exhaust opinion (4in...4in to 5 in..?)

[QuattroFever]

So using a gt3071 with full 3" system making 450hp you fit a 4" exhaust will increase power?

[euro_urs4]

Great info guys thanks you, I'm running a 3in DP with a seperafe dump pipe next to it, and I'm looking to obtain 500+ whp, so wouldn't you say best for me to run atleast 4in DP And full 4in exhaust to keep it as free flowing as possible and not restrict the exhaust at all?

And Merry Christmas guys

[willg54]

Theres so many more factors to account for than just pipe diameter. But I think that does give an indication of its affect. If it is restricted then yea, it will be reducing power.

I've been dealing with higher reving, higher boosted engines than anything the I5 will see and 3" is not the restriction!

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[willg54]

Merry crimbo too!

Surely turbo is going to be the largest restriction anyway....

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[euro_urs4]

Theres so many more factors to account for than just pipe diameter. But I think that does give an indication of its affect. If it is restricted then yea, it will be reducing power.

I've been dealing with higher reving, higher boosted engines than anything the I5 will see and 3" is not the restriction!

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should i worry about the 3in DP and 3 in exhaust for now, and go with 4in or run this for a while, while i do the other necessities to the car ?

[driftin jo]

Merry crimbo too!

Surely turbo is going to be the largest restriction anyway....

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You missing the fact that turbines work on pressure differential across the turbine,
by your statement we should fit a 2.1 inch exhaust pipe to an RS2 turbo as the turbine outlet is 55mm.
With a turbo you need the pressure drop post turbine to aid spool, hence standard pipe being 2.75 inch on an RS2.

[A80Avant]

Just to get an idea, my cat back is 2.75inch and I'm running safe 480-490hp. I would like to see if a 3inch would make a difference..

Standard piping on rs2 is 63mm. Not 70.



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[willg54]

You missing the fact that turbines work on pressure differential across the turbine,
by your statement we should fit a 2.1 inch exhaust pipe to an RS2 turbo as the turbine outlet is 55mm.
With a turbo you need the pressure drop post turbine to aid spool, hence standard pipe being 2.75 inch on an RS2.

Lol not quite that extreme! Just saying I'm sure we could get a lot out of a 3" without it being too much of a problem.

I do love a good engineering debate!

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[driftin jo]

Just to get an idea, my cat back is 2.75inch and I'm running safe 480-490hp. I would like to see if a 3inch would make a difference..

Standard piping on rs2 is 63mm. Not 70.



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S2 is 63mm , RS2 is 70

[A80Avant]

Have you ever measured that? Cause my rs2 had a 63mm downpipe and believe me my 70mm cat back was always bigger than my stock exhaust when replaced.

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[driftin jo]

Have you ever measured that? Cause my rs2 had a 63mm downpipe and believe me my 70mm cat back was always bigger than my stock exhaust when replaced.

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The downpipe on the RS2 is 70 then it splits to 2 smaller pipes before the cats ~ 2.5
the S2 is the same but the downpipe is 63 and splits into ~2.25 inch pipes before the cats.

[A80Avant]

Ah you are obviously referring only to the downpipe. I don't remember. But definitely cat back is not 70mm

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[QuattroFever]

I have posted this more than once before but here you go.



This chart is one I made years ago and was taken from S.E.A testing on an engine dyno, not theory.

Pipe bore size. BHP obtainable without
inch / mm pipe causing restriction.

2 / 50.8 --- 185

2.25 /57.15 --- 235

2.5 / 63.5 --- 285

2.75 / 69.85 --- 350

3 / 76.2 --- 415

3.25 / 82.55 --- 490

3.5 / 88.9 --- 565

3.75 / 95.25 --- 650

The above figures are based on petrol engines running a full length exhaust system 4 metres / 13 feet in length with free flowing silencers . Much more HP is obviously attainable from each size of pipe but beyond these figures the exhaust is increasingly restrictive .

Have those figures accounted for heat lost through the pipe walls affecting the effective diameter? Gas flowing nearer the walls is likely to be cooler and more turbulent than gases in the center which would mean if you heat wrap the pipe the effective diameter is increased. Also the further up the rpm band you go the higher the velocity of the gas and the hotter the pipe gets also improving things further.
Then exhaust design can come into play, some twists and turns 45's and 90's and you may have an exhaust that is 4meter but in terms of restriction is equivalent to a 6 meter pipe.

What were those test carried out on and with what conditons?

[Audi_CQ]

Easy test:
Use a short large diameter pipe out from turbo(remove bonnet and turn pipe upwards if neccesary ) and measure HP on a dyno. Then fit your own exhaust and measure again. THis will give you the loss. If this is a large number then maybe a new exhaust would be smart. If just a few % loss then it's OK

[driftin jo]

Easy test:
Use a short large diameter pipe out from turbo(remove bonnet and turn pipe upwards if neccesary ) and measure HP on a dyno. Then fit your own exhaust and measure again. THis will give you the loss. If this is a large number then maybe a new exhaust would be smart. If just a few % loss then it's OK