Exhaust size 2.5 or 3 inch?

[rar110]

Isuzu landrover brains trust, what is the best 2 1/2 or 3 inch exhaust for turbo and n/a 4bd1? My factory standard goes from 2 1/4 to 2 inches. I would rather go with 2.5 if it would be ok?

thanks

[Larns]

The 2-1/2 would be fine for the n/a. The turbo definatly benifits from the 3", and it sounds cooool:D

[rovercare]

The 2-1/2 would be fine for the n/a. The turbo definatly benifits from the 3", and it sounds cooool:D

^^^^x2:)

[Traveler]

i run 2.5" exhaust on ute no problems...except you cant hear a thing...

[EchiDna]

ok I'll bite... why not 3.5 or 4 inch doameter then? any benefits/downside in even less back pressure than 3 inch exhausts provide (4BD1-T)??

[andrew e]

ok I'll bite... why not 3.5 or 4 inch doameter then? any benefits/downside in even less back pressure than 3 inch exhausts provide (4BD1-T)??
The mob i got my car tuned at said 2 1/2 was big enough for a 4bd1t. They said that the landcruisers 1hz turbo diesels also made more power with a 2 1/2, than a 3. Dont ask me why though, i always thought the less back preassure, the better. I run a 3 inch all the way from the dump, but my muffler shuts it up heaps.

Andy

[rar110]

thanks all for the replies.

peter

[1103.9TDI]

G'day again Pete,
I ran a 3inch exit just in front of the rear left wheel, but it was too noisey with the pax window down, so extended as per standard exhaust, out the rear end. Now much quieter, in fact less noise than the old 2.25!. This fact alone makes the three inch the go, but it's also giving better economy, and the turbo is more free reving, so less lag from a standing start. These engines were designed for trucks and the exhaust manifold flange easily copes with 3inch.........

[DRanged]

Had a chat with an old school exhaust bloke. He said 2 1/2 with no muffler on a turbo 4bd1T is fine. he said unless your chasing a 10th of a second around Mount Panorama in a touring car 3" might benefit.


Justin

[rar110]

thanks Gerry and Justin.

it sounds like 2.5 would be ok. But might as well go with 3".

I am looking at putting the mig to some use and fabricating my own exhaust. Are the joins but welded, or is one side flared slightly so one end fits into another?

thanks

[lokka]

thanks Gerry and Justin.

it sounds like 2.5 would be ok. But might as well go with 3".

I am looking at putting the mig to some use and fabricating my own exhaust. Are the joins but welded, or is one side flared slightly so one end fits into another?

thanks


if your going to mig weld it do the flare and fit one into the other then weld this way u wont blow holes as easy unless your verry proficiant with the mig then go for the butt welds ...

I like to but weld exhaust's but i use a tig and purge the inside of the pipe with the same sheild gas im welding with this gives the best weld of the lot and has continus penatration and the weld looks the same on the inside of the pipe as it dose on the outside ;)

[stage1slave]

Hi all, we run a 3 inch with extractors on a n/a. Is noisy but what a sound...... keeps most tar babies at bay. We also tried that exhaust wrap stuff and found it to be excellent. Wrapped it all the way down to the 4 into 1 joint and it gave the 4bd1 heaps more grunt!
cheers all.

[123rover50]

If you use a muffler, what size etc. I guess it is a straight thru one if at all?
Didiman

[c.h.i.e.f]

If you use a muffler, what size etc. I guess it is a straight thru one if at all?
Didiman

What you need is a straight through resonator about a foot long just to take some rasp out of it....I run 3" turbine to tip I love it :D

[C H T]

Beaudesert Exhaust recomend 2.75" as optimum for the 4BD1T - the made a system for my 110 which has worked very well

They said if you go too big in diameter for the size of engine exhaust gas velocity drops below optimum reducing ultimate gas flow and therefore has a negative effect on power

FWIW

C H T

[Bush65]

Beaudesert Exhaust recomend 2.75" as optimum for the 4BD1T - the made a system for my 110 which has worked very well

They said if you go too big in diameter for the size of engine exhaust gas velocity drops below optimum reducing ultimate gas flow and therefore has a negative effect on power

FWIW

C H T
It was just a matter of time before someone would say something similar :rolleyes:

[isuzurover]

Beaudesert Exhaust recomend 2.75" as optimum for the 4BD1T - the made a system for my 110 which has worked very well

They said if you go too big in diameter for the size of engine exhaust gas velocity drops below optimum reducing ultimate gas flow and therefore has a negative effect on power

FWIW

C H T

Sorry Chris, but that is BS (as I have posted elsewhere with the maths to prove it).

