**Calculating Intake Manifold Runner Length** get out your vtak calculator Yo!
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From: Burnout Box, IA, U.S.A.
**Calculating Intake Manifold Runner Length** get out your vtak calculator Yo!
I just got done workin through this on another site and thought it should be posted here as well. I originally did the calculation for a stock cammed H23 making peak power at 6k rpm (guesstimate), but I'll figure it here for an H22 with skunk pro2's since thats whats going into my build. I'm also gonna aim for a length with a peak power at 7500 rpm (even though that may be a little high)
Basically were tuning the runner length of a manifold so that the resonant pulses coincide with the opening of the intake valve. These pulses happen when the a mixture is headed down a runner and the intake valve slams shut. A sound wave hits the closed valve and starts to travel a "pulse" back up the length of the runner. When it reaches the end of the runner it turns around (Idk how, haha) and heads back toward the valve. That is called a wave or ONE wave. The whole idea behind this is to tune the runner length so that this wave arrives at the intake valve at the moment it opens to help carry in the new mixture.
So first things first we need to know how long it takes from the time the intake valve closes to the time it opens again. This is where the particular camshaft and the desired peak power rpm point is considered. The duration of a cam is how long it is open, measured in degrees of a 720 degree engine cycle (2 rotations)
The skunk2 pro2 happens to have 270 degrees of duration, and my target rpm for peak power is going to be 7500 rpm. Get out the calculator!
60 seconds / 7500 rpm = .008 seconds per revolution
.008 X 2 (# of revs in a 720 degree cycle) = .016 seconds per cycle
Now since you know that there are 720 degrees in an engine cycle you can subtract the duration of the cam to find out how long the valve is closed. this is also the length in degrees from intake valve closing to re-opening.
720 - 270 = 450 degrees
Now we do a little cross multiplying...
take the sconds per cycle number (.016) and multiply it by the degrees that the intake valve is closed.
.016 X 450 = 7.200
then divide that number (7.200) by 720 degrees...
7.2 / 720 = .010 seconds (intake valve closing to re-opening)
Now we need to figure how far a sound wave can travel in that amount of time, so we use the "speed of sound" which equals 1128 feet per second at 70 degrees F and also at sea level. If you want to get more involved find out the speed of sound at your elevation and intake air temp.
We will convert 1128 feet per second to inches per second by multiplying it by 12 (12 inches in a foot)
1128 X 12 = 13536 inches per second
So we know how far sound can go in one second, but we need to know how far it can go in the amount of time from intake valve closing to re-opening. We already figured that to be .01 seconds at 7500rpm All you have to do for this multiply that number by the speed of sound haha (that just sounds cool)...
13536 X .01 = 135.36 inches
since the pulse will travel up AND down the runner, we can divide this number by two.
135.36 / 2 = 67.68 inches THIS IS THE MAGIC NUMBER!! well, sort of.
If you made your runner this length, measured from valve to end of velocity stack, the resonant pulse would travel up and down the runner and at 7500 rpm's the intake valve would open just as the pulse was hitting it giving the mixture a little "boost" as it enters the cylinder.
Obviously you can't fit a 68 inch runner under your hood so you want to tune the length of it to a multiple of that. This is whats reffered to as tuning to the 3rd wave, 4th wave etc. where the wave travels up and down the runner multiple times before the intake valve opens. Heres some scenarios...
68" = 1st wave tuned
34" = 2nd wave tuned
22.66" = 3rd wave tuned
17.00" = 4th wave tuned
13.60" = 5th wave tuned
Keep in mind that this is the distance from the intake valve to the end of the velocity stack. I'll guess that the distance from the valve to flange on an h22 is about 3 inches. Subtract that from the numbers above and that will be the length of the manifold assuming its a straight runner itb setup. Making the runner longer will shift the power band lower and shorter will shift it higher, UNTIL it comes into the range of the next "wave"
If you play around with different rpm points you kinda map out what works best and see how it changes the numbers you come up with. Keep in mind this is just a starting point though and factors such as feet above sea level and air temp will alter these results.
