Depending on the application, which is the driving factor in determining the design strategy, we now have several options available to us. So why are there so many “off-the-shelf” exhaust manifolds available, claiming to be the best?
No idea… As there isn’t one manifold to rule them all, forged in the fires of Mount Doom.
What we would consider is the following:
Is the car supposed to be primarily used on the street, track, strip, as a fun daily or the occasional ring-tool?
Each justifies a different design strategy, for example:
As a fun daily you want a fast responsive car, so fast turbo spool-up is something to be prioritised or in an N/A application a relatively wide torque band.
These two manifolds couldn’t be more different!!! As for the turbo manifold we would likely prioritise low exhaust manifold volume in order to present as much thermal and gas kinetic energy to the turbine wheel, resulting in faster spool-up, as well as trying to keep as much heat in the manifold as possible. In the case of an N/A can we would determine where the peak torque is and go from there, by either going tuned length primaries, stepped headers of tri-y (for a 4-cylinder) design. This results in more reflections, increasing the VE in more areas and thus widening the torque band.
Assuming the occasional ring-tool is a daily driven car as well as a daily, we most likely won’t deviate from the “fun daily” strategy, but as the Nurburgring is a monstrosity of a track (in terms of length) we do need to consider the thermal loading on the engine due to the extended period of time it will be used full load. This is more applicable to turbocharged platform and less so in N/A applications.
Considering a drag-race application, you want all the benefits you can get. In which case we would design a tuned length turbo exhaust in order to use the p-wave reflections to further increase VE. Taking into account that the car will likely not be using 100% performance for more than 6-13 seconds, we’re less concerned with heat management.
Similar to the “Ring-Tool” approach. Heat management is a serious consideration here, as the car will likely be using all of its’ potential for extended periods of time putting significant stress on all its’ components, especially in a turbo platform. Resulting in prioritising heat management over exhaust manifold volume.
Assuming gearing has been determined correctly a turbo race car shouldn’t spend a lot of time outside of its’ power band, where spool-up is a matter of milliseconds and not seconds.
We hope this gives you some basic insight into exhaust manifold design.
Next time we’ll discussing Direct Fuel Injection and Port Fuel Injection.
Have a good one,