You read it right although I have never seen it written anywhere. I usually hang around the RX-7 club forum but thought that since I am a rotary fanatic, and everyone here should know everything about the new RX-8, that this would be a good place to post this.

I've done alot of study on how intakes and exhausts are designed and why they work the way they do. I know how to design the proper length intake runners for any engine as long as I know the port timing and desired peak rpm desired. I also understand the effects of Helmholtz resonant tuning and how to design it too. So here's what I discovered through my study of the Renesis induction system.

First, I read on here somewhere that Racing Beat has a new RX-8 and has been testing different types of air filters and intake combos. After some experimentation they have found that the stock system works best. Why? Helmholtz resonant tuning! The air filter housing isn't just big because the filter is huge. It is this size for a reason. Here's how to design a Helmholtz plenum chamber and some basic rules for it. When someone gets their new car, go out and measure both intake ducts for length and compare them to these numbers. If I'm right we should be very close. My numbers are generalizations and intended as ballpark figures. It doesn't change the principle.

Since we are dealing with 2 rotors we will just assume that the engine compares to a 2 cylinder. This is only a number for intake runners sake and not the total number of combustion faces per rotor.

As a general rule Helmholtz plenum volume should be sized as follows:

2 cylinder engine: Equal in size to total displacement of the engine regardless of stroke. For a 2 rotor use 240 cu. in. (3.9 liters). )Please don't start a debate on this!!! I have 2 articles that I will post here later to show how this was deduced.)

4 cylinder engine: Helmholtz plenum volume should be 50%- 60% of the total engine displacement.

6 cylinder engine: Helmholtz plenum volume should be 65%- 80% of the total engine displacement.

8 cylinder engine: Won't get into it now.

Back to the rotary. Diameter of the air intake tube should be sufficient in size that at the peak power rpm, air moving through the plenum intake tube (not the intake runners) should not exceed 122.73 mph.

Now for the plenum intake tube length. Just taking a short cut and getting to the good part: The tube would have to be around 7" long for a 10,000 rpm tuning. If we want to tune it for a lower rpm, we add about 1.7" in length to the tube for each 1000 rpm we go down. For every rpm higher we subtract about 1.7" in length.

Now go look at a picture (or the real thing if you have the car!) of the intake tubes leading into the air box. I am speaking for the 250 hp, 6 port car. There are 2 intake tubes. One really long one and one short one. The short one opens up at 7250 rpm for a shorter air intake route. Why would that matter? You know why now! If we measure the long tube length and compare it to my above numbers we can pretty much determine what rpm the designers intended it to benefit. The same thing can be said of the short tube.

The air filter will play a small affect on what the chamber resonates at. The filter would act a little like poly fil added to a speaker box. It would make the box act as if it were slightly larger than it really is. This is negligible though.

So here's the lowdown on how a Helmholtz plenum works. It doesn't need to be the intake manifold plenum. The only things that affect the tuning of the system are; plenum volume, air inlet length, and air inlet diameter. Notice that it is not affected in any way by port timing. The whole point is to create a resonance at a certain rpm. In the case of the new RX-8, 2 different rpm's. This resonance is the point where the air in the chamber is at its lowest pressure. Ah, now you understand! Lower pressure promotes greater airflow into the chamber. More airflow into the chamber is more airflow out of the chamber and into your engine! The same principle is what makes the VDI system work. You could add a Helmholtz plenum the top of a Holley carb or a Weber carb and it would have the same effect!

Now you all know! Buy a drop in K&N filter but leave the factory intake design alone! Just say no to cone filters for the RX-8!

what do you mean by resonance? and whats a plenum?
________
WEBSITE DESIGN (http://hostndesign.com)

Thanks for the info! The automattic car has less power and a 7500 rpm redline, so I assume the intake tubes are different. Does this have anything to do with the AT car having less HP, but more torque? (164 vs. 159)

Originally posted by mazdabob
Thanks for the info! The automattic car has less power and a 7500 rpm redline, so I assume the intake tubes are different. Does this have anything to do with the AT car having less HP, but more torque? (164 vs. 159)

no, the torque issue has to do with the porting.

great stuff rotarygod. :) i've always wondered about the manifold-forward kinda stuff... any books (yes, heavy stuff is A-OK) that you'd suggest for more on all-motor tuning??

