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MG MGF Technical - SP exhaust, or 'empty vessels make most noise'!!

I have never been happy with the sound of my early (Falcon manufactured) SP exhaust system. (manufacture date of 1998) I like the DTM style of tailpipe but the noise that comes out has never been refined enough for me.

I have long standing doubts over the use of stainless exhaust systems for MGs as my experience over many years doesn't show them in a very good light. The problem seems very common and revolves around the silencing glass fibre wadding, burning, breaking up and being blown out of the tailpipe, usually within five years of new.

I have a rough and ready test to give an idea as to how effective a silencer will be by simply tapping the body of the silencer with the handle of a screwdriver. When I got the SP I did this and I did get some variations along the length of the silencer which was to be expected as there has to be several internal chambers owing to the point of exhaust entry and the positions of the exhaust exits. I was able to cross reference this with another new system and found similar results.

On the car the look is something I appreciate. The sound at low rpm and smaller throttle openings I also appreciate, but at wider throttle openings and higher rpms the sound has always been intrusive. Overall the ideal for me would be somewhere in the middle ground between the standard and SP positions.

Over the last few months I have noted that there has been an increase in the noise emissions when the engine and exhaust were cold, abrasive would be a good description. Once a few miles use introduced heat, the noise was softened. More recently I have noted that it has definately become slightly noisier at all times.

This has driven me to remove the SP and replace the standard system. What a difference, and not just in terms of the lack of noise, which is perhaps now just a little anaemic but much better than before. Now there is also a definate increase in torque in the lower rev range, especially at idle and just above.

The screwdriver test was done on both exhausts and with the SP a very tuneful ring was achieved. The standard system provided a dull thud! I didn't have any other thought than here we go again with the standard failure of a stainless steel exhaust system.

To satisfy my own curiosity, and to see if the silencer wadding refills I have had to do on other systems was possible, I have chopped open the top of the silencer.

Inside there are two bulkheads which create three chambers. The 'in pipe' passes through the first (left) chamber and empties into the middle chamber. In line with this 'in pipe', and about 70mm forward is another short section of pipe which passes through the second bulkhead into the third (right) chamber. Along the vertical centreline of the silence,r and passing between the two end chambers, are a pair of perforated pipes. Finally the two tailpipes collect gas from the centre chamber, but on the opposite side from the 'in pipe', with the two perforated pipes between them.

One confirmation I found on opening was that there was NO SIGN OF ANY SILENCER WADDING to provide any sound deadening. Really impressive for a supposed long life unit made in 1998 and subject to just 9,000 miles use!! Furthermore after studying the design for some time I am at a loss to see where any silencer wadding could go where it would firstly be of any use, and secondly could survive the blast and heat of the incoming exhaust gas.

It may be difficult to visualise the structure, so I have taken some photos which I will ask Rob to add to his site and exhaust section. (Rob??) But the normal structure is for wadding to surround the perforated pipes which allows the exhaust gas to expand, slow and cool through the holes into the exhaust wadding.

In this design this can't be done effectively because the hot gas from the engine comes into the same chamber as the perforated pipes AND is the same chamber for the tail pipe exits. Any wadding would be blasted to bits and blown through the tail pipes. ('would' can be replaced with 'does', from my experience!)

Further the connection of the small extra section of pipe between the middle and right chamber will also see that any wadding is blasted. There is a clear indication of exhaust heat blast projected from the 'in pipe' exit, straight through this short pipe and against the end wall of the right side of the silencer. I think many will be able to see this from the outside of their exhausts.

This leaves only the first (left) chamber where any wadding has any prospect of survival and since sod all exhaust gass passes through it will only function as an expansion chamber with access/exit through the ends of the two perforated pipes.