If you're up for a challenge you can take the runner lengths of our existing manifolds and backfigure at what rpm the resonant waves will line up the best at.
ENJOY
Basically were tuning the runner length of a manifold so that the resonant pulses coincide with the opening of the intake valve. These pulses happen when the a mixture is headed down a runner and the intake valve slams shut. A sound wave hits the closed valve and starts to travel a "pulse" back up the length of the runner. When it reaches the end of the runner it turns around (Idk how, haha) and heads back toward the valve. That is called a wave or ONE wave. The whole idea behind this is to tune the runner length so that this wave arrives at the intake valve at the moment it opens to help carry in the new mixture.
So first things first we need to know how long it takes from the time the intake valve closes to the time it opens again. This is where the particular camshaft and the desired peak power rpm point is considered. The duration of a cam is how long it is open, measured in degrees of a 720 degree engine cycle (2 rotations)
The skunk2 pro2 happens to have 270 degrees of duration, and my target rpm for peak power is going to be 7500 rpm. Get out the calculator!
60 seconds / 7500 rpm = .008 seconds per revolution
.008 X 2 (# of revs in a 720 degree cycle) = .016 seconds per cycle
Now since you know that there are 720 degrees in an engine cycle you can subtract the duration of the cam to find out how long the valve is closed. this is also the length in degrees from intake valve closing to re-opening.
720 - 270 = 450 degrees
Now we do a little cross multiplying...
take the sconds per cycle number (.016) and multiply it by the degrees that the intake valve is closed.
.016 X 450 = 7.200
then divide that number (7.200) by 720 degrees...
7.2 / 720 = .010 seconds (intake valve closing to re-opening)
Now we need to figure how far a sound wave can travel in that amount of time, so we use the "speed of sound" which equals 1128 feet per second at 70 degrees F and also at sea level. If you want to get more involved find out the speed of sound at your elevation and intake air temp.
We will convert 1128 feet per second to inches per second by multiplying it by 12 (12 inches in a foot)
1128 X 12 = 13536 inches per second
So we know how far sound can go in one second, but we need to know how far it can go in the amount of time from intake valve closing to re-opening. We already figured that to be .01 seconds at 7500rpm All you have to do for this multiply that number by the speed of sound haha (that just sounds cool)...
13536 X .01 = 135.36 inches
since the pulse will travel up AND down the runner, we can divide this number by two.
135.36 / 2 = 67.68 inches THIS IS THE MAGIC NUMBER!! well, sort of.
If you made your runner this length, measured from valve to end of velocity stack, the resonant pulse would travel up and down the runner and at 7500 rpm's the intake valve would open just as the pulse was hitting it giving the mixture a little "boost" as it enters the cylinder.
Obviously you can't fit a 68 inch runner under your hood so you want to tune the length of it to a multiple of that. This is whats reffered to as tuning to the 3rd wave, 4th wave etc. where the wave travels up and down the runner multiple times before the intake valve opens. Heres some scenarios...
68" = 1st wave tuned
34" = 2nd wave tuned
22.66" = 3rd wave tuned
17.00" = 4th wave tuned
13.60" = 5th wave tuned
Keep in mind that this is the distance from the intake valve to the end of the velocity stack. I'll guess that the distance from the valve to flange on an h22 is about 3 inches. Subtract that from the numbers above and that will be the length of the manifold assuming its a straight runner itb setup. Making the runner longer will shift the power band lower and shorter will shift it higher, UNTIL it comes into the range of the next "wave"
If you play around with different rpm points you kinda map out what works best and see how it changes the numbers you come up with. Keep in mind this is just a starting point though and factors such as feet above sea level and air temp will alter these results.
If you're up for a challenge you can take the runner lengths of our existing manifolds and backfigure at what rpm the resonant waves will line up the best at.