...resonance is typically an accoustical phenomenum that happens with air and sound at a specific frequency in an enclosed chamber (think pipe organs). Every pipe organ has a specific resonant frequency (how it is tuned for whatever tone (pitch or frequency) it was designed to produce; e.g. large piper organs produce low pitch (longer wavelengths) and tiny pipe organs produce high pitch (shorter wavelengths).

Another example of resonance is if you blow over a soda bottle half full - you can get it to "whistle". If you drink a bit of the soda, the pitch will be lower due to the chamber becoming larger.

BMW uses this principle on it's 7 series motor where the length of the chamber(the "plenum") is continuously changed for the matching RPM of the engine.

There is resonance for open ended tubes and close ended tubes. It is a great experiment to show people, how resonance works with an ordinary piece of PVC pipe, cut to the exact length to match the resonating frequency for a specific tuning fork. If you hit the tuning fork, you can barely hear it, but put it up by the open pipe of PVC and it sings like a pipe organ!

Resonance is almost like the air is perfectly tuned to the chamber and thus exits the tube perfectly, or with some added "umph!".

That's as much as I know. I have not heard of Helmholtz tuning before, but it sounds like the plenum lengths correlate to the valves on a trumpet (like the Renesis) where the BMW is more like a trombone (where you can slide the tube to increase or decrease the chamber length).

Man, you are making my head hurt on a Monday :o)

This is very cool stuff. I cannot wait to bring it up in a conversation....

All joking aside. Thank for the information.

And, does this mean the intake will "sing" at the right rpm?

Roachman

And, does this mean the intake will "sing" at the right rpm

That is what the rotary is best known for, and the RENESIS will be no exception.

What beautiful music it will make.

Mazda is no stranger to variable pitch intakes. I had the most fortunate opportunity to tour their IMSA shop in Charlotte before they bowed-out. Those engines had variable intake stacks. Beautiful!!! MAJOR BUCKS!
Resonance "packs" the intakes in a natural kind of way, so it doesn't rob HP in doing so un-naturally, like turbo's. Of course, turbo's definitely pack more for an even bigger bang, but if you don't have a turbo, then you better work on the resonance.

Most excellent info! Thanks!

Over the weekend I picked up the RX-8 Book sent to us a few weeks back -- it talks about how the air intake is set up, that once the intake port of the 1st rotor is closed off some percussion takes place, and it sends air back through the intake at the speed of sound, and actually helps charge with more air the 2nd rotor as it enters it's intake cycle.

They stopped short of calling it 'turbo'charging tho.

Originally posted by Racer X-8
I had the most fortunate opportunity to tour their IMSA shop in Charlotte before they bowed-out. Those engines had variable intake stacks. Beautiful!!! MAJOR BUCKS!

...years later, the R26B used very variable length runner trumpets (yes, the trombones) to the same effect. very wicked awesome.

I have to edit what I posted. The info is correct but the numbers given assume that the Helmholtz plenum is the main intake plenum which it is not on the RX-8. However, the principle and effect are still the same none the less and the volume of the air cleaner assembly is directly proportional to the inlet tuning frequency of either length inlet pipe. Still does the same thing!

The lower horsepower number from the auto car is two fold. First, the engine is only a 4 port! This gives it less total intake runner diameter as well as less total port timing. Is best suited for a lower tuning. It should be stronger up to its redline cutoff than the same rpm of the 6 port engine. Go look at the dyno charts Mazda published. It is!!! Also since the resonance effect that is given by the shorter inlet is tuned somewhere in the 7500-8000 rpm range, there is no point to having it on the lower horsepower car since its redline isn't even high enough to benefit from it. Tune it where the car will be driven most of the time.