Any exhaust shop who tells you that has NFI about fluid mechanics and turbo diesels.

[c.h.i.e.f]

There are 3 people that will dispute the above comment :angel: i do not have the maths to back it up but isuzurover will provide details once he reads this :D

[c.h.i.e.f]

Hahaha 2 of the 3 I was reffering to posted before I finished typing :D

[c.h.i.e.f]

Previously stated by isuzurover:

In fact velocity is largely irrelevant, it is the mass/energy flux and pressure drop across the turbine that is important..


Also:



Sorry, but this is a crock of bullplop, sprouted by idiots who don't understand maths or fluid mechanics. (not having a go at you lambrover)

Assuming two pipes (say 2.5" and 3" diameter) the same length, made of the same material, and both operating in the turbulent regime, then:

Pressure Drop = Velocity^2 / Diameter

So pressure drop (or back pressure) rapidly INCREASES as velocity increases due to the ^2 term. Increasing pipe diameter increases D and decreases V, both of which reduce pressure drop (back pressure).

So anyone who says smaller pipes give less back pressure has NFI what they are talking about.

[isuzurover]

Previously stated by isuzurover:

In fact velocity is largely irrelevant, it is the mass/energy flux and pressure drop across the turbine that is important..


Also:

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...

[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 :eek:

[isuzurover]

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 :eek:

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...

[c.h.i.e.f]

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...

[Shingleback]

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

[isuzurover]

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...
https://www.aulro.com/afvb/images/imported/2012/06/601.jpg

[Shingleback]

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.........:)

[Shingleback]

awsome photo by the way, that thing must go!

[Bush65]

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.

[ScottW]

Just to throw a cat amongst the pigeons, what about gas temperature along the length of the exhaust? As the temperature drops, the volume of gas decreases, hence the friction losses also reduce.
IIRC, 70's Fords had an exhaust on their V8's that decreased in size along the length. The idea was that using a smaller outlet from the muffler quietened the system down more, without sacrificing performance too much as the gas had cooled, hence less flow volume, hence a lower velocity, hence a lower friction losses.

[isuzurover]

Just to throw a cat amongst the pigeons, what about gas temperature along the length of the exhaust? As the temperature drops, the volume of gas decreases, hence the friction losses also reduce.
IIRC, 70's Fords had an exhaust on their V8's that decreased in size along the length. The idea was that using a smaller outlet from the muffler quietened the system down more, without sacrificing performance too much as the gas had cooled, hence less flow volume, hence a lower velocity, hence a lower friction losses.

It is only an issue if the flow changes from turbulent to laminar, which it doesn't.

[samuelclarke]

I've got a 200Tdi in my Defender (I know it's not an Isuzu 4bd1t - maybe one day!) and put a 3" exhaust in it. Two years on it's been great - smoother power and less fuel consumption when running 255/85/16's tyres. I can imagine that the 4bd1t would be great with a 3". :)

[Bush65]

Just to throw a cat amongst the pigeons, what about gas temperature along the length of the exhaust? As the temperature drops, the volume of gas decreases, hence the friction losses also reduce.
IIRC, 70's Fords had an exhaust on their V8's that decreased in size along the length. The idea was that using a smaller outlet from the muffler quietened the system down more, without sacrificing performance too much as the gas had cooled, hence less flow volume, hence a lower velocity, hence a lower friction losses.
I understand why you might think this.

For best performance it would be better to increase the diameter toward the outlet (better with a megaphone) so the gas velocity and pressure are closer to ambient conditions (i.e. zero velocity and zero gauge pressure). But the noise would be a killer.