ENJOY
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Re: **Calculating Intake Manifold Runner Length** get out your vtak calculator Yo! (Rosko)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Rosko »</TD></TR><TR><TD CLASS="quote">A sound wave hits the closed valve and starts to travel a "pulse" back up the length of the runner. When it reaches the end of the runner it turns around (Idk how, haha) and heads back toward the valve.</TD></TR></TABLE>
didn't have time to read the whole thing yet, but when a wave comes out into the plenum, it sucks air back into the runner in the same waveform, due to the negative pressure "wave" that coexists with the positive wave....
didn't have time to read the whole thing yet, but when a wave comes out into the plenum, it sucks air back into the runner in the same waveform, due to the negative pressure "wave" that coexists with the positive wave....
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From: Further down the spiral, TX, usa
Re: **Calculating Intake Manifold Runner Length** get out your vtak calculator Yo! (Rosko)
Very interesting.
Here's some more info from a howstuffworks.com article (if you don't mind me posting it):
How do tuned intake runners work on your car?
The intake system on a four-stroke car engine has one main goal, to get as much air-fuel mixture into the cylinder as possible. One way to help the intake is by tuning the lengths of the pipes.
When the intake valve is open on the engine, air is being sucked into the engine, so the air in the intake runner is moving rapidly toward the cylinder. When the intake valve closes suddenly, this air slams to a stop and stacks up on itself, forming an area of high pressure. This high-pressure wave makes its way up the intake runner away from the cylinder. When it reaches the end of the intake runner, where the runner connects to the intake manifold, the pressure wave bounces back down the intake runner.
If the intake runner is just the right length, that pressure wave will arrive back at the intake valve just as it opens for the next cycle. This extra pressure helps cram more air-fuel mix into the cylinder -- effectively acting like a turbocharger.
The problem with this technique is that it only provides a benefit in a fairly narrow speed range. The pressure wave travels at the speed of sound (which depends on the density of the air) down the intake runner. The speed will vary a little bit depending on the temperature of the air and the speed it is moving, but a good guess for the speed of sound would be 1,300 feet per second (fps). Let's try to get an idea how long the intake runner would have to be to take advantage of this effect.
Let's say the engine is running at 5,000 rpm. The intake valve opens once every two revolutions (720 degrees), but let's say they stay open for 250 degrees. That means that there are 470 degrees between when the intake valve closes and when it opens again. At 5,000 rpm it will take the engine 0.012 seconds to turn one revolution, and 470 degrees is about 1.31 revolutions, so it takes 0.0156 seconds between when the valve closes and when it opens again. At 1,300 fps multiplied by 0.0156 seconds, the pressure wave would travel about 20 feet. But, since must go up the intake runner and then come back, the intake runner would only have to be half this length or about 10 feet.
Two things become apparent after doing this calculation:
The tuning of the intake runner will only have an effect in a fairly narrow RPM range. If we redo the calculation at 3,000 rpm, the length calculated would be completely different.
Ten feet is too long. You can't fit pipes that long under the hood of a car very easily.
There is not too much that can be done about the first problem. A tuned intake has its main benefit in a very narrow speed range. But there is a way to shorten the intake runners and still get some benefit from the pressure wave. If we shorten the intake runner length by a factor of four, making it 2.5 feet, the pressure wave will travel up and down the pipe four times before the intake valve opens again. But it still arrives at the valve at the right time.
There are a lot of intricacies and tricks to intake systems. For instance, it is beneficial to have the intake air moving as fast as possible into the cylinders. This increases the turbulence and mixes the fuel with the air better. One way to increase the air velocity is to use a smaller diameter intake runner. Since roughly the same volume of air enters the cylinder each cycle, if you pump that air through a smaller diameter pipe it will have to go faster.
