OT - Which exhaust material
Turbo Mini Forums

Hi Everyone,

Hopefully a quick question regarding exhaust materials (Slightly off topic as not mini related).
I am having issues with our Formula Student car cracking exhaust primaries on the dyno, and we reckon it could be due to the materials we are using. We are running a KTM 510 EXC single-cylinder engine and are looking to turbocharge it once we get it sorted NA.

We currently have a mild steel primary and we have cracked two different designs of primary.
The first one was due to an EGT boss on the bend just outside of the head, but the second one cracked on a bend but nowhere near any welds.

I am wondering what is our best bet for an exhaust material, we were thinking 321 stainless, but it seems to be quite hard to get hold of. Unfortunately we don't have unlimited funds so inconel is probably out of our price range.

Cheers,
Jake



(Ignore the bandit silicone hoses these are now well clear of the exhaust)

Are the cracks radial or along the length? inside or outside of bend? Are they on the ERW seams? I assume you did the bends yourselves? How rigid are the fixings?

Edited by PhilR on 2nd Feb, 2015.

304 stainless should take care of all your issues... Just purge or solarflux it when welding.

It cracked as shown below, on the centre line of the bend, both top and bottom.
The bends were mandrel bends bought in from this place http://www.thebendshop.co.uk/Mandrel.html

The fixings were quite flexible the first time, so we stiffened them up, which we think could have contributed to the second failure.

is the exhaust able to flex as it heats up?

Yes i moved to the darkside

Instagram @jdm_brett

How well bracketed is it? Looks a lot hanging out from the picture.

In the picture above you can just about see a sheet metal support where the square flanges are (Near the lambda sensor),
but this was changed to one made from box section after the first failure, and we have changed it again so it is supported by, but not bolted too, the box section support.
The muffler is supported as well but you can't really see from these pictures.

Too far along in my opinion. Wants something near the second bend from the head that supports it. Obviously as you have alluded to it needs to support it whilst not restricting expansion.

That has more than likely cracked where the tube has been seam welded. It is always on the edge of the bend rather than inside or outside. If you look inside the bend, use probably see a small 'pip' along the bend where it has been welded.

321 stainless is bloody expensive, and needs specific rods too, and has to be back purged when welded. 304 stainless would be fine. How thick is the material you are using?

https://www.facebook.com/pages/Fusion-Fabri..._homepage_panel

www.fusionfabs.co.uk



1/4mile in 13.2sec @ 111 terminal on 15psi

those bends look expensive

We are currently using 16 gauge/1.6mm.
Any ideas of where to get 304 bends from at a reasonable price?

We also thought it might be due to the seam weld, but it cracked on the opposite side to the weld as well as on the side of the weld. We got an infra red temp gun on it today while running and it went off the scale hot, glowing red (scale ends at 650C). Will this be a problem for 304?

Cheers

Edit: tube diameter is 1.75"/44.5mm

Edited by jakejakejake1 on 4th Feb, 2015.

Do you know what grade the mild steel was? Did you notice any scoring or gouges left from the bending die? If the die was a touch too small, it may have left deep enough marks perfect for starting a crack, just where you've highlighted, on both sides.

The temperature itself shouldn't be problem for 304. When looking specifically for exhaust bends, most I found were 304 or 316.

Don't over constrain the exhaust fixings - Stainless will expand even more than mild steel so it's important to let it stretch. Try to use a fixing that has rubber at some point, and this will also help dampen out any harsh vibrations that singles create.

Edited by PhilR on 4th Feb, 2015.

Try this place, they'll make whatever angle you're after for no extra cost

http://www.custom-chrome.co.uk

http://www.jpexhausts.co.uk/product_listin...p?id=37&t=Bends

Never used them myself but there is quite a few suppliers about

Profusion have a good reputation and are a massive supplier of exhaust parts.

As phil says, have a look inside the pipe to see the quality of the bend. One of the main problems of mandrel bending mild steel is that the welding pip scores the inner die as its extracted, if it's an old die then it may of left marks inside the pipe.

We've been trying to get hold of some custom mild steel bends at work, but no one will do them, because of the inner die being damaged using mild steel.

https://www.facebook.com/pages/Fusion-Fabri..._homepage_panel

www.fusionfabs.co.uk



1/4mile in 13.2sec @ 111 terminal on 15psi