Exhaust size 2.5 or 3 inch?

lokka · 8th September 2007, 11:28 AM

Originally Posted by 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

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** · 10th September 2007, 05:21 PM

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** · 11th June 2012, 03:29 PM

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** · 11th June 2012, 04:13 PM

Originally Posted by 123rover50

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

**C H T** · 12th June 2012, 12:43 PM

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** · 12th June 2012, 01:09 PM

Originally Posted by 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

It was just a matter of time before someone would say something similar

**isuzurover** · 12th June 2012, 01:21 PM

Originally Posted by 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

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** · 12th June 2012, 01:31 PM

There are 3 people that will dispute the above comment

i do not have the maths to back it up but isuzurover will provide details once he reads this

**c.h.i.e.f** · 12th June 2012, 01:59 PM

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

**c.h.i.e.f** · 12th June 2012, 02:12 PM

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:

Originally Posted by isuzurover

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.

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