Originally Posted by DMRH

OK, its obvious no one understands what was proposed.

FYI, If I wanted a Revi or some other CAI I would just do it.

This excercise is about feeding the 4-port engine more air via the installation of the VFAD.

For reference, the 4-port 13B-MSP engine doesn't get a VFAD. It just gets the long tube.

The idea is to improve on that by swapping the tube for the VFAD.

I'll just go ahead & do this then test the changes on a dyno as this may be the first time anyone has tried it.

REgards

Originally Posted by Socket7

I thought the VFAD exists only to keep the engine quieter at low RPM. Why would it effect engine power on a 4 port vs a 6 port?

Originally Posted by Socket7

So it is increasing air velocity by decreasing the width of the air intake?

Originally Posted by SilverBullitt

Hi everyone,

I was watching the Renenis Module 03 video.

"They mentioned the VFAD opens above 5500rpm to add another intake air path. When open, it doubles the amount of air available to the engine and reduces restrictions."

"Is is not open all the times because because noise is also increased"

Originally Posted by rotarygod

Here's all you ever wanted to know about the RX-8 intake.

Aftermarket intakes only do away with VFAD. No there is no ecu retuning to account for this. There are gains (very small) though but they are based on total airflow vs resonant tuning. Somone needs to make a resonant tuned intake that flows good too. This would yield bigger gains that ANYONE else's intake system out there. I've said it here for almost a year now and still no one has done it.

I am not sure but I should at least hope that each valve and VFAD open at a set rpm regardless of load. Tuning is rpm based not load based. This would make the most sense.

If you look at the diagram very carefully you can see how everything works. The wavy lines are the air filter in the airbox. This set of diagrams is the 6 port engine. There is also one of the 4 port engine but not pictured here. It is very similar but lacks 2 intake ports and the VFAD duct on the intake.

Below 3750 rpm, only the primary (2 center ports) are receiving air. This insures that all of the airflow going into the engine is at a high velocity unlike the older rotaries where all of the ports were open at the same time. Mazda tried to overcome this problem with a 3 plate throttlebody and only opening the primary plate alone unit 20% throttle. It is a little complicated and I don't want to explain it all here. Let's just say this (Renesis) is superior.

At 3750 rpm when airflow is at its greatest and is starting to hurt power (intake airspeed around .6 mach), the secondary ports open up. Obviously this is why the fuel injectors are fired sequentially so there is no fuel in a nonmoving runner. Now we have a total of 4 ports being used until 6250 rpm. This again becomes a flow restriction and the tertiary ports open up.

At 7250 rpm the VDI (Variable Dynamic Intake as known on the 2nd gen RX-7) valve opens up which retunes the intake for higher rpm use. I'll explain it in a minute. Also at 7250 rpm the VFAD duct on the intake box opens up. This does a couple of things. It does allow a shorter more direct route for air to enter but that is really a moot point. More importantly it retunes the intake box. The stock airbox size along with the intake duct's length and width combine to acoustically tune the airbox to resonate at a certain frequency. This frequency is set to help the engine gain power at lower rpms. When the VFAD duct opens, it is retuning the intake box and adding its own area to the available area for air to flow through. It does a pretty good job considering that it is only a few horsepower less than the aftermarket cone filter systems but has the capability for way less ariflow. The total area for air to enter the stock box is less than the area of either the throttlebody or the MAF sensor. If Mazda would have given it a little more intake area with the same tuning, the aftermarket probably would have never outdone it. If you want to figure out the perfect size for the intake tube, figure out the diameter necessary for air entering the box at 8500 rpms to not exceed 197.5km/h. That will be the optimum size. Length will be determined based on tuning rpm and intake box size. There is a fun exercise for someone.

6 port "effective" intake runner lengths.

Primary VDI closed 50.3cm
Secondary VDI closed 51.8cm

Primary VDI open 34.3cm
Secondary VDI open 36.1cm

Tertiary always 43.9cm VDI does not effect these runners.

4 port "effective" intake runner lengths.

Primary VDI closed 68.8cm
Secondary VDI closed 73.9cm

Primary VDI open 37.6cm
Secondary VDI open 41.9cm


I forgot to mention another length difference between the 4 and 6 port engines. The intake duct going into the airbox is not the same between both engines. The 4 port engine does not just use the long pipe from the 6 port engine. It uses a longer pipe. Obviously it does not have the VFAD duct either. Different engine require different tunings.

Originally Posted by zoom44

but the actual intake manifold and 4 port configuration of the 4 port engine will not allow any gain by adding the 6ports VFAD. all you will be doing is adding length to the intake prior to the filter.

the long bit of the VFAD allows for quieter operation during low rpm. it then opens the short path for high rpm operation so there is no HP drop caused by staying with the longer path.


it doesn't add any power- it merely keeps the power from dropping in the 6port. you don't have the extra 2 ports to send that short path air to.

Originally Posted by zoom44

but only because there are 6 ports and not just 4

Originally Posted by Razz1

The VFAD is to reduce intake sound. NOT HP gain.