Well everyone always wants to know the mathematical way to figure out intake runner lengths so they can design their own manifolds. There are so many things to understand and the math seems to go on and on forever. Since there are books dealing with the subject in great depth I'll just get to the simplified math so you can figure out the perfect legth for your port style. Using this formula you will also learn why a halfbridge or full bridgeport engine utilizing the stock intake manifold has no top end but great midrange power. Here it is:

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

L= Legth of the runners. This is your answer.
EPD= Effective Port Duration (how long they are open for)
RPM= The spot where you want peak horsepower to be. (If you still have the stock gear ratio transmission and this is a streetcar there is absolutely no point in making this anything other than 6500-7500 rpm.)
RV= This stands for Reflective Value. The pressure wave reflects back and forth several times inside the pipe. For the intake the second wave is best so use the numeric value of 2. For a carburated car use 3 or 4 since the manifold may be too long. If you are figuring out exhaust length use 1. this will give you the proper length for a short primary collected system. If you want a long primary system, take the short length and multiply it times 4. OK lets plug in some numbers to prove this.

Let's look at just the primary ports in an '86-'88 n/a 6 port engine. 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, we 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-'88 n/a 6 port engine had a power peak of 6500 rpm this is what we will use for this example. Also since I said the second reflected wave is best to use, use the numver 2 for RV. 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 that slowly. Since the primary intake runner is 1 1/8" in diameter we must subtract half of this value from the length of the intake runner. .56". According to the calculations, the proper primary intake runner length for a stock port '86-'88 n/a 6 port engine is 17.04" The actual length as published by Mazda is 17.1"!!! Holy crap it works!!! The slight difference can be attributed to several small things. First we did not account for how fast the ports open and close. A peripheral port opens faster than a side port. accounting for this would give us shorter runner lengths. We also didn't account for the distance within the rotor housings that the air has to travel. This would add to our length. Basically these numbers almost cancel each other out so I don't worry about it. If you want to know how to figure out down to the last thousandth of an inch it will take some studying. If you look at the above number though we are within .06" of an inch from actual. Close enough. Your altitude where you live will affect it much worse than that.

If you need port timing specs for different year models I do have them as well as some of the Racing Beat port template specs and peripheral port specs. You will see something very neat when you type in the specs for a streetport but retain the factory intake manifold. Your horsepower peak will get lower! For my streetported GSL-SE I actually need a manifold with runners a little over 1" shorter just to retain the stock 6500 rpm peak number. This gets even worse with a half bridge or semi peripheral port. Many people think that since they ported it, their top end power falls off faster because lack of fuel. That may play a small role but they run rich up there anyways so big deal. The real problem is that the stock manifold they are using is too long. BDC if you are reading this it explains why Tony's half bridgeport car has much more midrange power than the streetport but falls off hard on the top end. Before you spend money on a new turbo, build a new intake manifold! Yours is too long. Yes it still matters on forced induction cars too!!!

If we wanted to tune the above engine to have peak power at 7000 instead of 6500 rpm the length would change to 15.75". For 8000 rpm it would be 13.75". Cool huh! Your low end will suffer the higher it is tuned and your power band will get narrower. You may quickly get out of you gear ratio range in which case all your new found power is worthless. If you used a Guru Racing transmission that has a shift point centered at 9500 rpm then you would want to tune for peak power at 9000 which would give a runner length of 12.15". The intake runner diameter may not flow enough air due to size though so this is something else to consider.

Now I suppose you want to figure out proper plenum size. I'll get into that later. I will just say that it gets SMALLER as the rpm limit rises! I'll let you all sleep on that one! If you have any questions about your porting style shoot me a pm and I'll help you out best I can. Have fun!

Cheers!

Fred

 

Most impressive. I have one question. Why do you use the number 720* as a base point for the EPD? Why not use 360* since that is a full circle, and the port is only open for 8* of the total 360*?

And in your equation, what does the 1080 represent, and what does the 650 represent?

 

Ok, working with your equation, I figured that 1080-728) 720 is the number you started with, plus the 8* that the port is open gives 728. 1080-728 is 352, the total amount that the port is closed. 352 +8 is 360, a full circle. I still dont' know what 1080 is, nor do I know why you multiply 352 by 650. Whats the 650?

 

Wow! That is some great info! I always wondered what the math behind it all is. You da man!

-Lee

 

I'm not sure if this is caused by another factor... but I have many FD dynos and you can almost always tell if someone has a street port because their power peak is HIGHER, not lower.

I suppose this might be due to the fact that a street ported car often has OTHER mods such as open exhaust, open intake, etc etc that will influence the numbers.

What do you think?