The downside to using smaller diameter intake runners is that at high engine speeds when lots of air is going through the pipes, the restriction from the smaller diameter may inhibit airflow. So for the large airflows at higher speeds it is better to have large diameter pipes. Some carmakers attempt to get the best of both worlds by using dual intake runners for each cylinder -- one with a small diameter and one with a large diameter. They use a butterfly valve to close off the large diameter runner at lower engine speeds where the narrow runner can help performance. Then the valve opens up at higher engine speeds to reduce the intake restriction, increasing the top end power output.
Here's some more info from a howstuffworks.com article (if you don't mind me posting it):
Originally Posted by howstuffworks.com
How do tuned intake runners work on your car?
The intake system on a four-stroke car engine has one main goal, to get as much air-fuel mixture into the cylinder as possible. One way to help the intake is by tuning the lengths of the pipes.
When the intake valve is open on the engine, air is being sucked into the engine, so the air in the intake runner is moving rapidly toward the cylinder. When the intake valve closes suddenly, this air slams to a stop and stacks up on itself, forming an area of high pressure. This high-pressure wave makes its way up the intake runner away from the cylinder. When it reaches the end of the intake runner, where the runner connects to the intake manifold, the pressure wave bounces back down the intake runner.
If the intake runner is just the right length, that pressure wave will arrive back at the intake valve just as it opens for the next cycle. This extra pressure helps cram more air-fuel mix into the cylinder -- effectively acting like a turbocharger.
The problem with this technique is that it only provides a benefit in a fairly narrow speed range. The pressure wave travels at the speed of sound (which depends on the density of the air) down the intake runner. The speed will vary a little bit depending on the temperature of the air and the speed it is moving, but a good guess for the speed of sound would be 1,300 feet per second (fps). Let's try to get an idea how long the intake runner would have to be to take advantage of this effect.
Let's say the engine is running at 5,000 rpm. The intake valve opens once every two revolutions (720 degrees), but let's say they stay open for 250 degrees. That means that there are 470 degrees between when the intake valve closes and when it opens again. At 5,000 rpm it will take the engine 0.012 seconds to turn one revolution, and 470 degrees is about 1.31 revolutions, so it takes 0.0156 seconds between when the valve closes and when it opens again. At 1,300 fps multiplied by 0.0156 seconds, the pressure wave would travel about 20 feet. But, since must go up the intake runner and then come back, the intake runner would only have to be half this length or about 10 feet.
Two things become apparent after doing this calculation:
The tuning of the intake runner will only have an effect in a fairly narrow RPM range. If we redo the calculation at 3,000 rpm, the length calculated would be completely different.
Ten feet is too long. You can't fit pipes that long under the hood of a car very easily.
There is not too much that can be done about the first problem. A tuned intake has its main benefit in a very narrow speed range. But there is a way to shorten the intake runners and still get some benefit from the pressure wave. If we shorten the intake runner length by a factor of four, making it 2.5 feet, the pressure wave will travel up and down the pipe four times before the intake valve opens again. But it still arrives at the valve at the right time.
There are a lot of intricacies and tricks to intake systems. For instance, it is beneficial to have the intake air moving as fast as possible into the cylinders. This increases the turbulence and mixes the fuel with the air better. One way to increase the air velocity is to use a smaller diameter intake runner. Since roughly the same volume of air enters the cylinder each cycle, if you pump that air through a smaller diameter pipe it will have to go faster.
The downside to using smaller diameter intake runners is that at high engine speeds when lots of air is going through the pipes, the restriction from the smaller diameter may inhibit airflow. So for the large airflows at higher speeds it is better to have large diameter pipes. Some carmakers attempt to get the best of both worlds by using dual intake runners for each cylinder -- one with a small diameter and one with a large diameter. They use a butterfly valve to close off the large diameter runner at lower engine speeds where the narrow runner can help performance. Then the valve opens up at higher engine speeds to reduce the intake restriction, increasing the top end power output.
