Exhaust Diameter conundrum...

[pantha]

So let me set the scene.

I have an A3 8P 2.0tfsi Quattro.

I also have (Sitting in the shed) a 3" APR Stainless DP with flex attached.

I want to have a custom exhaust installed, (Personal reasons) and here is what I'm thinking:

I install the 3" DP, into a 3" Cat, then shift/taper to a 2.5" catback system with hi-flow 2.5" mufflers. I like the brand Magnaflow as they have a good reputation and they pride themselves on a good throaty sound.

Note: I read that the majority of the power "gains" comes from the DP, and the Cat-back is purely a sound thing??? (Larger diam = louder)

My understanding of Physics is, when a fluid (Yes, air is a fluid) flows through a pipe, if the pipe increases in diameter, the pressure increases with it, at a cost of velocity. And complimentary to that, if the diameter of the pipe decreases, the pressure will drop, and velocity will increase.

SO....

Should I keep the 3" DP, and have a 3" cat as previously stated, to have "Pressure" from the Turbo outlet into the DP, to overcome flow restriction from the Cat... then, once past the cat, reduce the diameter, to aid in velocity of the gases. Not to mention that, by then, the gases would have significantly cooled, and would not require such a larger diameter to accommodate them.

Am I thinking clearly... OR ... Should I switch out the 3" DP for a 2.75" DP? (And if I did, what size Cat?)

Cheers!

 

[nluk100]

I ran a 2.75" DP to the standard catback (2.5") for over a year and it was OK, but it is usually better to go bigger for exhausts on turbo cars, especially when modified.

BTW, the Miltek DP is 3" and their own catback is 2.75".

 

[pantha]

I can understand why you would go bigger when it comes to turbo engines... but 3" seems excessive for any power below 200kW?

 

[S3Alex]

I can understand why you would go bigger when it comes to turbo engines... but 3" seems excessive for any power below 200kW?

I think this is true....

I've had 3" or 75mm systems on cars running up to 600bhp without problems.

 

[superkarl]

Diameter increase = pressure drop, which is why turbo cars benefit from infinitely bigger exhausts.
Turbine flow decreases with pressure increase.

Downpipe is more important though. Pressure and velocity isn't uniform through the exhaust system

 

[pantha]

So should I keep with the 3" DP? (Remembering its a K03)

Considering that the biggest restriction on these systems is the cat converter mounted ON the turbo outlet, then, another one shortly after...

Once those are replaced with a Hi-Flow cat, positioned at the end of the DP. then what would an ideal DP Diameter be? Or is it irrelevant when it comes to turbos, AS LONG as it isn't restrictive?

ie: if the potential power of the turbo is say, 180kw and it required a minimum (hi-flow) exhast diameter of 2-5/16". Any diameter above that is no change in power (better or worse) it would just be louder...?

 

[smurfworth]

Why not go 3" all the way, BCS is the only exhaust maker that keeps a 3" flow from the downpipe all the way to the exhaust tips and r tech have seen a 10hp increase on the BCS over competitors aftermarket "performance" exhausts.

Your trying to get the air in to the engine and out as fast as possible, the turbo is forcing the air through the exhaust so the bigger the pipe the faster it can escape.

 

[pantha]

Why not go 3" all the way

Because I don't want a very LOUD exhaust that sounds like it has tin cans for mufflers.

I've had 3" or 75mm systems on cars running up to 600bhp without problems.

TOTALLY agree, So why would I need a 3" exhaust if I have no intention of reaching 600bhp?

Your trying to get the air in to the engine and out as fast as possible, the turbo is forcing the air through the exhaust so the bigger the pipe the faster it can escape.

Based on that premise, you could assume that having NO exhaust would be best for maximum power. And yes, while I can agree on that for "Peak Power", one must also consider torque?

Would I be correct is saying the larger the exhaust, the less restrictive and LOUDER it is.

So, Inversely, At what diameter can I "shrink" down to, in order to not restrict power, and yet, keep the dB level to a suitable level? (NO drone either)

 

[Dobermansteve]

Because I don't want a very LOUD exhaust that sounds like it has tin cans for mufflers.



TOTALLY agree, So why would I need a 3" exhaust if I have no intention of reaching 600bhp?



Based on that premise, you could assume that having NO exhaust would be best for maximum power. And yes, while I can agree on that for "Peak Power", one must also consider torque?

Would I be correct is saying the larger the exhaust, the less restrictive and LOUDER it is.

So, Inversely, At what diameter can I "shrink" down to, in order to not restrict power, and yet, keep the dB level to a suitable level? (NO drone either)

I believe that a 3" system has always been considered the best on any turbo car regardless of your end power goals.

You do however need an exhaust as some back pressure is needed although I see your thinking behind that.

You could fit a 20" inch system and if it had silencers/mufflers/resonated it wouldn't be that loud, if that makes any sense?

(All statements are my opinion and can not be taken as fact!) Lol

 

[Gazwould]

Go for 3" and make sure the rest of the straight through silencers have adequate wadding/ packing .