Brian

 

A turbo car with a streetport will show a gain in a very close powerband to the original. If we were to take a completely stock T-II or a completely stock 3rd gen and just port it, leaving everything factory, you would see what I mean. When you add even a boost control you can change the tuning quite a bit. The turbo can overcome these deficiencies quite a bit if it is still in its efficiency range. A streetport on a 3rd gen motor will only shift the peak power number by about 250 rpm anyways. The peak hp number will be higher as in more power but the peak will be at a lower rpm. Again, this is assuming that everything is completely equal. Forced induction does change things around a bit. Look at a car with a supercharger (roots style). It is usually mounted as close to the engine as possible. Since there is positive pressure at all times it makes up for any lack of manifold. Peak power will be based on the efficiency of the unit. With a turbo we are not always under boost. When we are just cruising around we are basically n/a which is going to need some good length runners to be drivable. Forced induction cars have the luxury of being able to force more air through the manifold even if it is out of its peak efficiency tuning range. There are many very fast cars with bridgeports on stock manifolds. They are all turbos though! Even though the turbo can keep making power up past the runners tuning frequency, it doesn't mean that we can't get much more out of it with the proper manifold. If the turbo is a large single turbo that needs high flow, then we could in fact easily see peak numbers much higher since this is where the turbo is most efficient. It doesn't mean that the manifold is most efficient though. The small factory turbo units will give you results that are more manifold dependent. An n/a has no tolerance. If you put the proper length intake on a turbo imagine what your power would look like!

The number 1080 comes from how many degrees the eccentric shaft turns to cycle through all 6 rotor faces. It does 3 complete rotations. The engine fires every 180 degrees. 720 is the degrees from TDC to BDC that correspond with the total port open time. If the ports open after TDC, subtract this number from 720. If they open before TDC, add that number to 720. These numbers correspond with the port opening. The closing of the ports corresponds to BDC (not Brian Dean Cain!). If the port closes before BDC, subtract the number from the above figure. If it closes after BDC, add this number. The example port opens at 32* ATDC and closes at 40* ABDC. Therefore, 720*-32*+40*=728* There's your number! The number 650 is a simplification of a larger formula which had to convert time to degrees of rotation and also take into account the speed of sound. Don't worry about these numbers. The hard work is done. Here's the shortcut. Even the formula that this originated from is a very extensive simplification of a couple pages of equations. This just seemed easier.

Here are some numbers for you guys to play with. They are port timings for different models of engines and porting styles from Paul Yaw's website. I take no responsibility if they are 100% accurate or not.

IO= Intake opens
IC= Intake closes
EO= Exhaust opens
EC= Exhaust closes

US Model First Generation RX-7/European Model

IO 32* ATDC
IC 40* ABDC / IC 50* ABDC
EO 75* BBDC
EC 38* ATDC / EC 48* ABDC

2nd Generation 6 port 13B

Primary port
IO 32* ATDC
IC 40* ABDC
Secondary port
IO 32* ATDC
IC 30* ABDC
Auxillary port
IO 45* ATDC
IC 70* ABDC
EO 71* BBDC
EC 48* ATDC

Turbo II and 3rd Generation RX-7

IO 32* ATDC
IC 50* ABDC
EO 71* BBDC
EC 48* ATDC

Racing Beat Streetport template (Early 4 port)

IO 25* ATDC
IC 60* ABDC
EO 84* BBDC
EC 48* ATDC

Racing Beat J-Bridge Port

IO 115* BTDC
IC 72* ABDC
EO 88* BBDC
EC 57* ATDC

Mazda Factory Peripheral Port

IO 86* BTDC
IC 75* ABDC
EO 73* BBDC
EC 65* ATDC

Mazdatrix T-II Streetport (Intake side only)

IO 32* ATDC
IC 60* ABDC


These numbers should be quite accurate. If they aren't exact, they should be extremely close. This will give you guys hours of fun with a calculator and dreams of building your own manifold.

 

I thought I'd have some more fun by doing some more math. I'll skip the long stuff and get straight to the answers.

For a stockport exhaust on a 2nd gen n/a, the perfect length headers for short primary and long primary systems are as follows:

To tune at 6500 rpm (stock intake tuning frequency)
This is the best number to tune for based on stock gearing.
short primary: 23.1"
long primary: 95.4"

@7000 rpm
short primary: 21.37
long primary: 88.5"

@7500 rpm
short primary: 19.89"
long primary: 82.54"

If you used the streetport template on the exhaust
@6500 rpm
short primary: 21.8"
long primary: 90.2"

@7000 rpm
short primary: 20.17"
long primary: 83.69"

@7500 rpm
short primary: 19.89"
long primary: 82.55"

Notice how the streetport is tuned lower for the same length pipe than the stock port is. This only gets worse the larger you go. The intake numbers work the same way.

