Standard backbox with valve - mod project - MG-Rover.org Forums
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post #1 of 29 (permalink) Old 12-08-2016, 13:05 Thread Starter
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Standard backbox with valve - mod project

Background: I have a Toyosports, nice sound but awful quality. First the hanger failed and sliced open the back-box (soldered it shut and reinforced the hanger), recently it developed a rattle. Cut it open and found that one of the channel separators failed:

http://i67.tinypic.com/65r0jl.jpg

I soldered everything back, altered the separators to allow for less restricted airflow and reinforced them. Also, removed 60% of the wool. Will put it back this weekend to see how it sound. I'm expecting it to be quite loud and rewarding

However, I'm no more into extreme sounds, but I like the idea to have the car sound sporty, but silent while cruising. I have the original exhaust with noise control valve (it's a TF135) and am considering to alter it to make it louder but still controllable.

What I would like to do is to remove the separators (a-la-Toyo) and part of the wool just for the right tailpipe, the one with the valve on. The left one air path would be mostly left untouched.

Then I'll wire a 3-ways switch to the cabin to control the valve:
A - Automatic (controlled by the ECU i.e. valve opens at around 3k rpm)
C - Comfort (valve always closed)
S - Sport (valve always open


I'm pretty sure the Comfort mode will not substantially alter the engine breathing because I'm now using a 115 exhaust with small tailpipes and car behavior is not affected. And the car is designed to run with a single tailpipe up to 3k revs, Comfort mode will be for highway cruising at just about that rev.

The real question is: how would the sound be in Sport mode with a almost completely free tailpipe and a more restricted one? And how quite will the Comfort one be?

I now there's only one way to find out, but maybe if somebody already tried to modify the standard backbox I might have a hint

Would it work? Or should I just stick with the standard backbox and just wire the switch?
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post #2 of 29 (permalink) Old 12-08-2016, 16:21
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To be fair, the differences on the standard box between flap open or closed is hardly noticeable.
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post #3 of 29 (permalink) Old 16-08-2016, 00:20
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My experience with an aftermarket exhaust (an MGOC Standard S/S backbox) was that it was susceptible to a little drone at around 3000rpm. After doing some basic maths around the speed of sound, the wavelength of sound at 3000rpm (50Hz) which is 6.6m, I reckoned it was about 1/4 wavelength between exhaust pipes, if you take the measurement via the exhaust system.

So, that could form a resonator - with a reflection from the open end of either pipe reverberating back and forth between each pipe opening (oddly enough, sound reflects back both from the closed end of a pipe, and the open end of a pipe).

So, I stuffed a sock up one exhaust pipe and lo! The drone disappeared. Until the sock blew out.

All of which is by way of saying, I think you actually could have a worthwhile effect on any resonant drone, although I'm not sure if a hard surface on your valve would work. It might - it will reflect sound too, but in the opposite phase to an open pipe, which may have a cancelling effect. Worth a try.
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post #4 of 29 (permalink) Old 16-08-2016, 07:56 Thread Starter
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Thanks! This is quite interesting.

Last Sunday I fitted the modded Toyosports and found that after the modifications (less backpressure and more free-flowing design), the resonance at 3k almost disappeared, making it more comfortable to drive at cruising speeds. So I might keep it in the end, but still unsure about it. It is VERY addictive now. VERY.

My main concern is how the sound of a standard backbox can become by altering the internal structure, don't want to ruin a good (and rare!) valved-backbox if it's going to sound like a fart

I might consider sourcing a more common 115 exhaust and, although with smaller pipes, see how it turns out before altering the 1.8 one.
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post #5 of 29 (permalink) Old 16-08-2016, 08:30
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I had never heard of the exhaust valves until recently. My F is in Spain and got an advisory re noise. It has a standard cat and MS back box so not really considered loud. I have seen on ebay sound reducing valves, do they work, or can some one recommend something.
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post #6 of 29 (permalink) Old 16-08-2016, 15:21 Thread Starter
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Quote:
Originally Posted by campbosshassi View Post
I had never heard of the exhaust valves until recently. My F is in Spain and got an advisory re noise. It has a standard cat and MS back box so not really considered loud. I have seen on ebay sound reducing valves, do they work, or can some one recommend something.
The problem with sound reducing valves is that they need a very invasive work on the exhaust tips, cutting them, soldering the plates and fitting the valve.

