Posts Tagged 'exhaust'

Quite a mean stance!

vs the original BMW M exhaust

So the weekend before last, Tom and I set up to install a free-flowing exhaust on the M Coupe. I was pretty picky with what I wanted from an exhaust, the stock BMW M units are pretty good, albeit very quiet and thus quite heavy. I didn’t want a boomy american (or ‘rice’) sound just to shed some weight and add a bit of top end power, so I ended up getting bespoke exhaust makers Hayward & Scott of Essex (UK) to build me one. After a month of pounding on metal in sheds, the men from Essex sent me a hand-made exhaust. We waited for good enough weather to perform the install and got to work…

Flying M-Coupe

The process is easy enough in theory. In fact our biggest worry, seized bolts/joints was a non issue. The real challenge was fitting hands/tools in places to remove joints. We put the car up on four jacks, as high as we pretty much could. One of our jacks was low on hydraulic fluid, and was located at the back on the driver side, so we shuffled it to the front passenger side where it would be less of a determent. After playing with the jacks for awhile we were ready to begin. We began by removing the BMW back boxes. Each one was held on to two brackets, rubber mounts. Getting them off of the brackets was much harder than just removing the brackets from the body, so thats what we ended up doing. Each muffler was connected to the mid-section with two bolts and a metal ring, easy enough to remove. Removing the BMW mufflers, you really got a feel for how heavy they were, in excess of 30 pounds a pop.

Car parts scattered across the lawn like hillbillies

With those removed we could focus on removing the mid-section. The mid-section is guarded by several heat shields, which also keep it snug with the chassis. Removing those was trivial. The mid-section is attached to the rear cats (the car has four catalytic converters in the North American spec) with six bolts. Removing the top bolts proved to be exceedingly difficult but we managed to do so. At this point we felt pretty pleased with our efforts. We began to try and jiggle and slide the mid-section out from under the car to no avail. It was being blocked by two large pieces of the rear subframe that cross the back of the car and add to the overall rigidity. After hesitating and trying to partially undo and move them out of the way, we gave up and removed both cross bars. This made removing the mid-section a snap, and we put it on the lawn with all of our other parts.

The H&S mid pipes look pretty

The stock BMW M mid-section is an X-pipe and contain a mid resonator, and was also quite a heavy part. The Hayward & Scott mid pipes we were replacing them with were a straight stainless steel X-pipe. This change reduces back pressure, reduces overall weight (although not by more than 10 pounds) and produces an awesome crackley overrun noise on lift off and during engine braking… After a short break, we installed the mid pipe, having similar trouble getting those top bolts in. After putting those in, we reattached the cross bars, and then put the heat shields back up. From there we were able to begin installing the H&S back boxes. We rotated and snug them onto the mid pipes until they lined up with the brackets. This wasn’t particularly hard, though the mid-section was not perfectly symmetrical (an issue with any non robot made parts), and thusly made fitment on the drivers side a bit trickier than the other.

The heat shielding in the exhaust housings is impressive

We finished up, put the drivers side wheel, and lowered the car. I then proceeded to fire her up. The sound of a cold start was pretty impressive, you could FEEL it. Pretty soon the car warmed up and it became silent as it hummed a long in neutral. I took her out for a quick spin. Low revs are a bit deeper than I had anticipated, but the scream at high revs is just spine-tingling. Better yet is the sound between gear shifts, as unspent fuel burns up on the pipes and crackles out the back. This becomes more pronounced as the pipes heat up, to the point where a throttle blip with a fully hot exhaust is just incredible. Highway cruising is completely silent thankfully, however any throttle under full transmission load is quite a bit louder than I had anticipated. Superficial (though important, since you have to live with it) criteria aside, performance is fantastic. Though its really hard to say as the car has always had a ton of power put down to the road, so detecting a predictably small peak power improvement is impossible, we’ll have to book some dyno time to really know for sure.