Knowing the above numbers, somebody go measure the pipe length on a set of Racing Beat headers and tell me why the hell they are tuned so well? Are they? No! This only gets worse if you are porting the engine. They tune too low in the rpm range for a short primary and are not nearly long enough for a long primary. Also knowing the above numbers does anyone want to speculate on how well the Pace Setter headers tune your engine? I'll give you a hint: The front exhaust runner is over 8" longer than the rear!!!

The reason that a long primary works so good over the entire power band is that we are now utilizing the 1st, 2nd, 3rd, and 4th order waves twice each! This gives us a much broader power range.

All of you true dual guys are probably wondering how your systems work. Remember that there are 2 different kinds of waves in both the intake system and the exhaust system. We have the actual gas wave itself. This wave speeds up with rpm. We also have the acoustic wave which regardless of rpm always travels at the speed of sound. When we are tuning a runner to a particular rpm, we are tuning it to a resonant acoustic wavelength. On a collected system, the acoustic waves help to scavenge the exhaust gasses out by creating low pressure zones which help pull the gasses out of the engine. When the acoustic waves encounter the collector, the increase in area causes the acoustic waves to change in phase and direction. In other words, the length of the pipe after the collector no longer contributes to acoustic scavenging from the engine. Since these resonances have changed direction at the collector, any acoustic waves after the collector are now helping to pull the gasses away from the collector. There are now 2 points of acoustic tuning within our exhaust! The collector also utilizes the gas pressure wave from one pipe to strengthen the other. They will each pull each other out. A true dual system only has the very first acoustic resonance help. Only 1 spot of acoustic benefit and not 2. The longer the pipe, the lower the rpm where power is gained. If the pipes exit the rear of the car then low end power should be fantastic. If they exit midway under the car then you may have great midrange on up but no help on the low end. You get the idea. As you get higher and higher in rpm, you will hit a point where power falls off pretty hard where the collected systems might keep going. They lack the other scavenging benefits though. On a street driven car with stock porting the true dual may not be a bad thing. Power gain is very nice throughout the powerband due to length and it is still strong at the factory redline. On a ported car or anything even remotely designed around high performance, it should be laughed at. If you have a roots style blower, exhaust design is going to have minimal effect as long as it flows good. Sorry to make this long but I thought everyone should know this. I'm sick of the true dual vs. collected debate. Make your own conclusions now that you know this information.

 

2nd Generation 6 port 13B

Primary port
IO 32* ATDC
IC 40* ABDC
Secondary port
IO 32* ATDC
IC 30* ABDC
Auxillary port
IO 45* ATDC
IC 70* ABDC
EO 71* BBDC
EC 48* ATDC

Those number are off the Yawpower site, and are wrong. that is the Port timing of the gsl-se motor, second gen 6-ports close at 80 degrees after.

also why in the hell would mazda tune the manifold soley around the Primary ports, which are mainly used for crusing??????????, if they did tune around the primarys dont you think the number would be somewhere closer to a 3000 rpm tune. Also if you plug the REAL port timings of a 2nd gens 6-port's and runner length you get 5929 rpm, which i think is more than believable then tune on the primarys being 6500 ( with thier small cross section the primary ports are WAY out of there effeciency range at this point) also that equation is runner length for most effeciency(torque) at a given rpm, NOTHING more it will not give you peak hp numbers, if you tuned an rx7's runners to 2000 rpm i doubt VERY much that the peak hp will occur at 2000 rpm, you may get the peak TORQUE there. I was also under the impression that the tune on a S4 manifold is somewhere around 4000 rpm ( stock torque peak is stated at 3500 but most of the dynos ive seen of stock cars place it closer to 4), Maybe we should try the equation on a motor which we know for certain has a certain runner lenght that corisponds to a particular peak torque. r26b anyone?

 

I just used your equation for the r26b, aiming for a tune of 6000 RPM, which is the torque peak for the motor with its telescopic intakes fully extended. I got 273.79mm(10.7792 in.), which is WAY to short. each section of telescopic intake is 175mm long, that makes 350mm or 13.78 inches, also the telescoping section connects to another peice of pipe which is atleast 3 in. long itself, that makes 16+ inches total, atleast.

 

I think your second constant is wrong, and should be recalculated.

 

Thanks for the explaination rotarygod. Its not that I didn't believe you, I just wanted more of an understanding as to how the formula worked.

 

Like I said, I don't take any responsibility for the accuracy of the numbers on Paul Yaw's website.

