Milltek Cat Back Exhaust System info ( eg 2008 STI Hatchback )
06 November 2016 | 10:22 PM
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Milltek Cat Back Exhaust System info ( eg 2008 STI Hatchback )
Im making this thread as a source of info for anyone looking at the Milltek Cat Back exhaust systems as I recently installed one on my 2008 2.5ltr STI Hatchback. I went for the cat back as I already had a decat downpipe fitted, more info on this below as its very relevant to the purpose of this thread.
Ok first off, Milltek's website is a little misleading (in my opinion) for the 2008 2.5ltr WRX/STI but likely all the other Impreza cat backs too as they state the following in the title of their
Large Bore Downpipe and Hi-Flow Sports Cat. It says the following...
From their website:
To me that means their own downpipe and cat is required in order to fit the cat back system. Lets clear something up now...
THIS IS NOT THE CASE.
You CAN fit the cat back system (part number SSXSB017) to a standard OEM downpipe and cat OR full length decat pipe. By full length I mean the 'standard' one piece decat downpipes that people fit to replace the OEM downpipe and cat TOGETHER. Basically the overall length, design and flange arrangement of the OEM downpipe and cat is the same as a one piece decat downpipe.
Just remember the end/exit of the OEM downpipe is 2.5" and the one piece decat downpipes are 2.5" or 3". The Milltek cat back system is 3" throughout.
On the 2008 STI (and likely many other models of Impreza) Subaru use a donut gasket where the downpipe mates to the centre section pipe. This is definite on the OEM setup and probably all decat downpipes that exit with a 2.5" bore. I cannot confirm if you use the donut gasket with a downpipe that exits with a 3" bore. I did notice Milltek supply the cat back system with a 3" normal 'flat' gasket that would suggest to can mate a 3" downpipe to the 3" Milltek centre pipe without the need for the donut.
However, I thought the purpose of the donut gasket (and spring bolts) is to allow movement in the exhaust system on the rubber hangers. Without the donut gasket and possibly without the spring bolts, the downpipe and centre section would be bolted together rigid. Unfortunately I cannot give more info on this as it is not the setup on my car. My downpipe exits with 2.5" not 3".
I contacted Milltek before ordering the cat back system to get confirmation it would fit and they assured me their cat back systems (for any car) are designed to fit the OEM cat without modification, in this case the Impreza OEM downpipe and cat OR one piece decat downpipe.
That clears that up, now whats the system like?
In a nutshell...its awesome. Very good build quality, nice design, very quiet in the car and arrived well packaged with no damage. Why did I go Milltek?...
My car was fitted with a Magnaflow 3" cat back system that had no centre section resonator and it was very loud from inside the car. It was 'raspy' at low RPM and speed, was very loud and droned on the motorway. Basically it was too much for me and made driving - on the motorway especially - a chore. It was nice to begin with but the novelty wore off very quick.
Now, the Milltek is MUCH QUIETER, it has a centre section resonator (straight through like a motorbike silencer so no major restriction to gas flow) and the rear backbox is 'straight through' too. The pipes are not 'staggered' inside the back box so again, gas flow is very good. How they managed to make it so quiet I have no idea but the difference is night and day to the Magnaflow. Its interesting because if you look at the two systems in the photo below (Magnaflow on the left, Milltek on the right) the design is quite different and I would of thought the extra bends towards the rear in the Magnaflow would make it quieter but its not.
Magnaflow on the left, Milltek on the right
What is also interesting is when I was installing the Milltek I installed everything apart from the left and right dual tailpipes that poke through the rear bumper. I took the car for a drive to bounce everything around a bit and let it settle where is wants on the hangers etc. On this drive I was a little disappointed as although it was quieter than the Magnaflow, it still droned on the motorway. However, when I got back and installed the tailpipes I went for another drive and wow, it made such a difference, I could not believe the difference inside the cabin. I guess this is because without the tailpipes the exhaust was exiting underneath the rear of the car, under the spare wheel well but with the tailpipes installed the sound was carried out the back of the car.
I could not believe the difference between the two systems, I even checked the old Magnaflow system when I got back for any holes as the Milltek is so much quieter. The good thing though is the system sounds really nice from outside, it has a nice low basey rumble to it and sounds good when accelerating hard.
NVH? (Noise, Vibration and Harmonics)
A huge difference for sure. Most noticeable is I can hear road noise inside the cabin again instead of a constant drone on the motorway and I can honestly say its bliss. My bones dont chatter when accelerating at low speed in a high gear, for example 30mph-40mph in 5th gear where there is a high load on the engine. The car does not feel like its going to shake itself to bits. I understand noise level is a personal preference, some like a loud exhaust, others dont. What I would call loud, someone else may says its quiet. Its good to know you can have a completely decat exhaust (even the up-pipe), have the performance gain and still have a quiet exhaust.
Installation
Very easy and no modding required, removed the old system and installed the new one on the original rubber hangers in about 3 hours. This was on the driveway with the rear of the car raised on blocks. I used a new donut gasket and the original bolts/springs and nuts. Just to point out, even though the centre pipe is 3", the donut gasket does not disappear into the bore, the O/D of the donut is larger than the flange opening so seals nicely when tightened up.
In conclusion I'm really happy with the end result. Milltek systems are probably a bit pricier than other brands but when I consider the Magnaflow, in my opinion the extra cost justifies having a quality, quiet exhaust.
