I talked to Chuck from rotaryextreme and inquired about a titanium downpipe. He informed me that a titanium downpipe couldn't withstand the heat like a stainless steel downpipe could. I always thought titanium had a high melting point and good heat bearing characteristics, but I'm no metallurgist.

 

I thought they used titanium because of the weight advantage

 

Yeah, that's why I want a downpipe in titanium. I'm just saying, I thought it could take the heat but I guess I was wrong.

 

See I thought the same thing. I still think that titanium was supposed to be lighter and stronger with higher heat capabilities. Otherwise why would they make full titanium exhaust systems if it couldn't hold up to heat?? Doesn't make sense to me. If I'm wrong then please someon come straighten this out.

 

titanium can stand the heat and its lighter... but i can understand why they wouldn't use it on a downpipe because of thermal expansion... if you want the long winded engineering version, ask me, i'll copy it out of my text books, if not, it basically comes down to this. we all know metal expands when it gets hotter. Stainless steel expands at one rate, aluminum at another, and so on... the problem with titanium is that it expands probably about 10-15 times more than steel and "could" create some seal problems. i think the issue is that with the turbos, there is ALOT more heat generated in that region of the plumbing.

 

Ok, good enough for me.

 

ok, now that makes a lot more sense. Thanks pugg57!

 

Plus it's a PITA to weld.

 

The Z06 uses titanium, but I don't know how far up it goes. I doubt the headers are, but from the cat and back I believe it is. Only car that I know of that does.

 

This is not true. Why does everyone think this?

Generally speaking, there are three types of titanium alloys: Commercially pure (98-99% Ti), alpha-alloys (no more than 7% Al, and a very small amount of oxygen, carbon, or nitrogen), and alpha-beta-alloys (which also contain varying amounts of beta constituents, such as chromium).

Only alloys that contain a large amount of the beta phase are not easily welded. But a majority of titanium available for commercial use are either CP, or Alpha (which contain NO beta particles).

As long as you choose an appropriate welding wire, welding is easily done on most titanium alloys (usually TIG or plasma).

Rob

 

A SS downpipe only weighs 7 lbs. Why spend so much more money to save another 3 lbs? What a waste of cash.

 

How much more is the titanium? I didn't see a price..

 

Exactly. If you are so concerned about it, there are other, more effective ways of saving some weight.

 

With 1500+ degree F EGT's (exhaust gas temperature) on my Formula Continental racecar, I can't run titanium primaries because they would degrade from the heat and fall apart. Mild steel has the same problem. Therefore, I run stainless.
The rotary has similar high EGT's, and similar problems would occur.

 

EXACTLY, look at what burn stainless had to say.

"Titanium is not recommended for high temperature applications
( > 1600º sustained ) like high pressure turbocharger or rotary engine exhausts."

 

Would a midpipe even be ok or not?

Ty

 

Compared to Aluminum, Titanium has MUCH Better heat/strength characteristics, which is why it is used for High speed (mach 3 +) Aircraft skins. It is roughly equal to Mild steel (though its coeficient of expansion is about 10 times steel's, and much greater even than aluminum's) but it is not as good as Stainless Steel, or Inconel, (the Mach 6 capable X-15 was constructed primarily of Inconel) in the Heat vs Strength department.

 

A Stainless Steel downpipe would hold in the heat better than a Titanium downpipe (= cooler underhood temps). 321 SS or Inconel (best) is even better at holding in the heat, is more heat resistant, and stronger than 304 SS which most RX-7 downpipes are made from.

 

Lots of good info in this thread. Thanks everyone

 

If I had a part in stainless, and got a price on a titanium version, I'd budget 2x the money b/c of raw material cost. Plus add in the fact the market for a titanium version will be very small, so the production will be very limited and the cost goes up more.

I agree - even if someone bothered to deal with the varying expansion of manifold and titanium pipe, you have cost of 2-3x more than SS.

For me, I'll save weight by taking a dump before every time I race and save $600. That's like $300/turd.

Dave

 

And a PITA to bend, too.

 

Yeah... you could be spending that money on a carbon fiber hood where it would really make a difference...

 

Stainless steel is the best exhaust material. If you have money burning a hole in your pocket, use Iconel.

Titanium is not the "silver bullet" for this. Yes it is light and strong. However it is much more difficult to form, weld and drill. Its heat expansion is also a problem.

They say the SR-71 was over 18" longer at speed than at rest on the ground. The airframe expanded that much...

 

i've also heard that fuel leaks out of it like a sieve at rest because it's designed to have all the joints seal up at speed.

 

Fuel leaks out of it like a sieve when it's at rest because they could never invent a fuel tank sealant that was both resistant to the special JP-7 fuel and that would stay sealed at the temperature and expansion extremes the airframe saw. Therefore they chose a sealant that stayed sealed at high temps (at speed) but not at lower temps (parked or slow cruise)

Standard procedure was to put just enough fuel in the plane for takeoff and rendezvous with a tanker. Then it would top up and speed off. The tanker guys say you knew the plane was full when you saw fuel literally spill out of the top of the airframe and wings

If you look closely most pics of the plane at any speed other than supersonic show the fuel just raining out of it.