Wakeech: If you would like some good info on designing intakes and why they work the way they do, check out any books written by David Vizard. He has a couple usually sitting in the automotive section of most big book stores. They do require a bit of study though and can't be fully understood by just casually reading them. I have sat down and spent hours and hours of studying like I was being graded on it. I even have a couple of articles I have written if you would like me to post tem here. Even when you learn the formulas though you still need to know how to adjust them for the rotary engine. I know how! If you ever have any questions PM me and I'll be more than happy to show you how.


I forgot to fully mention the VDI thing and how it works. VDI is a brilliant system based on the speed of sound and acoustics that Mazda uses. The neat thing is that it is not affected in any way by port timing. It is only rpm based. If you want a boost from the effect at 7000 rpm it makes no difference what kind of porting style you have! It is only set at a certain distance to tune for a certain rpm and thats it. Typically you want the VDI effect set about 1000-1300 or so rpm below your tuned length intake runners peak horsepower point. Lets look at the Renesis. The high horsepower engine makes peak horsepower at 8500 rpm. this would mean that we would want the VDI effect to come into play at somewhere between 7200-7500 rpm. Where did Mazda open it? 7250! Hot damn the numbers fit! It is very easy to design an intake manifold that utilizes VDI. We only have to know what rpm we want it to work at and then calculate from there.

How could this knowledge be used to win a bar bet? ;)

Do all your research before you make the bet so you are 100% sure you are correct! Just don't tell the other party.

Originally posted by wakeech


...years later, the R26B used very variable length runner trumpets (yes, the trombones) to the same effect. very wicked awesome.

Exactly! Right on... I practically begged them for a job! Told them I'd be packed-up & back within 24 hours. Maybe I should have played hard-to-get. Er...uh...

Mazda has been using resonance tuning for a long time now. Even my Probe (w/Mazda engine) has a variable length intake controled by Mazda's VRIS (Variable Resonance Induction System) which opens up secondaries when the intake resonance reaches a certain point.

All I know is that total abstinence is the only sure way not to get VDI. One must be careful about these things. Good timing won't cut it either.

Originally posted by QuantumTheory08
All I know is that total abstinence is the only sure way not to get VDI. One must be careful about these things. Good timing won't cut it either.

Couldn't have expounded any more expediently if I do say so myself, Q.

By the way, which one of those fine gentlemen might you be, pray tell?

Oh yeah, well my Mazda with a Probe engine doesn't. (I think) Does it?

Checkout that snazzy race car previously eluded to via search engines like I did earlier. I don't tink you got dat.

...I'm the cop taking the photo in front of my squad car. (Note the dual port ram air inductance system this guy brought with him).:p

i know what resonance is, but had NO IDEA how it applied to engines.

so lemme see if i have this straight

when the frequency of the air matches the rpm they act in harmony without anything causing any disturbances so exit more forcefully?

edit:
that doesnt sound right, maybe i misread and RPM has nothing to do with it except for the frequency of the air vibrating so then it goes to a diff sized chamber or something?

edit:
ok, i have no idea how rpm ties in with this, i dont know enough about engines.
________
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wow, i feel like a moron...
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Originally posted by P00Man
i know what resonance is, but had NO IDEA how it applied to engines.

so lemme see if i have this straight

when the frequency of the air matches the rpm they act in harmony without anything causing any disturbances so exit more forcefully?

that doesnt sound right, maybe i misread and RPM has nothing to do with it except for the frequency of the air vibrating so then it goes to a diff sized chamber or something?

Hmmm. Think of a meatball on a rubberband. It boing-boing-boings up and down at a steady beat. That's the meatball/rubberband system resonant frequency.
A short rubberband or a smaller meatball = faster boing.
A long rubberband or a bigger meatball = slower boing, right?

