OEM Part Breakdown: 2.5L Skyactiv-G Exhaust Header

Analyzing an OEM part is usually our first step in creating a new performance part. We’ve been looking at the Mazda 2.5l SkyActiv-G Exhaust Header, and I wanted to bring you all along for the ride. It’s surprisingly complex for an OEM manifold/header and some serious engineering went into it.

If you’ve been paying attention to the CorkSport channels, you may have seen rumors here and there of a race header for the GEN3 Mazda 3 and Mazda 6 2.5L. While I can’t say too much on that just yet, but I can give you a breakdown of the OEM exhaust header that’s hiding in the back of your engine bay.

The OEM Exhaust Header

Stock Gen 3 Mazda 3 Exhaust Header
Stock Gen 3 Mazda 3 Exhaust Header

Excuse the dirty part, as this OEM header has had a hard life! I imagine many of you have not seen the stock header as it’s in the back of your engine bay surrounded by heat shields. Taking the heat shields off gives us a glimpse of the craziness that is the stock header. Mazda has gone with a true 4-2-1 design (also known as tri-y) with an integrated catalytic converter and what appears to be equal length runners. Stay with me, I’ll explain what all that means.

SkyActiv-G Exhaust Manifold Flow Path
Exhaust Flow Path

The image above hopefully helps you visualize the 4-2-1 design. Starting at the engine, there are four exhaust ports from the head. Each exhaust port gets its own pipe, known as a “primary”. The primaries then pair together to form two “secondaries”. Finally, the two secondaries combine into one collector pipe, in this case heading directly into the catalytic converter. The three unions or “y’s” are where the tri-y name comes from. The 4-2-1 design was chosen by Mazda for a very specific reason. Check out the image below and Mazda’s explanation HERE.

Mazda SkyActiv-G Exhaust Chart
Residual gas reduction by 4-2-1 exhaust system – From Mazda.com

Essentially, using a very high compression ratio causes very high exhaust gas temperatures. If too much of this exhaust gas is leftover in the cylinders for the next combustion cycle, knocking can occur. In addition, if you have a short 4-1 header or a log-style manifold you can suck exhaust gas into a cylinder before combustion as one cylinder can be on an intake stroke while another is on an exhaust stroke (see the upper image in Mazda’s diagram).

OEM Design Efficiency

The 4-2-1 has two benefits to fight this. First, the long length means the exhaust gas takes longer to traverse the pipes, so one cylinder sucking in another’s exhaust is drastically reduced. Second, the cylinders are paired correctly to one another (1 with 4 and 2 with 3). Since the firing order is 1-3-4-2, each secondary is receiving an exhaust pulse at a regular interval. If you paired 1 with 3 for example, you would receive two pulses quickly, and then a large gap as the other two cylinders fired. This helps with exhaust scavenging as the pulse from one cylinder helps “pull” the leftover exhaust from the cylinder it’s paired with. These benefits can also be present on a long tube 4-1 if designed well but, there is a good reason why Mazda did not choose this option.


2.5L Skyactiv-G
2.5L Skyactiv-G

Typically a well-designed 4-2-1 will make more power and torque in the midrange while a well-designed 4-1 will make more power way up at the top of the RPM range. Since normal driving does not involve being at the top of the RPM range all the time, it makes sense that Mazda went with the 4-2-1. We will likely do the same as we want to retain the low knock characteristics of the 4-2-1, high midrange power & torque, and because the SkyActive 2.5L is a fairly low revving engine.

OEM Exhaust Header 4-2-1 Design
OEM Exhaust Header 4-2-1 Design

It appears that Mazda also went with close to equal length runners. This means that each primary section is the same length and each secondary section is the same length. Having equal length runners ensures the exhaust pulses are arriving at the collector (or Y) at uniform intervals.

The easiest way to explain why this is a good thing is by visualizing the entrance ramp to a highway.  When cars entering the highway follow the “zipper” method for merging, the cars currently on the highway do not need to slow down. The highway and entrance ramp merge and flow in a smooth and consistent rate. However, if a surge of cars come down the entrance ramp to merge onto the highway you will get a back-up of cars on the entrance ramp and will disrupt the flow of cars on the highway.  If the cars are exhaust gases and the highway is the exhaust pipe, you can understand why equal length can help. Again, we will adopt this strategy with the CorkSport Race Header.