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Re: **Calculating Intake Manifold Runner Length** get out your vtak calculator Yo! (Hawkze_2.3)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Hawkze_2.3 »</TD></TR><TR><TD CLASS="quote">Very interesting.
Here's some more info from a howstuffworks.com article (if you don't mind me posting it):
</TD></TR></TABLE>
nope not at all, thats where I got the formulas from
Here's some more info from a howstuffworks.com article (if you don't mind me posting it):
</TD></TR></TABLE>
nope not at all, thats where I got the formulas from
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Re: **Calculating Intake Manifold Runner Length** get out your vtak calculator Yo! (Rosko)
Anyone know how to figure the speed of sound for different altitudes and air temps??
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Re: **Calculating Intake Manifold Runner Length** get out your vtak calculator Yo! (Rosko)
and also keep in mind that the length tuning only works for a very specific rpm range.. there is alot more to it thatn that
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http://www.grc.nasa.gov/WWW/K-....html
theres a good site for the speed of sound stuff, but it doesn't change a whole lot for us on ground....the only time they really worry about it is way up in the air, hence why its a NASA website....
jason what else do you mean? I would think getting runners that are big enough to flow as much as the head can plus tuning them for the right length would about cover it, no?
theres a good site for the speed of sound stuff, but it doesn't change a whole lot for us on ground....the only time they really worry about it is way up in the air, hence why its a NASA website....
jason what else do you mean? I would think getting runners that are big enough to flow as much as the head can plus tuning them for the right length would about cover it, no?
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From: Burnout Box, IA, U.S.A.
Re: (JDogg)
You're talking in terms of flow and more importantly volumetric effieciency though right? Or are you saying that those things will also have an effect on how sound travels?
I didn't mean to sound like this was an end all decision for runner length, just kind of a "wave tuning" how to. I think we've all seen from the euro-r vs usdm vs sk2 stuff that theres alot more to it than THAT, haha.
I didn't mean to sound like this was an end all decision for runner length, just kind of a "wave tuning" how to. I think we've all seen from the euro-r vs usdm vs sk2 stuff that theres alot more to it than THAT, haha.
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From: Burnout Box, IA, U.S.A.
Re: **Calculating Intake Manifold Runner Length** get out your vtak calculator Yo! (mgags7)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by mgags7 »</TD></TR><TR><TD CLASS="quote">
didn't have time to read the whole thing yet, but when a wave comes out into the plenum, it sucks air back into the runner in the same waveform, due to the negative pressure "wave" that coexists with the positive wave....</TD></TR></TABLE>
I read up on that a bit more, it seems like this would only exist in a manifold setup with a plenum and not completly apply to an itb setup? high pressure in the plenum vs. low pressure in the runner forces the "wave" back down to the valve. errr maybe I got that backwards?
didn't have time to read the whole thing yet, but when a wave comes out into the plenum, it sucks air back into the runner in the same waveform, due to the negative pressure "wave" that coexists with the positive wave....</TD></TR></TABLE>
I read up on that a bit more, it seems like this would only exist in a manifold setup with a plenum and not completly apply to an itb setup? high pressure in the plenum vs. low pressure in the runner forces the "wave" back down to the valve. errr maybe I got that backwards?
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Yeah that sounds about right, I pondered that for a while last night after I posted that....
Though I would say that I can assume a well designed runner would create a low enough pressure that the air right outside of the velocity stack or right around there could be at higher pressure, no?
Though I would say that I can assume a well designed runner would create a low enough pressure that the air right outside of the velocity stack or right around there could be at higher pressure, no?
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From: Burnout Box, IA, U.S.A.
Re: (mgags7)
yeah, I can see that. really though is there any pressure difference on a wide open itb manifold and a wide open plenum manifold?
Another thought I had was why tune the length so that it coincides with the monent the valve opens? Wouldn't it be better to let the valve open up a bit? What happens when the wave hits the piston thats dropping into the cylinder?
