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[Bojan]

I have some ideas about exhaust modifications. First, some intro:

1) I had my car decated and whole cat-back system made from aluminisied steel by a local exhaust shop – it is OK in quality, sound a bit louder which is OK, but it had a nasty knocking sound on hard acceleration (drove me nuts) – after a lot of investigation, I've concluded that it was because there was no cats, just a stright pipe followed by two near 90°bends. And, from what I've learned after next step – lost some low-down torque.
2) after that, I've had a pair of small mufflers installed in a position where cats orginial were – more quiet, and knocking on acceleration disapeared – it sounded very OEM like, and I've noticed an increase in low down torque. It was all nice for some time, but after a while, this mufflers, which are not not high quality TBH, started to have a bad sound on acceleration – I think that fiberglass in which the steel mesh is wrapped has burned out. I guess that main cause for this was LPG system that was overfueling for some time – now repaired (LPG deinstaled  ). So, it'll need to do something about this.

For now, I won't be needing cats (and most probably never, as I've found a loop hole for MOT here in Croatia), so I'm not considering going back to OEM setup ATM. Also, finding a decent second-hand cats for V6 here is almost impossible (and having them shipped from UK is to expensive due to the size).
I could redo the current setup, but with some more quality mufflers. But there is no guarantee it won't have the same probs.

[Bojan]

Now, this is what I'm thinking about:
- Decat (current muffler placed on cats position binned)
- Exhaust from the start of the cat (just behind the hanger) rerouted – now it goes straight for more than 500mm after the hanger, turns almost 90° twice – my plan is to make a 30°-45° bend just behind the hanger, than another 30°-45° bend when it comes side to the opposite side pipe (of course, other side is a mirror). And then join two pipes – H-pipe or X-pipe type.

Why?
Well, I'm hoping that in this way, I will get a indestructable and cheap system, while it will not produce any knocks or other sounds, as no sharp bends; but also that I wouldn't loose low down torque, as H or X joint would be in almost in the same position as OEM cats. From what I can see on the internet, X-pipe and H-pipe (especially) gives some low-down torque on dual exhaust.

What do you think?

[tunnie]

I think it's a lot less hassle to have an Eternal/ETS standard exhaust with OEM cats. No knocking, it's a cheap system and works well. But that's just me 

[Bojan]

This is how I've planed it:

https://www.dropbox.com/s/qgtjpf7l3p88gdp/18072008101-1%20-%20Copy.jpg?dl=0

[zirk]

Putting mufers where you  Cats should be, its going to get bloodly Hot there. 

[Bojan]

Putting mufers where you  Cats should be, its going to get bloodly Hot there. 

Yes, I think that mufflers that are on now got burned over the period of time, and started chattering. That's why I'm not keen on replicating this setup, as I belive same will happen sooner or latter.
Actually, better version of this setup would be resonated decat pipe - it's used to avoid nasty sounds, and I suppose it should keep the low-revs torque.

As for OEM setup - rest of the exhaust (mid-muflers and back box) are pretty well made. And yes, I could get the usual (either OEM second hand cats, or new pattern), but I also want to see what other options are available. One of the downsides of the omega v6 ownership is the lack of 'tunning and modification' knowledge among the people, so for the most of the mods you are on your own.

At this point, I really would like to find out if the decat + reroute with x-pipe or h-pipe would help keeping low-down-torque. I'm pretty sure that this setup would produce a knock and rattle free sound. 

[Ratboy]

what i can make up from your story, your trying to find a way to decrease your power and torque with all the welds and cuts. And at some point facing buying a pretty good new exhaust system after cutting it and welding your current one.
reading thru alot of posts about exhausts and personal expiriences in the past, these seem to be thru and tested mods by several forums.

the omega exhaust manifolds are cast iron and prone to cracks, so take the manifold from a cadillac CTS 2003/2004 and modify the flanges to fit.
these manifolds seem to be tube bend and have a larger inner diameter(preferably do a port to manifold match for best result.)

take the frontpipe(part between manifold and cat) from a omega and grind to fit the cadillacs manifold.

now what your cat is concerned you can do 2 things. one is decatting BUT take a piece of tubing to match the inner diameter to your frontpipe. take your cats, and carfully cut next to the weld at the cat side and NOT the flange sides. try to pry out whats left inside the cat's housing so its empty. slide this housing over the piece of tubing and weld it. now cut of the flanges and fit that to the tube so now you have a cat "look" straight pipe and your gasses have no hickup  on the way out. a plus here is also is less suspision as you have 4 new weldspots with no sign of rewelded seams

to prevent your lambda sensor to go bananas by the increased exhaust flow rate and temperature you can get a angled  adapter so the sensor is moved more away from the passing flow and should prevent giving error codes or overheated faulty sensors.