Mazda didn't tune solely off of the primary ports. They tuned the primary and secondary runners at different lengths because the port timing is not the same between them. The secondary runner length on the '86-'88 6 port manifolds is actually 16.2" long. When I figured it out I came up with is 16.09". Close enough. You can not tune one port to a lower rpm while the other one gets tuned to a higher rpm. You could only do this if each port were never open at the same time but rather switched off to each other like the Ford SHO runners do. When you are designing multiple runners with different port timings you have to tune each set of runners to the same rpm which corresponds to the desired horsepower not torque peak. Remember that at full throttle all of the ports receive air at the same time. If one set were tuned to 2000 rpm and one set at 6500 rpm, there would be some pretty bad things happening. Even the intake runners on a Honda V-tec engine are tuned for their horsepower peak and the fact that V-tec IS working at that point. I did the numbers on my Civic and came up with the exact same manifold that is on it from factory!!! I would just change the length of the pipe that the air filter is on to tune it where I want it. By running air only through 2 ports at lower rpm's, Mazda is utilizing inertia from the air to help keep low end power good. Remember that any runner that is tuned for a gain at 6500 rpm is also tuned for good power at 3250 rpm. Where was the torque peak on the '86-'88 6 port? 3500 rpm? Hmmm. See a pattern. That 250 rpm difference can be due to the fact that the secondary runner is tuned around the timing of the aux port and not the secondary port that shares the same runner. In fact it is! Plug in some numbers you'll see!

The formula figures out for maximum power not torque at that rpm. Remember that torque and horsepower are mathematically related. If you have more horepower then you have more torque. If you want to figure out the length where torque is the answer then plug in 5252 for your rpm. It doesn't mean that this will be the peak torque number though. Where did you get the port timing numbers for the R26B? Did you use the Mazda factory peripheral port numbers from above? They probably tweaked their runner diameter and timing around what was best for that race car. It doesn't mean we can buy the same housings or that the specs on the commercially available ones match. Even if we do use the numbers above, try a different RV. If you use 1 then you get a total of 19.5" in length. RV=2 is generally best to use for space reasons. Thats why I said to use it. If Mazda changed their numbers at all regarding port timing, the difference could be there. Somewhere your info of the R26B is inaccurate.

I didn't make the formula up. I just figured out how the rotary timing works so we can plug in the proper numbers. I do know how to mathematically arrive at this formula though. It absolutely does work. I even know how to design a VDI system using some other calculations. I went over all of this many times before posting it here to make sure it is accurate. It is! The formula does work!

 

I forgot to mention this. Note that nowhere does the formula take into account intake runner diameter in relation to length. It doesn't need to because it doesn't matter. The sound waves we are tuning around are the same length no matter what diameter pipe they are in. So you can't say that the primary runners are out of their efficiency range. They obviously are not. What does count is the total are of all intake runners feeding the same chamber combined. This total area is what will ultimately determine where the enigne will make most power at for the proper timing. I'll add the runner diameter formula in later but it has nothing to do with runner length! If you don't believe me go look at the new Renesis as a fantastic example. It has 3 sets of runners all at different lengths since all ports have different timing. This is how it should have been done all along. If a small port runner is out of its efficiency range then explain to me why the engine revs up to 9500 rpm but has individual skinny runners for the aux ports. If we add up the total area for all the runners we will see it is the sum off all parts and not the individual which matter.

If you really want to get technical I'll start to explain why the air filter housing on the RX-8 is really a Helmholtz resonator and is designed to boost power at different areas. The numbers work here too! Want to figure out perfect header design for the RX-8? I can even help here regardless of the Siamesed center port! The design is cool! I haven't seen it anywhere else yet.

 

Fine and dandy for the straight intake path. How about curves and such? I'm guessing you'd use the center of the path to calculate the length along a radius? How would curves and bumps in the intake runners alter this equation? Aren't sound waves influenced by the path they follow?

--(Disclaimer: Not pretending to be a rocket scientist or anything, just asking some common sense questions)--

 

Good questions! Yep you would just measure the centerline. Calculations for soundwaves follow this rule. I spent several years in acoustics and know this to be true. What curves will do is to slow down the flow since it can be a restriction. A tight curve will have the same effect as making the pipe smaller. Remember that the formula is not dependent upon pipe size. In a perfect world we would have purely straight runners (like the 787B). If we do have turns though it is important that they be as gradual and smooth as they can get. Bumps in the runners would have the same effect. They may play a miniscule effect if they were big enough and only if the soundwave bouncing backwards to the other side was strong enough to have a negative effect. Tuning wouldn't be effected whether it was purely used as cast or if it was extrude honed. These bumps may effect power due to actual airflow but will not affect the acoustic aspect as far as length is concerned. Just make sure you don't have any!