Another thought I had was why tune the length so that it coincides with the monent the valve opens? Wouldn't it be better to let the valve open up a bit? What happens when the wave hits the piston thats dropping into the cylinder?
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Re: (Rosko)
I think the wave is pretty well dispersed and broken up once is enters the cylinder, I doubt that it bounces off the pistons and pushes against any incoming air. It also seems logical to me that you would want it to enter just as the valve is openining so it could allow the air begining flowing into the cylinder more quickly at time 0+ almost as if being discontinuous and once that wave have passed the velocity remains more constant. Also just FYI for those wanting to tune their runners more specificly the flange to valve distance on the h22 head is 2.87"
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From: Lawrence, KS
Re: (Speedra500)
I've never looked up how to do this. Cool stuff. Is this the "Hemholtz" equations?
Just alittle addition from my understanding of waves, and I'm not sure if this really applies to real world tuning.
If you have a choice between 3rd order and 4th order harmonics of the engine it would be better to use the lower order. Since this would give you a 4th order bump lower in the rpm range someplace.
Now to find out how the air intake length, plenum volume, runner/TB/airintake diameter effect things lol
Just alittle addition from my understanding of waves, and I'm not sure if this really applies to real world tuning.
If you have a choice between 3rd order and 4th order harmonics of the engine it would be better to use the lower order. Since this would give you a 4th order bump lower in the rpm range someplace.
Now to find out how the air intake length, plenum volume, runner/TB/airintake diameter effect things lol
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Re: (M@)
you want the wave to build up on teh back of the intake valve before it opens so that when it does open there is positive pressure there to help start the filling of the cylinder, by the time the pressure wave is "done" the inertia of the air and the vaccum created by the piston moving down will continue to fill the cylinder, the effect of the pressure wave is negligable then.
a proper plentum will give you more power than itb's ever will, but again over a very narrow rpm range. itb's will create a wider power band.
a proper plentum will give you more power than itb's ever will, but again over a very narrow rpm range. itb's will create a wider power band.
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Re: (JDogg)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JDogg »</TD></TR><TR><TD CLASS="quote">you want the wave to build up on teh back of the intake valve before it opens so that when it does open there is positive pressure there to help start the filling of the cylinder, by the time the pressure wave is "done" the inertia of the air and the vaccum created by the piston moving down will continue to fill the cylinder, the effect of the pressure wave is negligable then.
a proper plentum will give you more power than itb's ever will, but again over a very narrow rpm range. itb's will create a wider power band.</TD></TR></TABLE>
good stuff
Is there an explanation of why a plenum will work better than itb's?
a proper plentum will give you more power than itb's ever will, but again over a very narrow rpm range. itb's will create a wider power band.</TD></TR></TABLE>
good stuffIs there an explanation of why a plenum will work better than itb's?
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From: Lawrence, KS
Re: (JDogg)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JDogg »</TD></TR><TR><TD CLASS="quote">sound waves dont bounce back with out something to bounce back off of</TD></TR></TABLE>
Well... just take a look at this. http://dev.physicslab.org/Docu...s.xml
I'm not saying one way or the other about the plenum, but closed ends behave differently then open ends, and open ends do have resonance. This isn't physics class so I'll stop since thats about all I know about.
Well... just take a look at this. http://dev.physicslab.org/Docu...s.xml
I'm not saying one way or the other about the plenum, but closed ends behave differently then open ends, and open ends do have resonance. This isn't physics class so I'll stop since thats about all I know about.
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Re: **Calculating Intake Manifold Runner Length** get out your vtak calculator Yo!
I just got done workin through this on another site and thought it should be posted here as well. I originally did the calculation for a stock cammed H23 making peak power at 6k rpm (guesstimate), but I'll figure it here for an H22 with skunk pro2's since thats whats going into my build. I'm also gonna aim for a length with a peak power at 7500 rpm (even though that may be a little high)
Basically were tuning the runner length of a manifold so that the resonant pulses coincide with the opening of the intake valve. These pulses happen when the a mixture is headed down a runner and the intake valve slams shut. A sound wave hits the closed valve and starts to travel a "pulse" back up the length of the runner. When it reaches the end of the runner it turns around (Idk how, haha) and heads back toward the valve. That is called a wave or ONE wave. The whole idea behind this is to tune the runner length so that this wave arrives at the intake valve at the moment it opens to help carry in the new mixture.
