Anyone made their own exhaust sections before?

bankz5152 · Sep 3, 2019

Considering making a 3" downpipe for my 110 (whether itll make any difference to standard is another question

any comments?)

Seems easy (and cheap) enough to buy the various bends, straights and flanges

Then just a case of welding it up good n proper.

discool · Sep 3, 2019

There’s a specific design involved with exhaust pipe work, and yes if design and build is correctly done then it should make a pleasing difference, if not it obviously won’t, may be no difference.

bankz5152 · Sep 3, 2019

How so?

TD5 downpipe seems pretty basic, tube with a couple bends or is there more too it than I realise?

Hicap phill · Sep 3, 2019

I’m lucky we have an exhaust maker round the corner. We take a motor & let them have the hard work.

On our racers we found pipes easy.
Get a flanged piece with that will fit on the down pipe & build from that.
No clamps but spring loaded clamps(from 1pipe to the other) & olives due to movement.
Don’t go bigger than 3” as you loose back pressure. Also we only ran a centre box.

Here’s a good place we’ve used for race items

https://www.merlinmotorsport.co.uk/s/exhausts/jetex-performance-exhaust-systems-parts

Nodge68 · Sep 5, 2019

bankz5152 said:
Considering making a 3" downpipe for my 110 (whether itll make any difference to standard is another question any comments?)

Increasing the size of the down pipe isn't going to be worth the effort.
What are you trying to achieve?

Hicap phill said:
Don’t go bigger than 3” as you loose back pressure.

You don't want back pressure in an exhaust. However you do want gas speed, which is reduced as the pipe gets larger.

flat · Sep 5, 2019

I thought back pressure was less of a concern on a turbo diesel and only an issue on a naturally aspirated motor (v8 etc?)

marjon · Sep 5, 2019

Exhaust design is complicated.
Chuck a turbo in and it actually becomes easy

before turbo you need velocity in the pipe to spin the turbo,
I.E small (ish) size, after that fill your boots and get rid it does no good

N/A is a different thing and as complicated as a 2stroke pipe

Yamaha made organ pipes before bikes

J

Nodge68 · Sep 5, 2019

flat said:
back pressure was less of a concern on a turbo diesel

A free flowing exhaust is best, turbo or NA. On a turbo, the greater the pressure difference between across the exhaust turbine, the faster it'll spool up the compressor wheel, so the boost will come on earlier.

flat said:
only an issue on a naturally aspirated motor (v8 etc?)

A 4 stroke NA engine doesn't want back pressure in the exhaust either. But a free flowing exhaust helps "pull" fresh air in through the intake.
Back pressure in the exhaust, reduces the amount of fresh air that the engine can draw on its intake, and increase pumping losses, as the pistons have to push the exhaust out down the pipe, which uses power.

Only a 2 stroke engine needs back pressure in the exhaust, and only a very carefully controlled amount.

Nodge68 · Sep 5, 2019

marjon said:
Exhaust design is complicated.

This ^^^^

marjon · Sep 5, 2019

Nodge68 said:
A 4 stroke NA engine doesn't want back pressure in the exhaust either. But a free flowing exhaust helps "pull" fresh air in through the intake.
Back pressure in the exhaust, reduces the amount of fresh air that the engine can draw on its intake, and increase pumping losses, as the pistons have to push the exhaust out down the pipe, which uses power.

Only a 2 stroke engine needs back pressure in the exhaust, and only a very carefully controlled amount.

I would disagree with this.
Tuned length headers scavenge exhaust out but also bounce back to stop intake charge coming out on the overlap.
Engine drive characteristic can be changed with size and length.
Same as a 2 stroke.
J

DanClarke · Sep 5, 2019

Just a memory from the 90's but.....
A mate of mine had a "Company Car" a Rover 216.
The exhaust fell off (at the Manifold to downpipe joint) while going along a motorway and the engine refused to run thereafter.
No turbo, Normally aspirated and definitely in need of back-pressure. True Story.
Low-loader trip to a garage for repairs. British Leyland ROVER quality assembly at its finest.

Nodge68 · Sep 5, 2019

marjon said:
I would disagree with this.
Tuned length headers scavenge exhaust out but also bounce back to stop intake charge coming out on the overlap.

