Removing the muffler - installing a straight pipe

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Chris901202
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#1
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#1
Hey!

I've seen a few posts on here, and people seem to know what they're talking about (on the whole) :p:

I'm getting the muffler on my car removed and having a straight pipe made up, i.e. from cat-back, so no legality problems with running with no cat..

Just wanted to know whether anyone had any idea whether running without a muffler is illegal, we've had the muffler off to test the noise, and it's much quieter than many of the aftermarket backboxes around here.

Any advice would be appreciated

Cheers
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gbduo
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You will probably lose a lot of back pressure if you don't do it properly and thus lose performance...
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lodzinski
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you would need to pass emissions and noise regs, and as stated above, you could end up losing a load of compression and blunting your performance.
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Chris901202
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True about the back pressure, and performance loss. However, I'm also planning on changing the header, which should prevent any major performance loss.

I'm having it made up so I can simply bolt on the stock exhaust when it goes in for MOT/Servicing etc, so no problems there!

The legality is the main concern, as I don't want to be stopped and slapped with a fine and points on my license If anybody with experience of this, or knowledge, could bring some light to this area it'd be great!
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prodriven
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(Original post by Chris901202)

The legality is the main concern, as I don't want to be stopped and slapped with a fine and points on my license If anybody with experience of this, or knowledge, could bring some light to this area it'd be great!
In theory the cops could do something if the cars noise could be seen as 'anti-social' or loads of people complain about it. But I think its extremely rare for this to happen. I know various people with incredibly loud cars (no silencers and no cats) who have never had any problems - just dont go revving the nuts of it around housing estates at 1am and you should be fine.
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Chris901202
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Thanks for the insight mate, I appreciate it!

No worries, I don't plan on terrorizing the local neighborhood, just wanted a bit more of a growl

Thanks again!
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JC.
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Don't believe for a minute all the guff spouted above about "back pressure". Most people haven't got a clue what it is and how it works.

It's far more important that the exhaust manifold is pulse tuned to opposing cylinders.

When I run my V8 MG at the drag strip, I take the exhaust system off completely and run on open headers.
Exhaust gasses are drawn out of the engine at the collector by the pulse of an opposite cylinder on each bank.

Anyway, do what you like. It's your car. I will say that having an extremely loud exhaust will piss you off on a long journey.
The exhaust on my V8 was basically 4 straight through cherry bombs - two on each bank.

Sounded great in the pub carpark but after about the 50th mile of nothing but [SIZE="6"*]"BLEM BLEM! BLEM! BLEM! BLEM!"[/SIZE] coming through the window (regardless of whether its open or closed) it starts to get a bit irksome.

-JC

*1 Rover V8's and Chevy V8's go "blem blem blem", Ford Flathead V8's go "potatoe, potatoe" which is an all together more satisfying sound.
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terpineol
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A good point is made on the noise.

I have taken to driving with ear defenders.
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Né Stig
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The MOT will be a problem if taken to some pedantic manufacturer.
They can strictly fail you on exhaust noise, as they did to me on a 3.5" tail end with cat. Ridiculous.
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JC.
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The rules for exhausts are that the vehicle shouldn't emit noticably more noise than a similar vehicle fitted with a standard system.
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Né Stig
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^ Precisely, hence why you shouldn't be undertaking an MOT at a manufacturer without a muffler.
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terpineol
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It seems the way to go with MOTs is to get friends with your local independant used car dealer. Give him the cash for the MOT, he goes and gets it done at trade rate, keeps the surplus, and all sorts of sins go unnoticed.
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JC.
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Or... just submit a car in good condition for testing?


I don't mind cars failing the MOT. It illuminates safety items that i've missed. Not that I miss safety critical stuff very often...
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Gypsy King
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(Original post by terpineol)
It seems the way to go with MOTs is to get friends with your local independant used car dealer. Give him the cash for the MOT, he goes and gets it done at trade rate, keeps the surplus, and all sorts of sins go unnoticed.
What, you mean that's not what you are supposed to do? :ninja:

It depends what you know to be wrong. If its a little bit of rust on something non structural, but its too close to load bearing point, then I say pass it. If it has less brakes than a Gordon Brown budget, it's gotta be pulled up!
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terpineol
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(Original post by Gypsy King)
What, you mean that's not what you are supposed to do? :ninja:

It depends what you know to be wrong. If its a little bit of rust on something non structural, but its too close to load bearing point, then I say pass it. If it has less brakes than a Gordon Brown budget, it's gotta be pulled up!
It did crack me up that with horrifically unbalanced rear brakes it passed as the car was so light that it only needed to generate a flea's worth of stopping force at the back.
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terpineol
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On a more interesting note it does seem to be impossible to get my headlights high enough to pass on the machine. In the end they let me sit in the car and brace myself against the adjuster rod, which got an extra few degrees of up.

I shall tap an extra few mm onto the rod at some point, and try and rotate the brackets a bit, see if that helps.
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rollover, pudding
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you should do this.

people will muffler tips are usually "badboy racers" and look like utter morons, 'cause you get a big beefy sound out of a teeny crappy 1.2 or 1.6. ;]

g'luck with that
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prodriven
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(Original post by JC.)