So first things first we need to know how long it takes from the time the intake valve closes to the time it opens again. This is where the particular camshaft and the desired peak power rpm point is considered. The duration of a cam is how long it is open, measured in degrees of a 720 degree engine cycle (2 rotations)
The skunk2 pro2 happens to have 270 degrees of duration, and my target rpm for peak power is going to be 7500 rpm. Get out the calculator!
60 seconds / 7500 rpm = .008 seconds per revolution
.008 X 2 (# of revs in a 720 degree cycle) = .016 seconds per cycle
Now since you know that there are 720 degrees in an engine cycle you can subtract the duration of the cam to find out how long the valve is closed. this is also the length in degrees from intake valve closing to re-opening.
720 - 270 = 450 degrees
Now we do a little cross multiplying...
take the sconds per cycle number (.016) and multiply it by the degrees that the intake valve is closed.
.016 X 450 = 7.200
then divide that number (7.200) by 720 degrees...
7.2 / 720 = .010 seconds (intake valve closing to re-opening)
Now we need to figure how far a sound wave can travel in that amount of time, so we use the "speed of sound" which equals 1128 feet per second at 70 degrees F and also at sea level. If you want to get more involved find out the speed of sound at your elevation and intake air temp.
We will convert 1128 feet per second to inches per second by multiplying it by 12 (12 inches in a foot)
1128 X 12 = 13536 inches per second
So we know how far sound can go in one second, but we need to know how far it can go in the amount of time from intake valve closing to re-opening. We already figured that to be .01 seconds at 7500rpm All you have to do for this multiply that number by the speed of sound haha (that just sounds cool)...
13536 X .01 = 135.36 inches
since the pulse will travel up AND down the runner, we can divide this number by two.
135.36 / 2 = 67.68 inches THIS IS THE MAGIC NUMBER!! well, sort of.
If you made your runner this length, measured from valve to end of velocity stack, the resonant pulse would travel up and down the runner and at 7500 rpm's the intake valve would open just as the pulse was hitting it giving the mixture a little "boost" as it enters the cylinder.
Obviously you can't fit a 68 inch runner under your hood so you want to tune the length of it to a multiple of that. This is whats reffered to as tuning to the 3rd wave, 4th wave etc. where the wave travels up and down the runner multiple times before the intake valve opens. Heres some scenarios...
68" = 1st wave tuned
34" = 2nd wave tuned
22.66" = 3rd wave tuned
17.00" = 4th wave tuned
13.60" = 5th wave tuned
Keep in mind that this is the distance from the intake valve to the end of the velocity stack. I'll guess that the distance from the valve to flange on an h22 is about 3 inches. Subtract that from the numbers above and that will be the length of the manifold assuming its a straight runner itb setup. Making the runner longer will shift the power band lower and shorter will shift it higher, UNTIL it comes into the range of the next "wave"
If you play around with different rpm points you kinda map out what works best and see how it changes the numbers you come up with. Keep in mind this is just a starting point though and factors such as feet above sea level and air temp will alter these results.
If you're up for a challenge you can take the runner lengths of our existing manifolds and backfigure at what rpm the resonant waves will line up the best at.
ENJOY
Basically were tuning the runner length of a manifold so that the resonant pulses coincide with the opening of the intake valve. These pulses happen when the a mixture is headed down a runner and the intake valve slams shut. A sound wave hits the closed valve and starts to travel a "pulse" back up the length of the runner. When it reaches the end of the runner it turns around (Idk how, haha) and heads back toward the valve. That is called a wave or ONE wave. The whole idea behind this is to tune the runner length so that this wave arrives at the intake valve at the moment it opens to help carry in the new mixture.
