Whispy after cat removal!!

phoenixv6quattro

phoenixv6quattro

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Hello gents! Had both the cats chopped off my 30v v6. At first the centre box was off too and just running a miltek back box (full milltek system from cats back). It was a bit too noisy so put the centre box back on. Now, when its under load it sounds a bit tinny/whispy! Does anyone know why this is? Is it because now the cats are gone the gasses are passing through quicker, and hitting the "Y" section on the milltek end? Short of putting the cats back on (which is not going to happen untill MOT time) does anyone have or reccomend a cure for this? Anyones input would/will be great!

Ollie....
 
Just a guess, but maybe not enough baffle in the system now.
 
All you can really do is try more and/or different silencers.

A lot of NA engines go the same way when decatted with a free flowing exhaust, which is why most of the chavvy kids cold hatches sound the way they do.

Its probably related to the design of the exhaust manifolds. Very short primarys and typically poor collectors merging into a single pipe far too early. My brother had a 1.2 nova on a big-bore exhaust back in the day which sounded horrible on its standard manifold, but once equipped with a nice tubular 4branch with long runners sounded so tasty he often got folk asking if it was equipped with an XE.
 
This is a very common problem that everyone moans about with all cars when they take the cats off. Just putting an idea out there, maybe some could invent some kind of baffling mesh that could be added to cat replacement pipe or somthing like that as the problem is purely down to increase in free flowing gases.

Suppose back in the early nighties when everyone fitted scorpians it was always going to sound better because there were no cats to remove so we didnt have a cat'ed sound to begin with.

My wifes 1ltr corsa had a new back box which sounded rank compared with the old one, turned the originals were double baffled and the after markets were single, so maybe like aragorn said try other silences, preferrably one which is known for baffling
 
Its not like your now flowing 20% more air thru the exhaust, the changes in flow are tiny, probably in the region of 5%, and this really wont change the sound by much.

The Problem is simply that the cats themselves act like a silencer, and that due to their removal you have less silencing. IE you can now hear the rubbish sound coming from the exhaust manifolds.
 
Its not coming from the exhaust manifolds. The whispiness/tinnyness Is originating from the Y split where it turns into a single pipe. We had the car up and its very clearly coming from there. True the cats were deadening some sound. The only thing I csn think of is to get rid of that Y split with a twin exit, or put a larger silencer on but not sure that will work as its after the Y split.....Oh well! Lol!
 
audi upgrades
All the noise comes from the manifolds ;)

Perhaps redesigning the Y so it offers a smoother transition might help? Does it look well made?
 
Yeah its a Milltek! Oh well ****** it......Its not the manifolds. Mate, its not the manifold. Aragorn, guess what, ITS NOT COMING FROM THE MANIFOLDS. THE SOUND IS NOT COMING FROM THERE. LOL
 
All the sound has to come from the manifolds :beerchug:
Where else can it come from? It cant get into the exhaust from outside can it!
 
audi upgrades
Yes, agreed.....Mate I'm on about where the tinny/whispiness is coming from, not where ALL zorst gasses come from buddy! lol!
 
Back onto this, If I had a system built (now very tempted) that ran two straight pipes INSTEAD of Y section where the two join does anyone reckon this would get rid of the tinny/whispy sound now it has been decatted. It sounds like, when standing underneath the car, that the gases are hitting the join in the Y section where both the pipes become one and thats where its coming from (yes aragorn,i know all exhaust sounds originate from the manifolds!lol!).....Also guys, my milltek system is resonated. This "resonator", is it part (is it inside) the centre box, or is it a seperate part built into the straight tubing?
 
Gotcha. Funny thing is I had a powerflow centre box put on there without a resonator in it, and it didnt make even a slight difference! Then I put the milltek original centre box back on and it changed nothing! What I'm after is something a tiny bit louder than what I have now (de-catted/full milltek system)without it being silly loud! I had the centre box bypassed with a straight pipr for a few days and the thing was so noisy it sounded like 10 saxos!lol! Will a differnece back box for instance change the sound/make it a bit louder? Was looking at this one instead of the milltek back box.......TRUST Exhaust Muffler Subaru Impreza GC8 WRX STI Ej20 on eBay (end time 05-Jul-11 10:48:05 BST)
 
On the S4, the "link pipes" are where the resonators are afaik, not the middle box itself. The link pipes sit between the standard downpipes and the Y section you mention.
 
