Port Matched Exhaust Yay or Nay?

Image: The nasty M156 cylinder head, exhaust port view.

As always, everyone is after the best set up and we had just watched a well respected builder-repairer bash and smash a manufacturer on mismatched flanges against cylinder heads exhaust port!

Image: A thoroughly welded one-piece headers. A true heatshielded header should have a fiber interface, however due to risk of fire and safety in mind, these are without.

A discussion since the beginning of ICE continues to recur and the one single mech. eng. math that keeps true is in keeping everything equal on the cylinder head side, what you do to one port, you must do to the other. Symmetry is key. When you explore the layout and surface area of the combustion within the exhaust ports, one thing is noticed, absolute symmetry.

You cannot bash one side of the exhaust flange for not matching up to the exhaust port. But if you must, the understanding that the ultimate goal of laminar exhaust gas flow bears very important guidelines.

Do not port the cylinder heads individually to match the headers. Instead, port the header flange side to match up against the heads. The flow on each cylinder should remain the same for the sake of scavenging and fluid dynamics and accurately accounting for lambda.

Image: Bead welds inside and out. The beautiful heat shielding in all its off-road glory.

Overdoing the flange on the header will cause an oversized surface larger than exhaust port volumetric efficiency. This is irreversible. While not the worst case scenario, this leaves a disallowance and inability to reach laminar flow. You cannot add missing material (easily) to build up the area to match the ports. If one desires to achieve laminar flow a scenario such as above prevents the enduser from achieving these fine results.

Image: Thicker flange will allow for port matching to turn down into the primaries.

It is expected since the dawn of fire with sticks that headers are to be marginally undersized to the cylinder head exhaust ports to allow such modifications of port matched (laminar) gas flow exit.

We ask our readers to research and pursue information.

TL;DR it is far more beneficial to look at the flow of gasses closer to the collectors and beyond downpipes than it is where the adjacent exhaust valves block flow for split seconds, intermittently at a time. The space immediately next to the valves (within single digit inch values) offer nothing in terms of laminar flow for the periods they remain closed.

Looking further down (the collectors) is more beneficial and offers further gains in terms of value returns. Don’t get us wrong, people definitely modify the flanges on tubular headers, but do this with the expectation that those that perform it receive a monetary salary for the cars they race.

We get it. Being a content creator is hard. Content is content, and the content people make may impress fledglings. The true story however is header ports are supposed to come undersized versus the cylinder heads ports. So its sus that people are saying otherwise.

We will tackle exhaust gas temperatures, scavenging and flow for a different day, facepalm. Expect some ideal gas law knowledge dropped and understand that the “tri-y”, “true long tube header” boys and believers will get butthurt once some real physics are dropped. Don’t worry, a Dyno will be included.