So first things first we need to know how long it takes from the time the intake valve closes to the time it opens again. This is where the particular camshaft and the desired peak power rpm point is considered. The duration of a cam is how long it is open, measured in degrees of a 720 degree engine cycle (2 rotations)
The skunk2 pro2 happens to have 270 degrees of duration, and my target rpm for peak power is going to be 7500 rpm. Get out the calculator!
60 seconds / 7500 rpm = .008 seconds per revolution
.008 X 2 (# of revs in a 720 degree cycle) = .016 seconds per cycle
Now since you know that there are 720 degrees in an engine cycle you can subtract the duration of the cam to find out how long the valve is closed. this is also the length in degrees from intake valve closing to re-opening.
720 - 270 = 450 degrees
Now we do a little cross multiplying...
take the sconds per cycle number (.016) and multiply it by the degrees that the intake valve is closed.
.016 X 450 = 7.200
then divide that number (7.200) by 720 degrees...
7.2 / 720 = .010 seconds (intake valve closing to re-opening)
Now we need to figure how far a sound wave can travel in that amount of time, so we use the "speed of sound" which equals 1128 feet per second at 70 degrees F and also at sea level. If you want to get more involved find out the speed of sound at your elevation and intake air temp.
We will convert 1128 feet per second to inches per second by multiplying it by 12 (12 inches in a foot)
1128 X 12 = 13536 inches per second
So we know how far sound can go in one second, but we need to know how far it can go in the amount of time from intake valve closing to re-opening. We already figured that to be .01 seconds at 7500rpm All you have to do for this multiply that number by the speed of sound haha (that just sounds cool)...
13536 X .01 = 135.36 inches
since the pulse will travel up AND down the runner, we can divide this number by two.
135.36 / 2 = 67.68 inches THIS IS THE MAGIC NUMBER!! well, sort of.
If you made your runner this length, measured from valve to end of velocity stack, the resonant pulse would travel up and down the runner and at 7500 rpm's the intake valve would open just as the pulse was hitting it giving the mixture a little "boost" as it enters the cylinder.
Obviously you can't fit a 68 inch runner under your hood so you want to tune the length of it to a multiple of that. This is whats reffered to as tuning to the 3rd wave, 4th wave etc. where the wave travels up and down the runner multiple times before the intake valve opens. Heres some scenarios...
68" = 1st wave tuned
34" = 2nd wave tuned
22.66" = 3rd wave tuned
17.00" = 4th wave tuned
13.60" = 5th wave tuned
Keep in mind that this is the distance from the intake valve to the end of the velocity stack. I'll guess that the distance from the valve to flange on an h22 is about 3 inches. Subtract that from the numbers above and that will be the length of the manifold assuming its a straight runner itb setup. Making the runner longer will shift the power band lower and shorter will shift it higher, UNTIL it comes into the range of the next "wave"
If you play around with different rpm points you kinda map out what works best and see how it changes the numbers you come up with. Keep in mind this is just a starting point though and factors such as feet above sea level and air temp will alter these results.
If you're up for a challenge you can take the runner lengths of our existing manifolds and backfigure at what rpm the resonant waves will line up the best at.
ENJOY
Trial User
Joined: Jun 2013
Posts: 2
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Re: **Calculating Intake Manifold Runner Length** get out your vtak calculator Yo!
man speed of sound ?? you cant take any number. you have to calculate air dense and know air pressure then you can get speed of sound calculation then multi by than number good post but i calculate a bit difrent this formula too works L=17.8 cm @ 10kRPM
L=17.8 cm + 4,3 cm @9kRPM L=17,8 - 4,3 cm @ 11kRPM
L=17.8 cm + 4,3 cm @9kRPM L=17,8 - 4,3 cm @ 11kRPM
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