Now, if your mouth was placed under the bouncing meatball, and everytime it reached the bottom of it's boing, you ate it (it's a magical meatball that doesn't go away when you eat it). If it's boinging too fast, you get bonked on the nose a lot, and that's not good. If it's boinging too slow, you go hungry, and that's not good either. If you boing it at just right frequency, you wind up getting fed more than just sitting there with a fork. Now, just change the meatball with intake air mass (blob of air), and your mouth with the intake ports. See?

OK, now, the intake air mass is kind of like water-hammer. You know, when you shut off a faucet, and when the flowing water reaches the new dead end, its kinetic energy, oops, I mean its weight gotta come to a stop fast. When it does, it "hammers" pipes & stuff. The speed that it does that (no, it's fast, but really not instantaneous) is totally dependant on the pipes & stuff that it's flowing in. If you practiced a lot, you could get good at opening the water faucet just at the moment the water's gonna hammer it. What you would get is a big splash coming out, fast (just like the comedy where the girl opens the door just when the guy's gonna smash it open. He he.)

You get it now? The intake ports open just when the air mass gets to it, and like the guy blasting through the open door, the air blasts through the intake ports. That's good.

Now, when the RPM's change, you'll want to change the frequency of the air/ports system, like the meatball/rubberband. If you ain't got the big gucks for variable pitch intake ports that get shorter for higher RPM's, then you gotta settle with one resonant frequency that's tuned to a strategically chosen RPM, like 7250.

Whew:eek:

ahhh
so it has nothing to do with acoustiscs so much as mechanics

the opening/closing of the ports is in perfect harmony with the intake of air so that where ever that leads to (rotors in the housing?) is constantly supplied with the optimum amount of air

i really dont know that much about engines, but im starting to peice it together pretty good i think, so when i put a "?" after a statement, could you say if im correct or not?
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Mazda has used intake pulsations to, in effect, 'supercharge' the rotary engine since the 13B powered GSL-SE model in 1983. The 'Dynamic Intake Chamber' on the SE helped the car make 135 hp and 135 ft/lbs of torque. This intake system was then refined for the 2nd generation RX-7, yielding a final hp of 160 by the end of the 2nd gen run.

Resonance tuning is used on exhaust manifolds (headers), as well. It's been said a good header is the best HP/dollar ratio you can get.

Originally posted by rxtreme
Resonance tuning is used on exhaust manifolds (headers), as well. It's been said a good header is the best HP/dollar ratio you can get.

You bet. Works the same, just backwards. It "pulls" the exhaust out by creating a low pressure at the exhaust port when it opens.

I dyno-tested a Fiat 850 engine in my senior IC Engines course (yep, showing my age again). Suprisingly though, the stock manifold performed much better than the racing one. I guess that says to be sure you know what you're buying...

The water faucet "hammer" is a good analogy. I think I read in Popular Science about the BMW 7 Series variable plenum. Part of the design is to have enough air "mass" moving in the plenum to help push air into the combusiton chambers when the valve opens (inertia, which = mass x velocity). I know of resonense in Physics and acoustics....I would like to think that there is an application here, used on intake manifolds, but I don't really know.

As for tune exhaust manifolds; I always thought that had to do with make sure that each exhaust port was the same length before joining together to form the final "one" exhaust pipe.

:)

Each pipe needs to be tuned the same as the others, basically, to the same tuned resonant frequency - or in accoustics, pitch.

Everything vibrates, and, from that fact, everything has its own resonant frequency, the frequency that it vibrates, basically after being excited by a single excitation. Your inner organs vibrate also, and that is used in MRI (magnetic resonance imaging), which comes out looking like a picture of your innards after a computer gets done with it. Galloping Girdie was a very sleek and slender bridge over the Verrazano Narrows. It didn't last very long because it didn't take very long until a wind at a certain velocity and direction excited the bridge to vibrate at its resonant frequency. I never saw concrete flex as much as that bridge did. A few minutes of that and it was all over. A new specialty in engineering hence was born - vibration analysis. (btw, the same bridge designer afterwards built the George Washington Bridge I think (NYC). Anyway, it was about the most massive bridge ever built by man. He he.)