So far so good then, as Mazda has put a lot of thought into making a high-quality stock header. However, as usual, there are a few areas we can improve on. That’s coming in a later blog though so you’ll have to stay tuned for more details! Let us know if you have any questions or thoughts down below.

-Daniel @ CorkSport

Exhaust Scavenging

In this blog, we are going to SHOW a demonstration of exhaust gas scavenging.  Instead of a lengthy blog full of text, we’ve opted to create a video that demonstrates the effects of exhaust gas scavenging for both good and bad designs.  

We will be comparing the prototype CorkSport performance exhaust manifold, developed for the Mazdaspeed 3 and 6, to the OE exhaust manifold.  

Exhaust gas scavenging within a manifold is the process of one cylinder runner, pulling (aka scavenging), the exhaust gas from an adjacent cylinder in a continual cycle.  Now enough talk, to see an awesome example and an awful example of exhaust gas scavenging check out the video below. BONUS! Not only do you get to see what optimal scavenging looks like, but this is also the first sneak peek of the CorkSport Performance Exhaust Manifold…

Video Link: https://youtu.be/RtydboDbwpQ

We hope you found this as interesting as we did!  Stay tuned as we continue developing the CorkSport Performance Exhaust Manifold for the Mazdaspeed platform.

 

-Barett @ CS

Performance Turbo Exhaust Manifolds – Tubular or Cast?

If you have been in the car scene for a while, you have probably seen or heard of performance exhaust manifolds.  Like any other component on the engine that affects flow, performance exhaust manifolds can have a significant improvement to the engine’s peak performance and power under the curve amongst other aspects that the exhaust manifold can affect.  

You have also probably asked the question. “What type of exhaust manifold do I need?” In this blog, we at CorkSport would like to help you better understand the differences between cast and tubular so you can make the best decision for your Mazdaspeed.  

There are two main styles on performance exhaust manifolds; tubular and cast. Both have their pros and cons to consider as an end user (that’s you the enthusiast) and as the designer/manufacturer (that’s us at CorkSport).   

First, let’s look at tubular as it’s the most common in the performance industry.  Tubular is the most popular option because of it manufacturing flexibility.  Unlike casting, tubular does not require expensive molds to develop even a single prototype. Not needing expensive molds allow great flexibility in design and manufacturing, which lends the tubular manifold as an exceptional option for one-off builds.  

To fabricate a tubular exhaust manifold you need just the raw components: flanges, tubular sections, collector and fabrication supplies, and of course the expertise to fabricate the manifold.  Let me emphasize the necessity of fabrication skills here. To produce a reliable and performance proven tubular exhaust manifold takes the correct skills, tools, and patience…it’s honestly a work of art.  

With this work of art does come some compromises.  To create the necessary runner routing, many tubular sections will need to be welded together.  This increases the chance for weld impurities and slag which can later result in cracking and poor performance.  Any reputable fabricator should be able to avoid this, but it does come at a premium due to the many man hours that must go into each and every manifold.  

Next up is cast.  Manufacturing via casting provides a different set of opportunities and difficulties to overcome.  The process of casting alone has restrictions that must be considered; such as mold design and molten flow in the casting.  Assuming these issues are overcome casting can provide unique opportunities for the design to improve reliability, performance, and packaging.  

A well-designed cast exhaust manifold can have great reliability due to its one-piece design.  There are no welded joints that can crack or fail and casting typically has a higher threshold to heat before issues arise.  The wall thickness of the casting can also be defined for the application which can improve strength if the exhaust manifold is the only part supporting the weight of the turbocharger.  

Overall performance can also be affected due to the casting design flexibility with each runner.  Unlike tubular, a cast is not restricted to standard tubular elbows and straits. The runners can bend and change profile as desired to aid in performance and packaging.  

Speaking of packaging, casting can really change the game here.  Since each runner does not have to be accessible for welding, the entire design and each runner can be tightly packaged together to reduce the overall size of the exhaust manifold and better retain heat which aids with turbocharger response.  

Lastly, comes the cost to you the enthusiast.  Although the upfront cost of a cast manifold can be high, typically the unit cost and necessary man-hours are low which helps keep cost down for you.  

As a designer and manufacturer of performance parts for you Mazdaspeed, these are all things we have to consider providing you with the best parts possible.  We’ve explored both and are happy to stick with casting as we feel it provides the best balance reliability, cost, and performance. Keep a look out for future projects and updates!

Thanks for tuning in with CorkSport.

-Barett @ CS