Stainless Exhaust Made From Pre-Bent Pipes

malcolm · Apr 4, 2010

I'm slowly building an exhaust for my Renault from pre-bent pipes. Wouldn't have posted anything on the forum but Si seemed very keen to see some photos, and it could be of use to others who have accidentally fitted the wrong engine to their car.

The bits

Impossible to find at first, but you guys helped. Here's a pic of the bits:

The front bit

I started a bit late in the weekend and have since been inundated with visitors. I'm going from front to rear. Just cutting bits and tacking them together at the moment. The front bit (weird shape as the exhaust is in the wheelarch and I need to clear the wheel) is tacked together. I'm nearly there at the back - should have everything at least tacked up tomorrow.

Here's a pic (the Jetex bends are quite handy - mandrel bent and have sleeves that fit over the next pipe if you fancy a joint. I'll mostly be butt welding and the sleeves have been cut off, but there will be at least 2 joints in the system):

More tomorrow.

gavuk · Apr 4, 2010

Looking good Malc :clapping:

,did you get the box from a different place to the jetex pipes ?....do you mind sharing how much it all cost?

malcolm · Apr 4, 2010

The silencer is from Birchills Automotive (pipes were from Merlin Motorsport).

I think the silencer was about £75 inc VAT (it was over the phone and I can't find where I've written it down). They make them to order next day and offer a choice of pipe diameter, silencer diameter, length, and pipe position. It's a weld it on yourself thing - the perf pipe is supplied loose in the silencer, but that's handy for me as I want to push the solid pipe that I'll join on some way into the silencer to reduce noise.

I think all the pipes were about £120 inc VAT which seemed a little pricy.

gavuk · Apr 4, 2010

Was there not a jetex supplier closer to you? i had assumed there would be ,i qoute Merlin because they are local to me ....you didn't like any of the the jetex boxes?

malcolm · Apr 4, 2010

The Jetex boxes were all a bit small (boy racer high noise stuff). As a rule of thumb the silencer area ought to be at least 6 times the pipe area, but I just went for the biggest silencer that would reasonably fit the space, 13 times pipe area. Allows me some margin to adjust the distance the pipes go into the silencer to reduce noise further.

There is a local Jetex supplier. Not sure what their prices are like. Bends seemed fair, but the straight pipe bumped up the price. zx55 are way cheaper, but don't stock 1.75 inch pipe.

gavuk · Apr 4, 2010

malcolm said:
As a rule of thumb the silencer area ought to be at least 6 times the pipe area, but I just went for the biggest silencer that would reasonably fit the space, 13 times pipe area. Allows me some margin to adjust the distance the pipes go into the silencer to reduce noise further.

Good info cheers Malc

malcolm · Apr 4, 2010

Are you building an exhaust? I scribbled a few notes down for another site:

I can't resist mentioning something about silencer design. It can get a bit involved if you get into the maths, so here are Frederick the Frog's top tips on silencer design:

The best place for a silencer is at the back of the car (that's because when the air cools both the flow rate and the speed of sound reduce, so the silencer can be shorter and more restrictive without affecting noise or performance).

A rule of thumb is the silencer area should be at least 6 times the pipe area. That's just a rule of thumb, but much less isn't really all that effective for silencing, and adding a lot more silencer gives decreasing returns. I'm assuming someone worked out a swet spot.

Longer silencers will reduce lower frequency noise more than short silencers. Lower frequency stuff would be the booming noise from the tailpipe while cruising on the motorway. Standard silencers tend to be tuned to a sensible length to deal with that sort of thing.

A simple silencer would just be a change in section - small pipe -> big expansion chamber -> small pipe and relies on expansion and reflection to reduce noise. A straight through silencer (with perforated tube and glass stuffing) works in a similar way, and whole it is less effective for lower frequency noise the glass stuffing takes out the higher frequency noise.

Silencers reduce noise by expansion but mostly by reflecting noise waves backwards so they cancel themselves out. That only works at some frequencies, but extending the ends of solid pipe into the silencer can help fill in the gaps and filter out some other frequencies. Sticking the entrance pipe in by 30mm and the outlet pipe in by 70mm will likely result in a quieter silencer than having the solid pipe end at the entrance to the silencer.

Tailpipe length and diameter make a lot of difference. If noise reduction is important the tailpipe should be at least 250mm long (including the solid pipe within the silencer) and should not exceed the diameter of the rest of the pipe in the system. A short large diameter tailpipe like the boy racers have is a waste of weight - the same silencing could be had with a sensible tailpipe and a smaller silencer.

Don't assume exhaust manufacturers know anything about the acoustics of exhausts. The science tends to done by the car manufacturers, and aftermarket exhausts are generally developed by trial and error.

