Exhaust Fabrication

[facthunter]

 Is your system made of stainless.  Nev? 

[skippydiesel]

 

"You are better to only flatten one side if you're going down that path."

 

I took that as a given Nev - heating to plasticity (probably cherry red) would make deformation of one side, without impacting the other, easier but I am concerned that the heat may significantly damage the integrity of the metal - what say you & others?

[onetrack]

Heating will not significantly damage the integrity of the metal, exhaust systems often glow a dull red with high power outputs.

[skippydiesel]

39 minutes ago, facthunter said:

 Is your system made of stainless.  Nev? 

Yes

[T510]

2 hours ago, skippydiesel said:

"In an exhaust pipe it's the sonic pulses which determine engine power rather than gas flow, hence length being critical, cross-section less so.

By squeezing the pipe oval where it fouls the frame, you don't change your length and you dont reduce your cross section."

 

Good point - The Rotax exhaust is 25mm. I need to improve clearance by somewhere between 10-15 mm to be sure that engine movement has been accommodated - particularly over time, should there be any "softening" of the engine mounts .

 

Questions:

• If I introduce a 10mm deep "depression" in the exhaust pipe how will this impact on a 25 mm pipe?
• What shape should the depression take - cone? - elongated flat bottomed flattening of one side? - elongated shallow/ deep /shallow?
• How to create the above dent ? - I have oxyacetylene, with which I can heat one side of the (removed) pipe and push/tap it to forme a 10 mm depression?
• Will heat to plastic stage have a negative impact on the SS pipe?
• Cold working (ball peen hammering)- always thought this negatively impacted on SS  integrity?

This is an automotive application but you will be surprised how much you can flatten an exhaust header pipe without compromising power

 

https://www.facebook.com/watch/?v=152724102049876

[skippydiesel]

1 hour ago, T510 said:

This is an automotive application but you will be surprised how much you can flatten an exhaust header pipe without compromising power

 

https://www.facebook.com/watch/?v=152724102049876

Hmm! Reassuring . I would point out that the percentage "dinged in" (flattened) to the ID of the pipes, was quite small but then of course they did lots of dings, where I will only have the one - any comments from the Brains Trust????

Edited February 6 by skippydiesel

[Thruster88]

I would go with the dent. Try squeezing your garden hose down to half diameter with your thumb, minimal flow reduction. I would form the dent in a press using 50-60mm bar stock on the same angle as the engine mount tube.

[T510]

I would try with the dent, If you are worried about it you could compare egt on the RHR and LHR cylinders before and after.

 

This would give you an indication if the dented pipe had a negative effect on that cylinder. You could even check it with an infrared thermometer during a ground run. 

[skippydiesel]

Good thinking Thruster88 & T510. 

 

I will come up with a method that hopefully results in only one side being depressed jst enough to clear the contact with a bit to spare for future changes to engine mounts.

 

Thank you all, for your constructive and very helpful suggestions - I now have a cost effective way forward,

[facthunter]

Pack it VERY tight with  dry sand before you press the dent in or it might cave in a bit in the centre. Put short wooden bungs in each end.  Nev

[Blueadventures]

Could weld a side pipe on the mount area and cut that original section out of the mount; that will give necessary clearance.  Looks like the header passes through a 'V' shape of mount piping.  Limited view in the image I saw.

[facthunter]

You could do that but it seems more than just moving an exhaust pipe less than 1/2 inch IF the ex pipe IS stainless it will need the correct  welding rod and TIG and be out of any wind when you weld it. Nev

[Thruster88]

1 hour ago, skippydiesel said:

Good thinking Thruster88 & T510. 

 

I will come up with a method that hopefully results in only one side being depressed jst enough to clear the contact with a bit to spare for future changes to engine mounts.

 

Thank you all, for your constructive and very helpful suggestions - I now have a cost effective way forward,

Be aware that denting the pipe may cause a slight bend, good in that it will give more clearance, just be careful if this will this will impact the alignment into the cylinder head or muffler.  

[Methusala]

Packing the pipe with sand will defeat any attempt to deform it inwards obviously. This method is used when pulling a bend and maintaining the round section. Ask any plumber.

[facthunter]

Plumbers don't do much pipe bending these days. Copper pipe has disappeared. You can't pack the sand hard enough to achieve what you suggest  You can depress a section without it caving in in the middle.  Heat only the bit you want to move and that's the only bit that will depress Knock it down with a bigger radius bar than the minimum and it will look good.. I've done a bit of pipe bending in my time and can't stand bends that look like $#!t

[T510]

I would be making a jig to hold the pipe so the ends can not move and then support it either side of where you plan on putting the dent.

 

I'd mark up area before removing it then find a piece of round bar stock or steel pipe about 50mm diameter and place it over the area at a 90 degree angle to the pipe centreline. Then give it a sharp hit with a decent size hammer. Creep up on it, it's always easier to dent it more than try to pull the dent out. 

 

See if you can get some similar sized exhaust pipe from your local exhaust shop or scrap metal merchant and do a couple of test hits to get a feel for the best way to do it. 