 

I admit i was streaching the r26b thing a little, i used mazda factory specs, but even if the ports were slightly differnt, i dont think it would work out to 7 inches of differance. I dont think using 1 would work, woulnt 1 be no reflection, which would be a negative wave?, if not then 2 would be a negative wave and couldnt be used, which could be why the equation didnt work.. Let me redo the calculations, but instead of using a set rpm ill use the differance between say 6000 and 9000rpm in runner length, which i do know is 175mm. I still, however am assuming the motor is close to a mazda factory P port, it may well not be however, because the housings were completly custtom cast. Also if the equation is accurate, it is for runner length tunning to determine at what rpm your manifold is most effeciant, and gives the largest bost in ttorque. it has nothing to do with peak HP, you could tune your runner length all you want to 11,000 rpm, but i gaurentee you a stockport motor will still have peak hp somewhere between 5000-7000, you may have boosted torque production at 11,000 rpm by as much as 20%, it still wouldnt be enough to overcome the flow restritions of the ports, thus this equation is for a manifold induced torque increase, at a certain rpm, NOT peak hp. Also, the primary ports are way outa of there efficency range at 6500 rpm, not there pressure wave effeciency range, but the port itselfs limitations. Airflow is most effecient when it flows at .6 mach, and the primarts are much to small to have a velocity at or lower than this at the cfm the motor is pulling at 6500 rpm. I was not aware the rx8 used a helmholtz set up, i havnt really looked closely at the rx8's intake, i would love to see however, if youve got a link or anything.

 

6000 rpm
rv:
1=21.558
2=10.779
3=7.186

8500 rpm
rv:
1=15.218 in.
2=7.609
3=5.072

Differance:
1=6.34=161.036 mm
2=3.17=80.518
3=2.114=53.9
an rv of 1 get us very close to the correct value, the r26b's ports being slightly more agressive in timing could easily make up for the differance, aswell as a number of other things.
This however means that you were using the wrong rv. for intake and exaust calculations, both of which require atleast 1 reflection, so im guessing rv of 1 means 1 reflection, which would mean that you could not use 2 as a value, unless you wanted a negative wave returing to your port sucking some intake back out. Exaust too would have to be odd numbers for the rv. or youd have positive waves returning and forcing exaust back in.

 

Also very interesting, if you use the stock manifold runner lenghts you've provided, and an rv of 3 you get a number around 4000 rpm ( a lil more), which corrisponds better with the torque curve. second, yout thing about the manifold being tuned for 2 differnt values, doesnt work unless the motor has VERY VERY high redline, because the second tune would not occur untill 3x the rpm of the first one, which is why we have such things as variable length intake runners, and vdi's. If the stock manifold had a tune on it of 6500, and 3500, mazda wouldnt need vdi because the manifold woul allready have midrange and top end, but it doesnt. If you look at a stock s4 NA rex's dyno run youll see torque it falls off much after 5000 rpm, if the manifold was tuned for 6500 rpm, youd expect to see the rise of another curve around this point, which simply doesnt exist. and the rx8's aux ports are much larger than the primarys on s4 na's, which is why they are still in their effeciency range at high rpm, also i believe the runners are larger diameter than our primarys aswell. I also dont understand how the helmholtz on the rx8 would work given all the differnt port timings, is it only for one set of ports or soemthing?

 

I understand what you are saying. However the formula is for figuring out where you want your horsepower peak to be not your torque peak. There is no debating this. If you can honestly tell me all of the math variables, how they fit together, and where torque comes into play, then I'll believe this. I'll give you a hint: Torque is not one of the variables! Torque IS however directly related mathematically to horsepower. One horsepower number at a certain rpm will always give us the equivalent torque number for this rpm. We can't add horsepower at this same rpm with out adding torque or vice versa. You have the mathematical proof that the formula figures out horsepower peak right in front of you!

The primary ports would only be out of their efficiency range if they were the only ports to be used and no others were added in. However it is the total combined intake runner area that matters for total power potential and not just one runner (primaries). The only time a single runner area matters is if it is a peripheral port and only has one runner per rotor. The total runner diameters of the Renesis added up should be more than the 2nd gens. It has a higher rpm limit. It doesn't mean that they are alot bigger. Since you have 3 runners per rotor, they are still each going to be small in diameter so size in fact is not a tuning issue.

Yes the stock S4 torque starts falling off at 5000. It should based on the hp peak at 6500 rpm. Torque can not be greater than horsepower above 5252 rpm. By this point horsepower is still going up so torque is going down. Renesis peak torque is at 7500 rpm with a hp peak at 8500 rpm. Torque is still lower than hp above 5252 though. The example isn't valid since just because they have to cross at a certain point doesn't mean that they still can't be going up above that point.