You often see examples of this with large ventilation fans where the performance gains/lower power consumption is worthwhile.

[c.h.i.e.f]

I understand why you might think this.

For best performance it would be better to increase the diameter toward the outlet (better with a megaphone) so the gas velocity and pressure are closer to ambient conditions (i.e. zero velocity and zero gauge pressure). But the noise would be a killer.

You often see examples of this with large ventilation fans where the performance gains/lower power consumption is worthwhile.

Just throwing my 2bobs worth in with what you said John...my old boy plays around with old motorcycles in particular his nortons (Manx,international,es2 etc etc) one of these he has manufactured almost everything over the last billion years :angel: and I remember him telling me something about megaphones on the exhaust and for the particular models he plays with were found to have gains from a reverse megaphone exhaust approximately finishing inline with rear axle...not really important just thought I would say it though...

the supposed optimum exhaust for a turbo is short like isuzurover has posted and tappering out on a 10.5-15* angle this is slightly indicated on the turbine housing of my Garrett turbo

[Lotz-A-Landies]

I know what to do!

Put a hiclone in the exhaust, that'll save fuel, double the Kw, halve the noise and reduce CO2 to boot! :twisted:

[isuzurover]

Just throwing my 2bobs worth in with what you said John...my old boy plays around with old motorcycles in particular his nortons ...

Part of the problem in this discussion is the application of NA petrol engine theory to turbo diesels.

They are completely different animals!
e.g. - this is a performance 2-stroke MB exhaust!
https://www.aulro.com/afvb/

[c.h.i.e.f]

Part of the problem in this discussion is the application of NA petrol engine theory to turbo diesels.

They are completely different animals!
e.g. - this is a performance 2-stroke MB exhaust!
http://www.jcmotors.com/images/PRODUCT/medium/JCM-01-2010DG.jpg

Yeah sorry just throwing it in there though that's why I covered it by saying not really important with what we are discussing....

[Bush65]

The megaphone exhaust as used on old 4 stroke racing motocycles is specifically designed for utilising pressure pulses for scavenging benefit (increased V.E.) over a required power band and not applicable to our turbo diesels. The greater the diverging angle (up to a point) the greater the gain but at the expense of narrowing the power band. From memory, the small reverse cone on the end of a megaphone helps with the power band. With 2 stroke expansion chambers there are basically 2 cones the 1st diverges to help scavenging, then the 2nd converges to help reduce loss out of the exhaust port after the transfer ports have closed. Before the exhaust valve/port opens the pressure in the cylinder is much higher than in the exhaust port. When the valve/port opens the sudden pressure change causes a +ve pressure pulse to travel down the exhaust system at the speed of sound - it travels as a sound wave (longitudinal wave) superimposed on the normal pressure and flow of the exhaust gasses. When the pulse reaches the diverging cone it is reflected as a -ve pulse and travels back to the valve/port at the speed of sound. The length of pipe and cone (and speed of sound in hot exhaust gas) determine when the -ve pulse reaches the valve/port and at some particular engine speed it will help scavenge the cylinder. This also happens with the 2 stroke expasion chamber, with the the further complexity that the initial pulse has continued until it reaches the converging cone, which reflects it as a +ve pulse, that goes back to the exhaust port, hopefully to prevent loss of fresh air and fuel from the cylinder. The quicker the exhaust valve/port opens and the greater the pressure difference at opening, the greater the magnitude of the pressure pulse.

What I was talking of in my previous post is different. It was not a recommendation just a small technical point for those who might find it interesting. The issue here is that when the exhaust gas travelling at some speed and pressure exits the pipe it 'bumps' into the stationary ambient air that it has to merge with. There will be less resistance to flow from the pipe if the difference in velocity and pressure from ambient is smaller.

This is similar to the principles used in designing nozzles, such as for steam turbines - they converge the flow to increase flow velocity, then have a diverging outlet (this is where I claim similarity) for reducing losses (Ben or Dougal, et al should know more about this as it has been much longer since I studied this subject).