...I know what you are talking about. The bridge in Washington State that failed due to a small gail wind was the "Tacoma Narrows Bridge". I don't know if the same bridge builder was also responsible in designing the Verrazano Narrows bridge in New York; I'll look into it.

I found this interesting document, which was only recently published at to why the Tacoma Narrows Bridge fell.

http://www.math.umbc.edu/~gobbert/teaching/math101.s2003/reports/Group1Tacoma.doc

Leon Moisseiff was the Tacoma Narrows Bridge designer and Othmar Ammann was the master designer of the Verrazano Narrows Bridge in New York

Here's a before and after as I'm sure you've seen before.

I love the meatball analogy!

Don't think of an intake or exhaust system having only a mechanical or only an acoustic effect. The mechanical aspect should be considered the direct speed of the intake or exhaust pulses as they pass through each runner. There is a high and low pressure zone traveling with these waves at the same speed. There is also a smaller acoustic wave. This wave resonates back and forth within the pipe, obviously at the speed of sound. Remember that a sound wave has pressure (both high and low). This is why loudness is measures in SPL (sound PRESSURE level). A sound wave is just that, a wave. Compare it to an ocean wave. the peak of the wave is the high pressure zone. The trough of the wave is the low pressure zone. The difference is that sound waves don't travel in an up/down relationship like this. Rather they travel in a back and forth motion. The peak of the wave is coming towards you, the trough is moving away from you. By tuning to a certain length we are trying to time the intake port closing time so that just before it does close, one of these troughs of low pressure enters the engine past the port to draw in air while a peak of high pressure behind it helps push in more air. Mazda found on the early 6 port engines this was good for as much as 2 psi additional air! It takes careful consideration on air pulse timing to coordinate it with the acoustic pressure zones. This only happens over a very narrow zone. The disadvantage is that while it helps at some rpms it hurts at others. There is no way around this though and all engines suffer from this phenomenon whether intentionally designed around it or not. There are several reflections though each getting shorter in length. A good analogy to look at to confirm this is with water. Watch the ripples emanate out from a point. Each successive wave behind the primary gets smaller in height (intensity) and they get closer together. We can tune around any reflection that we want. The lower the reflective value (earlier reflection) the longer the wave. The same effect can be had at specific ratios though to tune for a different reflective value. You are about to learn how to calculate proper intake runner length for a rotary engine at a particular rpm regardless of porting style. I have already done the piston engine to rotary engine conversions for you. This is it! Go get a note pad and a cold drink because this will take a while to fully understand. Some of you guys are engineers so you'll probably think duh?!

The formula is as follows:

L= ( (1080-EPD) X 650) / (RPM X RV)

L= Intake runner length. This is your answer.
EPD= Effective Port Duration. This is how long in degrees the ports are open for.
RPM= Desired peak power rpm.
RV= Reflective value. Which reflected wave you want to use i.e. 1,2,3,4,etc... The second wave is the strongest! Use 2!

Lets plug in some numbers shall we!