Manifold length tuning is an exact science, and so much more complicated than simply slapping some random pipe on the engine. However once the exhaust has got passed the manifold and all the exhaust is in the pipe, then back pressure will cost engine torque.

marjon said:
Engine drive characteristic can be changed with size and length.

Don't forget that manifold tuning only works at certain points in the rev range.

DanClarke said:
No turbo, Normally aspirated and definitely in need of back-pressure.

Anything that happens to the exhaust, is reflected in the intake system. So it's very likely that the engine was calibrated to run well with a restricted system, however when the engine was freed of its restricted exhaust, the calibration went off and the engine ran lean.

David Vizzard did extensive research on this and many other subjects of car engine tuning throughout the 70s, 80s and 90s.

flat · Sep 5, 2019

Interesting stuff thanks nodge

marjon · Sep 5, 2019

Nodge68 said:
Manifold length tuning is an exact science, and so much more complicated than simply slapping some random pipe

I didn’t say just slap any old size on. It needs calculating for engine/customer wants.
I have run different sized header pipes on my bike and can see a difference in sizes.

Nodge68 said:
However once the exhaust has got passed the manifold and all the exhaust is in the pipe, then back pressure will cost engine torque.

Agreed, after the tuned point you want free flow as it will effect the tuned length characteristics, maybe silencing for regs.

Nodge68 said:
Don't forget that manifold tuning only works at certain points in the rev range.

Yes and at that point it takes off, “where would you like your power sir” it’s all tuned for the engine, but there are best points in the rev range of any engine. Real pipe tuning is only beneficial in a fairly small range.

Nodge68 said:
Anything that happens to the exhaust, is reflected in the intake system

Now we can really get into NA and tuned length with variable intake lengths

J

Nodge68 · Sep 5, 2019

marjon said:
Now we can really get into NA and tuned length with variable intake lengths

Rover did, on the KV6 engine. That used variable length intake runners (called VIS), to increase both low and high Rpm torque. It makes a huge difference too, very noticeable when driving it. It's got lots of low Rpm torque, but plenty of grunt up the top of the rev range too. However out the factory, the exhaust manifolds were truly hideous, it's a wonder it made the power it did.
They were literally 3 pipes squished together about a foot from the head face. It's difficult to conceive a worse design.
Front manifold

Front to rear collector.

Primary collector.

Dumping these and fitting decent length tuned manifolds, instantly gives an extra 15 Ftlb of torque, without any other mods to the engine. With a decent filter and cold air pickup and a re-map the KV6 will happily see an extra 30 Ftlb at 6K Rpm.

marjon · Sep 5, 2019

Those exhausts are atrocious.

Nodge68 said:
Rover did, on the KV6 engine. That used variable length intake runners (called VIS),

Be interested to see how they did that properly, or they were fooling the system? ( I don’t know)

J

Edit. Ok got my goggles on and had a look around. Ok they have tried hard in some places.

marjon · Sep 5, 2019

@Nodge68

Just thought I would leave this here for you to enjoy/mull over you might actually enjoy the read, on the real way to do it, discuss.

https://jalopnik.com/mazdas-all-conquering-700-horsepower-four-rotor-le-mans-1825651052

Loads more around but this came up first.
J

bankz5152 · Sep 7, 2019

So from reading on here plus speaking to a few people in the know I should get some benifit from a 3"downpipe and considering id be making it myself its cost effective too.

Just needs as fewer bends as possible.

Idea being to make a sort of timber template then buy the appropriate bends/lengths.

Use a standard flange at the turbo end which would open up from standard 2.5" to 3" down to another standard flange for the mid box (2.75" i think)

Should only cost about £60 in materials

marjon · Sep 7, 2019

@bankz5152
Yep after a turbo get rid of it simple

It does no good after.
Consider the 2.75 mid box you may change the reverse pulse at that point

will it make a diff you see

probably not.
From 2.5 to 2.75 the rest is probably good enough. Or if you really want 3” then consider a different mid box.

But at the end of the day you probably won’t see much change on your engine anyway.

Making an engine breath better is only beneficial if you feed it better.

.

Just read the first 2 lines the rest is rambling

J

Anyone made their own exhaust sections before?

bankz5152

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