wildman10

I'm building an exhaust manifold to hold a single turbo by designing and cutting myself and using professional welders. I've had a look at the 4 main material options and would like some second opinions to help me choose (cost and other factors in the home made turbo spirit) - please will you give me the benefit of your advice?

What is it for?
I'm building an exhaust manifold for my Mazda RX8 with an RX7 FD3S 13B REW engine and single turbo (Greddy TDS07 25g, effectively a T67). None of the RX7 (FDS3) manifolds sold by various companies will fit as the available width in an RX8 is roughly half that in an RX7.

Rotary engines generally have a hotter exhaust and greater flow than piston engines.

Manifold Design
The manifold is going to be pretty simple. There are 2 ports at the bottom of one side and I've sketched in 2" runners for each going straight in to a 90 deg bend upwards, a straight run of 12"-15" and a 40 deg bend with the ends shaped to join a standard T4 twin scroll flange. I've not yet decided on the waste gate locations. The pictures show the bodge the engine builder put in for the previous owner (I'm having the 40 deg bends to accommodate my new turbo's tangential entry as opposed to the centre entry of the original turbo).


The Options I See
Stainless Steel 321 - the best material option but I can't find 90 deg bends here in the UK.
Stainless Steel 304 - easily obtainable but will it survive the life of the car?
Carbon Steel with Ceramic coating - steel easily available and easy to manipulate
Stainless Steel 304 with Ceramic coating - more expensive and harder to manipulate than steel.

Matt Cramer

I'd go with thick wall carbon steel (schedule 10 hydraulic tubing in the US - not sure what it's called in the UK). For a high heat application, I'd rather have a lot of material than better material - unless, of course, Inconel or other nickel alloys are an option.

wildman10

Matt, thanks.