Lets look at the primary ports for an '86-'91 n/a 13B. The ports open at 32* ATDC (after top dead center) and close at 40* ABDC (after bottom dead center). We use 720* as our base point to start figuring out EPD. Since the port opens after TDC, subtract 32* from 720* to get 688*. Since the port closes 40* after BDC, add 40* to 688* to get a total EPD of 728*. You now have one number to plug in to the above formula! So far the formula is (1080-728) X 650 = 228800. Now we need to know what to divide this by. Since the '86-'91 n/a engine has its peak power number at 6500 rpm this will be our rpm tuning number. Also since the 2nd wave is always the strongest we shall use an RV of 2. There are the rest of the numbers for you. Take 6500 X 2 = 13000. Now we have: 228800/13000. The answer: 17.6" There is one other thing to consider though. The reflection doesn't take place at the very end of the intake runner pipe but rather at a distance 1/2 the diameter of the pipe out away from the end of it. (What the hell did he just say?) Go back and read it slowly. Since the primary intake runner is 1 1/8" in diameter we must subtract 1/2 of this value from the length of the intake runner. .56" According to my calculations using this formula the proper intake runner length for the '86-'91 n/a engine is 17.04" The actual length as published by Mazda is 17.1" !!!! Holy crap it works!!! The slight difference can be a matter of many small things. This formula gets very close and then it is just a matter of fine tuning on the dyno. Remember your altitude also plays a very important part. Now you are probably wondering where the speed of sound comes into play for this formula. I simplified it quite a bit but it was one of the variables in the original equation. This is the length pipe necessary for this intake port style to have everything come together at the right time (air velocity, sound pressure wave, etc.)at this rpm. Notice that at no time did I ever mention the area of the intake runners. Since this is far too long for a carburator we merely use a smaller reflective value, say 3. The answer would come out at 11.17". For RV of 4 it would = 8.25". You get the idea. You can use any of these lengths for this rpm but as the reflective value goes up, the benefit goes down. The exception is that RV=2 is strongest. Now go take any known port timing numbers and susbstitute them into the above formula to determine proper length. Also change the rpm peak power number around. It does some pretty cool stuff. If you have any questions on a particular port style just let me know. I'll tell you.

Now back to the plenum. As I stated earlier it does not matter where this plenum is utilized at. It can be the main collection point for the intake runners (use the sizing rule at the beginning of the post) or it can be an air filter housing. The principle is still the same regardless of location. Earlier I said that the maximum speed for incoming air into the intake should not exceed 122.73 mph (180 fe/sec). At this speed there can be only one intake pipe length in a ratio with the appropriately sized plenum volume. Yes we can tune the pipe in a smaller diameter or change the plenum volume to get the same resonant frequency. Velocity through the pipe will also change though. Now we see that while several different designs can be made to work, only one will work perfectly! The others may have the same effect but lesser intensity. This velocity does not correspond with the max velocity through the intake runners though. They should have a max airspeed velocity of .6 mach to be perfect. Again as with the plenum, other sizes can be made to work but the intensity of the air is affected.

If you study all of the numbers I have given out in the total post you should even be able to figure out how far away from the engine a VDI actuator needs to be to work at a certain spot!

Got a headache yet?! Have fun with this one guys! :D

Thanks Q, for correcting me. I had a feeling I wasn't quite right on that. Tacoma Narrows is the one.

And many thanks to Rotarygod. U da man!

And it was Gallopin' Gertie, not Gallopin' Girdie, or so I judge by the name of a pub near the narrows (and it's newer, resonance-compliant bridge.) Though I suppose Girdie would be a more appropriate name for a steel structure.

ah, nice stuff RG. so, exhaust gas resonance tuning relies on mostly the same equations, or do there have to be concessions made for the differences in pressure, temupature, etc??

that was a very lovely depiction of the "point of reflection" for the negative intake wave... if no one "gets it", i could draw a pic... when i get home from work :D

Yes it relies on the exact same equation. The difference is that you use the first reflective value so RV in the equation will be 1. This will get you proper exhaust length to place a collector at on a short primary system. A long primary would multiply this final number by 4. Don't know how this would benefit the Renesis though since 2 of the ports have collected already before they leave the engine. We'd have to find a way to seperate them.

http://www.mae.wmich.edu/faculty/hathaway/classes/me468/Lecture/Lecture08.ppt

It is a Power point ..just click on eash slide to see the next slide

Originally posted by Racer X-8




Oh yeah, well my Mazda with a Probe engine doesn't. (I think) Does it?




If you have the KL series 2.5L V6, then you do have Variable Resonance Induction System... oh yea, the secondary actuator also has a reputation of sticking closed <: smacks head :>


more info... http://www.geocities.com/mikey9t6/car_uvwxyz_vris.htm

Excellent Thread about the RX-8 Intake.