PowerFlow was the first mistake, the dealers don't have a flipping clue about even the basics of exhaust design. They opt for the throw generic boxes on with pipes done on a compression bender and make it sound "phat" approach which usually yields a loss of power over the original standard item.

Anyway I digress, my experience with my car (albeit a 1.8T) the centre box seems to do most of the silencing. When swapping the back box for my custom one when I first got the car we ran it round teh block to see how loud it was and it was next to silent!

I would be inclined to either put up with it or speak to a proper custm exhaust place like Hayward & Scott about having a system properly spec'd and built for you. The later of which won't be cheap but I can promise (having seen their work) that it will yield the results.
 
Interesting point. Yes the middle box on mine sure as hell does all the silencing. When I took it off it sounded like a ****** off hornet which was not nice (I have anaphalaxis!lol!)......The only thing I can think of now (shy of getting another system built from scratch) is to swap out the resonated mid box for a non-resonated one whilst still retaining the midbox itself.....

Aragorn, you knowledgeable old bean, I think I know what your on about. There is a section about 4 inches long that "bulges" on the link pipe. Always wondered what it was? I will get a pic up........
MILTEKFITTING.jpg


Its where the arrow is pointing to. Its a small cylindrical kind of chamber on one of the pipes. Any ideas?
 
That could be it.

I've never seen them personally, but a few folk on the S4 forum have mentioned that miltek sell the non-res link pipes for about £50 if you want a bit more noise on a resonated S4 system.
 
Slight hijack, but i'm toying with putting my decats back on and having a straight out exhaust from each bank.
Exhaust-1.png

Would this work?? Or will it need an 'X' pipe in place so it can draw gasses from each bank??

Dan
 
It can work, but i'd imagine it'll probably benefit from an X pipe.

Twin pipe systems are available for the S4, but they typically share boxes, which presumably has some of the effect of an X pipe.

Also, there really isnt much room under there for the second pipe on the drivers side with its own backbox, the fuel tank and spare wheel well are in the way.
 
aragorn said:
Also, there really isnt much room under there for the second pipe on the drivers side with its own backbox, the fuel tank and spare wheel well are in the way.
Click to expand...

How do other people fit dual exit exhausts then on the V6 Q's??

The garage who fitted my S4 stainless system didn't mention the X pipe bit but said he could fit dual exhaust to it. I'm sure he knows what he is doing though. As i've seen three Ferrari's in there and an Aston Martin on the two visits i've made.
 
Obviously both pipes would run into the middle in the tunnel and then seperate again to either side onto the backboxes. They just wouldnt be like your picture. Sort of a cross between the two but still using seperate pipes from each bank and no x pipe.
 
What i mean is that you cant fit two backboxes. There isnt room.

On a Quattro, the fuel tank sits on the drivers side where the backbox sits on the passenger side, and the wheel well is in between the two.

If you want two tailpipes, you'd have to take a branch off the existing backbox and route the pipe round the wheel well, ending in a tailpipe with lots of insulation so the tank doesnt melt.
 
well I don't want to melt the tank and have a leak. The car uses enough fuel as it is!!! lol

Ok, so might just look to get a system from cat back with twin exit like standard but have S4 Slant cut tailpipes
 
Heres a photo, but bear in mind this car has the spare wheel well REMOVED.

You can see the outline of where it would have been, and hopefully can understand why you couldnt fit two backboxes.

You MAY get away with MRC's approach, with a single middle box and carefully route the pipework around the spare wheel well, but there really isnt much room to tuck it up out the way on the drivers side, which would significantly reduce the ground clearance.

SDC12158.jpg
 
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