As you know, the rotor receives charge air from both sides. The front rotor receives the primary charge air from the rear and the secondary charge air from the front. Consider the typical streetported motor with an untouched primary and a slightly enlarged secondary is way out of balance as to equal flow from both sides. i prefer to have increased flow from both sides to encourage swirl. Swirl encourages atomization. Better atomization leads to a more linear burn. Which leads to more power and less detonation. Lots more flow is needed from the primary side of the runner.

Of course more fuel is then needed at the primary position. Fortunately modern ECUs and modern fuel injectors (Bosch EV14) check the box. No problem at idle with large primary injectors today.  

A completely revamped Primary port and runner should be job one.  And no, significantly uprating the flow will not effect drivability or MPG. Less resistance makes for higher efficiency. I routinely see 21 MPG on the highway. 

My high flow primary:

My port system is dual purpose. Maximum power "under the curve." My "curve" is from around 4000 to close to 9000. I want it linear so it is driveable out of a corner on a road course as well as fun around town. I have no interest above 9000 rpm nor above 600 rwhp. Peak torque with a properly set up system is 6300. Happy rotary idle is around 1100. With specific tuning it can easily idle at 850 but it doesn't sound like a rotary. 

I currently run a rather large brand new tech Garrett G40-1150 and recently made 536 rwhp/404 torque at 15.8 psi/6967 rpm/10.38 AFR at 2650 ft altitude. 12.7 degrees lead, 93 pump, 32% ethanol and 700 cc of methanol as AI. My egts were 1625 and 1624 and my IAT was a real 71.8. Boost 15.8 and EMAP/backpressure was 18.1.

Nice metrics for dual purpose ports. 

When we think about ports or porting we generally are thinking about power. 350, 400, 500, 600 or...

When i think about ports i think about lowering the resistance to the intake charge air.

Better ports will deliver benefits ALL the time the motor is running.

Better ports deliver benefits in vacuum.

Better ports deliver better MPG when cruising.

Better ports deliver power AT LOWER BOOST.

Why is this especially important? Because lower boost means lower turbo outflow temperature. It is common to see turbo outflow temperature (before the intercooler) to be around  175 F when cruising. At the end of a 20 psi run temps will be between 350 and 400, or more. If you can make your target power at a lower boost level your intake air temperatures will be less.

Note re IAT. If these temps seem high to you re your logged data you are using the wrong type of temp sensor. All thermistors, including the "Fast" type generate incorrect data. A thermocouple is the only sensor that will generate accurate data. If the temp out of the turbo at 20 psi is 375 F and you have a great intercooler that removes 67% of the rise from ambient 125 F (which is measured at your air filter not outside) your IAT temp will be... 

                                                                        208 F

But your logs indicate 76 F.... Is it any wonder why so many rotaries are lost to detonation? Better ports allow the turbo to flow the same amount of air at a lower boost level. Lower boost level might generate the same flow at 50 F lower intake air temperature. ‚Äč

Lower intake air temps =s more oxygen molecules per volume to burn =s more power and less detonation.

Great ports pay real dividends all the time.

They will, of course make more power:)

The Mazda 13 BREW motor, found only in the FD RX7 is the high point as to the 2 rotor Mazda rotary engines. This is primarily because of the peripheral exhaust port although the secondary intake ports were greatly improved versus the FC series.

The primary intake ports are TINY and remain a mystery of sorts because much of the shaping, which was good, was later kneecapped by additional flow limiters both as to reduced opening size/ shape and the runners.. Clearly Mazda decided to hit the brakes on the important primary ports. Probably emissions.

Most streetporting services that are offered do nothing with the primary ports. I consider them to be the most important part of my total port system. They also require the most time which may be the reason they are often untouched.

Let's take a step back to ensure we all properly appreciate the totality of the roles ports play in our rotary motors.

The rotary has no valves to hamper intake and exhaust flow. Pistons engines have a valves impeding the flow. Piston engines have an approximate 90 degree turn on both the intake and exhaust. 13 BREW rotaries have no turn for the exhaust. The nature of the ports and their flow ability enable the 13 BREW to significantly out flow a piston engine. Add a turbo to supply the flow and you better have a great set of brakes.

As you probably know, besides flow capability the other key feature is that port location determines timing as to intake and exhaust open, close and overlap. Port location generates intake and exhaust events exactly similar to the cam on a piston engine.

Change the cam, you have an entirely different engine. Change the ports and you have an entirely different rotary engine. 

When i initially designed my ports i fixtured a degree wheel to the crankshaft so i could extract both Mazda's intake and exhaust timing but also fashion my own timing. This was not new to me as I have designed my own valve timing previously. When i raced piston engines (long time ago) I started with blank cam billets and worked with Competition Cams to get what i wanted... 10,000 RPM and 210 hp from a 2 valve single overhead cam 2 liter motor. NA piston power at 1.72 hp per cubic inch back in the day. (4 valves make 27% more power... port area in action.) 