Everyone should read about this before modifying the intake on the RX-8.

[BUMP]

thanks Smoker, nice to see you 'round again :)

My previous car - Acura CL-S - used the same principal. They used a dual stage intake runner to develop the Helmholtz supercharger effect.

A funny thing happened between myself and the Service Mgr at Acura Carland. The actuator motor that changed the butterfly valves in the intake runners died. Well, this is one of the major things that makes a type-s into a type-s. Without it you lose about 30HP. I tried to explain this to the Service Mgr and he was clueless. I had to take him out to a new car and show him the actuator flipping at the key rpm. What a goob!

Alright I know that I'm not the only one that really doesn't care about the techy stuff, so I'll be the brave one and ask. So what does this exactly mean in terms of HP and intake? I don't care how or why things work but I want more HP/torque- so what should (and shouldn't) do to get it, in terms of this thread?

The Mazda 787B engine that won le Mans in 1991 used this type of intake system. The velocity stacks were a straight shot into the engine. The Intake tube was variable in length, telescopic in operation with one tube sliding within another allowing the intake runner to double in lenght. The lenght of the intake runner would change in lenght dependant on the power requirements of the engine. All this was controled by the ECU and changed via solenoids.

http://www.mrccfl.com/images/428.jpg

http://www.mrccfl.com/images/427.jpg

The resonance issue can be explained by looking at the Trombone. The instrument is played by moving the sliding portion of the instument in and out. As you slide it out the effective lenght if the tubes increases and produces a lower harmonic. Move the slide in and the tube length decreases and you will have a higher harmonic tone.

That's all fascinating, but I take exception to your final conclusion (which I included here at the bottom). The Rotary Extreme intake appears to be giving the car some extra power without altering the power or torque curves in a negative way. I don't know if Chuck took all this into account, or just lucked out...but I don't think you can say, despite Racing Beat's comments, that you can't improve on the stock intake if you're willing to compromise on some of the factory requirements like noise. I do think it makes sense to keep in mind that the stock intake was designed the way it was for a reason, and don't just assume that a pipe with a cone filter on the end of it will be better.

jds

Originally posted by rotarygod
You read it right although I have never seen it written anywhere. I usually hang around the RX-7 club forum but thought that since I am a rotary fanatic, and everyone here should know everything about the new RX-8, that this would be a good place to post this.

...Lots of interesting stuff deleted...

Now you all know! Buy a drop in K&N filter but leave the factory intake design alone! Just say no to cone filters for the RX-8!

Beareau13: I'm suggest the analogy of the difference between the trumpet and the trombone. For harmonic resonance, the intake tube, the diameter and the velocity of air have an effect on the resonance. Just as a Trombone has infinate positions to create perfect resonanse, the trumpet has the ability to apprioximate the resonating frequency using "valve" to open and close ant different throttle settings to create the same resonance.

Resonance is the same principle for either instrument, just acheived by different methods. So is the valves on the Renisis engine and the one you illustratied in your photo.

-jcs

Please realize that this thread was started before most people here even had their cars yet and it was written based on the known facts at the time. Racing Beat's findings were the only ones available. Saying that however, Mazda did design the intake to function as I said it does. I am sure that it does tune for certain spots in the power curve. However, if the physical diameter of the available intake pipes is not large enough that the airflow is still not as fast (or too fast) this tuning will be negated. Chuck, K&N, and others are obviously gaining power because their intakes, however tuned, are providing more air to the engine. I am sure that if someone were to redesign the intake in the style of the stock system to account for greater airflow (larger air filter, greater area intake tubes), we would see even higher numbers.

RotaryGod:

I'm confused by your last comment; help me here:

I could not find a member nambed "racing beat's" - or - were you refering to something else?

On your comment about get more horespower via larger intake filters or bigger pipes - yeah I suppose it's allowing more air into the rotary chamber-thus giving more O2 to mix with gas for more power....but is it "tuned" as the renesis was designed?

