Exhaust System Flow Capacity 1

[140Airpower]

Just a question. Is there a rule of thumb for sizing exhaust systems for passenger cars? I assume the main concern is the avoidance of condensation by keeping velocities up. Second would be low back pressure.

 

[patprimmer]

Thank you.

Most helpful.

Confirms a few thoughts and creates a few ideas.

Regards
Pat
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[SomptingGuy]

It is rumoured that some of the tier 1 guys are suggesting a DPF operates as a noise reduction device, good enough to eliminate an upstream muffler.

- Steve

 

[Tmoose]

"250 mbar at peak power" ~ 3.6 psi

I seem to recall David Vizard was suggesting if the back pressure measured at a pipe wall bung was 2.5 psi or less, leave it alone.

Dan T

 

[Marquis]

TMoose I know Vizard, he's concerned mainly with tuned aftermarket cars.

My figures are vehicles that meet emissions, complete with catalytic converters and meeting drive by noise regulations

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[140Airpower]

Tmoose, Vizard also said exhaust back pressure is about 1/25th as important as intake pressure drop. That also is in rough consistency with these figures.

 

[SomptingGuy]

Intake pressure drop starves power.
Exhaust back pressure melts pistons.


- Steve

 

[patprimmer]

And melted pistons kinda kill power also as well as ruining your day.

Regards
Pat
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[Tmoose]

Hi Marquis,

I guess I should have said I was offering David Vizard's number as a number I recalled, maybe incorrectly, just as another, interesting (to me, anyway) data point.
(Does he still sport the hairdo he wore in "How to build horsepower". For a while I thought he and Clive Tricky were the same person)

I'm hoping to persuade/convince an impatient young engineer friend to weld a few bungs onto his old Corvette's exhaust before re-sizing the exhaust system for his passenger car.
The high performance values you provided will be quite useful.

thanks

Dan T

 

[Marquis]

Tmoose I spoke to him a few times on the phone.
I didn't ask about his hairstyle ;-)

I think he lives in NC.

I think for your own project his figures look good and low, the lower you can go the better.
Back pressure isn't really good for anything- although some folks get confused and talk about BP being better for low speed tuning- what they probably mean is that smaller diameter pipes benefit low speed tuning which just happens to have higher back pressure unfortunately.

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[Lou Scannon]

Recalling from my heavy duty engine days,... 4"Hg I think was the maximum allowed, typically for an automotive heavy duty application (e.g. full size bus, Class 7 & 8 truck). So, 2psig, or 14 kPa.
Stationary engines typically have even lower backpressure requirements. One application comes to mind, IIRC, in the vicinity of 10" H2O. This is due to the free-floating turbocharger - wastegates are not accepted in the mainstream powergen market.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[140Airpower]

hemi, A wastegate is needed for installations where boost is required at the bottom of an engine's wide operational range. An engine optimized to run in a narrow range can have a turbo sized correctly for that operating range. It can be much more efficient. 10"H2O exhaust back pressure is impressive. What ratio of intake/exhaust pressures are common?

 

[Lou Scannon]

The ratio I don't recall off the top of my head, but it is in favour of intake for stationary engines. For automotive engines, it is in favour of the intake in a range surrounding peak torque, before too much energy is being "wasted" across the wastegate. I even see this on my own daily driver, during the latter part of spoolup, before the wastegate gets involved.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz