I've been seeing quite a few people mention that they prefer welded tubular manifolds over cast manifolds because they are less prone to cracking. Over the past 10 years, I have dealt with hundreds of manifolds and the tubular manifolds crack much more often than cast manifolds. The ratio is close to 5:1. Anyone want to clear this up for me?

 

Cast manifolds are more likely to crack if you drop them off a cliff...

 

LOL thanks for clearing that up for me. That's one aspect of turbocharging I haven't put much thought into but that makes sense.

 

i dont think i have an answer but here are some pros/cons i see in them.

casting: better process control (doesnt rely on talent of welder), exotic designs are easier to reproduce reliably, design changes can be incorporated into molds, cost is lower, metal has lower tensile strength (since it is a material that can flow into the casting)

welding: poor process control (highly dependant on welder), exotic designs complicated to fabricate, simple designs can be 'stronger' than similar casting (stronger meaning more resistant to stress cracks from weight support combined with thermal cycling), material can have higher tensile strength

which one is 'better' for our applications? i think it depends on the complexity of the design.

i am not a mechanical engineer, if i said something wrong, please point it out.

*edit* more:
i think welded tubular manifolds crack more often when they are an exotic design (trying to have equal length runners and whatnot). i think these manifolds experience harmonic vibrations with the engine vibration at certain speeds/conditions, and since they are a 'harder' material, they tend to vibrate/resonate, thus promoting failure. cast parts are 'softer' and can absorb this vibration instead.

 

Sounds about right to me falconGSR. Good job.

 

Cast Iron manifolds will survive heat much longer than a steel tube one. i think they are also better for emissions, by not absorbing as much heat. I would go with a cast(unless I had to build custom).

 

I am selling a cast DRAG3 manifold. PM me for details.
Sorry I had to jump in, maybe someone will buy it. Sorry for being a troll.

 

how much?

 

Tubular is the way to go if your seeking performance PERIOD. They tend to crack if they're not braced correctly BUT when braced correctly they will last a long time and you will be very happy with the performance gains over a cast piece.

The reason why Tubular is perferred over cast is because with a turbo manifold you want long runners (usually about 12 inches) equal length of course. Long runners create great amounts of velocity and this is what will spool your turbo. You also want stainless steel tubing. Stainless steel tubing is good for about 1~1.5 percent hp gains because it will keep the heat in the pipes where you want them.

The only con's of tubular manifolds are Cost, there usually one-offs and custom pieces so they tend to be expensive. And Design, Not everyone knows how to properly design and weld manifolds. But with a good fabicator, tubular manifolds could be mass produced with good quality just like any other header.

 

IMHO, the only advantage to going tubular is to save weight, and to get into some really exotic designs that aren't easily reproduced in mass quantities.

 

exactly, it all depends on how its designed and what materials are used to make the manifold. most manifolds that crack are made of either mild steel or 304 ss. what you want to prevent cracking is 321 ss, which is the perfered metal in high heat turbo apps.

 

How/ why does the length of the runners affect the velocity?

 

I figured exhaust gas velocities would be faster the closer to the head you get, but I figure the he knows something I don't? Can you please elaborate AE86? Thanks.

 

The air will move with more velocity in a thinner, longer runner over a shorter fat one. Same theory applies to CAI's... A shorter, fatter runner would take more air to get moving fast. Just my crazy opinion

 

Makes sense to me.

 

Exhaust velocities should be the same throughout the entire exhaust, unless the diameter changes. What goes in must come out, right?

 

I guess I didnt fully state what my thought was, what I meant was that given the same diameter piping, the velocities would be greater closer to the head. I just think you cant expect a mannifold to work better just because it has long runners......Later!