The way our exhausts are designed (diagonal tubes), it would be VERY difficult to precisely cut the tip and fit the valve without altering the position of the tip in the rear bumper. It's something that needs to be done with precision cutting tools and millimeter-exact positioning of the pieces before soldering, i.e. something that falls outside the usual DIY tools/skills
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post #7 of 29 (permalink) Old 16-08-2016, 23:01
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Thanks for the reply I will find a friendlier mot station or swop the exhaust once a year.
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post #8 of 29 (permalink) Old 17-08-2016, 07:23 Thread Starter
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Well, if you just need to pass the MOT, you can very well fit a DB-killer to one or both the tips
(like this one: Adjustable Volume 4" Car Exhaust Silencer Baffle DB KILLER - UK made .SIL.003 | eBay)
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post #9 of 29 (permalink) Old 17-08-2016, 10:15
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Thanks for that I have just ordered on.
If it reduces just a bit I will be with in limits and will get an MS 4-2-1
manifold.
and a K-MAPS remap. its expensive as I have the old 1.9 ecu, but the mg is my only hobby what the hell.
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post #10 of 29 (permalink) Old 17-08-2016, 14:56 Thread Starter
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Hope you checked the exhaust tips tube size before!
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post #11 of 29 (permalink) Old 20-03-2017, 10:03 Thread Starter
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Well, I'd like to update the forum about the findings. I successfully modded 2 exhausts, a 115 and a 135/160 one. I cut them open and removed baffles and tubing to make them similar to the Toyo (2 lateral chambers), although the Toyo has solid chambers separators instead of the OEM drilled ones. It took some careful angle grinder action and dremel finishing touches to remove everything while making a neat job. Then welded the lid back on and secured the air tightness with some high temp paste. Here is a picture of the 135 backbox just before putting the lid back on:

http://oi63.tinypic.com/2qnwf2d.jpg

Now for the results. The 115 exhaust benefits from the mod in terms of backpressure, the car is more responsive, the exhaust pops a lot. The sound is better but not nearly as sporty or loud as any stainless steel backbox. This is probably due to the limited diameter of the tubes, the main one and the ones going to the tips. Throttle response on a 115 car is definitely better, especially at mid-high range.

The 135/160 exhaust mod went GREAT. Thanks to the larger diameters, the sound after the mod is almost the same as the Toyo. It does not drone at 3k rpm like the Toyo does (although I modded mine so it does way less). The perceived volume is only slightly lower but the sporty note is quite rewarding and it growls, pops and bangs. I LOVE the sound, less intrusive but still very sporty and definitely a head turner.

Despite removing a lot of metal (at least a couple of kgs) from the original unit, the Toyo is still way lighter than the modded OEM one.

However, I did this mod to be able to use the right tip valve, so in the next weeks (or months, time is always scarce here) I will create a circuit to control the valve for silent cruising.
It will be a simple knob, positioned on the "flap" over the ignition, with automatic, sport and comfort settings.

To do so, I will feed the valve controller with:
1) Nothing (valve always open, Sport)
2) Permanent 12v live (valve always closed, Comfort)
3) Original ECU feed to the valve actuator (valve closed at low rpms, open at high, Automatic)

Now I must find the right rotary switch, a low profile one with sufficient amperage/voltage tolerance. Any suggestions?
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post #12 of 29 (permalink) Old 20-03-2017, 11:33
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You could use a very small/discreet switch and use relays to handle any current.
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post #13 of 29 (permalink) Old 20-03-2017, 12:48 Thread Starter
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You could use a very small/discreet switch and use relays to handle any current.
Well, I believe the involved current should be low, the valve is controlled by what seems like a low-current electric actuator, I was thinking of powering it using a ignition live so that the circuit only works with the engine running.

I already use the mirror heating ignition wire to power 2 usb chargers (1 for my self-built Android onboard computer in the parasol, 1 for a USB port), adding a 3d flow might be not so easy on the loom, so will need to source another ignition live
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post #14 of 29 (permalink) Old 20-03-2017, 12:58
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Please do not overload existing circuits in the hope of success and convenience.

The cabling is sized to suit the application, as I am sure you are aware.

An electrical fire (anywhere) is not nice...

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post #15 of 29 (permalink) Old 20-03-2017, 16:26 Thread Starter
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Please do not overload existing circuits in the hope of success and convenience.

The cabling is sized to suit the application, as I am sure you are aware.

An electrical fire (anywhere) is not nice...
I see your point, that's why the extra gadgets (USB and OBD totalling less than 5 amps) are hooked to the wing mirrors heating, which is a hi-load circuit with good tolerance. The addition didn't even blow the standard fuse so it's within operating range.

For this one I think I'll go with a switched live taken from the stereo, in my case it's a 4x50w system, while the TF should support up to 4x80w with no amp, so I believe there's plenty of margin for an electric actuator not too different from a relay ;-)
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post #16 of 29 (permalink) Old 20-03-2017, 17:57
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The addition didn't even blow the standard fuse so it's within operating range.
That is not how fuses work.
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post #17 of 29 (permalink) Old 20-03-2017, 19:34
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I don't know if the F/TF box is the same, but removing or losing exhaust packing from some exhausts loses performance. It tends to be ok until it reaches a certain rpm and then the power just flatlines.
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post #18 of 29 (permalink) Old 20-03-2017, 20:51
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Unless you want to be able to change the valve when driving can you rig something up close to the actual valves control unit.
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post #19 of 29 (permalink) Old 21-03-2017, 09:50 Thread Starter
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Unless you want to be able to change the valve when driving can you rig something up close to the actual valves control unit.
I'd very much like to control the valve while driving
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post #20 of 29 (permalink) Old 21-03-2017, 09:52 Thread Starter
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That is not how fuses work.
Please explain, I was under the impression that they protect from overloads and that a 10A load on a 5A fuse would blow it.
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