I had a program that did the maths, but it is now defunct (something about licencing). Pity as it was very effective.

Welderpaul · Apr 4, 2010

Mig or Tig Malc?

malcolm · Apr 4, 2010

I was feeling impoverished last year and returned the TIG gas, so it's going to be MIG. Also I'm planning to be naughty and use argoshield light which has too much CO2 (2% CO2 is best and I've got 5%). So it's a bodge. I do plan to purge a bit.

gavuk · Apr 4, 2010

The mid section on my car has a hole so i was considering putting on a cheapy (if i can find one )and using the old mid as a pattern for a stainless replacement ,,,,so yes :laughing:

.......i do wonder if purging is overkill on an exhaust though?....great for food industry but would an exhaust last 10+years if you didn't? seen vac pipes and process pipes which haven't been purged put in service but i have not seen any fail yet ?

piman · Apr 5, 2010

Hello Malcolm,

I notice the straight bits are swaged, which you said you would cut and weld? Most decent motor factors will get basic straight pipe, at least in my area, which could have reduced the cost?

Alec

malcolm · Apr 5, 2010

My local motor factor can only do mild steel pipe. But the swages came in very handy for trial fitting bits of pipe together to figure out angles. I've left 3 swages in there for joints.

Not a lot of progress today. The old exhaust (32mm) fitted neatly between the anti-roll and the torsion bar at the rear. The anti-roll bar bends down to form a handy gap. The new exhaust is 47mm and looked a little tight so I routed it around. You can see where it would have been routed in the photo.

Lot of time wasted trying to figure out how to get the silencer in - it's an inch bigger than the original in every dimension.

piman · Apr 5, 2010

Hello Malcolm,

my mistake, I wasn't thinking stainless.

I see your axle stand has lost it's pin? :-)

Alec

Robotstar5 · Apr 5, 2010

piman said:
I see your axle stand has lost it's pin? :-)

No, that's an exhaust stand, the axle stand is holding up the silencer behind :whistle:

Rick · Apr 5, 2010

Nice work, its amazing how hard it is to get an exhaust to line up nice, 1mm of movement at the front makes it 20mm out at the back!

I don't know if its any help to any1 in the future, but we make any size silencer pretty much, ovals and round. All made from 316 stainless steel, and cheaper than the above places

gavuk · Apr 5, 2010

Hi Rick I may be interested soon ,for my old golf cab ,don't want anything too loud with roof down though?...do you work off sizes only or by manufacturer/model ?....can you supply bends and pipe also? many thanks

malcolm · Apr 11, 2010

I've just about finished the exhaust - took a lot longer than I had imagined it would, but partly that's because I changed my mind on routing about 3 times.

Here's the completed exhaust (just needs hangers). The little brace is a feature found on the originals - it's where the handbrake cable runs a bit close to the exhaust and the brace holds it off a little.

The welds were a pain - I was trying not to get excessive penetration so was using quite a low voltage setting, but that made for tall stop starts using the 0.8mm wire. Obvious solution would have been to up the voltage and travel faster, but it's pipe and fiddly and slow is easier. I'll try TIG next time.

shenion · Apr 11, 2010

Nice work Malcolm.

Some nice welds that PortaMIG is doing.

Those sway bars look like they are just intentionally bent to be in the way.

TIG would have been nice, but a lot more work. With all that fitting, spending more time welding something that won't be seen is not worth much to me. I do sometimes fire up the TIG to flow over some cold starts on MIG.

I say those cable-tie hangers will last about 3-minutes

Will need to redo my Jeep again as need to replace clutch and still have the cat the idiot welded onto the bolt flange. Need to cut off to get tranny out.

malcolm · Apr 11, 2010

The sway bar (anti-roll bar) was only fitted to a small proportion of cars, and they bent them to clear the standard exhaust. The new exhaust is about 15mm wider and the clearance was too tight.

I wasn't happy with the welds. Operator error was the main thing - I'm rubbish at pipes and the last one in the photo was a lot better than the first, but I didn't get on with using 0.8mm wire on 1.5mm pipe. I would have been much happier with 0.6mm.

Good point - might need to re-engineer the cable tie exhaust mounts. :laughing:

MG David · Apr 12, 2010

Did the perf pipe in slip into the solid pipe or did you have to but weld it?
When you say 6 times area I assume it is silencer cross section area versus pipe cross section area that you are tlking about. I that correct?

Are you going to use modern rubber band type mounts or traditional cotton real type mounts? Do the rubber bands give better isolation?

Stainless Exhaust Made From Pre-Bent Pipes

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