And in the interest of best flow, you want a metal that retains heat. Stainless steel is better than aluminized steel, not sure what the heat transfer rate of titanium is. I will have to check.

I found it - the heat transfer rate of titanium is 50% better than 316 stainless steel.

So all things being equal (wall thickness and pipe diameter), a titanium exhaust will have slower flow rate but lower weight than a stainless steel exhaust. (Thx MacGyver for pointing out I had it reversed).

 

If Ti has a better heat transfer rate, then the exhaust gases cool down more before they exit the pipe. Cooler gases flow more slowly, so it seems like Ti would work against you (in minor amounts).

 

It's my understanding that Ti gives you a substantial weight advantage, but no advantage in gas escape velocity.

Regarding the singles vs duals, it would depend greatly on which vs which.

I agree with Road Rage, great responses guys.

 

[QUOTE]Originally posted by MacGyver
If Ti has a better heat transfer rate, then the exhaust gases cool down more before they exit the pipe.

 

[QUOTE]Originally posted by Road Rage
I have to say, the responses in this thread have been some of the most thoughtful and accurate I have ever seen on a web Forum. I think it speaks well of the posters, and this Forum. Nice job, gentlemen!

Some BMW's have an electrically controlled exhaust valve in the muffler.

 

I found this about the M62 engine:
Low-back-pressure exhaust system

Exiting the engine through double-wall stainless-steel exhaust headers (as on the M62 engine), exhaust gases then flow through one catalytic converter per cylinder bank. There are four oxygen sensors: two ahead of the converters, two behind. A pressure-equalizer passage connects the two exhaust streams behind the converters, enhancing low-speed torque and contributing to the engine's wonderful exhaust sound. Aft of the catalytic converters, the full dual exhaust system includes two mufflers and two resonators and ends in four glistening stainless-steel outlets.

 

Exhaust back pressure is bad period. If anyone tells you otherwise, they're preaching a myth which has been proven incorrect like how the world isn't flat. The engine is basically a huge air pump, more air in = more power made. How can more power be made if the exhaust is seeing restriction?

Exhaust manifold/header design if made efficiently can help the motor pull in more of a fresh intake charge by scavanging spent gases from adjacent cylinders. This has nothing to do directly with back pressure at all.

People have told me that the engine requires a load (like backpressure) to generate power. My beef with that is where the source of the primary load is... it ISN'T the exhaust system, it's the transmission which applies the car's weight and such. Moving 2800+ pounds is a much larger load than the backpressure some may think is required on top of that.

 

For WOT and peak horsepower yes, but if you want a wide powerband and driveability than it can be a good thing.

[QUOTE]Originally posted by Hyper-X

[B]Exhaust manifold/header design if made efficiently can help the motor pull in more of a fresh intake charge by scavanging spent gases from adjacent cylinders.

 

[QUOTE]Originally posted by l8brakr

 

I am not saying we need backpressure for the motor to run. But for most street driven vehicles a little is not a bad thing. I also doubt that anyone has open headers shooting out of the hood on their car because by your logic this will give you the most power therefore equate to the best exhaust system. I'm glad that we agree scavenging is something good and should be utilized but what you don't get is that if you put a 4" exhaust system on your car it will have to rev to the stratosphere before it could fill the amount of volume in the pipe for scavenging to become efficient.

After rereading some of this thread we may be using a different scale on which to measure backpressure (like celsius vs. fahrenheit). I am using wide open header as zero and you are using a decent flowing full system as zero. I will never advocate stuffing bananas in your tail pipe to make more torque (a la Beverly Hills Cop) and I'm sure you won't put a diesel exhaust on you car either.