Alfa Romeo Forum banner

Benefits of decatting/straight piping a JTD?

8K views 20 replies 9 participants last post by  joeymannero 
#1 ·
My rear silencer is a bit rusty, so figure I should replace the whole exhaust. Wanted to get a ragazzon dual exit but theyre 400? + 400? shipping. Will probably get a custom one made.

Are there any benefits of doing this? Will the turbo behave better or worse? Or should I get a whole exhaust made with a catalytic converter?

Not really expecting power gains, just looking to do something for my car, if it will help the car breathe more.

Engine is a 81kw from a 156, swapped into a 147.
 
#2 ·
I have a 2005 156 JTDM 150 ( remapped to about 190), and I needed a new exhaust at 110,000 ( on the original !) so I opted for a Cybox stainless steel single exit system with a decat ( the 'cat' is just a small 'silencer' ). I actually took the car down to The Wirral from Co Durham to have it fitted at the factory workshop. I was very impressed with the service, and it looks great. I must admit that I was a little disappointed in that I expected it to be a bit louder than standard (!) but that aside, there was no noticeable increase in power or mpg. So I am not sure that it actually does affect the engine performance..... However, I wanted a stainless system and IIRC ( it was a couple of years ago now ) it cost about £320 fitted. That was a good deal, especially compared to an Alfa OEM system !
 
#3 ·
Would you say there is a benefit to doing that, or should I just replace the rusty rear silencer? I just want a bit more grunty sound, if thats possible with such an exhaust.

Also, does anyone know if the exhaust made for a 8v jtd will fit a 16v engine? And if not, can the part connected to the manifold simply be replaced without touching the rest? Was thinking of swapping in a 16v eventually.b
 
#4 ·
I my case, I decided that I would need a whole system as it had done 10 years/100,000 miles on the original one....including 2 track days at Croft Race circuit ! I am sure that Cybox could have made the system louder if I had asked them, but I did not know how it was going to sound until it was finished ! Also, I intended to keep the car and could see no reason to get an OEM complete system as it would have been a lot more expensive ( IIRC in 2013)

By the way, on my previous Alfa 156 1.8TS, I managed to buy a secondhand Cybox back box via on of the Alfa forums, and I was so impressed with that one that I went with them for the diesel one. The TS engine did sound so good with that S/H box on, a bit louder but not too loud, so I was looking forward to the same difference with the JTDM fitment...

So if you are going to keep the car I would get it done, otherwise maybe try and source a used one ?
 
#5 ·
Was looking previously at a few used ones, but the car culture here is so basic and weird. Replacing the exhaust just means replacing the rear silencer, or even just the tip to make it sporty-looking. Definitely no complete exhausts for sale. Even so, my engine seems to be very rare - 81kw from 156. Upon some digging around, found out it was only made during 2001. It's a bit of a hassle when looking for parts, because I have to explain everyone that the car is a 147 but the engine is from the 156.
 
#6 ·
Hi. My car is straight piped from the turbo all the way to the back. What I can say,well , it's mostly benefits. When idling, it sounds deeper, it has some grunt to it, it's miles better than a stock exhaust. It presumably sounds great at speed ,but I haven't really heard it from the outside, I have been told so.

Negatives...well the lack of any cat means the gasses go straight to the back and sometimes at a stop light, with windows down, I can start smelling the diesel being burned. It's not a leak anywhere ,it's just from the exhaust. I've checked numerous times and have talked to people ,who share the same modification with the same results.
 
#7 · (Edited)
==sorry, wrong section==
Hi Guys,
Just a quick question. I have 159 2,4 210cv. DPF-off, EGR-off, remaped to theoretical 243hp/470nm. Now thinking about decat (mostly to get deeper exhaust tone). Do I still need to make software changes if DPF allready off? Whats better - remove internals of CAT (You can actually sell those for about 100EUR/pce) or straight downpipe?
Thanks!
 
