[kilogram]

Originally Posted by kern417

Every piece of the system has an effect. I just use illustrations with the manifold because he said that scavenging isn't important for turbo applications. Everything has to do with exhaust gas velocity, including the downpipe.

Skyhigh is right though. The best exhaust post-turbine is no exhaust. Velocity is the enemy here; don't take my word for it, look at the equations for head loss in a pipe (the math supports the practical data) in any fluids textbook. There is no scavenging effect post turbine because the turbine kills the pressure pulses. Scavenging is crucial in front of the turbine, but it all goes out the window after it.

Higher velocity means higher head loss along the length of the pipe, all other variables being equal. Flow is driven by pressure differential: the more head loss through the pipe, the more flow you lose.

The real kicker is this: It also increases the pressure at the outlet of the turbine, since ambient pressure in this case is fixed. Turbine work is defined as the change in enthalpy across the turbine, and higher pressure at the turbine outlet means less deltaE. This affects spool. Lower head at the turbine outlet means more turbine work for the same flow rate.

There's a point of diminishing returns on size though, which is why you're right about the 5in exhausts being ridiculous. Look at the BMW/Megatron F1/GTP motor from the mid-80s. It's got a ~3in exhaust and moved upwards of 1400bhp worth of air through it just fine.

[Supr3me]

Originally Posted by kilogram

The best exhaust post-turbine is no exhaust.

I'm sold.
BRB, disconnecting my downpipe

[aadum]

Originally Posted by OzBMR

Interesting idea, although the second O2 sensor is between the two monoliths?

not sure....someone would have to take a risk and open it up

[OzBMR]

After checking out all the options I've ordered a CG Precision DS-1 catted downpipe as it has the same 5" cat diameter, but a single 200 CPI race cat, so guaranteed to flow a lot better than the stock DP. I also really like the design, purged/passivated welds with no annealing colors ensuring more durability.

I'll be posting up datalogs from before and after vs the stock DP. I hope to have it by the end of next week and installed ASAP. Now that I'm running higher boost on top of the MHD Stage 2 E30 OTS map and E34 fuel some additional flow will benefit.

Thanks Mike at Extreme Power House for sorting it out so quickly for me

[OzBMR]

Got the CG Precision DS-1 down pipe delivered today and ... they sent me the wrong pipe

It looks like someone's old pipe that must have been sent in for warranty repairs as it had a new catalytic converter, but the pipe itself looks like it's been on a car for a few years and 30K+ miles of wear and tear.

Sending it back and expect it'll be several more weeks before I get the replacement.

[eljefe30]

Does anyone know the weight of the stock b58 downpipe? I know it's not a big deal, but I'm still curious...

[Skyhigh]

Originally Posted by eljefe30

Does anyone know the weight of the stock b58 downpipe? I know it's not a big deal, but I'm still curious...

Here you are
Above - stock.
Below - Akrapovic (note they declare lower weight which is wrong)

[IMG]

View post on imgur.com

[/IMG]

[eljefe30]

Originally Posted by Skyhigh

Here you are
Above - stock.
Below - Akrapovic (note they declare lower weight which is wrong)

[IMG]

View post on imgur.com

[/IMG]

Thanks for the info skyhigh!