MKV Flapper mod completed

[Nelson_R32]

Decided as I had nothing better to do tonight I would give it a go.

As I am fairly young I thought I would do it without the aid of my Jack. Unfortunately after completing the mod I do feel allot older So I would recommend using one.

There are 3 main ways this mod can be carried out.

Firstly by putting a screw in the pipe; I wasn't happy with this as it meant a pipe lying around.

Secondly; You can remove the valve itself, tape up the small hole that closes the valve and reseal. Looks OEM.

Thirdly, and my route. Remove pipe, put a ball bearing into the pipe, refit pipe Looks OEM is easier than playing with the value.

Now allot of Americans are saying that this mod will reduce your low end torque. But I fail to see how this can be as the general purpose of these systems is to reduce the noise for EU legislation etc It would be interesting if a tuner would one day to a test on this though to give the definitive answer.

Anyway, end result is the car now sounds a little more 'boomy' (in a good way) in normal traffic and allot more 'metallic' around 3500-4000rpm. So basically noticeably noiser

I've yet to see what it will do to MPG but I shall report tomorrow on my Morning MPG run

[Calm Chris]

I imagine the Americans (are they getting Mk5 now?) feel that the flapper applies a certain amount of back pressure below the 3k rpm when it opens.

The engine map could be designed to work better with the back pressure and hence power is lost below 3k.

It made jack all difference to the Mk4, until you got near a tunnel

[Nelson_R32]

To be honest I did think it would probably be the Americans putting a negative on it...

Well so far I've only seen benefits MPG is now up 3mpg has a nice burble below 3000 rpm

The downside (and IMHO feel what its designed for) is that it tends to warble/resonate a little on downshifts and when coming to a stop? Perhaps VW thought that leaving the exhaust without this valve could lead to older Customers complaining about the noise?

Anyway, I'm more than happy with it

[Maxyboy]

Without wantin to come across as stupid, or, more stupid than usual. where is this pipe? how long did it take to do and if its not to my taste is it east to return to normal?

[Oli]

Its on the right hand rear exhaust pipe, and very easy to change back if you dont like it - Or you could just stick a MILLTEK cat back on it - sounds far nicer

[Maxyboy]

This is true, but ball bearings are cheaper.

Also at a time when i'm thinking of changing the little devil so not wanting to throw money at it.

[DanG]

Can someone explain how back presure (or exhaust restriction) helps??

Thanks!

[RedRobin]

[ QUOTE ]

Perhaps VW thought that leaving the exhaust without this valve could lead to older Customers complaining about the noise?

[/ QUOTE ]

....

At 58yo when I bought my VeeDub, I'm an "older customer"! The first mod I did was change to a Milltek because the Mk5 cabin is far too well soundproofed and I want to hear my engine as a driving aid whatever I drive (and cos I enjoy xzorst music!).

[Nelson_R32]

[ QUOTE ]

[ QUOTE ]

Perhaps VW thought that leaving the exhaust without this valve could lead to older Customers complaining about the noise?

[/ QUOTE ]

....

At 58yo when I bought my VeeDub, I'm an "older customer"! The first mod I did was change to a Milltek because the Mk5 cabin is far too well soundproofed and I want to hear my engine as a driving aid whatever I drive (and cos I enjoy xzorst music!).

[/ QUOTE ]

58? Were you meant to order a 1.4 Match and ticked the wrong box?

Just kidding But you know what I mean The average R32 customer is a little more mature. God knows why because it's a riot!!

[Calm Chris]

[ QUOTE ]

Can someone explain how back presure (or exhaust restriction) helps??

Thanks!

[/ QUOTE ]

I'd like to know as well, or is it an urban myth

[Mook]

Back pressure is caused when the exhaust gases "hit" bends and restrictions in your exhaust as they leave your engine and go through the exhaust system. This means that the gases can't escape as quickly, so, moving back up the exhaust into the piston chambers, it's harder for the exhaust gases to be pushed out, so the engine has to work harder, so your power and torque are lessened.

