duffro

So I'm looking at the big difference between the size of the ports on my intake manifold and the ports on the side of my motor.

The stock manifold on my 12a has much smaller openings than the housings.

Should I try and match these up a little better, or is there some need for the turbulence caused.

I did a few searches and came up with no answers.

Thanks

duffro

EDIT-- According to Paul Yaw "1. Port matching the intermediate runners on a stock 12A intake manifold. This absolutely ruins the flow. Additionally, the velocity is reduced, and so both high, and low rpm power is reduced."

Keeble

what about filling in the runners on the plates so thet are the same size as the runners in the manifold? wouldn't this produce more torque

j9fd3s

if you do a search on here, someone airflow tested all the different side housings. i think the tall runner plate flows the best of the stock ones?

Lynn E. Hanover

If you ask a very specific question, that includes cross sections, velocities RPM degrees of overlap, amout of back pressure, and about 100 additional facts, this or any answer may be completely accurate.

You would want to know why this mod was being done and so-on.

So here is an answer that we can assume is correct, because a famous engine builder with a dyno and flow bench and lots of experience says it.



The stock 12A irons have chamfers around each port. The manifold in all casses has openings that are just a bit smaller than the edges of those chamfers. Why would that be you might ask.



In the case of the manifold, it is probabal that more than one manufacturer has been producing manifolds. The pieces are sand castings. The part that forms the runner, is a column of sand pounded into a mold made of wood probably. The plenum and runner forms in sand are stood up in the bottom of the mold form called a drag. The sand form for the upper half of the manifold is lowered over the drag and encompassing the sand forms, and the top piece is called the cope. This is a very old system for making complex aluminum pieces, however it can be set up in a small shop, and the pieces made by unskilled workers. It is possible to adequatley inspect such a piece, and failed pieces are just dropped back into the melt. Having a number of shops provide this and similar castings provides a wide manufacturing base, as a defense against labor trouble, and acts of God cutting off supplies of pieces to Mazda. In sand casting there is a good chance that an interior core will move a bit when the melted aluminum runs into the sand. So a runner may be 1/16 off in that direction, and the one next to it may be off in this other direction. In some manifolds you will see tooling marks where an end mill has touched the end of the runner in a few spots. In manifolds that hit the center of the spec dead on, you will see no tooling marks. The chamfer in the irons can account for a wandering runner.



So long as the missmatch favors the irons very little is lost, power wise. So here comes Speed racer armed with a new Chinese die grinder and flame bit to create street racing history with a runner to port match. Everyone sees this same thing and does grind out the irons to match the runners in 12As and even later engines. The specific problem with the 12A is that the center irons have the biggest ports available in any rotary. Since the center iron has the primary ports and runners, and is used far more than all 4 holes together, screwing up these runners will hurt you all of the time.



So, the ports and runners were already just a hair too big for good low speed performance. Then we match up the runner face on the manifold with the outer edge of that chamfer, and that looks real bad, so we do away with some of the chamfer, and sand a bit here and there, and it looks real good.



Let us review what just happened. We changed the cross section of the runner in the manifold near the face. We enlarged the runner into the iron to match the manifold runner. So the slightly poor velocity of the too big a runner now gets to the Matched area and slows some more. That mismatch that caused a bit of turbulance and kept the gas atomized, is gone and we see fuel droplets precipitate out of suspension, and the engine sees this as a leaning mixture. Then the runner size returns to stock just past the port face, and all is well except for the velocity. Since the cross section remains stock, would not velocity return to stock also?



No......Since you changed velocity (slowed) some energy was expended, and the flow changed temperature. The flow now cannot return to the premodification velocity if no additional energy is installed.



But people don't do things like this to manifolds (in general) in order to screw up off idle performance.

More typically, Speed Racer is grinding in the hopes of improving HP.



Do builders port match the center ports (and the end (Secondary ports). We sure do. But when you do it. You try to maintain a constant cross section (area wise) so that the velocity does not change over the length of the runner, Or in the case of the all out screamer, a runner that has the cross section change at a constant rate (decreasing) from the bellmouth to the rotor. Also there is an understanding with the owner that this thing will not pass a smog test, and the allready dismal mileage will get worse. The torque that got you off the traffic light is now 1,000 RPM further up the rev range, never to return.



The lower vacuum may trick the vacuum secondaries into opening much later, so Injection, a TB or mechanical secondaries or similar must be figured in.



So there is some basis for only playing with the secondaries, and leave the primaries alone until such time as you only move this pig around on a trailer. Then it could actually get stock mileage, and yet have a giant bridge port on hogged out secondary runners, and be dynamite from 7,500 to 9,500 RPM.

For use in off the road trials only, and never on a public highway. So, the Paul Yaw quote is true....But it seems not to make sense until you read all of his tech articles. Which everyone should already have done. WWW.YAWPOWER.COM



Lynn E. Hanover