Bigger Is Better?

[G60Rallye]

So roughly speaking you are gaining 10" squared for every 1/2" increase in diameter. Obviously the best exhaust for a turbo car is not to have one at all, but I am dubious how much you would gain from going 4" instead of 3.5" on a realativly small turbo.

[Jonnybravo]

A friends ex Schance Rallycross Rs200 runs a 3.5" Downpipe and that was dynoed at 780hp.

Unsure what turbo but capacity is similar.

[Audi_CQ]

This is my 3.5" DP / 2" WG + first part of exhaust: Dynoed at 584hp and 520lbs/ft @wheels. Larger injectors needed for next year

[terrybullwon]

Thats Great work Roger!!!!!!

I must take a photo like that!

[david]

This is my 3.5" DP / 2" WG + first part of exhaust: Dynoed at 584hp and 520lbs/ft @wheels. Larger injectors needed for next year

that's a work of art you've got there mate

what turbo are you running with that dp mate?

[Audi_CQ]

that's a work of art you've got there mate

what turbo are you running with that dp mate?

Thanks
I'm running a Single ball bearing T66 with Q-trim AR 0.82. Custom 6" inlet to help airflow:

[david]

where did you get the turbo from and how much?

[Audi_CQ]

where did you get the turbo from and how much?

From here: http://www.krbtrading.no/ (only in Norwegian I'm afraid )
I bought it 4-5 years ago. Was ~1500euro then.
The custom filter attachment was an old K&N part that I found at a local tuner. Some work with a lathe and a couple of spotwelds was all that was needed
Another picture:

[G60Rallye]

A friends ex Schance Rallycross Rs200 runs a 3.5" Downpipe and that was dynoed at 780hp.

Unsure what turbo but capacity is similar.

Regardless of the capacity being smaller, a 5 cylinder would still need to burn roughly the same fuel and air to make the same power?

I am planning to use 3.5" for my exhaust when the times comes. Seems its not causing any issues at huge power levels.

[Jonnybravo]

I tend to use cars like that as a basis of whats needed for that sort of power as they do build these cars with no real budget, it just needs to be right, its also a Julian Godfrey engine who IMO knows enough on whats required at those power levels but ofcourse its not quite the same engine.

4" downpipe on a S2 would be a bit of a mission to fit regardless of wether its needed anyway !

[Jamo]

This is my 3.5" DP / 2" WG + first part of exhaust: Dynoed at 584hp and 520lbs/ft @wheels. Larger injectors needed for next year

How does this fit in terms of the wastegate pipe? Seems to be a high separate pipe

[G60Rallye]

I would say the wastegate pipe goes over the driveshaft?

[driftin jo]

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

Errr , NO! Area is pi x radius squared.

So the area of a 2.5'' pipe is 1.25 x 1.25 x 3.14= 4.90625 square inches,
as you can see you calculation is out by a factor of 4 !

3'' is 7.065 square inches
3.5'' is 9.616 square inches
4'' is 12.56 square inches

[Audi_CQ]

How does this fit in terms of the wastegate pipe? Seems to be a high separate pipe

I've got a custom transmission mount and the WG pipe passes over the mount and then down to the DP.

[goodall]

Oops, thought would got away my maths error, I did notice it the yesterday too

Think I was (pi*D^2)/4 and forgot the dividing by 4 bit

Corrected

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" = 4.91"²
3" = 7.07"²
3.5" = 9.62"²
4" = 12.56"²

if you assume cross sectional area is representative linearly to power output, using jamos figure that 500bhp=3" we can assume a 4"=(12.5/7)*500=893bhp

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