That's probably not a fair question to have to try to answer. I'd think the bigger filter would create less air restriction, and therefore, more would get in. I'd think a whole new port manifold would have to be designed to be bigger and give more power - including boring out the intake at the chamber at the block (trichoid casing, or what ever you what to call it). Leaving that "stock" IMO, does not allow the ability to truely get more power - there would be a restriction right at the end of the manifold.

BTW, I like your base kit project; I'm very interested in how you will "open end" tune that (refering to open and closed resonance). Very cool project and thanks for sharing.

...just my two cents worth.

-jcs-

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.

RotaryGod:

...obviously I need to read up on Beat Racing's work. I had no clue about the .6 Mach rule or other things that you mentioned.

I know enough to get into trouble I suppose.

Thank you for your insight and I'd be curious as to what your results are.

-jcs-

The .6 mach speed you will really have to dig hard to find. It isn't on the forums though. I was first introduced to this number in a phone conversation with Paul Yaw (the same one on here) several years ago, long before this forum or any rumors of an RX-8 existed. It was years before I actually saw this number published anywhere. I do have a book that you can get on amazon.com. It is called "The Scientific Design of Exhaust and Intake Systems". You'll probably find it quite interesting although it isn't light reading. This book is a wealth of knowledge even though it was written in the '50s. Check it out.

I understand most of what ya'll are talking about relation resonation to the intake system. I know a little about resonation and the physics about it so I'll post a little info. on it maybe it will be helpful.

Lets examine a cord that is attached to the wall at one end and held in your hand on the other. If you shake that end, a continuous wave will travel down to the fixed end and be reflected back, inverted. As you continue to vibrate the cord, there will be waves travelling in both directions, and the wave travelling down the cord will interfere with the reflected wave coming back. Usually this creates quite a jumble. But if you vibrate the cord at just the right frequency, the two waves will interfere in such a way that a large-amplitude standing wave will be produced. It is called a standing wave because it doesnt appear to be travelling. The cord simply appears to have segments that oscillate up and down in a fixed pattern. The number of nodes (lowest position of the waves) and antinodes (highest position of the waves) determine the frequency. The frequencies at which standing waves occur are called resonant frequencies. Also, because resonant frequency waves don't interfere with each other, the wave will not die out quickly as opposed to the jumble mess that was talked about earlier. If you associate this concept with waves created from vibrating air, this is what is being talked about on this thread and related to musical instruments.

Therefore, getting a new intake might not be so beneficial if the stock one uses resonate frequencies and the new one does not. If the new intake has a longer pipe/bigger diameter, the air waves might be thrown off their resonating frequencies and have the jumbling effect I talked about earlier and these air waves will lose velocity and a smaller volume of air will travel down the intake. I don't know if compaines take this into consideration when developing their intakes, though.

I think this is what your talking about in layman's terms but if what I'm saying is wrong please let me know.

Part of the design is to have enough air "mass" moving in the plenum to help push air into the combusiton chambers when the valve opens (inertia, which = mass x velocity)

Inertia is the measure of the resistance to a change in motion of an object. It has nothing to do with time and therefore no relation to velocity. Rotational inertia has units mass times distance squared (this can be found by taking the integral of the mass of any infinitesimal particle of the body times the perpendicular distance of this particle from the axis of rotation. The integral is taken over the whole body). It can also be found by dividing the net torque by the angular accelleration. Yes, angular accelleration has to do with time, but its time units cancel out the time units in torque and once again give you mass times distance squared as units. Maybe you are talking about momentum which is mass times velocity.

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.

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! :eek:

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

almost a 6 year thread ressurection! :eek:

at least he's doing his research.

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

That's exactly what I thought! :lol:

Easter is coming!!!

at least he's doing his research.

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

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 :banghead:

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

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!

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

We do know that what it does utilize tuning in the intake system, it isn't for power. It's for noise control only.

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.

almost a 6 year thread ressurection! :eek:

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)?

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


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!

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