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Fairthorpe Electron Minor (Based On Triumph Herald)

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My Electron Minor - first registered in 1959 - uses an 1147 Triumph engine re-bored to 1174cc. The original exhaust system was replaced in 1973, for reasons that I have mostly forgotten, and the silencer that I then used was bought off the shelf to fit the space available under the driving seat (but outside the car, of course). When rebuilding the car recently, the silencer was copied in dimensions but using stainless steel. 

I have no idea what the internals consist of but I do know that it is very noisy and gives a max speed of about 60mph, much less than the silencer it replaced. Nothing else has been changed.  I have been trying, through various other online forum sites, to find out what actual dimensions the original Triumph 1200cc engine had but although I have had a lot of informative replies about manifolds, pipe sizes, carbs, etc. nobody has actually given me the length and outside diameter of the 1200cc silencer that was used on production vehicles. Even a part number would be of value if it is possible to find a conversion to a modern equivalent so if there is anyone able to provide me with either or both of the answers, I'd be grateful.

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Hello Brian and welcome.


Seems a bit strange that your new silencer is both noisy and apparently restrictive.  I don't remember the silencer on my 1200 Herald being anything very special - it got replaced with an Austin Healey Sprite back box.  This made it sound fast (!) and didn't actually slow it down, though it probably didn't speed it up much either.


There are two main types of silencer.  Baffled ones have some kind of physical barrier inside to forces the gases to take and indirect path through them and loose energy in the process.  I'm pretty sure the OE Herald one was this design. Absorption ones (also sometimes called "straight through") have a straight piece of perforated tube through the middle of them and the rest of the can stuffed with some kind of heat resistant fibre.  These are usually seen as being less restrictive.  As the name implies, you can normally look straight through them, whereas you can't with the baffled type.


Something else to consider is that the exhaust may now be less restrictive, allowing the engine to flow more air, but this has upset the calibration of the carb(s) making the engine run lean (especially at higher rpm) and reducing power.  Are you still running the Solex downdraft from the 1200 or do you have twins SUs?


I would have thought you should be able to get hold of a Herald 1200 exhaust box, the Mk2 version at least. Do you have any pics of the silencer that was replaced?  A pic of your car would good too.


From the Canley Classics website:


Rear box 948 / 1200 Mk1  GEX 3197

Centre box TH9483


for the Mk2 1200 there seem to be 3 numbers listed, all marked NLA

305635, 304515 & 308284


For the 13/60 GEX3198 (which I suspect is near identical to one of the above)

I suspect that as a '59 car yours will have the Mk1 chassis so the Mk2 exhaust parts may well be a red herring.  My technique back in the day was to rummage in the skip at one of my local exhaust centres (I did ask first!).  These days I have a friendly motor factor who lets me wander the exhaust section of his warehouse with a tape measure.


Hope this is helpful





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Nicks thought about back pressure may be appropriate, did you keep the old box to compare?


Maybe take a look at the MK1 Spitfire exhaust boxes that use a similar engine, boxes for these seem to be available from the likes of Rimmer.


NB if your new box is straight through as per Nicks description, then you could try introducing a temporary baffle or two using bits of tin can in the entry or  exit to the box to see if that makes a difference.



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Yes, I still have the old box and will be re-fitting it for comparison. I'll also check both silencers for being straight through or not.

Your comments are appreciated.


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Bordering on the weird perhaps, one piece of info that might be of interest is a 'formula' for calculating the necessary volume of the important parts of the exhaust system.

It is suggested that volume, i.e.cubic capacity, of the silencer PLUS that of the pipe between exhaust manifold and entrance to the silencer should be between 2500 and 3000cc.

(If readers need to be reminded of how to calculate the volume of cylindrical objects such as pipes, a private message to me is OK.)

Whether the previous capacity statement is meant to be used with the formula is unknown but here it is.

Cubic capacity (silencer plus inlet pipe) = 1.8 times engine capacity in cubic centimetres.

So, for a 1200 engine, the combined volume of the silencer and inlet pipe is 1.8 X 1174cc which gives 2113cc. (My engine is a re-bored 1147cc.)

This is below the suggested value of 2500 to 3000cc and that is where an element of doubt creeps in.

The figure of 2113cc, using an inlet pipe of 100cm length and 3.8cm diameter means that the silencer volume is approximately 1000cc.

A 1000cc silencer of 8cm diameter will have a length of about 20cm - look at a typical 1litre soft drinks bottle. This length seems unrealistic!!

So, if any reader has come across the above 'formula' and suggested volume statement, perhaps some light could be thrown on the apparent contradictions.

I have to admit that I can't place much credence on them - perhaps an error arose in transmission to me?


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