Billowing Hot Air: A Brief Analysis of Fluid Dynamics in Exhaust Systems

[DocWalt]

Yay physics

[VR4Rob]

Vouch for OP's knowledge. I have witnessed him watch at least 500 Star Trek episodes.

[BigTyla]

Vouch for OP's knowledge. I have witnessed him watch at least 500 Star Trek episodes.

As a matter of fact I watched some TNG on SyFy last night.

[AdamVR4]

A pipe too large will result in insufficient fluid velocity, causing inertial forces in the pipes to become less dominant over the viscous forces.

You're really just saying that the Reynolds number will go down with velocity, which in and of itself isn't a bad thing. To a point.

Pressure drop through the system is proportional to velocity^2 and friction factor, however the friction factor is inversely related to Reynolds number (and velocity), so there is indeed a point where the exhaust is too large. Have you ever taken the time to find this point given some reasonable engine conditions?

[BigTyla]

You're really just saying that the Reynolds number will go down with velocity, which in and of itself isn't a bad thing. To a point.

Pressure drop through the system is proportional to velocity^2 and friction factor, however the friction factor is inversely related to Reynolds number (and velocity), so there is indeed a point where the exhaust is too large. Have you ever taken the time to find this point given some reasonable engine conditions?

That's the point I was trying to make, I just didn't word it quite as elegantly as I should have. I should have discussed the friction factor a little more deeply as opposed to merely throwing the Darcy-Weisbach link out there.

As far as finding a sweet spot, this is something I would like to do. Really, I'd like to get into some platform-specific discussions given the comparative lack of aftermarket support for this platform when it comes to tuned exhaust products. I think we could really benefit from a set of exhaust headers proven via flow bench testing. The trick is actually getting ahold of a flow bench.

[IPD]

Vouch for OP's knowledge. I have witnessed him watch at least 500 Star Trek episodes.

STTOS: 79 episodes
STTNG: 178 episodes
STDSN: 176 episodes
STVOY: 172 episodes
STENT: 98 episodes


wow! only 203 episodes left to go!

[Permanent grin]

Just a question regarding the internal diameter and volume of an exhaust system.

Am I right in believing that we should be trying our best to match our total down pipe internal surface area to our total through pipe surface area?

For example, take a typical down pipe of say 2.5" internal diameter, it's internal surface area is 4.90 square inches. Of which we have 2, therefore the total internal surface area would be 9.80 square inches.

Where the two downpipes feed into one, should we be aiming to match this with a single pipe whose internal surface area is as close to 9.80 square inches as possible?

A 3" pipe's internal surface area is 7.07 square inches, a 3.5" pipe has an ISA of 9.62 square inches and a 4" pipe has an ISA of 12.57 square inches.

For the above calculations, I used Pi x r squared.

From the above figures, would we cause less turbulence and back pressure with the 3.5" or 4" pipe?

Would the exhaust gases have cooled sufficiently in the down pipes to mean that the 0.18 square inch difference between 9.80 and 9.62 not matter due to the gas now occupying less space due to decreased temperature?

Great write up BTW

André

[IPD]

i'd say that you're also forgetting what turbos do to an exhaust. if the equation was only for n/a's, you might be more correct. the more boost you put through the engine, the more a bigger pipe will help you to scavenge it quicker. see #7 above.

[MR2]

Would very short not so equal length headers make much difference for turbo vs considerably longer equal length?

I don't really get on with all these huge manifolds people seem to make, so much more heat in the engine bay, so harder to cover with shields etc.

[BigTyla]

Would very short not so equal length headers make much difference for turbo vs considerably longer equal length?

I don't really get on with all these huge manifolds people seem to make, so much more heat in the engine bay, so harder to cover with shields etc.

Empirical evidence shows that it makes a dramatic difference.

Equal Length vs Log Manifold Test

RESULTS: SWAPPING MANIFOLD = ~68.3 whp peak gain before tuning, ~48 whp after tuning, ~49.7 ft/lb before tuning, ~44 ft/lb after tuning

We recently conducted a complete test detailing every possible aspect of the manifold experiment. Using a Stock GSR bottom end, with a JG intake manifold and Full Race Stage 3 turbo kit with 10 psi wastegate spring, no boost controller connected.

Full Race fabricated a traditional log manifold which placed the tubine flange and turbo in the EXACT SAME spot as our ProStreet manifolds. The purpose of this is that we were able to swap manifolds without even having to pull the turbo off of the car. As a result, we used EVERYTHING the same, except for the manifold. same turbo, same wastegate, same downpipe, same exhaust, same intercooler, same oil lines, same EVERYTHING.

We videotaped the test for anyone who needs to see it. Time lapse, done by evan (eluder200k). This video is taken for 1 second every 30 seconds. For anyone who doubts us, we will be happy to post the video.

The results were quite astonishing, but do make a lot of sense. The log manifold spooled the turbo about 250-300 rpm SOONER than the equal length. From ~4700 rpm on the log manifold couldnt even come close.

The solid line is the log manifold, the dotted line is the equal length manifold.