 

I asked a mate who builds custom motorcycle exhausts and he says he always bends or shapes stainless cold. He said you need to be heating it to between 450 - 800c to get it hot enough and then it compromises the corrosion resistant properties of stainless steel

[facthunter]

The exhaust should take red heat in normal operation. They make them double skinned to stop them blueing.  Nev

[turboplanner]

6 minutes ago, T510 said:

I would be making a jig to hold the pipe so the ends can not move and then support it either side of where you plan on putting the dent.

 

I'd mark up area before removing it then find a piece of round bar stock or steel pipe about 50mm diameter and place it over the area at a 90 degree angle to the pipe centreline. Then give it a sharp hit with a decent size hammer. Creep up on it, it's always easier to dent it more than try to pull the dent out. 

 

See if you can get some similar sized exhaust pipe from your local exhaust shop or scrap metal merchant and do a couple of test hits to get a feel for the best way to do it. 

 

I asked a mate who builds custom motorcycle exhausts and he says he always bends or shapes stainless cold. He said you need to be heating it to between 450 - 800c to get it hot enough and then it compromises the corrosion resistant properties of stainless steel

how practical would it be to take the pipe off and send it down to him? You'd have a hitter with the correct feel.

[T510]

1 hour ago, facthunter said:

The exhaust should take red heat in normal operation. They make them double skinned to stop them blueing.  Nev

His comment was not regarding how much heat it could take, more that there is no need to heat it to work stainless. 

 

1 hour ago, turboplanner said:

how practical would it be to take the pipe off and send it down to him? You'd have a hitter with the correct feel.

He only does bikes and he is typically booked 6 months in advance

[RFguy]

for these sort of flow velocities and volumes and viscosities of the fluid  and size of pipes, 30 maybe even 50 % of pipe dia change over a few inches is not going to be measurable.

[Methusala]

2 hours ago, facthunter said:

Plumbers don't do much pipe bending these days.

Well, I certainly don't after 45 years in the trade with 11 years teaching in TAFE. But I have done a bit of sand loaded bending and know what it entails. BTW I was able to fabricate my exhaust systems without endangering any part of the aeroplane. My point is simply that we are involved in amateur flying for recreational purposes and I am very supportive of Lizzard's posts. He has the background and depth of experience that comes from having worked through his flying. Don

[skippydiesel]

 Lizzard's posts ??????

[LoonyBob]

On 06/02/2024 at 10:45 AM, turboplanner said:

Rolled hollow section.

 

In an exhaust pipe it's the sonic pulses which determine engine power rather than gas flow, hence length being critical, cross-section less so.

By squeezing the pipe oval where it fouls the frame, you don't change your length and you dont reduce your cross section.

Not so much for a four stroke... sonic in hot exhaust gas is around 1,400 fps, so at 5,400rpm - 90 revs/sec, 45Hz - 1 wavelength will be 31.1ft; a 2ft pipe will experience ~15.56 traverses, whereas a 2ft 1" pipe will experience ~15.0 traverses. The muffler end of the pipe will reverse the sign of the shockwave, and so will the still-open exhaust valve on the cyinder end, for the second and possibly third traverse. Normally, as long as the first (inverted) return arrives while the exhaust valve is still open, it will help scavange, with a small power reward.

If you have longer engine pipes (say 6ft effective), or much higher rpm, the shockwave is less attenuated; but still the major work the shockwaves do, is help pump the gas out - until they hit the muffler!

 

In any crankcase-compression two-stroke, a returning compression wave is required, both to stuff spilled fuel-air mixture back into the cylinder, and to achieve some boost in the process. The Rotax 2-stroke zorsts are set up for snowmobiles, and are goodly at avoiding "falling off the pipe". The Ultrabats used custom exhausts, which (with pumper carbs) substantially improved the performance; they were of simple design, using basic physics. If the carb was out of tune, they did fall off the pipe quite drastically.

 

Skippy's muffler is NOT designed to benefit the gas flow or resonance in any way, and - as Nev observed - the pipes are of different length anyway.

 

The traditional way to form thin-walled steel tube (zorst) is to fill it with dry sand, and plug the ends; then heat to dull red, and gently and slowly bend it. Wander around with the flame so you don't stretch any hot spots too thin, or - if on the compression side - don't incur a wrinkle. Ifr you do get a wrinkle, you can shrink it by the normal panel-beating trick of heating a spot, then chilling it with a wet rag. 4130 is piss easy to form freehand, even without sand in smaller diameters, but prolly not great for zorsts. SS is meant to be easier than mild steel to heat form, but it doesn't conduct heat as well, so tends to hot spot.

 

Why not make up a dummy out of car bog and pvc, then hand it to your fabricator?

[turboplanner]

5 hours ago, LoonyBob said:

Skippy's muffler is NOT designed to benefit the gas flow or resonance in any way, and - as Nev observed - the pipes are of different length anyway.

Yes but you're talking a simple calculation. Rotax have a reputation for exceptional reliability with their different length pipes. I wouldn't change that tune.

[facthunter]

They are NOT tuned length. Also the way that engine is installed with little pipes everywhere a cracked exhaust pipe is not something you should encourage by a suss  repair . Weaken a section of pipe and that's where the crack will happen You don't even draw lines on them with a graphite pencil.   Nev