Bigger Is Better?

[david]

i was just wondering whether a bigger downpipe like 4"inch would be any good on s2.
i know the normal thing to do is to go down the 3"DP route,after all the bigger the better.
or this option is only for serious track cars?

[Jamo]

It is better yes but you pay a slight price off boost and also it's far noisier!

Only applicable for 500 bhp plus applications

[lind4hl]

As said, less backpressure, but also more noice

If youre planning to upgrade later and you can live with the noise, i wold consider a 4" dp and then just change/ remake the top of the DP after the new exhaust mani/ turbo.

[david]

It is better yes but you pay a slight price off boost and also it's far noisier!

Only applicable for 500 bhp plus applications

so what your saying is,it only applicable for higher hp really.but what if i wan't to bolt one on now for future upgrade?
at the moment i'm contemplating putting 3"DP,but if i can get away with 4"DP then why not.atleast i'll be saving myself time and money in the long run

[Jamo]

As said depends on your future bhp plans and if your car is a daily, 3" will support most needs

I looked into getting 4" but it's hard to fit and also not many places will custom make them.

[david]

As said, less backpressure, but also more noice

If youre planning to upgrade later and you can live with the noise, i wold consider a 4" dp and then just change/ remake the top of the DP after the new exhaust mani/ turbo.

at the moment i've got 2.5" LONGLIFE ss with no back box,just twin outlet exhaust pipes and that's sound quite boomin enough for little pipe's.so a 4" DP sound will be something else.

[david]

As said depends on your future bhp plans and if your car is a daily, 3" will support most needs

I looked into getting 4" but it's hard to fit and also not many places will custom make them.

well Jamo i'm teaching myself how to TIG weld at the moment
i'm a bit useless at the mo,but giving time i just might get the hang of it.

[Jamo]

Speak to messers Terrybullwon, S2spike & Lind4hl

[GazzaG60]

id excho the 3" is easier than 4. i can remove my 3" pipe in not a lot of time. id expect 4" would be tight maybe need mods down below.

one option is 3" plus atmosheric dump of gate

[goodall]

If gas velocity drops too low in the exhaust the gas cools and gets thicker/heavier and makes extra back pressure due to higher viscosity and loss of inertia, meaning engine has push harder expel the exhaust

heres some numbers to help understand how much each size pipe changes cross sectional area, iv assume the sizes are the internal bore even if it isnt
2.5" = 19.63495"²
3" = 28.27433"²
3.5" = 38.48451"²
4" = 50.26548"²

if you assume cross sectional area is representative linearly to power output, using jamos figure that 500bhp=3" we can assume a 4"=(50/28)*500=893bhp

so unless your planning for that peak power and peak is your only concern, bigger isnt always better

[Audi_CQ]

About backpressure: Size isn't everything
A 4 inch DP with a lot of "bumps", small turns and so on because of lack of space could flow less then a "clean" 3.5" DP.
Just my $0.02

[terrybullwon]

Speak to messers Terrybullwon, S2spike & Lind4hl

Will to help

About backpressure: Size isn't everything
A 4 inch DP with a lot of "bumps", small turns and so on because of lack of space could flow less then a "clean" 3.5" DP.
Just my $0.02

Yes i totally agree 100% and in fairness i think a 3.5" pipe would be very easy compared to a 4" one.

It took me almost 2 days to get mine correct. This was tacked and not welded but now i can move mine in and out with just undoing the V-band at the turbo and at the bottom. The waste gate also has a seperate pipe that runs down and exit's at the gear box.

I had to cut my sub frame and re-weld it and also removed the lump of aluminum from the casting next to the starter on the gear box.

If you need help just ask, i suffered

Cheers James

[lind4hl]

About backpressure: Size isn't everything
A 4 inch DP with a lot of "bumps", small turns and so on because of lack of space could flow less then a "clean" 3.5" DP.
Just my $0.02

Understandable.

BUT in the car that is spoken about ( s2 )

i´only see 1 way to go down, and not that different from a 3" dp.

so im almost 100% sure there wont be more backpressure with an 4" dp..

EDIT: Help is here also

[david]

If gas velocity drops too low in the exhaust the gas cools and gets thicker/heavier and makes extra back pressure due to higher viscosity and loss of inertia, meaning engine has push harder expel the exhaust

heres some numbers to help understand how much each size pipe changes cross sectional area, iv assume the sizes are the internal bore even if it isnt
2.5" = 19.63495"²
3" = 28.27433"²
3.5" = 38.48451"²
4" = 50.26548"²

if you assume cross sectional area is representative linearly to power output, using jamos figure that 500bhp=3" we can assume a 4"=(50/28)*500=893bhp

so unless your planning for that peak power and peak is your only concern, bigger isnt always better

mathematical genius, goodall

[david]

Understandable.

BUT in the car that is spoken about ( s2 )

i´only see 1 way to go down, and not that different from a 3" dp.

so im almost 100% sure there wont be more backpressure with an 4" dp..

EDIT: Help is here also

yeah lind4hl,what dp are you using at the moment.3" or 4"?