VDI is designed to utilize the same phenomenon that we tune our runners around to give us a boost at other rpms as well. We don't have changing length runners but we can open up a valve that simulates it! You want your VDI to open about 1000 rpm below your hp peak to help broaden the powerband. It just happens to correspond with the torque peak of the Renesis at 7500. The S5 uses runners which are too long to tune at the proper high rpm. This gives them more average power throughout the rpm range. The VDI opens up to restore and even give a boost to the top end. Ingenious design. The variable length runners of the R26B are designed to maximize power throughout the usable rpm range. They use the term of flattening out the torque curve. If you stabilize the torque curve, you also affect too. They get more usable hp throughout the powerband.

Helmholts resonant tuning uses resonant frequency of the length of a pipe to create a low pressure zone in the plenum. There are only 3 variables to figure out Helmholtz effect. 1. Size of the plenum. 2. Intake pipe diameter (not intake runners). 3. Intake pipe length. thats it! Notice that nowhere is port timing a factor. Think of it in ported speaker box terms. If we have a 1 cubic foot box with a 3" diameter port (intake pipe)that is 6" long installed in the box (plenum), it will tune this box to a particular frequency. The size of the speaker (engine displacement) that is to be installed in it has nothing to do with what the box is tuned to. No matter what size we install, it will always be tuned the same! It doesn't mean that this will be the perfect combination but there are other totally independent figures that work all of this out. The Helmholtz plenum on the Renesis is the factory air filter box. It is a certain size. There are 2 intake pipes (high power version). The long one makes the box tuned to a frequency benefitting a lower point in the power band. At 7250 the short tube opens up and retunes the airbox to a higher frequency. This tuning causes a low pressure zone in the airbox which helps draw more air into it from outside. If more air is drawn into the box then more air will enter the engine. Port timing had nothing to do with this. Just for info sake, the shorter intake pipe and VDI both open at 7250 which really helps give us a boost in the 7500 rpm spot since we have 2 independent spots both taking advantage of an acoustic effect. Where is the torque peak? 7500 rpm!

You sound eager to learn and I do welcome all questions. We should all help each other here! For some very good reading go to Amazon.com and buy the book Scientific Theory of Exhaust and Intake Design. It is a wealth of info. 257 pages or so worth!

 

This equation does not give you peak hp however, it cant. if you toke a pport, and tuned the runners for 2000 rpm, it would still not have a peak hp at 2000 rpm, and i know that hp and torque cross at 5250, that hp will be higher than torque past this point has nothing to do with the equaton. The equation has nothing to do Directly with HP, or torque, but when your intake manifold will preform most effeciently. The reason i express this in torque peaks and not HP peaks is that torque is an Instantaneous meassure of rotatonal force, where as HP is a measurement of power, or force over time, therefore, the equation is more directly related to torque, which is in turn related to HP.

-*Yes the stock S4 torque starts falling off at 5000. It should based on the hp peak at 6500 rpm. Torque can not be greater than horsepower above 5252 rpm. By this point horsepower is still going up so torque is going down.*-

Because the HP is goind up, does not mean the torque has to go down, you could have a motor which makes more torque at 6000 rpm than it does at 3000 rpm, and still have a HP peak of 6500 rpm HP going up DOES NOT correlate to torque going down, if the manifold were tuned for 6500 rpm you would see a raise in torque around there, look at any s5 dyno, you can see the vdi open because the curve starts to raise again after 5500 rpm, you would see atleast a flatening of the torque curve on an s4 if the manifolds were tuned for 6500 rpm, probably starting at about 5000 rpm, which doesnt not exist.

-*The primary ports would only be out of their efficiency range if they were the only ports to be used and no others were added in. However it is the total combined intake runner area that matters for total power potential and not just one runner (primaries). *-

I am not disputing that total intake runner volume matters for total power, what im saying is that the runers and ports are so small that they will not flow very much and still be kept in there peak velocity of .6 mach, thus there not pushing much air at high rpm

My comments on the manifold not being able to be dual tuned, were not directed twards the vdi system, i am aware that it has a dual tune, due to its change in geometry at 5500 rpm, i was refering to a s4 manifold which cannot have a dual tune within its stock rpm range.


-*Remember that any runner that is tuned for a gain at 6500 rpm is also tuned for good power at 3250 rpm. Where was the torque peak on the '86-'88 6 port? 3500 rpm?*-

This is NOT true, at 3250 rpm you would have LESS torque if the manifold were tuned for 6500 rpm because it would be utilizing the second reflection, which would be a negative wave beign reflected back which would be exactly converse to what you want, which is why there is VdI as i stated earlier, to have long runners at lower rpm for good low end, and shorter runners at high rpm for top end. If, like you state one manifold length tunes for 6500 rpm, and 3250 rpm, we wouldnt need vdi now would we?

