Exhaust Theory?

[JTaylor2005]

Been to find out about a custom exhaust today. Wanted a mandrel bent on so went somewhere that I knew that did them.

The man said that there will have to be a cat or at least a place for a cat to be (Not sure if the cat is provided or it uses the origional and then a pipe is made to replace it)

He also said there is no point going too big as it might actually lower the performance. Now I thought turbo engines you just want the easiest flow as possible. Also he said it might be worth thinning the pipe nearer the end to keep flow speeds up :S
The work they do looks amazing to be fair as I have seen some but I thought maybe a proper 2.75" or higher straight through turbo back would be best for this car. I am not looking for the sort of "it will do"  I actually want something really nice that I know will last as well haha.

What do you guys think? Does any of that sound right. They must know a thing or two as they are very highly rated so are there any exhaust gurus on here that know about this sort of stuff?

Cheers

[bashbarnard]

If its an xud you want it as free flowing as possible. 2 1\2 inch will do. Not much point going any bigger.

[JTaylor2005]

Its not the XUD its the HDI one. But I was thinking at least 2.5 but if im gonna go for that may as well go 2.75 or something haha.

[Toms306]

You want free flowing for top end. You want 'some' restriction for low end torque. It will be a compromise as everything is with engines.

Try driving a HDi with a decat, noticeably less bottom end torque so I always the cat myself. In regards to the cat, it is needed for MOT which is why they want you to fit one.

[JTaylor2005]

Ohhh haha. I have always heard that it was only really on non turbo cars that you want decent high speed gas flow and on turboed engines you just want as little restriction as possible haha

---

This is from a Garrett engineer apparently. And it says what I thought was the idea all along. Maybe removing the cat could cause low torque down low due to other reasons? A turbo should theoretically spool up a lot quicker and lower down in the RPM range when there isn't a cat present.
http://www.tercelreference.com/tercel_in...heory.html

[Eeyore]

er.... N/A needs a certain amount of back pressure so you dont go massive...
turbo you want to have few restrictions so as big as possible. Ideally you wouldnt have anything?

Depends if you have a friendly MOT tester as to whether you need a cat or not...

[JTaylor2005]

Thats what I thought. So Toms306 where he says he lost a lot of low down torque there must have been some coincidental thing that happened at the same time that could have caused that to happen

[Eeyore]

I know on the 1.8 the decat doesnt really add too much and you lose some torque. Didnt think that was the case for the turbdervs.

[Iceman299]

What are the actual regs regarding decats? Some say it's smoke test only so a decat is fine. Even online autosport on ebay who sell hoffman decats state the car will still pass an mot in their description. 

[JTaylor2005]

As far as I am aware now the regulations basically say that if a car came fitted with a cat then it must have one at the MOT

[Eeyore]

oh god... and this weeks discussion of cat legality begins...

[Seb_Ryan]

As far as I am aware now the regulations basically say that if a car came fitted with a cat then it must have one at the MOT

This.

Or it must at least look like it has a cat fitted..

[JTaylor2005]

Top post does ring a bell http://www.landroveraddict.com/forums/th...adID=82061

[Poodle]

The bloke you talked to is dead right. Anything HDi this side of 180bhp will be fine with 2.5", get a 2.75" downpipe and cat if you're concerned. Going too big can supposedly allow for some strange resonant frequencies, plus it'll be heavier and sound like a wet fart due to the total lack of gas velocity. Look at some of the petrol turbo exhausts, they've got be heading for 300bhp before they go above 2.5".

[Ruan]

Yeah but tbf Poodle, they're running 12-13:1 AFR @ WOT... Diesels are running 17-18:1 AFR to keep smoke levels down, even more at stock calibration...

Diesel needs more air for a given power level. That's why a 90hp Diesel has the same size exhaust as a 167hp GTi6.