now the muffler part, where you decide if you want it loud antisocial and a magnet to the eyes of the blue elven that hold the law. or be civilised to a certain extend  when your driving relaxed but giving a mean growl when you get into "pedal to the metal' modus, my personal favorite by SO FUNKIN FAR. the flowmaster deltaflow 40 series. these come at arround 150€/piece in different diameters and "in"out" configurations so see what matches best to you stock tubes you will reuse. the biggest benefit is that they will not change sound over time due to not using glasswoll to reduce sound. this muffler uses a "delta" shaped inner room design that by bouncing of the soundwaves to all the delta shaped walls. another benefit as your using LPG is that the condens from the LPG will not be absorbed by the glasswool  dampening what always leads to less dampening and rust forming on the inside.
when using this muffler you dont need a second one, as its a great sound.
you can "tune the sound" a little by either placing it more where your middle mufflers normally are so the sound is a bit deeper, or where your rear silencer should be making it more "in yur face"
and what all those X and H connections are concerned, unless your serious into HP gaining adding 100HP or so, not worth the time spending

[Doctor Gollum]

Or you could fit straight pipes...

 

[Bojan]

After a lot of consideration, and talking to several peoples, I've came up with a conclusion that a 'single pipe' (two exhaust pipes joined into one after the cats) exhaust would be most suiutable for me - I want a exhaust that is simle, gives nice and smooth sound, and it doesn't bother me if it's loud (as long as it doesn't rattle on acceleration).

The system would go:

downpipe - 54mm pipe with 45° turn (one one each side), joining into one 63.5mm (2.5") pipe in center of the car - flow-trough resonator (optional) - 63,5mm pipe - STD 3.0 backbox

More-or-less, I'd copy the systems from links:
http://www.omegaowners.com/forum/index.php?topic=54996.msg830517#msg830517
http://www.omegaowners.com/forum/index.php?topic=103756.msg1299433#msg1299433

BTW, thanks to Mr.OmegaMan, VXL V6, minifreek for a feedback on your systems and experiences 

=========

WARNING - BORING STUFF

I see that V6 omegas with single mid section was quite common thing in UK several years ago, and that feedback is mixed:
- ones that are made by random exhaust shop - using Y-connection, single mid pipe and one mid box - usually claim some power loss - IMO, this is probably caused by poor design which comes with many exhaust shops when they do custom work (sharp turns, Y-part at to big angle making exhaust pulses coliding, not enough diameter for single pipe)
- others (like one in links fiven before), that have better design, where flow is an key factor - claim no power loss (or even some increase)

Why I think that single pipe is a good option:
- Our V6 engines, when used in other applications (Vectra, Saab...) have this setup, and it works and sound nice
- only other RWD application of this engine was in Cadillac CTS - and it had cats on each bank, resonator with 2 inlets and 1 outlet - and it had no power loss compared to Omega
- less weight, cheaper to make
- I'm quite sure that purpose of original twin pipe setup was purely to obtain quieter sound (suitable for that kind of car), and not to give extra performance over the single pipe setup
- most of the V6 cars use single exhaust setup
- most of the modification to the V6 omega exhaust system end up with rattle and nasty sound - this is, IMO, due to the nature of the twin exhaust setup - until it reaches the back box, it's basically a big 3-cyl engine - and they all sound nasty
- on the other hand, FWD V6 Opels - which have single pipe - are not that sensitive - you can do various 'chav' mods to the exhaust, which will make it loud, drone and similar, but will not rattle and knock under acceleration

[Doctor Gollum]

3.0 Back box has two pipes in and out

[Bojan]

3.0 Back box has two pipes in and out

I know that...I'll put a Y-piece there. Or maybe even replace the backbox with single-inlet one.
But, the backbox is quite irrelevant in the flow-part of the story, IMO.

[tunnie]

Just my view, but a lot of hassle. A ETS/Eternal exhaust for £75 and it's sorted, sounds fine, goes fine, lasts well, but that just me 

[Bojan]

Yes, it may seem as a lot of hassle, but I hope to get a simpler exhaust system, which is easier to tune, modify and maintain (single cat and single central muffler), opposed to twin exhaust (where, whatever you do, you need 2X items).

For those who had bad experience wiht single mid section exhaust:
 - did the mid section had a larger diameter than downpipes (2")?
 - was the central muffler baffle plate type?

[2boxerdogs]

Always had OE exhaust on my V6 when I had it replaced never had any problems , why fart about with it ?

[Lincs Robert]

Just my view, but a lot of hassle. A ETS/Eternal exhaust for £75 and it's sorted, sounds fine, goes fine, lasts well, but that just me 

Well said, I really don't know why people bother with modding exhausts. There are quite a few modified cars in this part of the world so you get used to hearing cars which are louder than they should be.

All IMHO of course .....