Link: http://www.millteksport.com/exhaust....?variantid=283
EDIT - I'll update with some more piccys soon.
Ok first off, Milltek's website is a little misleading (in my opinion) for the 2008 2.5ltr WRX/STI but likely all the other Impreza cat backs too as they state the following in the title of their
Large Bore Downpipe and Hi-Flow Sports Cat. It says the following...
Must be fitted with the Milltek Sport cat-back system and requires a Stage 2 ECU remap
To me that means their own downpipe and cat is required in order to fit the cat back system. Lets clear something up now...
THIS IS NOT THE CASE.
You CAN fit the cat back system (part number SSXSB017) to a standard OEM downpipe and cat OR full length decat pipe. By full length I mean the 'standard' one piece decat downpipes that people fit to replace the OEM downpipe and cat TOGETHER. Basically the overall length, design and flange arrangement of the OEM downpipe and cat is the same as a one piece decat downpipe.
Just remember the end/exit of the OEM downpipe is 2.5" and the one piece decat downpipes are 2.5" or 3". The Milltek cat back system is 3" throughout.
On the 2008 STI (and likely many other models of Impreza) Subaru use a donut gasket where the downpipe mates to the centre section pipe. This is definite on the OEM setup and probably all decat downpipes that exit with a 2.5" bore. I cannot confirm if you use the donut gasket with a downpipe that exits with a 3" bore. I did notice Milltek supply the cat back system with a 3" normal 'flat' gasket that would suggest to can mate a 3" downpipe to the 3" Milltek centre pipe without the need for the donut.
However, I thought the purpose of the donut gasket (and spring bolts) is to allow movement in the exhaust system on the rubber hangers. Without the donut gasket and possibly without the spring bolts, the downpipe and centre section would be bolted together rigid. Unfortunately I cannot give more info on this as it is not the setup on my car. My downpipe exits with 2.5" not 3".
I contacted Milltek before ordering the cat back system to get confirmation it would fit and they assured me their cat back systems (for any car) are designed to fit the OEM cat without modification, in this case the Impreza OEM downpipe and cat OR one piece decat downpipe.
That clears that up, now whats the system like?
In a nutshell...its awesome. Very good build quality, nice design, very quiet in the car and arrived well packaged with no damage. Why did I go Milltek?...
My car was fitted with a Magnaflow 3" cat back system that had no centre section resonator and it was very loud from inside the car. It was 'raspy' at low RPM and speed, was very loud and droned on the motorway. Basically it was too much for me and made driving - on the motorway especially - a chore. It was nice to begin with but the novelty wore off very quick.
Now, the Milltek is MUCH QUIETER, it has a centre section resonator (straight through like a motorbike silencer so no major restriction to gas flow) and the rear backbox is 'straight through' too. The pipes are not 'staggered' inside the back box so again, gas flow is very good. How they managed to make it so quiet I have no idea but the difference is night and day to the Magnaflow. Its interesting because if you look at the two systems in the photo below (Magnaflow on the left, Milltek on the right) the design is quite different and I would of thought the extra bends towards the rear in the Magnaflow would make it quieter but its not.
Magnaflow on the left, Milltek on the right
What is also interesting is when I was installing the Milltek I installed everything apart from the left and right dual tailpipes that poke through the rear bumper. I took the car for a drive to bounce everything around a bit and let it settle where is wants on the hangers etc. On this drive I was a little disappointed as although it was quieter than the Magnaflow, it still droned on the motorway. However, when I got back and installed the tailpipes I went for another drive and wow, it made such a difference, I could not believe the difference inside the cabin. I guess this is because without the tailpipes the exhaust was exiting underneath the rear of the car, under the spare wheel well but with the tailpipes installed the sound was carried out the back of the car.
I could not believe the difference between the two systems, I even checked the old Magnaflow system when I got back for any holes as the Milltek is so much quieter. The good thing though is the system sounds really nice from outside, it has a nice low basey rumble to it and sounds good when accelerating hard.
NVH? (Noise, Vibration and Harmonics)
A huge difference for sure. Most noticeable is I can hear road noise inside the cabin again instead of a constant drone on the motorway and I can honestly say its bliss. My bones dont chatter when accelerating at low speed in a high gear, for example 30mph-40mph in 5th gear where there is a high load on the engine. The car does not feel like its going to shake itself to bits. I understand noise level is a personal preference, some like a loud exhaust, others dont. What I would call loud, someone else may says its quiet. Its good to know you can have a completely decat exhaust (even the up-pipe), have the performance gain and still have a quiet exhaust.
Installation
Very easy and no modding required, removed the old system and installed the new one on the original rubber hangers in about 3 hours. This was on the driveway with the rear of the car raised on blocks. I used a new donut gasket and the original bolts/springs and nuts. Just to point out, even though the centre pipe is 3", the donut gasket does not disappear into the bore, the O/D of the donut is larger than the flange opening so seals nicely when tightened up.
In conclusion I'm really happy with the end result. Milltek systems are probably a bit pricier than other brands but when I consider the Magnaflow, in my opinion the extra cost justifies having a quality, quiet exhaust.
Link: http://www.millteksport.com/exhaust....?variantid=283
EDIT - I'll update with some more piccys soon.
Last edited by scoober101; 06 November 2016 at 10:40 PM.