angryhillbilly

Okay so ive been reading, learning and comprehending the Acoustic Supercharging or ram air theory idea.

i read this thread ( https://honda-tech.com/forums/showth...ram+air+theory ) and wanted to ask a question in it but didnt know the rule on posting a new thread vs bumping one from 3 years ago.

so i made a new thread and referenced the old one =]

i have also found this thread to be very educational
http://www.team-integra.net/forum/bl...culations.html

sorry its not from H-T

So i understand the spring-mass idea that waves of air generate a frequency and if a motor is timed correctly, you can get the wave to strike the intake valve as it is open and actually achieve positive intake manifold pressure generating real live "Boost" while N/A

in the second thread i referenced, it speaks of choosing 1 rpm speed at which this boost happens and that would be your peak tourq rpm.

my question is, why does he insinuate that this action can only be done once within a given rpm range. why not twice? or multiple times? or every single time the intake valve opens?

is it possible to time a motor in such a way that every single time the intake valve opens, the positive pressure wave pushes air into the cyl.

or, is this the secret of custom cam makers.

GhostAccord

It's also called intake resonance tuning or Helmholtz resonance tuning. The time at witch the waves hit the back of the valve is based on the length of the intake and the time in which it takes for the sound wave to bounce back and forth down the runner length. Based on the PRM range and intake length you can hit a specific resonance wave.

In order to hit an N/A boost using this you would have to a strong wave resonance. IIRC that would be a 1st or second wave. Or you need to have your intake and exhaust valve overlap and exhaust scavenging all timed within that same RPM range. It is because of all of these things having to be aligned that the rpm range is so small.

Here is a good thread on this topic as well; https://honda-tech.com/forums/honda-prelude-4/%2A%2Acalculating-intake-manifold-runner-length%2A%2A-get-out-your-vtak-calculator-yo-1963201/

It discusses the speed of the sound wave and what lengths are optimum.

angryhillbilly

thanks for the thread post. it helped alot. but this brings a new question to rise.
in the thread you posted he found that the wave at the speed of sound at 70 degrees F and at sea level and 7500 rpm engine speed travels 67.68 inches in the time that it takes his valve to close and re-open again.

so he insinuates that the wave will in turn travel up and down a 13.60 inch runner 5 times in the EXACT same amount of time. and hit that valve perfectly as it is opening.

my question or "doubt" as i would call it is this. we know from laws of physics that in order for an object moving at speed "X" to bounce and go back in the opposite direction it must first come to a complete stop. this would involve a time for it to slow down, stop, and accelerate in the opposite direction. these slowing, stopping and accelerating actions take time. time that i do not see this person accounting for.

interested to what you think about this.

also, he calculated all of this out for one specific rpm. is there any way to get this "boost" more than once through out the rpm range? or can you just tune it once where u want it and maby if your lucky youll hit another.

GhostAccord

Sorry I'm not a physics major, I cannot tell you what the slowdown/stop time of the sound wave is each time it banks off the intake wall and the back of the valve.

On the other question however, These sorts of formulas have constants and variables. You can input/change the RPM number becuase it is a variable. Therefore you can calculate the optimum intake length for any given RPM range.

So if you have a cam that makes it's peak tq @ 5500 rpm. That is the number you would generally want to calculate for.

Formula in that thread that he posted for figuring out the speed of travel for the sound wave. (Not sure about the dead times, sorry) 60 seconds / 7500 rpm = .008 seconds per revolution

or

60 seconds / (Input peak tq RPM here) = .XXX seconds per revolution

60 seconds / 5500 rpm = 0.0109 then you use that answer in the formula.

angryhillbilly

cool thanks for the help. hopefully someone who has done this before can stop in and help me out more.

GhostAccord

You may be waiting a while for someone else like that around here!

N.Balauro

Runner length, Runner Diameter, and for the sake of this conversation valve timing, all remain constant. With these 3 variables static, the strongest pulse of energy will only perfectly arrive at the back of the intake valve at a certain rpm. That certain rpm being a function of runner length, diameter, and valve opening. If none of these change during operation - this super charging effect will only be most effective at one event in time. (Thats why we have dual plane intakes like on the gsr We can tune for both non and vtec valve opening, providing more useable torque.

And only at this perfect time of arrival will you shove the most amount of air into the cylinder and produce the most possible torque. Thus all the "pick where you want peak torque and build from there..."

The stoping and change of direction at the ends of the runner creates a spring like compression and expansion of air.

Note also that reverse supercharging will also occur at another rpm, because of these same principles.

N.Balauro

Length and diameter of an intake runner will create a certain frequency of oscillation in the air mass. The time it takes for the air to change direction defines the shape of the cure at the peaks (If we were looking at an oscilliscope). This is called saturation time. Although for our purposes you shouldnt be concerned about this. The saturation is still considered in the frequency. 2khz still = 2000 cycles per second, regardless how long it takes the wave to turn around ya digg?

angryhillbilly

Here you said "the saturation is still considered in the frequency." But the thread I posted a link to had no calculation which considered frequency. He used the amount of distance in inches the speed of sound would travel and divided it into more realistic runner lengths. He also didn't consider runner width or taper. Is there a better way to calculate this? That the guy in the thread didn't do?

N.Balauro

Well your right - diameter actually has no effect on the resonance or frequency. But it does need to be considered when considering gas velocity and airflow requirements.

Frequency is a function of the speed of sound. I dont have much time atm. Im gonna send you in the right direction!

http://www.brighthub.com/engineering...les/84316.aspx

http://speedtalk.com/forum/viewtopic...=1163&start=15

DDTECH

Its called "Naturally" supercharging..essentially the motor is being over efficient..
Its also a term used in F1 racing, you'll see alot of those motors creating positive manifold pressure..

I actually had this happen on my last motor, I was seeing around .5-.7psi of boost or "positive" Manifold pressure,depending on how cold it was outside and whether or not the intake was in the hood, or out of the headlight.

This is the theory behind sticking the intake, with a Vstack out of your headlight, because you're "pushing" air in.. this would create a "boost like" result.. But there's more to it.. along with induction system, proper cam timing, manifold / runner length, header length, head flow, everything has to work together to help create this..
Volumetric efficiency is generally a ratio of air mass trapped inside the cylinder when the valves close, to the volume of the cylinder under standard atmospheric conditions.

In an FI motor, the whole cylinder is pressurized, so naturally the VE rises above 100%.

In a naturally aspirated motor, well designed intake systems, exhausts, and cam profiles can cause a "natural supercharging" effect. The air flowing into the cylinder has enough velocity / momentum that it continues to cram its way in, resulting in a slightly higher than atmospheric pressure, and > 100% VE.

angryhillbilly

Thanks much man. i will be sure to give these a read while im ever so had at work tomorrow =]

idk man, im no expert as i have only recently been learning about this. but from what i understand, this natural supercharging effect happens, as atleast one time in most engine setups. even if they dont specifically tune for it. but the purpose of tuning for it is to use this effect exactly where you want it (at the rpm of peak tourq) and to its maximum efficiency.

also what i would be interested to test out and crunch some numbers and see how many times you could get this effect to happen with a constant runner length.