My knowledge of silencer design is limited and covers mainly normal straight through designs. However it is worth noting that the better quality systems use stainless steel wire wool wrapped around the central perforated pipes in these silencers to act as a porous heat shield to keep the scorching blast of the exhaust as it enters the silencer from frying the wadding, which leads to it breaking up and being blown out of the tailpipe. There was no sign of any of this, but in fairness I couldn't see where this could go to achieve any positive effect anyway.

Overall from my examination this is an apparently very poor design. I wonder whether the Miltek made versions have a different inner configuration? The sound levels indicate this may not be the case.

I would love some exhaust specialist to come along and explain too me the finer operating parameters of this design to dispel my sceptism, but I will take some persuading that there isn't a design flaw of major proportions that allows complete emptying of the silencer within a 2 year and low mileage period.

This is a new angle on the common exhaust issue and I ask that as many of you as possible go and tap along the underside of your silencers and report back on this thread what you find, and any other comments. I am ceratin that if the design is as flawed, as I suspect, then the current suppliers will be more than interested to rectify current products and avoid headaches for themselves in the future. SP of course are not around to comment directly, although Mr SP is still with the MG scene.

Rog
Roger Parker


Hi Rog,

Nice to read about Your findings on the SP system, about 2 years ago I did some modifications to the original system as to save some money but still gain some performance and a slightly more pleasant sound. It seems to me that what You describe in terms of number of chambers , perforated walls and pipes is exactly how a original system looks like. Will be interesting to see pict. later on ! There is NO filling in the original system either !! Only some steel-wool (NOT stainless!) is present at each sidewall stuffed between sidewall and first / last chamber. These 2 chambers are very narrow - I guess just enough wool to keep sidewalls from resonating at specific revs. Due to the very small drilled holes in walls and pipes on original system there is no way wool will find itīs way out of the system - just a pity that they didnīt use stainless steel there as well.
When tuning systems for track use , (roadsport with MoTīed cars over here have hard restrictions on sound level) it is often used stainless stell pads used in resturant kitchens to scrub pans etc. These are made of rather broad bands of stainless steel and is the size of a hand. When a system is "stuffed " with a few such it is still fairly free flow but resonances and sound can be controlled. There is no way they deterioate or get blown out of system ! They just turn blue in colour ...
Anyway , my bet is that many of the "sport" systems doesnīt differ too much internally to original system, drilled /pressed holes in baffels and pipes may be a bit larger - pictures of Your system will tell!

PS, with a airdriven hacksaw and some other tools it is no big deal to open up a system and also close it and seal with straps and SS selftapping screws!

Regards, Carl.
Carl

Rog, I'd be delighted to host the pictures. I am very curious to see what you have described.

Rob
Rob Bell

Hi Roger,
I have a SP exhaust which I bought in April 99,so it is probably the miltek version,I have done 12,000 miles with it and have noticed it getting louder
over the past few months especially when I am de-acelerating it becomes more intrusive into the cabin,
It also reverbarates more when the exhaust is cold.
I have also noticed less power lower down the rev range
or is it me?? What I have also noticed is the silencer
has now concaved inwards in the the middle I thought I had damaged it on a speed bump at first,but since reading your thread I am thinking otherwise.
Thanks for the info though.

Stuart
Stuart

Stuart,

Yes thats exactly the scenario, plus when the engine is cold you get the popping in the exhaust on a trailing throttle as unburnt fuel ignites in the exhaust. (Over run fuel shut off is inoperative at coolant temps below about 35deg C and at speeds above about 6mph or so.)

Rog
Roger Parker

Hi Gang,

My SP exhaust was installed in April 1998 and has covered about 12,000 miles. I am quite confident in saying that there has been no change whatsoever in it's perfomance. It has always 'Popped' while cold, and the overall power has remained constant throughout it's short life. Personally, I am more than happy with it's durability.

Rog, have you noticed how SP exhausts without a K&N induction system (element or cone) sounds a darn sight more tinny than with??

Nick
Nick

This thread was discussed between 07/05/2000 and 17/05/2000

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