The 3 factors you list for the helmholtz are the correct 3 factors, however, the banks of intake runners must be "even firing" in piston terms, or else the pressure wave will arrive and have no where to go, a rotary with 3 seperate intake port timings would not be "even", as the ports will open at differnt times, which are not evenly spaced apart, thus the resontor would be creating pressure waves for some ports that were not open yet, or allready closed at some points. [[edit: the duration of the 3 ports would have to be the same, not the opening and closing times, as each port would make its own wave for the resonator, but if there durations are differnt the wave could reflect back when the port is closed or in the middle of its cycle, which would not be optomal]] also its the Positive pressure wave that is utilized by the resonator to make power, not the negative wave. The resonator reflects the positive wave into the plenum to "ram" air into the runner which is open at the time, a negative pressure wave in the plenum at this time would be counterproductive, causing a slight vacume in the plenum the motor would have to work against. I can quote many sources which state the positive wave is used, not the negative one.





Manifold tunning lenght is ALLWAYS related to torque, not HP as it is a much more direct correlation

 

I'll look into that book, do you think they'd have it at a regular bookstore?

 

L = ((ECD × 0.25 × V × 2) ÷ (rpm × RV)) - ½D

Here is the unmoleted piston version of the equation, i did the calculations myself, and a7 25 degrees C, sea level, and 0% humidity,(SAE standards) 1087.48 ft/s would be the correct number for V, 543.75 would be the number to use as the first constant if you were going for SAE standards, however your version with 650 would be much closer to the average of where most of us live, so it is a good value to use. Other than that your adaptation of the equation too rotaries is Flawless. That equation Is listed in all the sources ive found as the equation for optimum intake runner lenght for a given rpm, not peak HP, and basically increases the VE of the motor at that rpm, increasing torque, which increases HP. I also beleive i was WRONG in stating that only odd values could be used for RV, as i believe RV is refering to a set of reflectins, not induvidual reflections, thus any value could be used.
You were also right about the manifold being able havie dual harmonics, however the It would be utilizing more bounces, be less ideal and not add as much VE as the lower RV bounce. I still think the stock S4 manifolds are tuned to 4000 rpm however. Do you happen to have the runner length of a S5 with vdi open? the full lenght(one port to the other) and the port too vdi lenght.

 

Also have you ever noticed how the runners to the rear rotor are about 1-2 inches longer than the front rotor due to the LIM?

 

What temp, humidity and altitude did you use for your speed of sound calculation?

 

Damn our responses are going to turn into small books!

I love a good healthy debate! Its helps us all learn This is cool dude! I'm not trying to be a punk (although I am one!).

Actually the exact formula that I used is :

L= ((1080 - ECD) X .25 X 1300) / (RPM X RV) - 1/2D so it does match yours. This is good.

This formula plus the formula for intake runner diameter combined let us tune for any particular rpm tuning that we desire. We can in fact design a runner that is tuned for 2000 rpm. It is easy but we have to shrink down the total runner diameter if we want peak efficiency at this point. Notice I said efficiency and not power peak. Power peak is there. Since the port runners on our cars are designed at a certain size from factory, it is logical to assume that they designed the total area of the runners to correspond with the desired horsepower peak and corresponding rpm limit to obtain the efficiency that they wanted. Like the Renesis, they would use a total area of all runners added together to determine the max speed of flow through them at their max desired rpm. Since the Renesis redlines 2000 rpm higher then it makes perfect sense that the total area of all ports combined is greater. It is also because of the greater total runner area that low end torque is less. If we wanted to optimally tune for a lower rpm then we would optimally want less total runner area. However I use the term optimally. Runner length is what determines power peak and not diameter. Intake runner diameter is what determines how efficient the powerband is for a particular length at the desired rpm. We can tune an engine for peak power at any rpm even after the velocity has passed .6 mach. This doesn't mean however that it couldn't make more power with a different runner diameter. Obviously the stock 13B runner size is too large for a runner tuned to 2000 rpm to be at its max efficiency but it would still be tuned to 2000 rpm.

I am aware of the .6 mach number because I am the one who posted it on here. The reason I know it is from a phone conversation I had several years ago with Paul Yaw. He didn't write it anywhere. I am also the person who first posted the importance of a 3" straight section out of the exhaust due to flow disruption reasons. Again this came from Paul Yaw several years ago. One conversation with him is a virtual plethora of knowledge. The one thing I did make an error on when posting the importance of the .6 mach number originally is, coincidentally enough, making the same argument that you are about its importance. I did use to think the same way but there was one thing that I wasn't taking into account, total runner area combined. I used to state the EXACT same thing that you are now thinking that the correlation has to be a direct function of intake runner length to intake runner diameter. I used to swear by this rule! This just isn't so. Where it is true however is in the Helmholtz tuning but we'll deal with that later. The formula for total runner area and the formula for total runner length are 2 completely independent formulas that are not dependent upon each other. The output efficiency however is tied in with proper calculation of each of these factors. We can tune any port combination to any rpm that we want solely by changing the length of the runners. This doesn't mean that a smaller or larger diameter runner wouldn't help or hurt power at this rpm though. For this particular equation just ignore the .6 mach number. It will come in to play when we use the formula to determine the the optimum runner diameter to determine max efficiency at the desired rpm. It is a concept that is a little hard to grasp at first.

