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I can get a pic of mine over the weekend, but yes I get what you are saying now

the main reason I got these cats removed was because when accelerating and revving I would get this “whisp/flutter” from the mid cats where the two exhausts joined together
932012
 

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I can get a pic of mine over the weekend, but yes I get what you are saying now

the main reason I got these cats removed was because when accelerating and revving I would get this “whisp/flutter” from the mid cats where the two exhausts joined together View attachment 932012
I have a custom 2.5 cat back exhaust running through a pair of straight through flowmasters. Its sort of loud and rumbly, people have told me it sounds powerful and expensive.

Its a bit too deep and grunty for me though , i want to swap the straight through for some baffled mufflers to elimate the deepness. i do like the wizard exhaust, sounds ferrari like
 

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Discussion Starter #63
I have a custom 2.5 cat back exhaust running through a pair of straight through flowmasters. Its sort of loud and rumbly, people have told me it sounds powerful and expensive.

Its a bit too deep and grunty for me though , i want to swap the straight through for some baffled mufflers to elimate the deepness. i do like the wizard exhaust, sounds ferrari like
I don’t mind that at all! What’s it like when you aren’t toeing it, when cruising?
 

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I don’t mind that at all! What’s it like when you aren’t toeing it, when cruising?
not that bad, just a deep rumble, no drone, but on acceleration it howls like a banshee, it crackles and pops off acceleration as well. It's really hard to determine what it sounds like outside of the car, but i know does turn a lot of heads.

I retrofitted a pod filter in place of the factory airbox, and it made the exhaust more grunty, i had to change it back to factory because it made it harsh.

as i mentioned, id like a baffled muffler, to elimate the deeper tones a bit, to put some detail into the exhaust note.

id like to take some driveby videos in the coming days, to get a better idea. i'll be on a hd cam and tripod. i'll put them on here when i have..
 

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Discussion Starter #65
"it crackles and pops off acceleration as well." - That'll be your timing shifting when you come off throttle [opinion]. Lifting causes drag from the transmission onto the engine/crankshaft, and standard oil pressure is not enough to keep tension on the chains - particularly the lower primary. It is a fundamental reason why the timing chains stretch/fail, not just Alfa's, Holden's, Chrysler's, Vauxhall's/Opel's and Saab's.

So long term ownership invariably means regular timing chain system overhaul. But the message has been getting through and confidence is growing and more people are willing to D.I.T.
 

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I don’t mind that at all! What’s it like when you aren’t toeing it, when cruising?
The Wizard catback system for Brera & Spider V6s is pretty quiet on cruising - no drone or boom anywhere in the rev range. Volume is very linear to throttle position and engine rpm, so low rpm, low throttle is very quiet, high rpm, full throttle is when it is at its loudest. At some point we will have to re-develop it to suit the 159 V6.
 

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Discussion Starter #67
The Wizard catback system for Brera & Spider V6s is pretty quiet on cruising - no drone or boom anywhere in the rev range. Volume is very linear to throttle position and engine rpm, so low rpm, low throttle is very quiet, high rpm, full throttle is when it is at its loudest. At some point we will have to re-develop it to suit the 159 V6.
Sounds good (no pun intended). But before I do anything, I need to address the current issue. As Ken said, he has no centre box but still has the drone at Seventy in Sixth, so we are in the same boat so to speak. And the work 939 has done on his seems to suggest the problem does lie with the Siamese cats. Attached below is a picture of my spare. I thought it had a 2 mm. gap down the centre, but it must have been some other I saw.

None the less, that "Reed" needs grinding off. and re - welding. My experience with Mass Flow Controllers suggests, the capillaries of the cats are linearising the flow, in a similar fashion to M.F.C.'s, which eliminates turbulence; which enhances their accuracy. In this instance however,the alternating linear flow from the cats are cutting across the "Reed" and may well be the source of the drone. Blowing across a milk bottle top has a similar effect, with the bottle determining the pitch of the tone. In this case the pipework performing the same function.

If the edge is ground into a irregular shape and metal welded on to create an rough profile, which breaks up the linear flow across it, it should prevent it from generating a harmonic, which translates into the drone.

That's the theory and it is worth a try!
 

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Its a very cheap way of making a collector. The twin cats they used in the GTAs although very similar to look at didn't use that type of collector where the two pipes are crushed together and sleeved into a third. Here's a pic of the GTA cats. A good fabricator could cut off the poor collector on the twin cats and then fabricate a better one similar to that which Alfa used on the GTAs.
 

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The twin cats themselves I very much doubt are restrictive, they certainly aren't on the 3.2 Busso, even modified examples don't seem to make any more power with the middle cats removed. Getting the manifold cats out and the operating temperatures down I would say should be the priority for someone who uses one of these engines hard.
 

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Discussion Starter #70
The twin cats themselves I very much doubt are restrictive, they certainly aren't on the 3.2 Busso, even modified examples don't seem to make any more power with the middle cats removed. Getting the manifold cats out and the operating temperatures down I would say should be the priority for someone who uses one of these engines hard.
No, I don't believe they are particularly restrictive. I believe you said, they had 400 capillaries in each which is great. However, the capillaries remove turbulence at the output where the peculiar collector is fashioned. And I have no man - cats, and engine temperature has dropped like a stone.

So yes, to emulate what has been done with the GTA makes sense. It probably means modifying the pipe length of the centre box input as well, as the outlet of the 159 Siamese unit is pretty short. Or even doing away with it all together and just fitting a section of straight pipe.

There are lots of technical articles on the "Reed" phenomenon, which heavily researched and employed throughout industry/science, so it is worth investigating further. But, there is little point in trying to re - invent the wheel. If, as articles suggest, this is a well understood characteristic, the simplest thing to do is to modify the 159 cat to prove it.

And, the centre silencer connection! To eliminate all other factors, this would be the proper way to analyse the problem. Otherwise, some twerp would say it was x, y or z. Change one thing at a time by way of isolating the problem.

So, I suppose the question is, can you do it?
 

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I removed the twin cats on mine, leaving the main cats in place and it certainly feels like the car revs and performs better

I am getting the car on a dyno in feb so will see then what its actually producing
 

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Discussion Starter #72
I removed the twin cats on mine, leaving the main cats in place and it certainly feels like the car revs and performs better

I am getting the car on a dyno in feb so will see then what its actually producing
It will be difficult to judge, what is responsible for what, when more than one element has been changed. Your cat replacement element is certainly going to be an improvement. Your centre box gone as well, so their will be a little there also. However, a Dynamometer plot will confirm one way or another, what gains you have made.

For me, eliminating that knife edge/output pinch is my first step. Good thing I have a spare to work with, so the job can be thought out properly without fretting over a garage's time. and final cost. I would like to have the work done before I have mine dyno - ed, But, my hat is still stewing nicely - just in case!
 

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A place near me will dyno it for £50 so no idea if thats good value or not but I'd like to see if it is producing anything more than stock
 

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Discussion Starter #74
A place near me will dyno it for £50 so no idea if thats good value or not but I'd like to see if it is producing anything more than stock
I paid £100 first time around. But, the guy started to take a genuine interest in what I was doing and next time around it came down to sixty. And he overlaid the plots, with hard copies and to my email address.
 
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