Competition Cams now offers "Turbo" cams. They have unique timing versus a NA cam and it is all about overlap. Overlap is the period when both the intake and exhaust valve, or in the case of the rotary intake and exhaust ports are open. Sounds complicated but it isn't. 

let's assume you are making 20 psi of boost. Around 5000 to 6500 the exhaust pressure driving the turbine wheel exceeds the boost pressure as it takes around 100 hp to drive a turbo at around 70 pounds per minute.

If both the intake and the exhaust are open and the exhaust has 30 psi and intake 20 psi which way is the flow going to go? The exhaust would go into the intake.

The exhaust, which is 1600 F and contains little oxygen will pollute the charge air. The entire intake side of the motor has been engineered (hopefully) to deliver as cold an oxygenated charge air as possible. It is easy to see that you want to do everything possible to isolate the cool charge air from exhaust pollution.

Increased charge air temps encourage detonation. Gasoline auto-ignites just above 450 F! Hello pre-ignition and a dented rotor. 

Delaying intake port opening or advancing exhaust port closing will decrease overlap and therefore decrease intake air pollution. Power will be reduced.

Opening the intake earlier and closing the exhaust later will increase power but increase intake charge air pollution.

 A careful balance must be struck as to the amount of overlap.

 I do not change the exhaust close. The exhaust port is peripheral. It is a huge port and it is open a long time. It does not need to be bigger. Numerous porting templates extend the close allowing more exhaust into the intake. No thank you. 

Let's take an actual look at the ports starting with my favorite, the primary intake port. This is the port that is working all the time. It takes you to the store and it helps taking  you to turn one.

And it is dreadful.

If your motor has been "ported" chances are the primary intake port hasn't been touched. It is mis-shaped and tiny. Ditto the primary runners. Both have been neutered. A very close look at the totality of the primary port intake system ends with the conclusion that initially Mazda cast a reasonably descent port/runner and then was ordered to back it down, a bunch.

here's the port... it always looks bigger in a picture but it seems about the size of my thumbnail:




While it is relatively easy to settle on a particular porting template, creating an optimum port system is the other 75%. 

The configuration of the port nearest the coolant ring is thick and has an abrupt edge. This entire section should be thinned/tapered and shaped like the trailing edge of a wing to promote laminar flow. This shaping should be done on all four intake ports.

The shoulders of the runner, where they turn 90 degrees to discharge into the motor, should be greatly widened to smooth the turn. Smoothing the 90 degree turn properly takes lots of time but is important.

Finally, a bevel should be added in the area where the leading edge of the sideseal meets the closing portion of the side iron face. 


The secondary ports, like the secondary runners, show Mazda's intent of promoting flow. They are of pretty good size and shape, for a 255 hp. We want more and we want it efficient, as in low restriction.

The exhaust port on the 13 BREW makes the engine. No valves, no turns, just straight from the explosion out of the engine. As such it is a simple port. Sort of a rectangle that turns into a circle. Porting opportunities reside at the rectangle. The bottom is the open and the top is the close. Don't go near the top or you will add to overlap and intake dilution from the exhaust. Don't touch the sides any widening gives up crucial support to the apex seal. I do modify the open just a bit. A fraction of drive is diverted from the rear wheels to the turbo turbine which helps midrange/spool.

Just as with the Secondary port there is a lot of metal removed. There is one area where it is important to NOT remove metal. Note the inner edge of the port. The steel spring loaded  oil rings ride over this part of the side iron. The side iron face supports the oil rings. 

Mazda extended the side iron in this area and it presents a .1 inch ledge out over the port wall that reduces flow. This 90 degree ledge exists on both the primary and secondary ports.

It is very tempting to grind this ledge away as it presents a clear barrier to flow. This entire ledge is missing from most of the ported motors i receive.

Not only is the ledge missing but crucial oil ring support is missing.

Better for flow. Bad for oil ring support. Great for increased blow by.

i retain all of the support area but change the 90 degree ledge to 45 degrees. Great for oil ring support and better for flow.

This is my catch can that was fabbed in 2013. I have never cleaned the sight gauge. As you can see zero blowby in 10 years. Partially due to my ports.   

The runners carry the same theme. Choke the motor. Note the speed bumps and thickened walls.t

The primary runners are pretty straight forward although quite a bit of time is required to port match and open them on all four sides. One item that requires a special longer stem porting stone is some sort of casting plug located on the floor near the end of the port. No more speed bumps no where. 

After the tune up

Mazda went pedal to the metal when they cast the secondary runners. They are HUGE but do have a quite rough surface.