New Renesis utilizes Helmholtz tuning!
11-17-2003, 12:55 PM
#51
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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.
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.
11-17-2003, 01:43 PM
#52
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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.
Thanks for the info. I find this stuff pretty interesting.
11-18-2003, 02:39 PM
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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.
11-18-2003, 05:54 PM
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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.
03-26-2009, 02:32 PM
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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.
03-26-2009, 02:41 PM
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almost a 6 year thread ressurection! 
03-26-2009, 03:37 PM
#64
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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.
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!
03-26-2009, 03:39 PM
#65
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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
Ninja, grats on searching, this was one heck of a find
03-26-2009, 03:42 PM
#66
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.
03-26-2009, 04:23 PM
#68
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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)?
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)?
03-26-2009, 04:27 PM
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03-26-2009, 05:49 PM
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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.
05-01-2012, 07:14 PM
#74
Voids warranties
I was referring to Racing Beat the company. They have been in business since the early '70s. At the time of the original post they were they only ones who had stated any results in regards to an aftermarket intake not giving any more power over stock. Now we can go through the forum and find that this could be due to a number of things. One of them being the absolute perfect mounting of the mass air flow sensor in the new intake tube. there is a thread about this right now.
My personal hunch is that there are some good aftermarket results based off of the fact that the filters they are using have more area and are less restrictive. This seems to be overcoming any advantage the stock tuned system had just purely on an amount of airflow basis. When I say I want to make the intake pipes bigger that isn't entirely the whole thing. I want to essentially remake the entire system right up to the throttlebody. Maybe I'll be going to alot of trouble for nothing but then again...? I would entirely redesign the air filter housing, readjusting the size for a larger better flowing air filter but also calculate its volume to account for the new intake tube sizes. Then the intake tubes would be a larger diameter. Since the area has now changed I will also have to calculate their length according to where I want it tuned. The total area of each tube will be different and will correspond with max air speed through them at their desired efficiency point being 122 mph. Since the long tube is going to peak out at a lower rpm than the short tube, it will physically be smaller in diameter. I may try to use a butterfly valve to open up the larger tube. Its area will also be calculated according to its desired peak rpm. Unlike the factory I do not intend to open a trap door on the longer tube to permit a shorter intake tract. I just want a 2nd location with a butterfly valve running it. I also don't have any intention of marketing it even if it works well. It is just something I want to try. If it works then hooray. If not then I guess it is one of the aftermarket systems for me too.
I don't see a need to go any larger or change any of the intake runners in any way. As long as the air velocity in them does not exceed .6 mach at their peak rpm they are big enough. Once you go over this speed, efficiency drops off pretty fast. The air filter housing is what I want to tune to. If it can be designed in a way that it resonates at a certain (or a couple) rpms then it would help to attract and suck in more air into the air filter housing itself. More air there is more air entering the manifold and going into the engine.
My personal hunch is that there are some good aftermarket results based off of the fact that the filters they are using have more area and are less restrictive. This seems to be overcoming any advantage the stock tuned system had just purely on an amount of airflow basis. When I say I want to make the intake pipes bigger that isn't entirely the whole thing. I want to essentially remake the entire system right up to the throttlebody. Maybe I'll be going to alot of trouble for nothing but then again...? I would entirely redesign the air filter housing, readjusting the size for a larger better flowing air filter but also calculate its volume to account for the new intake tube sizes. Then the intake tubes would be a larger diameter. Since the area has now changed I will also have to calculate their length according to where I want it tuned. The total area of each tube will be different and will correspond with max air speed through them at their desired efficiency point being 122 mph. Since the long tube is going to peak out at a lower rpm than the short tube, it will physically be smaller in diameter. I may try to use a butterfly valve to open up the larger tube. Its area will also be calculated according to its desired peak rpm. Unlike the factory I do not intend to open a trap door on the longer tube to permit a shorter intake tract. I just want a 2nd location with a butterfly valve running it. I also don't have any intention of marketing it even if it works well. It is just something I want to try. If it works then hooray. If not then I guess it is one of the aftermarket systems for me too.
I don't see a need to go any larger or change any of the intake runners in any way. As long as the air velocity in them does not exceed .6 mach at their peak rpm they are big enough. Once you go over this speed, efficiency drops off pretty fast. The air filter housing is what I want to tune to. If it can be designed in a way that it resonates at a certain (or a couple) rpms then it would help to attract and suck in more air into the air filter housing itself. More air there is more air entering the manifold and going into the engine.
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