[welshpug]

its not "backpressure" its flow, for a given gas speed you need a certain flow, get it wrong and yes you create 'backpressure" be that from too large a pipe which causes gas to stop dead, or from a small bore throttling it.

[JTaylor2005]

So a nice 2.75" from the turbo down to the bottom of the origional cat and then a nice reduction down to 2.5" to keep the noise nicer do u reckon?

[Poodle]

Yeah but tbf Poodle, they're running 12-13:1 AFR @ WOT... Diesels are running 17-18:1 AFR to keep smoke levels down, even more at stock calibration...

Diesel needs more air for a given power level. That's why a 90hp Diesel has the same size exhaust as a 167hp GTi6.

Yuuuuup, hence why i said what's good for a 300bhp petrol should be adequate for up to 180bhp on tuned diesels. If you don't believe me go and look at the exhaust on the 250+bhp BMW 30/35D engines, and that's bearing in mind they're packed with restrictions.

The point is at stage 2, like what JTaylor is aiming for, anything over 2.5" is unnecessary, expensive and heavy.

[Dum-Dum]

I'd been told that on a turbo car the turbo provided the back pressure and none was needed post turbo so you can go as big as you like.

I had a full 3" system on my old HDi and it sounded epic.

[Jonny81191]

Yes you can go as big as you like but you also need to factor in gas velocity. If you go too big, you'll slow the gases down. It'll more than likely cause resonant frequencies causing your ears to bleed on the motorway. Too small and you'll obviously cause a restriction. 2.5" or 2.75" would be fine. TBH I've made a 3" for a stage 2/3 and whilst the gas velocity was high and it sounded good, I'd run 2.75" if I was to make another one. Also consider the fact that 3" really doesn't fit in a 306 exhaust tunnel.

[Poodle]

Gas velocity on yours didn't sound high mate, was perfectly adequate though tbf, wasn't like you had any problems from it being too big. That was on 2056 and 180bhp+ though, for those that don't already know.

[Jonny81191]

Sorry, I meant high enough. My bad

[zx_volcane]

For what its worth

I'm running a homebrew 3 inch downpipe to 2.5 system with 1x barely restrictive backbox

Thats with about 240bhp

Is it restrictive? Maybe, maybe not. Be interesting to dyno with and without the system connected.

It has crush bends which no doubt take it down to nearer 2inch in places

One day I'll have them cut out and replaced with mandrel.

I would have thought a 3inch downpipe to 2.5 inch unrestrictive system would be fine for near 300bhp on a diesel - but am keen to see some real world evidence to back any of this up.

[Rippthrough]

Whilst you can go much bigger on a turbo car after the turbo as gas speed isn't much of a concern, you can actually slow the gas flow down so much at low rpms, that it slows down, cools and increases the pressure rather than reduces it - which is probably why the guy is suggesting a tapered system, 2.75" from the turbo feeding into a 2.5" system after the mid box would be more than enough.

[silverzx]

Just go with whatever you want..

2.25", 2.5", 2.75", 3", 3.25", 3.5", 3.75", 4".

Personally I'd go with 3" downpipe to quad 4" (one exit either side and two at rear).

[JTaylor2005]

Haha. Might just try and get a 2.75" or 2.5" mandrel bent one lol

[Rippthrough]

Yeah, even with the airflow a diesel needs, you should be able to push 180-190bhp through 2.5 before it's worth going bigger.

[todd1983]

For what its worth

I'm running a homebrew 3 inch downpipe to 2.5 system with 1x barely restrictive backbox

Thats with about 240bhp

Is it restrictive? Maybe, maybe not. Be interesting to dyno with and without the system connected.

It has crush bends which no doubt take it down to nearer 2inch in places

One day I'll have them cut out and replaced with mandrel.

I would have thought a 3inch downpipe to 2.5 inch unrestrictive system would be fine for near 300bhp on a diesel - but am keen to see some real world evidence to back any of this up.

I have 3" into 2.75" downpipe on my 330d with 285bhp