy when driving carefully (you still get the rv8 rumble) but when you open the taps, its quite anti social
Here are two vids, one of a drive off (a Ferrari on there too!) And another just idling then revving:
http://youtu.be/5z1WETrLap8
http://youtu.be/TzGeEP92mG4
Since posting that science-y bit, and reading about Zig's sax.....
I should have said at the end that -
FOR A CAR ENGINE, EVEN A STRAIGHT PIPE WILL CREATE SOME BACK PRESSURE.
I'm pretty sure that's true, given the pulsed nature of exhaust gases......
Zig, you can always plumb in your sax to the exhaust pipe !!
I should have said at the end that -
FOR A CAR ENGINE, EVEN A STRAIGHT PIPE WILL CREATE SOME BACK PRESSURE.
I'm pretty sure that's true, given the pulsed nature of exhaust gases......
Zig, you can always plumb in your sax to the exhaust pipe !!
gmw9666 said:
superwedge said:
have you got youtube video on this?
Turn it up LOUD scroll down for the vid http://www.pistonheads.com/gassing/topic.asp?h=0&a...mrzigazaga said:
...Back pressure is a myth..
Hmmnn?? I don't think so! It depends upon the required end use.For a typical RV8 tuned with all the exhaust boxes intact as it left the showroom, then Back Pressure is a by-product, designed into the system using baffles, to maximise efficiency and help reduce noise. So back pressure is not a myth.
For best efficiency, the back pressure needs to be tuned in order to match the outgoing pulses of energy from the exhaust manifolds.
At the same time, tuning for minimum noise output requires that the system is not allowed to resonate (sing) at it's natural harmonic rate.
If noise output is allowed to maximise and back pressure is to be minimised then maximum flow means that all boxes and baffles should be removed.
But then, the most efficient exhaust system, is no pipes, no baffles and no boxes. Try it sometime, with exhaust manifolds removed. - ie - No Back Pressure at all.
Mr Plod dictates, however, that certain noises are unreasonable.
So for social reasons, some pipes and/or baffles and/or boxes have to be retained and thus the LAW dictates that all systems will therefore, also have back pressure.
Thus as mentioned, depending upon 'end use' it becomes necessary that the noise is maximised for unsocial reasons yet minimised for social reasons.
And the happy medium can only be achieved by managing back pressure to optimise both noise and flow for maximum resonance but minimum social disruption.
Just my two-pennyworth really, but for the whole quids-worth one can study my full essay on exhaust systems for the RV8 - you know where to find it!
Happy New Year!
superwedge said:
maybe good to fit something in line on the pipes ,where you can open the pipes when needed,or shut the pipe when higs are around,then you got the best of both worlds,
Mine gets through the mot just fine. Admittedly it has an advisory saying "exhaust slightly loud" ;-)And my mot man is proper strict as well
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