Why use the split port manifolds instead of the oval port manifolds?
When using the split port manifolds you can generate a tapered neck-down in port size starting at the beginning of the head inlet. This is something that can not be done with the oval port manifolds. Not only that but the oval port manifolds start out with a significant disadvantage in this area as well.
There should be a 2 to 2.5 percent port size reduction per inch based on the flow area of the intended valve size. This creates higher velocity at the back side of the valve and aids in cylinder filling potential.
{All sizes in mm}
So 2 valves of 35mm = 962.3 (x2) = ~1925
However you will only get about 85% flow around the valve so that bring it down to roughly:
1636 {Effective Area}
The maximum head port length is nearly 11”
So the optimum inlet area would start at (1636 + 360-450) or 1996-2086
My setup is roughly 70mm x 33.5mm at the inlet.
However since it is “ovaled” it can’t just be calculated from length & width.
It equates to ½ a 33mm circle + ½ a 34mm circle + the ~36x33 rectangle between them.
427 + 453 + 1188 = 2068
That is a 2.4% taper which is within the optimum range. It’s on the high velocity side actually.
The taper in port size even starts in the LIM. It is not nearly as dramatic but the effects are the same.
Now by comparison the small {stock} oval port inlet is far from ideal and if you use the stock plastic LIM then it is stuck that way.
Stock oval port starting point is only: 60mm x 25mm
Using the equation from above that is an effective area of only: 1366
That is only 83.4% of the flow area of the 35mm valves.
If you factor in the stock non-ported head’s reduction in flow around the valve it’s still only ~90% of the valve flow area. Talk about constricting the 35mm valves!!!
That also means the airflow is slowing down significantly as it travels towards the valves because the port itself is getting larger to accommodate the 35mm valves. That is counter productive.
There is a point of contention for the oval port manifolds. Mainly the UIM. It has greater port volume then the split port manifold and likely a SH SVT UIM.
This would only really benefit a 3L engine as the SVT UIM has enough port volume for even a hot 2.5L
Also maximum extrude honing the split port UIM makes it close if not superior in port volume. This is a big bonus on a 3L engine. The effects will not be very significant on a 2.5L engine because of its smaller CFM requirements. A hot 2.5L will show some benefits in the high rpm range.
) Even if you fabricated some large copper external return drains and ran an Accusump they are still inferior to the oval (tumble) port design. Plus you would have to spend money to swap over the valves as well.