Thread: Exhaust size 2.5 or 3 inch?

Putting in my two cents worth here. From what I understand there is far more to consider in the calculation of back pressure from an exhaust if you were to do it properly. In simple and summarized form (I'll try not to get carried away)

1. Velocity of flow, Exhaust Diameter and Kinematic Viscosity of Fluid (exhaust gas) all contribute to the first step of the process, calculating Reynolds Number.

2. Reynolds number is then used to find flow state, either Laminar or Turbulent.

3. Depending on the flow state the friction coefficient can be found using two different equations which involve Reynolds Number. Turbulent flow state takes into account the roughness of the pipe.

4. Finally using the friction coefficient the Pressure Drop in the exhaust pipe can be calculated. Resistance coefficients such as gravity effects due to elevation changes (negligible) and elbows in the exhaust can be taken into account if accuracy is required.

So in theory there are many variables to take into account, much more than Velocity and Diameter.

But assuming the Kinematic Viscosity and Velocity of Flow is kept equal for two different size exhausts (different diameter, same length, same roughness and same resistance coefficient) the Reynolds numbers would be higher for the larger size exhaust. Then assuming they are both laminar flow the friction coefficient would be smaller for larger exhaust.

A smaller friction coefficient is then used in the pressure drop equation therefore assuming all variables are equal a smaller friction coefficient would cause a smaller drop in pressure hence less back pressure in a larger exhaust.

But because the final equation for pressure drop takes into account friction coefficient and diameter (in the same equation) there would be a point of optimization where the diameter is too big and would start increasing the pressure drop again.

If I have got this horribly wrong please let me know. I only did fluid kinematics last semester so there should be at least something correct in there

Originally Posted by isuzuscott

Putting in my two cents worth here. From what I understand there is far more to consider in the calculation of back pressure from an exhaust if you were to do it properly. In simple and summarized form (I'll try not to get carried away)

1. Velocity of flow, Exhaust Diameter and Kinematic Viscosity of Fluid (exhaust gas) all contribute to the first step of the process, calculating Reynolds Number.

2. Reynolds number is then used to find flow state, either Laminar or Turbulent.

3. Depending on the flow state the friction coefficient can be found using two different equations which involve Reynolds Number. Turbulent flow state takes into account the roughness of the pipe.

4. Finally using the friction coefficient the Pressure Drop in the exhaust pipe can be calculated. Resistance coefficients such as gravity effects due to elevation changes (negligible) and elbows in the exhaust can be taken into account if accuracy is required.

So in theory there are many variables to take into account, much more than Velocity and Diameter.

But assuming the Kinematic Viscosity and Velocity of Flow is kept equal for two different size exhausts (different diameter, same length, same roughness and same resistance coefficient) the Reynolds numbers would be higher for the larger size exhaust. Then assuming they are both laminar flow the friction coefficient would be smaller for larger exhaust.

A smaller friction coefficient is then used in the pressure drop equation therefore assuming all variables are equal a smaller friction coefficient would cause a smaller drop in pressure hence less back pressure in a larger exhaust.

But because the final equation for pressure drop takes into account friction coefficient and diameter (in the same equation) there would be a point of optimization where the diameter is too big and would start increasing the pressure drop again.

If I have got this horribly wrong please let me know. I only did fluid kinematics last semester so there should be at least something correct in there

I have to ask... what mark did you get???

You were doing OK until you said:

assuming the ...Velocity of Flow is kept equal for two different size exhausts (different diameter

and then

assuming they are both laminar flow

Calculate exhaust flow rates and then Re (Donaldson have good calculators). To get you started, 20 m3/min is about right for a healthy 4BD1T at 3500 rpm.

You will find that Re varies ~8% between 3" and 2.75" and ~17% between 3" and 2.5", however is always well and truly turbulent.

If you look at a moody diagram you will find that for complete turbulence, there is no change to the curves with changing Re.

You can then simplify the energy equation (bernoulli + viscous forces/losses) and you will end up with DeltaP=[is proportional to]V^2/D.

SO, that tells you that a 2.75" exhaust will have 1.5* the pressure drop (back pressure) of a 3" exhaust and a 2.5" exhaust will have 2.5* the pressure drop of a 3" exhaust, provided they are all of the same length, with the same number and radius of all bends etc...

Originally Posted by isuzurover

That bitcould be misconstrued if taken out of context. V^2/D shows that velocity is VERY relevant. I think I was replying to (BS) comments that you needed to maintain an "exit speed" for exhaust from the turbo...

That is true the stupid thing didn't post properly to show the full post sorry....I think the equation solves the debate for everyone though...

Originally Posted by isuzurover

SO, that tells you that a 2.75" exhaust will have 1.5* the pressure drop (back pressure) of a 3" exhaust and a 2.5" exhaust will have 2.5* the pressure drop of a 3" exhaust, provided they are all of the same length, with the same number and radius of all bends etc...

OK, sorry if i've missed something but I'm still confused as to which size exhaust is best! I understand that the bigger the exhaust the less back pressure, but is a little back pressure good, or is the least the better for the turbo? As I'm sure you know this topic is like tyres, everyone has a different idea! But I'm interested in your valuable experience.

And if say a 2.5 or 2.75 is the best size, will going a 3" give less performance or just be no advantage? If so a good reason to go 3" may be just for a good exhaust note?
Cheers, Andrew

Originally Posted by taylorslandy

OK, sorry if i've missed something but I'm still confused as to which size exhaust is best! I understand that the bigger the exhaust the less back pressure, but is a little back pressure good, or is the least the better for the turbo? As I'm sure you know this topic is like tyres, everyone has a different idea! But I'm interested in your valuable experience.

And if say a 2.5 or 2.75 is the best size, will going a 3" give less performance or just be no advantage? If so a good reason to go 3" may be just for a good exhaust note?
Cheers, Andrew

With a turbocharged engine you will get best performance by minimising back pressure.

So a 3" exhaust is better than a 2.75", and so-on...

For performance, the best option would be this (A very short, large diameter pipe) if you could stand the noise...

Thanks, that's what I always thought, 3" is better for turbo cos it flows better. How simple was that? Now lets talk about tyres.........

awsome photo by the way, that thing must go!

Originally Posted by taylorslandy

OK, sorry if i've missed something but I'm still confused as to which size exhaust is best! I understand that the bigger the exhaust the less back pressure, but is a little back pressure good, or is the least the better for the turbo? As I'm sure you know this topic is like tyres, everyone has a different idea! But I'm interested in your valuable experience.

And if say a 2.5 or 2.75 is the best size, will going a 3" give less performance or just be no advantage? If so a good reason to go 3" may be just for a good exhaust note?
Cheers, Andrew

With a turbo, the turbine will produce more torque if the back pressure is reduced. More torque from the turbine enables the compressor to spin faster across the range of engine rpm.

In a previous life I have had jobs involving modifying the exhaust from turbines from minimum back pressure to vacuum, which greatly increases the power output to cope with plant upgrades. The principles are little different to the turbines used in turbo chargers.