 

[pantha]

I believe that a 3" system has always been considered the best on any turbo car regardless of your end power goals.

For K04 S3's... you're probably right, but are you going to say that for a 1.4TFSI?

What I'm getting at is, I have 3 options:

1. Use a 3" DP, with full 3" system

2. Use a 3" DP, with a 2.75" System

3. Use a 3" DP, with a 2.5" System

Now... I believe option 1, is OVERKILL for the A3 with a K03 Turbo... Correct me if I'm wrong? (Without SERIOUS internal work, and $$$$$$ spent)

So, which of the next options is best for an A3 with a K03?

OR.... Do I get a 2.5" or a 2.75" DP and start again??

 

[pantha]

Go for 3" and make sure the rest of the straight through silencers have adequate wadding/ packing .

Me thinks this car wished it ran on AvTur?!

 

[Dobermansteve]

For K04 S3's... you're probably right, but are you going to say that for a 1.4TFSI?

What I'm getting at is, I have 3 options:

1. Use a 3" DP, with full 3" system

2. Use a 3" DP, with a 2.75" System

3. Use a 3" DP, with a 2.5" System

Now... I believe option 1, is OVERKILL for the A3 with a K03 Turbo... Correct me if I'm wrong? (Without SERIOUS internal work, and $$$$$$ spent)

So, which of the next options is best for an A3 with a K03?

OR.... Do I get a 2.5" or a 2.75" DP and start again??

Why not people run toyota starlets/galanza's etc with mahoosive exhausts, goes back to what smurfworth said about getting the air in and out faster.

I'd keep your 3"dp and just add a decent 3" system and forget about it, it's future proofed then. And I doubt you'd notice the noise difference between them.

 

[Gazwould]

Start again ,
be creative , be unique ...

 

[pantha]

Everyone knows, if you're going to do multiple exhaust tips, you have to do AT LEAST 20... Fail!!!!

 

[downtothebone]

Start again ,
be creative , be unique ...

Brilliant!!

 

[Sayam]

Because I don't want a very LOUD exhaust that sounds like it has tin cans for mufflers.



TOTALLY agree, So why would I need a 3" exhaust if I have no intention of reaching 600bhp?



Based on that premise, you could assume that having NO exhaust would be best for maximum power. And yes, while I can agree on that for "Peak Power", one must also consider torque?

Would I be correct is saying the larger the exhaust, the less restrictive and LOUDER it is.

So, Inversely, At what diameter can I "shrink" down to, in order to not restrict power, and yet, keep the dB level to a suitable level? (NO drone either)

Far as I know the BCS system is quiet and comfortable daily driving and only gets loud once the valve opens

 

[smurfworth]

Because I don't want a very LOUD exhaust that sounds like it has tin cans for mufflers.



TOTALLY agree, So why would I need a 3" exhaust if I have no intention of reaching 600bhp?



Based on that premise, you could assume that having NO exhaust would be best for maximum power. And yes, while I can agree on that for "Peak Power", one must also consider torque?

Would I be correct is saying the larger the exhaust, the less restrictive and LOUDER it is.

So, Inversely, At what diameter can I "shrink" down to, in order to not restrict power, and yet, keep the dB level to a suitable level? (NO drone either)

Have you ever heard of a screamer pipe? Normally it's a bit of exhaust tubing, normally around 3", that when on boost becomes active. It's insanely loud as people normally have them coming out the bonnet, side exit or under the engine but it is best for power.

But in response to your "torque" question.. The whole point in the BCS system, which is 3" flow all the way through, is to have low end torque when the valve is shut and once open allows maximum flow with very little restriction.

Tbh, based on your requirements id say don't cheap out on a custom system reducing down to a 2.5".. Just buy a BCS powervalve. Quiet when you want with no drone retaining low down torque, and maximum power when you want.

 

[Rick @ Unicorn Motor Dev.]

Do whatever is easier/cheaper, keeping a 3in down pipe. After that all your options are more than adequate.

Rick

 

[S3Alex]

Have you ever heard of a screamer pipe? Normally it's a bit of exhaust tubing, normally around 3", that when on boost becomes active. It's insanely loud as people normally have them coming out the bonnet, side exit or under the engine but it is best for power.

But in response to your "torque" question.. The whole point in the BCS system, which is 3" flow all the way through, is to have low end torque when the valve is shut and once open allows maximum flow with very little restriction.

Tbh, based on your requirements id say don't cheap out on a custom system reducing down to a 2.5".. Just buy a BCS powervalve. Quiet when you want with no drone retaining low down torque, and maximum power when you want.

A screamer pipe is a pipe from an external wastegate.

An internal wastegate,i.e. integrated into the turbo as on the K03/K04 and most other OEM fit turbos will open and divert exhaust gas that would pass through the turbo,adding to boost,when the ECU senses a set boost level is reached,preventing it from doing so.

Most externally wastegated turbos also release excess exhaust gas into the exhaust downpipe,but a screamer pipe is simply routed from the wastegate to atmosphere.
It's loud as there is no silencer,and usually also no cat.

You can see the wastegate pipe on this pic....