Exhaust size 2.5 or 3 inch?

**isuzurover** · 20th June 2012, 12:08 PM

Originally Posted by 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.

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

**samuelclarke** · 20th June 2012, 12:20 PM

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** · 26th June 2012, 08:34 AM

Originally Posted by 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.

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** · 26th June 2012, 11:36 AM

Originally Posted by Bush65

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

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** · 26th June 2012, 11:56 AM

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!

**isuzurover** · 26th June 2012, 12:14 PM

Originally Posted by c.h.i.e.f

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!

**c.h.i.e.f** · 26th June 2012, 02:01 PM

Originally Posted by isuzurover

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!

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** · 27th June 2012, 09:00 AM

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).

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