Reddit reviews Scientific Design of Exhaust and Intake Systems (Engineering and Performance)

It's worth emphasizing that this strongly ties in to camshaft design.

For more detailed reading, read The Scientific Design of Intake and Exhaust Systems

I’m no new comer to this, I’ve been building and dyno tuning bikes for a decade and a half. the issue with open pipes hurting bottom end is not up for debate, it definitely will hurt bottom end power if they aren’t matched to the characteristics of the engine, what is up for debate is the fact that back pressure helps maintain it which is simply not true.
What occurs when you remove baffles is an increase in pressure at the port due to the reversion wave not being defused.
The little washer on a bolt trick or the various other baffles aren’t there to add back pressure (even if that’s how they advertise) they scatter the reversion wave which delays it’s arrival back at the exhaust port.
The other side effect of removing baffles is a reduction in gas speed due to the increase in pipe volume, physics tells us that a slower moving fluid (exhaust gas flows which makes it a fluid) is under higher pressure. When the exhaust gas slows it will lose momentum which is required to achieve scavenge (negative pressure), scavenge is required to make power since it aids in cylinder fill.

Back pressure will, especially at low engine speeds, push exhaust gas back into the combustion chamber, this is evident on a dyno when tuning. When exhaust gas is pushed back into the chamber it displaces fresh oxygen, with less oxygen in the chamber there needs to be less fuel added or the mixture will be rich, which is exactly what the o2 data shows, the reduction in fresh air and fuel causes a loss of torque
As engine speed increases so does the gas speed which carries more momentum to keep it flowing in an outward direction, reversion waves travel at the speed of sound along the exhaust which means that it is essentially a fixed amount of time to reach the exhaust port, as engine speed increases the exhaust valve will close earlier in relation to the reversion wave allowing it to hit the port on a closed valve so it doesn’t push gas back into the combustion chamber. When reversion is timed right it aids in reducing port pressure (lowering back pressure) and increases scavenge.

This is a scientific study on exhaust and intake design showing the detrimental effects of back pressure
https://www.omicsonline.org/open-access/analysis-of-exhaust-manifold-using-computational-fluid-dynamics-fmoa-1000129.php?aid=77193

This is an article by rb racing, they have been designing some exceptionally high performance bikes and manufacturing exhaust systems for Harley’s in particular for an awful long time
https://www.rbracing-rsr.com/exhausttech.htm

This is an enlightening read on exhaust and intake design where they monitor exhaust and intake pressure at fine increments of engine rotation while testing its effects on performance, higher exhaust pressure repeatedly shows a reduction in power and fuel economy, this is performed many times over with lots of different intake and exhaust designs.
https://www.amazon.com/Scientific-Exhaust-Systems-Engineering-Performance/dp/0837603099

I’m basing my opinion off of the data of several scientists within fluid dynamics who actually understand the physics of what is happening in an exhaust system and have measured exhaust pressure and performance on precision equipment, as well as my own real work data as tested in a repeatable way on a dyno, rather than a someone who sells Harley’s.

My mum does not drive. Her pimp provides all necessary transport.

Milo tins actually detract from the vehicle performance. The GReddy decals and stick on aluminium look fuel cap covers probably contribute more to performance.


Edit: you should read this book mrscienceguy. Spearwood library used to have a copy.

http://www.amazon.com/Scientific-Exhaust-Systems-Engineering-Performance/dp/0837603099

This is the one correct answer, and well stated.

Philip Smith's Scientific Design of Exhaust and Intake Systems is also an excellent reference. It's older, but the science really hasn't changed, and you don't really need an engineering degree to get through it. The book and research were long before digital modeling, so it's all dyno experimentation using mechanical measuring systems, no CFD or modern voodoo.

The whole "back pressure is necessary for low end torque" idea is 100% bullcrap.

Scientific Design of Exhaust and Intake Systems