It's far more important that the exhaust manifold is pulse tuned to opposing cylinders.

When I run my V8 MG at the drag strip, I take the exhaust system off completely and run on open headers.
Exhaust gasses are drawn out of the engine at the collector by the pulse of an opposite cylinder on each bank.
Changing a standard pipe for a similar but non silenced replacement will not affect the pulse tuning, afterall the whole point of pulse tuning is to set the correct lengths of the entire system from head to tailpipe. Provided the replacement pipe is not of a different length then you will be fine.

Pulse tunings also a bit pointless on a road car really. You get the advantage for such a small rpm band that you will hardly ever be able to take advantage of it.
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JC.
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(Original post by prodriven)
Changing a standard pipe for a similar but non silenced replacement will not affect the pulse tuning, afterall the whole point of pulse tuning is to set the correct lengths of the entire system from head to tailpipe. Provided the replacement pipe is not of a different length then you will be fine.

Pulse tunings also a bit pointless on a road car really. You get the advantage for such a small rpm band that you will hardly ever be able to take advantage of it.

I don't think it's pointless. If you're pottering around in a little hatchback then the effect won't be a huge gain of power. However, on these little engines all gains are comparatively smaller.
It's generally accepted that you need a gain of around 10bhp to feel a difference. Fitting a manifold with tuned length primaries over a cast iron manifold on something like a 5 litre chevy lump could make for a difference of over 50+bhp which is certainly nothing to be sniffed at.
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Lara C.
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(Original post by JC.)
Don't believe for a minute all the guff spouted above about "back pressure". Most people haven't got a clue what it is and how it works.

It's far more important that the exhaust manifold is pulse tuned to opposing cylinders.

When I run my V8 MG at the drag strip, I take the exhaust system off completely and run on open headers.
Exhaust gasses are drawn out of the engine at the collector by the pulse of an opposite cylinder on each bank.

Anyway, do what you like. It's your car. I will say that having an extremely loud exhaust will piss you off on a long journey.
The exhaust on my V8 was basically 4 straight through cherry bombs - two on each bank.

Sounded great in the pub carpark but after about the 50th mile of nothing but [SIZE="6"*]"BLEM BLEM! BLEM! BLEM! BLEM!"[/SIZE] coming through the window (regardless of whether its open or closed) it starts to get a bit irksome.

-JC

*1 Rover V8's and Chevy V8's go "blem blem blem", Ford Flathead V8's go "potatoe, potatoe" which is an all together more satisfying sound.
yup. been reading some of Dave Andrews posts on the other forum i use (he even once gave me advice on porting my manifold ) and thats exactly what he says. opposing cylinders should be connected in a 4-2-1 fashion to draw out the gasses from the next fireing cylinder.

what ive heard about back pressure is that if you lose it you get turbulance which reduces the efficiency of the flow.

edit;
"An engine is basically an air pump, if you had continuous airflow through the engine at constant pressures and temperatures then the configuration of the exhaust would be irrelevant, however you do not have this perfect state, you have air and exhaust gas that is constantly being accelrated and decellerated at varying temperatures and pressures. Knowing a little about the dynamics of an engine helps to understand what works and why.

The exiting exhaust gas travelling down an exhaust primary pipe (manifold) carries with it a large amount of momentum, so much so that if it passes another pipe of static air it will tend to pull that air along with it, a sort of Bernoulli affect. Cojoining pipes from cylinders at opposite ends of the engine's cycle will allow one cylinder's exiting gas to assist the evacuation of the cojoined cylinder when the cylinder is at the scavenge point, I.E when the exhaust valve is closing and the inlet opening, this helps to fill the cylinder better and hence you will get a larger bang.

Without this pull from the opposing cylinder when the inlet and exhaust valves are on overlap, the exhaust gas pressure in the cylinder will cause exhaust gas to go out of both the exhaust and inlet (this is called reversion) giving poor cylinder filling and therefore poor torque/power until very high RPM where the harmonics of the cylinder on it's own provide proper scavenging.

Depending on the speed (size) and volume (length) of the exhaust primary and its gas load, the pull affect will happen at a particular RPM spread, with a 4-1 this is concentrated nearer the top end of the engines rev range and there is only one 'pull' shared across the other 3 cylinders for each exhaust 'pulse', only one of the three cojoined cylinders will benefit directly, the others will be out of step and the pull will be wasted. With a 4-2-1, there is a single direct large 'pull' on the opposing cylinder and a secondary pull on the other two timed a little later, This tends to give better efficiency in the mid range.

Too large a pipe will move the affect up the rev range as will the wrong length pipe. So a 4 inch bore manifold will most likely lose you a stack of mid range torque.

There are similar affects and gains to be made by carefully sizing the induction and selecting its length so that the incoming charge has suffiient momentum to keep piling into the inlet port even after the valve has closed rather than stalling. This is why TBs and long trumpets can dramatically improve mid range torque.

Dave"

http://forums.mg-rover.org/showpost....3&postcount=25
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