Back in 2016, when we decided we were going to create a manifold that fit all of our personal needs for our personal cars, we acquired a variety of flanges and spent a lot of time deciding which would be the most ideal for us. We went into our search knowing that each runner, from the exhaust head flange to the turbine inlet, is an extension of the exhaust port and is designed for the smoothest transitions possible in order to shuttle each exhaust pulse with careful violence to the turbine wheel. In making our decision, and helping to solidify our initial thoughts, of which flange we were going to go with, we did some hands-on research. Here’s what we found and why we use the oval port flange here at Morrison Fabrications.

Externally and from the runner entry, they can both look very similar so we cut them in half vertically and horizontally to see what happens immediately after the exhaust port (2g in this case, with 1g and Evo being the same, as well). This was a stock exhaust port; however, even a ported exhaust port really shouldn’t have a much larger outlet or be vastly different in the overall shape. Each flange style was inside welded, ported, and finished to the same level of attention for the best comparison we could give.

Left, upper and lower: Oval port flange, 1.5” NPS (Large runner)
Right, upper and lower: Transition flange, 1.5” NPS (Large runner)


1.5” NPS (Large Runner) Transition Flange

1.5″ NPS (Large Runner) Transition Flange cut in half against exhaust port.

A requirement, for all large runner (1-½” sch10 pipe size) 4G63 DSM and Evo transition flanges, is to offset the (1.68” ID) round runner down from the (1.25”) oval exhaust port in order to clear the stud just above it. This minimum offset distance is ~3/16” (or more). More offset will allow a closed end wrench onto the nut and room for the fillet weld without intruding onto the nut and washer seat. A cutaway viewed from the side gives the best illustration.


Transition flange cut in half against exhaust port, top view. 1.5” NPS (Large runner)

The top view shows the transition from the wider oval port (2.00”) to the 1-½” sch10 pipe diameter (1.68” ID).


1.5” NPS (Large Runner) Oval Port Flange

Oval Port Flange cut in half against exhaust port, side view. 1.5” NPS (Large runner)

A side view with a 90* bend up immediately off the flange (i.e. a bottom mount setup). This shows the worst case transition on an oval port flange, but is important for a direct comparison. We would avoid such a short straight section before the bend unless absolutely necessary for fitment sake since it doesn’t allow much room to get to the top nut. When the bend is facing downward (i.e. a top mount setup), there is no nut to worry about below and the bend can be brought into the flange. This allows the direction change to initiate in the oval port flange itself, allowing the turbo to be placed closer to the engine (and further from the radiator and fans) without using a tighter radius bend.


Oval port flange cut in half against exhaust port, side view. 1.5” NPS (Large runner)

A side view of an oval port flange and straight. The straight section is just over 3” long and allows from a very gentle change from oval to round. Although some manifolds won’t be able to use a straight this long off the flange, it follows the path of the port shape extremely well. The runner entry is slightly larger, accounting for small misalignments or an exhaust port that has been ported (this difference in a ported exhaust port should generally be quite small).


Oval port flange cut in half against exhaust port, top view. 1.5” NPS (Large runner)

Top view of an oval port and straight. Again, following the port shape nicely.


1.5” NPS (Large Runner) Side-by-side

Top: Side view 1.5” NPS (Large runner). Transition left, oval port right, cut in half
Bottom: Top view 1.5” NPS (Large runner). Transition left, oval port right, cut in half


1.25” NPS (Small Runner)

Left, upper and lower: Transition port flange, 1.25” NPS (Small runner) with straight
Right, upper and lower: Oval port Flange, 1.25” NPS (Small runner) with bend


1.25” NPS (Small Runner) Transition Flange

The small runner (1-¼” sch10 pipe size) transition flanges don’t require the same offset to clear vertically as the inner diameter (1.44”) is somewhat close to the height of the factory port (1.25”). The width ends up having the largest transition (2.00” to 1.44”), inherent to a large oval and smaller pipe. A cutaway as viewed from the top best illustrates this.

Transition flange cut in half against exhaust port, top view. 1.25” NPS (Small Runner)


Transition flange cut in half against exhaust port, side view. 1.25” NPS (Small Runner)

The side view shows the better vertical alignment that can be kept with the port (vs the larger runner transition flange) and more gradual transition. There is also much more room to get a closed end wrench on the top nut.


1.25” NPS (Small Runner) Oval Port Flange (with our own transitions)

Oval flange cut down centerline against exhaust port, top view. 1.25” NPS (Small runner)

This is the worst case scenario with an oval port flange and our own transitions- a bend needed almost right off of the flange. Typically we would aim for another inch or so of straight to get a more gentle change. Transitions going in the more natural direction (larger), should be gradual to best maintain exhaust energy. Transitions smaller are even more important, especially when the whole premise is to see gains in the low and midrange, while hopefully keeping the top end the same. Get it wrong and it won’t help anywhere.


Note: A cut side view of the oval port 1.25” NPS small runner wasn’t done since we ran out of oval flange ports to cut (4) and it really wouldn’t show much of interest anyway- the side cross section stays very close to the port height throughout. The interface will look the same as the large runner oval port cross section but only expand slightly to match the small runner height.


1.25” NPS (Small Runner) Side-by-side

Pictured left: Transition flange cut in half, 1.25” NPS (Small runner)
Pictured right: Oval port flange cut in half/along centerline, 1.25” NPS (Small runner)

In short, after careful consideration of several, we feel very strongly about keeping an oval port flange.

While several other manifold fabricators – whom we have the utmost respect for – use the transition flanges on their pieces, they simply are not for us.