Side running exhausts

[PatClarke]

Originally posted by icireland:
I've noticed a few teams running side exhausts with the muffler exit pointed directly into the oncoming air flow (i.e. in the direction the car is travelling).

It is my understanding that you want as little resistance as possible for your exhaust flow, so wouldn't this hinder it? Or is there some kind of underlying witchcraft going on that makes this preferable?

As a new Forum member, you should introduce yourself and identify your team.

As for your query, think for a moment about the pressure in the exhaust gas flow Vs the air pressure when the car is moving forward, even at top speed.
Realistically, there is no real restriction to gas flow, even if a forward facing exhaust doesn't look too intuitive.

PC

[JWard]

A colleague did a hand calc at the time and it came out in the tens of mBar. Anyone care to stump up some numbers?

[Simon Dingle]

Four-Stoke Performance Tuning (3rd Ed.) by A. Graham Bell has a table on page 24 with pressure rise against vehicle speed. This table lists 100kph as having a 5mbar pressure rise.

He was looking at pressure rises in an inlet manifold due to ram air, but I don't see why the numbers wouldn't be roughly applicable to a forward facing exhaust.

[Z]

Dynamic pressure (= stagnation pressure) = half-rho-V-squared.

So if,
Rho (air density) = ~1.2 kg/m^3,
Velocity (say ~max for FSAE) = 30 m/s = 108 kph,
then
Pressure = 0.5 x 1.2 x 30 x 30 = 540 Pa.

And since
1 Bar = ~100,000 Pa,
the stagnation pressure is about 5 mBar (or 1/200th of atmospheric pressure),
as before.

Z

[JWard]

Wait, I remember, he made the joke to a doubting Thomas that if it were a ram air exhaust (sidepod shroud for the fwd facing exhaust) it was in the tens of mBar.