But back-pressure is a good thing as well, because it slows the gases down, hence reducing the noise. Not everyone wants a noisy car. When I say not everyone, I guess I'm not in that list, because my toy is quite a bit louder now than it used to be thanks to Mr Milltek

In an ideal world, you would have a dead straight pipe after your exhauts manifold, rather than having twists and bends to get the exhaust under the car, because the exhaust could get straight out of the engine. Take a look at at F1 exhaust next time you get a chance.

One thing you will notice on an F1 exhaust is that it looks like a freaky bunch of bananas. That's to ensure that there is equal back pressure for each chamber.

You could argue that for drag cars, a lot of the V8s just have one pipe per cylinder (but you'll notice they're all the same length) - this is because the gains in power you'll get from engines putting out the amount of horses you get from ethanol powered beasts is just stupid, so it would be minimal.

Do modern engines benefit from some back-pressure? Yes, they do. More of this is to do with ensuring that there's equal back pressure for each cylinder, though.

I'm sure there's a load of stuff linked to on Google, but hopefully this helps a bit?

[RedRobin]

....Cool overview, Mook

[DanG]

Cheers Mook,

Next question.

With the exhausts with flaps that open above certain revs, so initially restricting the amount of gas flow (R32 and most BMWs I think), what is this benefit?

I read that free flow exhausts cause a loss of low end power and tourque, however once up at WOT they work better.

Would I be right to say that the exhaust system needs to be 'charged' with presure and then, as the extra 'out' pipe is opened, the gas is channelled in the right direction, so creating through flow and potentially slight negative presure as the gas exits to the air?

[Mook]

It's only about sound and keeping it below whatever the Euro numpties say it should be - they test the noise of the car at X,000 RPM. Anything above that, the manufacturers can do what they want.

Check out an Aston V8 Vantage for how to do it properly

Re the rest of your post, no, that's not why they have them - the flaps are there for noise reduction.

With the flap closed, the exhaust is routed via more baffles in the silencer. With it open, it has a quicker exit from the rear silencer (so is noisier).

HTH

[Calm Chris]

So the flapper alters sound but not back pressure?

[zharca]

[ QUOTE ]

Back pressure is caused when the exhaust gases "hit" bends and restrictions in your exhaust as they leave your engine and go through the exhaust system. This means that the gases can't escape as quickly, so, moving back up the exhaust into the piston chambers, it's harder for the exhaust gases to be pushed out, so the engine has to work harder, so your power and torque are lessened.

But back-pressure is a good thing as well, because it slows the gases down, hence reducing the noise. Not everyone wants a noisy car. When I say not everyone, I guess I'm not in that list, because my toy is quite a bit louder now than it used to be thanks to Mr Milltek

In an ideal world, you would have a dead straight pipe after your exhauts manifold, rather than having twists and bends to get the exhaust under the car, because the exhaust could get straight out of the engine. Take a look at at F1 exhaust next time you get a chance.

One thing you will notice on an F1 exhaust is that it looks like a freaky bunch of bananas. That's to ensure that there is equal back pressure for each chamber.

You could argue that for drag cars, a lot of the V8s just have one pipe per cylinder (but you'll notice they're all the same length) - this is because the gains in power you'll get from engines putting out the amount of horses you get from ethanol powered beasts is just stupid, so it would be minimal.

Do modern engines benefit from some back-pressure? Yes, they do. More of this is to do with ensuring that there's equal back pressure for each cylinder, though.

I'm sure there's a load of stuff linked to on Google, but hopefully this helps a bit?

[/ QUOTE ]

I'm not really sure about this "back pressure" stuff being a feature of exhaust design.

F1? Bananas? Equal back pressure?

Surely not?

Whenever I hear “back pressure” mentioned in relation to 4-stroke engines, I start to worry.