#8 ·
This should be in the 159 Brera section, but no you wont need any remap, the probe is just a temperature sensor. I removed the internals of the cat on my 1.9159, I did notice the turbo spooled up quicker. A proper pipe would be better than an empty cat as the gas will flow much more smoothly than when going into a larger chamber and then back out again.
Iwas thinking about doing the same to my 2.4 Brera but heard from a well thought of tuner that it can cause turbo overspeed issues.
 
#9 ·
Contrary to popular belief, de-catting wont really give you any benefits, maybe literally a couple of BHP more, the only thing it will do is make your car spew out more crap into the air that you and the rest of us have to breathe.

the 'cat' is just a small 'silencer' !
No - there are 2 "cats" the first (under the bonnet) is actually a heat sink that keeps the exhaust gases warm to enable the catalytic process that change some of the harmful exhaust gases in the second (middle) cat into less harmfull gases (heat helps the process), the third (the last one, at the back of the car) box is a silencer.
 
#10 ·
Contrary to popular belief, de-catting wont really give you any benefits, maybe literally a couple of BHP more, the only thing it will do is make your car spew out more crap into the air that you and the rest of us have to breathe.



No - there are 2 "cats" the first (under the bonnet) is actually a heat sink that keeps the exhaust gases warm to enable the catalytic process that change some of the harmful exhaust gases in the second (middle) cat into less harmfull gases (heat helps the process), the third (the last one, at the back of the car) box is a silencer.
I completely get what you mean. I don't even like the sound of a de-cat diesel. It sounds very unnatural and weird. And decatting would be a hassle since I would either have to bribe the inspections guy (which is easy) or I'd have to re-fit it everytime I have to get it insured.

But I was figuring why not do it if I'm getting a complete exhaust.

May just get the rear silencer, tho, since the original one is rusty. Would just a rear silencer deepen the sound at all? No HP gains, obviously, but at least the sound?
 
#13 ·
Worth saying that all this depends on the level of boost you are runnning..

If the boost is high and thd exhaust diameter or flow is not up to the job you will get very high egt's leading to the ecu reducing fuelling..

The result

Flatspots at high boost..

There ard a couple of ways around it..a freer flowing exhaust and cat removal being one..

The other ways are too reduce the temp of the charged air...so either a fmic or water meth.

If you are running normal boost then I think the pre cat is the only one worth removing as it will help with earlier turbo spool.

Main cat leave it in place...unless of course you are running very higb boost(3 bar)
 
#15 ·
Thanks for answer guys. What do You mean by main cat? As I said DPF is already empty - straight pipe welded inside) Now thinking about cat that comes right after turbo:
see attached - 04.4184.
Main reason is to get deeper tone (as some of members here said that after decat it sounds more deeper) and quicker spool.
I have booked those jobs for this wednesday together with new turbo, i will let You know the results :)
 

Attachments

#21 ·
agree with 99% of that...

for me though running 225 bhp on a stock turbo ..? on a cf3 I am not sure its possible (although I will take your word for it ) and if it is probably not the best idea...regarding the intercoolers, yes we both seem to be of the same mind on this , I think throttle response is more important than a few extra ponies and would always go for driveability over a number that doesn't really mean much in the real world.

water meth is a more elegant solution to charge temps!


regarding modifying the flywheel for an SMF, well you don't ''need'' to but both Damien and I found clutch options for the DMF were either unsatisfactory in character or couldn't hold the power at peak torque and in addition were very expensive... my car has had 2 clutches in the time I have had it with the first unable to cope with the power.

Damien's has been through 3..

.Ned suggested the GTA clutch kit and smf as the only only option that would hold the 600nm of torque we are pushing whilst still maintaining OEM pedal feel and driveability in traffic.


He was spot on...we both have those now and mine is 10k in with no issues.

it means subsequent clutches are cheaper and you remove the DMF as a potential failure point..you also get a freer revving engine. win,win,win.


I am just making sure the OP understands the potential pandoras box he is opening by tuning the car.


most people want more power and over a period of time lots of time and money gets spent in the pursuit of this!
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top