What you are referring to is the fact that a certain frequency has a certain wavelength. If I interpret you correctly you seem to think that by making the intake tubes longer and wider to accomodate more air that we will mess up the tuning since we will change the wavelength due to the distance being farther. While that is true in relation to acoustic ramcharging within the intake runners itself, it does not apply here.

Think of the airbox and intake tubes as a speaker box. Since the Helmholtz principle is an acoustic phenomenon, lets use a ported speaker enclosure to demonstrate. The airbox is essentially a speaker box of a certain size. We want to tune the resonant frequency of this box to a certain frequency (or 2) in order to create a lower pressure zone within the box at this frequency. In order to tune this box to a certain frequency we need a certain size hole (port) at a certain length. However more than one diameter port can be used. This doesn't mean that they all would be the same length though. It would be easy to just guess it if this were the case. Let me use a known speakerbox airspace as an example. This is not the volume of the RX-8 airbox, it is just an example:

Goal: to tune a 1 cu. ft. speaker box to 35 hz. Heres how:

2" (diameter) port = 5.43" long

2.5" (diameter) port = 8.95" long

3" (diameter) port = 13.33" long

3.5" (diameter) port = 18.57" long

As you can see all of these tune the enclosure to the exact same point. The length of the port increases with diameter. This is because the the tuning frequency of the port is not determined by the wavelength through it (35 hz would be very very long!) but rather through a volume/velocity through the port relationship. Now lets go back to the RX-8.

The RX-8 airbox (speakerbox) is a certain size and the intake tubes (ports) are a certain length. If I change the volume of the airbox then the stock length intake tubes will no longer be tuned the same. If I change the diameter (area) of the intake tubes their stock length will not stay the same. If I change the stock length the diameter can not stay the same. It is merely as easy as figuring out how big the airbox is, measuring the area of the intake tubes and then using their distance to determine what frequency they resonate at. All I have to do is adjust length and area accordingly to maintain the same tuning principle at the same spot. The next thing is that there is no need to physically make the intake area larger once the airspeed through it has dropped below 122 mph. That is the spped of max efficiency. Right now it is probably too small and the airspeed is higher where it is less efficient. This is why the bolt on cone filter systems that people are making are giving more power. The greater amount of flow has outdone any benefits that tube tuning has given. If we can get the flow and the tuning to where they need to be, power will probably go up nicely. Unfortunately until someone does it we won't know for sure. If the available area is not available, then a good compromise is to keep the tuning the same like it is now. Go look at racecars that are restricted on the intake side. The Panoz race cars are restricted on the intakes so that they can only breathe through 2-1.75" holes yet they still make 700+ hp. If you stare at these intake pipes carefully you can see that they are a certain length and go into a sealed plenum of a certain volume. Could it be they know about this too? You bet they do.

Hopefully if you read this far it explained how the tuning principle works a little better. I'm about to get carpal tunnel syndrome from all this typing.

 

Hahaha

Thanks for the info. I find this stuff pretty interesting.

 

Rotary god or to whom it may concern. I was thinking, what would happen if the plenum wasnt really a factor and the second intake port was ALWAYS open? Increase in torque possibly? If anyone has any insight on this topic, please let me know.

 

If the 2nd intake port was always open then what I suspect would happen is that a little low and midrange power would be lost. Probably not much but we want all we can get. Alot of the 2nd gen RX-7 guys somehow think that leaving the auxillary engine ports open all the time gives them more power when all it does is to lose them some low/mid power and gas mileage. It's there for a reason and I wouldn't change its operation as long as it remains stock. If you change the entire air intake system to something else that is a different story.

 

Thanks for clearing that up rotarygod, much obliged

 

This is for RotaryGod or anyone else who has a good understanding of the resonance factor. I think I understand the idea presented here, my question is does air speed at the inlet effect the necessary air inlet length to hit this frequency? I guess the only reason I can't figure this out on my own is because the equation isn't in front of me. My thinking is that while preserving the length if you could reroute the tube coming off of the VFAD so that air were being forced in, say like a ram intake, would the resonance still be there or would that increase in inlet speed change the necessary length. I guess asking if cutting a hole in the bumper to force air in (a mod I saw in the DIY section) would actually work or throw everything off. Sorry if this is a stupid question, I'm applied math but I'm also a noob.

 

almost a 6 year thread ressurection!

 

damn - thought this was new info for a breif moment - for the "new" Renesis ...

 

at least he's doing his research.

 

That's exactly what I thought!

 

Easter is coming!!!

 

Ha ha no kidding! no 'search noob' for this guy

i got half way through the first post before i realized the thread was old; i got all excited about info on the 'new' renesis

 

Sorry guys, didn't think before posting here. Made a little note in my head to do that before next time.

 

In the 6 years that it's been since this thread was started, we've learned alot about the engine that wasn't known then. We do know that what it does utilize tuning in the intake system, it isn't for power. It's for noise control only.

FWIW: This thread marked the very first post I ever made on this forum!

 

holy crap, legoninja is instantly promoted to epic status for bumping RG's FIRST ever post.
Ninja, grats on searching, this was one heck of a find

 

Huh?
I presume you are talking about the VFAD only. The other valves are DEFINITELY for torque improvements.

To answer the resurrectionist, yes: changing velocity will change pressure which will change resonance because of density.
How much it will actually affect power is not known, but it is likely to be very little since the air path is convoluted. The air column will decelerate to whatever speed the engine requires and the "ram effect" at the front bumper is small at terrestrial speeds.

 

chrism would be proud/jealous!

 

Good to see my initial hunch being confirmed rotarygod.
Cobb Tuning had similar conclusions, that the stock intake design is as good as it gets.

Just have a Green Filter and an AP for now.

Could similar principle be at work on the backend (exhaust)?

 

Of course I'm only talking about VFAD!

 

Thanks guys, I guess if it's just for noise control then tweaking it should at the most make it noisy. I'm just trying to think of ways to improve it a little bit with some elbow grease. I'm pretty broke most of the time and the CAI and Ram Intakes for this car are costly. Not that I'm knocking them, I know they've put more time and know how into it than I have, just can't afford them yet and I'm getting ancy to tweak something.

 

/\ have a look at the stock intake DIY thread

 

So should I toss out my RB intake?

 

Epic!!!

I can't believe it was RG's first post. Epic status for the new guy who bumped this thread!

 

Did you ever do the intake redesign you talk about here?