01: Exhaust Tech, an intro

Alright, let’s talk shop! 

Exhaust manifold design in a naturally aspirated engine drag-race application. (200-300 BHP)

As you might’ve gathered from, hopefully, looking around on this website, we spend a great deal of time understanding the why-part of engineering. 

During one of those endeavours, analysing the exhaust manifolds of F1 powertrains, we realised a number of things, which are to be considered when looking to upgrade your exhaust system. 

The first thing is: what is your goal?

Is it track-racing, drag-racing, sport (street) and so on? A specific power goal?

 Each of these goal requires different engineering approaches, when going for the optimum. Obviously personal wishes in terms of aesthetics are to be factored in, although this might not be optimal in terms of engineering…

         It is your car, your dream. 😉

 Assume we’re building a drag car, normally aspirated, 200-300 BHP. What would our first considerations be?

RPM and Peak Torque would be two very important considerations. The reason being that the vehicle will be used in a drag-racing environment, short (sub 15 seconds) 402 meter runs. So you want to get the most possible VE (Volumetric Efficiency).

 Basically higher VE means more trapped air and fuel mass, thus: more power. (with power being a function of torque and rpm)

 One way to increase the VE is by means of using an exhaust manifold with, so-called, “tuned-length” runners. The length of these runners is based on the harmonic effects, pulse-waves, within the exhaust gases being expelled from the cylinder by the opening and closing of the exhaust valve(s).

 Showing in the images to the right is a set of, so-called, Schlieren photographs, showing a pulse wave (P-wave) exiting a pipe.

 
  
What isn’t shown is the reflection of the P-wave moving back into the pipe. Because of these reflections one wants to have “tuned length” runners. Ideally you want to time the reflected P-waves in order to have a relatively lower pressure when the exhaust valve open. This would increase the pressure ratio across the valve at the time of opening, which in turn helps “extract” more residual gases from the cylinder, post-combustion obviously. In doing so you increase the VE because the amount of fresh charge entering the cylinder is increased and the residual gas is reduced.

 More fresh charge (fresh air-fuel mix) = more power!!!

 Things to be mindful of:

Tuned length runners don’t work all over the rev range, hence why it influences where in the RPM-range peak-torque occurs.

The diameter of the exhaust manifold runners determines how much mass flow rate can pass through and in turn determines at what power level the exhaust becomes more of a restriction than an improvement. The term “choke” applies here.

More on exhaust tech-topics next time!

We’ll be looking at things, such as:

  • Scavenging
  • Choke
  • Different engine configuration

VPR-Engineering