Exhaust questions

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rcx106

Original Poster:

188 posts

118 months

Wednesday 29th October 2014
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I'm thinking of making up my own exhaust because the standard exhaust on my 4x4 (Vitara) is heavy, bulky, obtrusive, and knocks around when off roading. Plus I like a little bit of exhaust noise, helps feel what the engine is doing.

I've never done exhaust work before, apart from welding in new sections to pass an MOT. Questions:

I've measured the outside diameter which is 48mm at the cat, and then 43mm outer diameter at the rear section (there's only two sections). What do these sizes equate to in nominal exhaust size? Online shops list 1.75", 2", 2.5".... I am planning on running the same diameter all the way through on the new one.

The section with the cat joins to the rear section using a flange. What are these flanges called so I can Google and buy?

Are there any glasspack silencers which are road legal? Not sure about cars, I ride motorbikes and on bikes they need to be E marked. On bikes it's possible to buy an E marked silencer with a removable inner baffle, so you get a straight through exhaust and still have the E mark.

Just to check the obvious, the longer the silencer the better it silences right? I see universal one in 12" and 18". I have space for upto 21".

Any tips for which silencer to use if I just want a bit of exhaust noise? Remember the main aim here is to have a light and unobtrusive exhaust. I don't want to be too antisocial. And remember that my car only has ONE silencer.

Thanks!

CrutyRammers

13,735 posts

197 months

Wednesday 29th October 2014
quotequote all
43mm is 1.69" so I guess you'll need to go with whatever's closest. IIRC they are sold on internal diameter rather than external.
For legality, I believe that it's technically illegal to modify a silencer at all under construction and use regs - there have been a few threads on here regarding it. That said, straight-through silencers with wadding are not illegal as such (most kit cars have them), but in theory you'd have to go through an IVA process to get them approved (i.e. check that they're quiet enough). I'm sure as long as it's not super loud, in practice it won't be a problem. As a guideline, current noise limit for IVA approval is 99Db, which is likely to be louder than you'd want anyway.

The larger and longer a silencer, the quieter it'll be. A straight through type will need to be very large or it'll be very loud, so I don't think will fit your "light and unobtrusive" criteria tbh. I've got two massive Wunof cans on my Dax and it's still bd loud.

rcx106

Original Poster:

188 posts

118 months

Wednesday 29th October 2014
quotequote all
Aha, it's the internal diameter we go by. I've checked an online exhaust calculator which says I only need 1.7" pipe. I will go with 2" as it's a standard size and it's the closest to the pipe size at the manifold and cat.

Just dawned on me that if I'm making my own exhaust then I don't need to use OEM type flanges. I can use whatever joint method I fancy! What's the best or most convinient type of joint? Simple clamp type?

rcx106

Original Poster:

188 posts

118 months

Wednesday 29th October 2014
quotequote all
Just read topics on law on exhaust noise. So I need to keep noise to be comparable with the original system. I'm not a by-the-book person, so a system that gives moderate noise would be fine by me.

Pumaracing

2,089 posts

206 months

Wednesday 29th October 2014
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rcx106 said:
Aha, it's the internal diameter we go by.
No it isn't. Exhaust tube is always specified by outer diameter which is the only thing that can be conveniently measured for a system that is already on the car.

However clamp-on silencers are specified by the diameter of tube they fit OVER so the I/D of the flanges on the silencer will be the same as the O/D of the tube.

In turn this means that the size of clamp required will be 3mm (1/8") larger than the tube O/D to allow for the wall thickness of the silencer flange.

So a 45mm (O/D) exhaust tube system will require 45mm (I/D) silencers and 48mm (I/D) clamps.

Standard wall thickness are 1.5mm for metric tube and 1/16" (1.59mm) for imperial tube. Near as dammit most of the sizes are interchangeable so 1.75" imperial tube (1.75 x 25.4 = 44.45mm) is a swap for 45mm metric tube. 1 7/8" imperial tube (1.875 x 25.4 = 47.6mm) is a swap for 48mm metric tube etc.

CrutyRammers

13,735 posts

197 months

Wednesday 29th October 2014
quotequote all
I stand corrected smile