Decat Pipes, which way?

[LawrieS]

My decat pipes made up by a local friendly exhaust specialist were slightly too long, as witnessed by anyone following the sparks on the high peak run.

I've now received what I hope to be the correct length made from jig, i can see where the local place went wrong, they guessed at a length then flanged(?) the ends, resulting in an inch lower exhaust. These look like two seperate peices (one slightly larger in diameter) have been flanged then welded at the correct length.



Should I fit them with the wider diameter pipe at the top? Looking down looks like this, note the step in size.



Would it be beneficial to smooth out this step with dremel before fitting? Or fit smaller diameter towards top, if that makes sense.

I have engine mounts to do at the same time.

Thanks
Lawrie

[glenrobbo]

Hi Lawrie,
It begs the question: Why didn't they use single lengths of one diameter of pipe for each side? I fail to see why they've used two diameters of pipe sleeved one inside the other like that? Are they not confident of accurately measuring the correct lengths between flanges?

[LawrieS]

I see the sense in the method following the mistake from the original set, a quick offer up looks like they'll solve my problem.

However thinking about it, I agree, what you do every day, you just know.

Tthe manufacturer in question did come highly recommended by a trusted forum member

[phillpot]

Obvious..... fit one each way round

[ijbd]

ah, this is theoretically interesting.

For the section itself the answer is the wider side first.
Because expanding flow has more resistance than reducing dito.
The dameter changes where the new and old flanges meet may have the other effect.

Of course it is calculable. But no doubt also negligible.
Crane 410M, Flow of Fluids, has the formulae.
My 5th printing 1983 has this on page A-26.

regards Boudewijn

[glenrobbo]

In the early hours of this morning, I was trying to remember the gas flow equations from when I was studying thermodynamics. Unfortunately this was back in 1964/65/66, and was an impossible ask of my poor atrophied brain, and I soon nodded off to sleep.

As Boudewiyn says, the impact either way up will probably be minimal. Lawrie, I'd go with Mike's suggestion.