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Discussion Starter #1 (Edited)
Hi Guys,

Thought I'd share my feedback here after fitting a custom cat-back exhaust on my 645Ci, as this is information I was looking for when I bought my car.

Full cat-back system.

Overall, first impressions are great, you can definitely hear the V8 note more and at higher revs it turns into a vicious roar and howl and a raspy supercar type sound when accelerating.
I was pleasantly surprised, as the car is still dead silent at cruising speeds (50mph-80mph and above...), there is no difference to stock, you can only hear the tyres rolling.
Only thing I could pick on, is there is a droning noise when driving in the city - only at low speed high gear acceleration - however once you reach your target speed that noise stops.

Also recorded a video on the GoPro (1080p HD) to give you an idea of how it sounds.

AFTER CAT-BACK
https://www.youtube.com/watch?v=aWe1U0-ulGw&feature=youtu.be

BEFORE - OEM EXHAUST (maybe not clear on the video, but OEM is much much quieter live)
https://www.youtube.com/watc
 

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I like the sound of that!

And I'm particularly impressed by the way it sounds pretty normal when cruising.

Do you mind if I ask how much they charged you?
 

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Discussion Starter #3
Sure, I was quoted £800, but negotiated a further 10% off, so £720 in total - all in.

Other places quoted from £900 to £2000 for a cat back and £500-600 for just the back boxes.
 

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That's a great sound - but the droning is a bit of a nuisance and is something I hate on my 840ci. The problem comes when the centre resonator is removed as it is this that removes the drone. I have had excellent results on just rear boxes, that solution gives a great sound without the drone.
 

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Discussion Starter #5
The droning isn't too bad. Only at low speeds and light acceleration.

Do you think the sound would be equally good if you left the resonators?

Fortunately, if it gets annoying it's an easy fix to get a centre silencer welded in, but I don't think it'll get to that.
 

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Discussion Starter #7
Hi Timm,
After a couple of weeks of driving, I must say I'm missing the deep V8 burble with this setup.

It's very raspy and it's missing the V8'ness at lower revs, it's just droning. It sound amazing when gunned though.

I'm thinking of getting a pair of Magnaflow or Vibrant resonators fitted back. Partially to get rid of the drone completely, and partially to try and get the deep V8 burble back.

Have you ever fitted resonators back on a cat-back system? I'm curious of the results.
 

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Discussion Starter #9
Timm, going back to our conversation, I have had quite a few changes to my system since and had the whole Longlife crap take off and scrapped (never go with Longlife btw.). I replaced it with stock/OEM down pipes including front OEM bottle resonators, connected to a Magnaflow x-pipe, 2.5" piping and two Magnaflow 14816 mufflers. Sounds great now, like a proper big V8 bruiser, BUT the droning around 1500-2000 rpm is highly annoying, to the point, if it doesn't go away after a while, I'll be fitting the centre resonator back on.

I ordered one from eBay and to my surprise it's a straight through design! There are no chambers inside, just perforated pipes. This means that:

a) The OEM system is not x-piped/balanced, it's (virtually) just two separate exhaust systems for each bank.
b) There is HARDLY ANY benefit in fitting a full Cat-Back system! As the OEM system is straight through, the only benefit would be changing the back boxes, as those are chambered, i.e. fitting an Axle-Back system.

Will let you know once/if I get middle resonator fitted and what the results are.
 

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The resonator is the same on the E31 - straight-through perforated pipes in a chamber which is devoid of anything else! But, that design does what it was designed to do, drag the exhaust gasses through using the depression formed when the pulse of gasses from the other bank passes. The use of the resonator doubles the frequency of the exhaust note and the resonant frequency of the cavity is above the audible drone that is so annoying.

In short, the resonator does quite a lot to the exhaust tone!

I fully agree though - you only need to fiddle with the rear boxes to get the sound of a powerful V8.
 

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Discussion Starter #11
Ahh, so it's empty inside too. I thought it's filled with sound deadening materials, like an after-market straight-through muffler. In that case it does allow exhaust scavenging, similarly to an X or H pipe.

I'm curious of the end effect once I get this fitted in, hopefully no drone and still a brutal sound.

I went from OEM to Longlife(total crap!)-Full-Cat-Back to Magnaflow-Semi-Cat-Back and now soon to Magnaflow-Axle-Back.

This has been an expensive learning curve :)
 

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I've been through the same learning curve - except, on my first E31the Longlife rear boxes were really great! Unfortunately, Longlife rear boxes with tapered tail pipes, X-pipe and 200-cell cats creates an embarrassingly loud noise whenever I give the throttle the slightest poke. It really is the LOUD pedal now......
 

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Discussion Starter #13
In my case, the Longlife boxes per se, weren't the issue, just the quality of workmanship of the branch I had the work done (Longlife Surrey) was awful and I ended up replacing their system completely.

I'll report back once I get the middle-resonator fitted back, hoping it'll do the trick :)
 

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Discussion Starter #14
Timm, so I finally got the centre resonator back in and WOW! What a difference! Not only the drone is gone, but the sound of the car has changed completely. In a positive way.

It sounds like a proper V8 again, from idle all the way to the top of the RPM range, burbles beautifully, even when slowly accelerating from the lights.

With the x-pipe it was just a boom noise up until 3k revs - highly annoying and even embarrassing.

IMHO opinion if enhancing the V8 sound is what you are after, an x-pipe is a bad idea, as it completely alters the V8 sound of the car... Just swapping the back boxes more than enough does the trick.
 
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