Like I have said (and you have understood!) it is the total runner area combined that is important. The primary ports and runners being so small would be out of their efficiency range extremely quickly if they were the only ports. Once the area for the other runners in the same housing is added in, velocity through any single runner is no longer an issue. Air will not flow faster through these ports, in relation to the other ports, since it is also flowing through the other ports as well. Velocity with the other ports working will never exceed .6 mach until a very high rpm. It doesn't matter if we have a primary port that is 1.25" in diameter and a secondary port that is 1.5" in diameter. Since they are tied into the same chamber, they will flow at the exact same speed as each other since airflow will average itself out over the sum of the total available flow area. Now that we know this it makes absolutely no difference if we have:
1.25" primary + 1.5" secondary
1.38" primary + 1.38" secondary
1.94" single port
or even 6 port runners .8" in diameter.!

It is the sum of the total but airflow through any of these runners will always yield the exact same speed at the same rpm regardless of configuration! Different port timings fo different runners will effect this minimally since it is the average speed over the total duration.

If we look at the max efficiency rpm as to how it works in the Renesis (I love that motor so I use it as an example often!), only 2 runners are being used at low rpms. The runner length still has to be tuned for the peak power rpm regardless. Since we are only flowing all of our air through these 2 ports, we are relying on air speed efficiency to help us. When these ports reach an rpm where airspeed reaches max efficiency (.6 mach?) then a valve opens to add in 2 more runners. Now that their total is added into the mix, airspeed in the first runners decreases and matches the speed of the additional runners. As the engine rises in rpm these runners soon hit their peak speed efficiency range somewhere around 6250 rpm and the third set of ports opens. Again since more area is added in, the velocity in the runners averages out and the first two sets of ports slows down a little. Now that all of the runners are open we will again encounter the same air speed problem that the other runners had as rpm rises. We will come to a point where total area will hit result in max air velocity. Luckily this corresponds with where redline is. Where the power peak is desired is where all of the runner lengths in proper ratio with all of the port durations are tuned to. In this case it is 8500 rpm. So now after reading all of this you are probably wondering how, with 3 different length intake runners, can air possibly be flowing through them at the same speed at the same rpm. It is really quite simple. If all of the ports had the same duration of timing this would not be so. If we compare a short runner to a long one and the air is moving at the same velocity to the same spot then there must be another factor coming into play. The shorter runner will be on the port that has the longer open time which sounds kind of weird. Luckily we can back this up with the above formula. It doesn't need as long a runner to tune to the same rpm since timing makes up for the difference. If we were to average out the speed of the incoming air into each respective port in relation to how long each port is open, we will see the same velocity per time. It's really cool how it all ties together.

You know I had an S5 manifold until about a month ago. I'm kicking myself for not measuring the damn thing! The total runner length itself does not change when VDI opens. What does change is the distance that the returning acoustic wave has to travel before it reaches the other port and helps it. Since speed of sound is constant this obviously has to be a shorter path as rpm rises.

If you look at the factory intake manifolds, you'll notice that the back sits up higher in relation to the engine than the fronts do. The front runner slopes backwards and looks longer but the back goes up higher and makes up the difference. Saying that though I found this to be true on the T-II manifolds and not the n/a manifolds. You never know why manufacturers do the little things they do when they spend so much money to design it. Look at the factory exhaust manifold as a prime example.

Looking back to your response, yse the second wave is not as strong as the first. This doesn't mean though that there isn't help in this range. By no means was I implying that peak torque is solely based on a reflection point of a particular wave. It is a function of several things including engine displacement, runner length, port timing, and rpm but not solely dependent on any one factor. The S4 torque curve just happens to go down by 5000 because the horsepower curve is leveling off. Give me any horsepower curve and I can transpose the torque curve on it using only the horsepower information and be 100% accurate.

Your Helmholtz information is correct provided that the plenum itself is the actual plenum collecting all of the intake runners together. However, the phenomenon can be used in locations other than as the main plenum if you know how!

Go check out at a Barnes and Noble or a Bookstop and see if they can order the book. Its not exactly light reading but I find it very informative. My girlfriend doesn't agree however and she's a math major!

Here's a cool website that shows powerpoint notes from a Mechanical Engineering class on this exact subject. Warning: the math is very big!

http://www.mae.wmich.edu/faculty/ha...e/Lecture08.ppt