Isn’t the point of designing exhaust manifolds to create NEGATIVE PRESSURE pulses travelling back down the pipe to the cylinder to completely scavenge the exhaust gasses and, if really done right, for the pulse to reach right back to the intake tracts. So, in a properly designed system, you can say the exhaust energy is used to “pull” the gas through the cylinder. They look like bananas to make all the primaries the same length and to join the right cylinders up to match exhaust pulse to intake stroke.

This stuff has got very complicated now, with computer modelling, but the simplest calculation I know came from Bob Gayler many years ago when he was with Baldyne engineering:

Manifold primary pipe length (from the exhaust valve seat)

L=(950x(180+E)/R

(Divide by 2 for a 4-2-1)

E=exhaust open bbdc

R=rpm. Choice here, but normally peak BMEP

The answer's in inches. Said it was old!

Modern stuff gets very complicated and starts taking in peak egt’s, thermal as well as sonic pulse etc, but that’s beyond me.

For noise reduction and efficient flow in exhaust systems, I guess you’d use wavelength tuning, very crudely:

(1100x60)/rpm x no of cylinders. So the point is to keep the velocity UP, but avoid multiples of the exhaust pulses to keep the noise down. These days, noisy aftermarket exhausts are just lazy design. Efficient silencing and performance can be done, it’s just much harder to get right.

Yes, bends in pipes do slow down flow but the pipe diameter is adjusted to compensate.

Isn’t the choice for the system after the manifold to provide enough gas velocity so that the gas still has enough energy to clear the system – ideally the opposite of “back pressure”? If you slow down the gas flow, it has more time to cool, loses energy and stalls, so you do then have to start “pushing” it out. Road exhausts do this to some extent, but that’s an evil of having to make the system long enough to get to the back of the car, rather than by design.

“…for drag cars….one pipe per cylinder …..the gains … from ethanol…..would be minimal.”

Noooooooo noooooooo no!

These are hugely supercharged engines and all the rules change. The exhausts are effectively “dump pipes”, although again still tuned to a resonant frequency, like an organ pipe. Take away the supercharger and you’d need pulse tuned exhaust manifolds.

[Mook]

[ QUOTE ]

So the flapper alters sound but not back pressure?

[/ QUOTE ]

Primarily for sound, yes. But by allowing the exhaust to take a quicker route through the back box you'll naturally reduce the back pressure, but not by very much.

You've doubtless heard everyone waxing lyrical about the wonders of Milltek exhausts and how the car feels more responsive. The main reason for this is that the exhaust bore is bigger and there is less restriction in the silencers (hence they're a bit louder), so less back pressure.

[Mook]

You an engineer, by any chance, Zharca?

[Calm Chris]

Thanks guys

zharca [ QUOTE ]

Manifold primary pipe length (from the exhaust valve seat)

L=(950x(180+E)/R

(Divide by 2 for a 4-2-1)

E=exhaust open bbdc

R=rpm. Choice here, but normally peak BMEP

The answer's in inches. Said it was old!

[/ QUOTE ]

I'm just off to measure my lawn mowers pipe and work out the calc for a Briges 5.5 bhp 150cc unit

[Mook]

[Nelson_R32]

Well after 3 months I've removed the bearing and reverted back to stock.

It wasn't that I wasn't pleased with it. But many people in my village wern't; I actually got a couple of V signs from two Pensioners on there bicycles because I accelerated up to speed up a street and the noise reverberated through the buildings either side. Now normally this wouldn't bother me, except I'm concious that it does sound a little Barry Boy and as one of my friends said yesterday. What have I got to prove? I only need to be seen in it for people to know it's a special car

The other reasons, and probably more importantly, was the 'boom' from the exhaust on my morning commute to work was getting a little unbearable after a while. I know my first post said it was brilliant but unfortunately I forgot the point I would have to live with it on a daily basis. After seeing some youtube videos recently I also wanted that 'rasp' back that you loose with this mod.

Lastly, the DSG actually changes gear at different times with the car back to stock. Whilst that wouldn't damage the car in anyway it is nice to know that it's back to OEM