It's a bit of a gamble really. I've got no silencers and no cat and mine barely makes any whistling sound. It sounds quite tinny almost at idle, my Dad keeps moaning at me as "your exhaust is blowing" but it's not! haha

 

Sounds great when really going for it though.

 

http://www.youtube.com/watch?v=LykXFEEyfVI&noredirect=1

yeh thats what im after

Was considering a bigger turbo plus decat at one point (several years ago). Was at that point I read/realised that cat converters do a reasonable job on a diesel. It's not all bolox. So if doing it now, would try to engineer a sportscat into it somehow.

 

Back on topic:

 

Improving the pressure differential across the turbine will have some effect, definitely. I've always thought anything to reduce back-pressure would be a good thing if fitting/mapping a bigger turbo. It's not quite that simple though. As peak back pressure won't be present in the exhaust system at boost threshold or spool from cruise (if that even applies in a turbo diesel).

 

But you can't improve the pressure differential because the exhaust gas is being throttled by the VGT vanes, energy is being extracted from that hot gas stream by the turbine and the resultant slower, cooler exhaust gases are flowing freely into a non-restrictive system.

 

It's like sticking a piece of 1" garden hose over the end of your 1/2" garden hose and expecting it to flow more water. Think about it.

I've done a bit more thinking.

 

But you can't improve the pressure differential because the exhaust gas is being throttled by the VGT vanes, energy is being extracted from that hot gas stream by the turbine and the resultant slower, cooler exhaust gases are flowing freely into a non-restrictive system.

 

It's like sticking a piece of 1" garden hose over the end of your 1/2" garden hose and expecting it to flow more water. Think about it.

 

In my world view, the hose has two kinks in it downstream of the 1" section (the cat and the exhaust/back-box).

 

Regardless of the VGT, a turbo always transfers energy from the exhaust gasses into the turbine. Those gasses flow from a high(er) pressure area to a lower. The lower that low pressure area, the more energy can be or is extracted from any measure of those exhaust gasses.

 

The sticking point would be: As the VGT lowers spool RPM and the engine processes less air at that lowered RPM, is it possible to measure a worthwhile reduction in back pressure at the critical RPM?

I've done a bit more thinking.

 

 

In my world view, the hose has two kinks in it downstream of the 1" section (the cat and the exhaust/back-box).

 

Regardless of the VGT, a turbo always transfers energy from the exhaust gasses into the turbine. Those gasses flow from a high(er) pressure area to a lower. The lower that low pressure area, the more energy can be or is extracted from any measure of those exhaust gasses.

 

The sticking point would be: As the VGT lowers spool RPM and the engine processes less air at that lowered RPM, is it possible to measure a worthwhile reduction in back pressure at the critical RPM?

 

That's a fair point, but the two 'kinks' you mention are what is limiting the flow capability of the entire system and the entire system is good to about 220-230 BHP, removing the cat and the back box might raise that threshold to 250 BHP but that doesn't mean that doing so will add 20 to 30 BHP, it simply means you've increased the potential of the system and in order to get that extra power you're going to have to do a whole host of other mods.

 

Ok, so ambient pressure in the exhaust system is atmospheric pressure which doesn't change until back pressure starts to build and this doesn't happen until the system reaches full flow potential, it doesn't matter what you do to the system if you aren't reaching that point, you can't beat the atmosphere.

 

In a naturally aspirated engine the exhaust system geometry can be tuned so that negative pressure waves created by combustion pulses can be used to create a slight extraction effect at a particular engine speed, unfortunately this technique doesn't work in a turbo because the exhaust turbine itself absorbs and damps those pulses producing an effectively linear gas stream, this is why turbo dragsters and racing cars simply use the shortest straightest piece of plain pipe they can and stick it out of the side of the car somewhere behind the front wheels, by contrast look at the tortuous nest of vipers at the back of a N/A mid-engined racing car.

Edited 21 November, 201312 yr by sepulchrave

That's a fair point, but the two 'kinks' you mention are what is limiting the flow capability of the entire system and the entire system is good to about 220-230 BHP, removing the cat and the back box might raise that threshold to 250 BHP but that doesn't mean that doing so will add 20 to 30 BHP, it simply means you've increased the potential of the system and in order to get that extra power you're going to have to do a whole host of other mods.

 

Ok, so ambient pressure in the exhaust system is atmospheric pressure which doesn't change until back pressure starts to build and this doesn't happen until the system reaches full flow potential, it doesn't matter what you do to the system if you aren't reaching that point, you can't beat the atmosphere.

 

In a naturally aspirated engine the exhaust system geometry can be tuned so that negative pressure waves created by combustion pulses can be used to create a slight extraction effect at a particular engine speed, unfortunately this technique doesn't work in a turbo because the exhaust turbine itself absorbs and damps those pulses producing an effectively linear gas stream, this is why turbo dragsters and racing cars simply use the shortest straightest piece of plain pipe they can and stick it out of the side of the car somewhere behind the front wheels, by contrast look at the tortuous nest of vipers at the back of a N/A mid-engined racing car.

 

 

I'd say that back pressure is likely to be measurable under any operating condition (compared to engine off) - using a water manometer. However, I'm aware that back pressure is very unlikely to be "linear" and that back pressure increases markedly as load approaches the limits of the system - call it exponential if you like.

 

What I'm aiming towards is this. With the stock turbo, it's quite unlikely that any great difference will be apparent when "playing" with the exhaust. Especially if done without other enhancements and a remap. Once someone is running a larger turbo however, it's my opinion that anything that can be done to help aid spool and offset lag and produce as near an OEM experience as possible, should be explored.

 

Having written the above, I have previously posted links to a series of Autospeed articles where "improvements" in the intake and exhaust were made and measurements made to assess their impact. It was only after a remap that any power enhancements were apparent. There was a noticeable effect on fuel economy (positive).

 

http://www.briskoda.net/forums/topic/229131-powering-up-the-19tdi/

 

J.

This is for anyone that says you cant make diesels sound good 

 

http://www.youtube.com/watch?v=uLnx-Jley10

 

 

..... i had a trouser accident