Free-flo exhaust systems - Alfa Romeo Forum
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(Post Link) post #1 of 8 Old 31-05-09 Thread Starter
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Hmmm Free-flo exhaust systems

If you remove the cat, what has to go into it's place? Can the two pieces just be bridged. This is going to sound stupid but what's a cat back? Before branching the manifold I would like to a) remove the cat and b) put two twin back boxes at the back of my GT (more for esthetic reasons). Would it drop the back pressure too much or would it be beneficial to the power?
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You can get de-cats to replace a cat or a sports cat with 200cells per square inch to allow the gases out quicker. A cat back is an exhaust system that replaces everything from but not including the cat, to the back box.
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(Post Link) post #3 of 8 Old 01-06-09 Thread Starter
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South Africa does not currently have laws on catalytic converters. Can i not just take it out?
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I think you'd need a decat pipe with lambda sensor mountings? The 3.2 V6 may also have two pre-cats in the header pipe, again with lambda sensors. I'm trying to remove the two pre-cats on my Twin Spark and I hope the ECU won't trigger fault codes because of that.
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If you have multiple Lambda sensors, these are normally in front or ahead of the cat they are sensing for, oddly!

The good thing is that if they are ahead, you can often get away with a very free flow section instead of the cat, even a bridged tube with nothing to it whatsoever.

However, the complexity of the ecu and its trigger sensors would lead me personally to use a performance silencer type, like a shalllow resonator box instead of a straight through pipe.

If used along with the lambdas in their original boss position, you are very likely going to get away with any fault codes or running issues from lack of back pressure too, BUT also this type of silencer box will help stop vibration and resonance intruding into the cabin.

Ive dropped that clanger myself before, but now find that this arrangement if properly scaled will give nothing but benefit, no negatives at all, and work just as well as the straight tube option.

If you're going to go with twin exits, then keep that as free flowing as possible, i.e no silencer boxes prior to them.

So, if it were mine, i'd go De-cat the front with a perfromance silencer, a small volume performance box in the centre section, and then tubed straight out to the twin exits.

If thats not fruity enough, drop the centre section box altogether aswell, but you'll be dropping back presssure slightly again too.

Voila, that V6 can sing its heart out as she was supposed too, and a glorious noise it is too.

Ive just done similar to this on a 2.5 V6, and the note is Erotic, and so much nicer than the YouTube examples i heard with Ragazzon's and Supersprints etc etc...

Custom made all the way, every time for me, and tailor the noise level as you build it, perfect!

Good Luck, let me know when its on YouTube!!!

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I would recommend going to powerflow. They did my freeflow, branches and decat on my 147.

The way they do it is between the lambda sensors they put a cat fooler, not sure exactly aht it is or how it works, to fool your ecu into thinking that the cats are still present. Just be prepared for your car to get alot louder. People will hear you for miles. Not really a bad thing
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(Post Link) post #7 of 8 Old 02-06-09 Thread Starter
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Makes sense, thanks.
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on the V6 GT, there are 4 cats: 2 'precats' in the manifolds, and 2 'main' cats underneath the car. each is associated with a lambda sensor.

i believe that removing the manifold cats does not trigger the CEL (check engine light) as long as the lambas are refitted properly. so you could use non-cat GTV or 164 24V manifolds, or aftermarket like Supersprint or Unicorse.

this is what the factory manifolds look like (these are actually off a 147GTA) vs GTV non-cat versions that have been sprayed and wrapped (photos courtesy of ALFA_001):

changing the manifolds should apparently bring some of the biggest benefits, BUT is also the most labour-intensive. removing the manifolds is major work from what i hear; official workshop manual suggests removing the engine, but it can be done without this, but still requires much of the surrounding piping (cooling, aircon) to be dismantled.

thus far, i have kept the original manifolds. i am not chasing maximal power, so i'm reluctant to go so far. but i have changed the main cats, to 200-cell high flow versions like mentioned. these are built in Australia by HiTech Mufflers, Sydney, who are somewhat local to me. saved 2.5kg in weight too.

by law, my car must have these cats. i should be able to get away with new manifolds though, in the future if i so decide. if you remove these main cats, you are VERY likely to trigger the CEL. does this result in the engine going into 'limp home mode' (major cut in power)? not sure. but there are adaptors you can screw into the ends of the sensors to trick them into think everything is 'normal'; Glenwood sell them i think, from reading around here.

my new cats have added a very small amount of power to the car. around 4kW at the wheels on pre and post dyno runs. now, following more 'time on the road' and an ECU reset, i think it would be a bigger improvement if i put it on the dyno again. it 'just' feels noticably better when driving; nothing huge. but it HAS improved the sound, which was as important a priority to me as the power (i mostly wanted better sound, and no loss of power, then i would have been happy).

i have kept the original centre pipe. i believe it is straight-through and stainless, so is good quality with no impedence of flow or pressure. there is a resonator there, but it does not affect flow. my backbox is also from HiTech mufflers; the smaller 'cannister' makes it fairly loud, but also 'drony' under load at around 2500rpm. i don't mind it, and am used to it. my wife doesn't like it when the drone occurs. but under hard acceleration, it sounds great.

YouTube - Alfa GT 3.2L Mille Miglia high flow cats + rear muffler (1)

quad pipe at the back is a matter of taste, i'm happy with my twin 70mm tips.

anyway, i hope my experiences help guide your choice. i think having straight through rear pipes (no backboxes) would be very, very loud. probably not legal in australia, but might be in SA.

GT . 3.2 . Q2 . Kyalami Black - Red . Eibach . KONI . ASSO . ATP 19" . TAROX . FERODO . Mille Miglia exhaust
155 . 2.0 8V TS . Grigio Grey - Grey . H&R coilover . Enkei 17" . TAROX . GOODRIDGE . REMUS
75 . 3.0 . Alfa Red - Grey

Last edited by shiny_car; 02-06-09 at 23:10.
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