The weekend after we did a bit more fitment work on the drivers side, as the pipe still sits crooked, more so when the exhaust heats up and expands. We’ve not quite gotten it perfect. I’m also not 100% happy with the volume level, it’s a bit TOO loud for my taste, though it sounds fantastic and raw, not synthetic and big-tippy. Never the less it’s been a fun project, however it turns out! Thanks to Tom for his hours upon hours of work.

The following is a video I made with a cold exhaust, the overrun is not nearly as noticeable as hot pipes, sadly. I had made a better one earlier but had completely forgotten to TURN THE MICROPHONE ON (as was the fate of my ‘before’ video). I might make another one one day, but I generally don’t like sitting and revving the engine like a child in place, its rather anti-social :)

Gettin' my Myspace on: the cell-phone-in-a-bathroom-mirror-style self-portrait

One of the hardest things to capture when it comes to cars is the sound of the engine. Engine sound is made up of several components, but the majority of the sound is induction and exhaust. Induction is the howl produced when the cylinders suck air in, exhaust is when they push post-explosion hot air out. Capturing induction isn’t particularly difficult, the frequencies seem to pick up pretty well on everything from cell phones and better. Exhaust is a much trickier thing to capture accurately, though being the louder sound on most cars, is what people desire to record.

The reason exhaust is so hard to capture accurately is two fold. Firstly the frequencies are quite low, which poses a problem for most smaller mic pickups. To that end, I don’t believe its ever *really* possible to capture and play back an accurate exhaust of say a Ferrari or a Le Mans car, but we wan’t to get as close as possible… As much of the sound is ‘felt’ as it is heard.

Boom!

Secondly, and the major problem when recording exhaust notes is the shear amount of sound. A showroom stock high performance car will produce upwards of 80 dB at full throttle at a distance of about 15 meters (50 ft). Thats not particularly a problem for most microphones, but the sound levels go up dramatically as you get closer making it impossible to get any sort of accurate recording. Race cars on the other hand, such as ALMS cars produce 110dB as close as 15 m. Good luck trying to capture it from a far, let alone from right at the apex of a turn. Once a microphone is over exposed, it begins to clip, and not all frequencies are lost evenly. This produces very unrealistic audio, and is the reason that most of the exhaust videos on the youtube are terrible… Anyone who’s tried to record race cars will attest to this.

The 5Dmk2 revolutionized the world of HD video recording, with its full frame sensor’s amazing low light performance. As good as its video quality is, its audio quality isn’t. The tiny onboard mono mic clips early, produces very non linear tones and always ends up sounding echoy. I’ve never been happy with the sound on the videos I’ve produced of our own cars, and at the track its completely useless. Upon doing some research into the budding SLR microphone market, I decided to pick up a RODE Videomic on Tuesday. The mic provides recording down to 40hz (with an optional high pass filter to cut it off at 80hz, though thats the opposite of what I need), and can record 143dB with less than 1% distortion. On top of that, theres dips for -10 and -20db pads. More than enough for the race track. The mic is light and snaps directly into the hot shoe mount, plugging into the mic port on the left of the body.

I gave the setup a test run, which more than vetted the recording resolution of the mic. However I had set gain up wrong on the camera body, which didn’t really show it off. I whistled in the parking garage and watched the dB meter, deciding upon an audio gain of 3/64. This was much too low while blipping the throttle to about 4500 rpm, with the sport button off, that both slows down the opening and prohibits the throttle bodies from opening fully at low RPMs.  (I’ve since learned humans can whistle in excess of 120dB!) I doubled the audio in iMovie ’09, and it was still too quiet. Though despite being boosted by 200%, the reproduction quality was still good, you can hear the muted throaty-ness of the exhaust, and the wurr of the inline-6 feeding it, pretty accurately to being behind the car. Next time I’ll probably go with a gain setup on the camera body of 15/64, keeping the HPF off and not using any of the on-mic pads.  Fun ahead!