Racer X-8

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...

QuantumTheory08

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.

Racer X-8

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.)

QuantumTheory08

...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/te...oup1Tacoma.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.

rotarygod

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

Racer X-8

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!

8_wannabe

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.

wakeech

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

rotarygod

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.

gettingan8

http://www.mae.wmich.edu/faculty/hat.../Lecture08.ppt

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

r0tor

iTrader: (1)

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

Smoker

Excellent Thread about the RX-8 Intake.

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

[BUMP]

wakeech

thanks Smoker, nice to see you 'round again

Chrisbert

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!

Goldenhue22

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?

Rotary Nut

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.





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.

bureau13

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

QuantumTheory08

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

rotarygod

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.

QuantumTheory08

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-

rotarygod

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.

QuantumTheory08

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-

rotarygod

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.

GooOnYou

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.

GooOnYou

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.