You might be wondering why I made this topic. Well, maybe the intake manifold could be modified to suit our needs better than the stock one. The stock throttlebody is restrictive, although the ACIS design is great. Oh well, it's just something to talk about. Onto the information... here's an article I found
II. CALCULATIONS
How do we calculate and design the IM dimensions so that the stacked columns of air waves arrive at a certain rpm ?
There are 2 ways to calculate the dimensions for an IM. Using:
1. Variable length runners formulas
or
2. A Helmholtz resonator method
II A.) Variable Length Runners Formulas
From the header tech article you have learned that longer tubes create peak torque at an earlier rpm. This is true whether you are looking at air flow in terms of a fluid or in terms of a sound wave.
By choosing the length and diameter of the runners, an intake manifold can be "tuned" for optimum performance at a certain RPM range.
Longer, narrower runners favor lower RPM's because they have a lower resonant frequency, and the smaller diameter helps increase the air velocity.
Shorter, wider runners favor higher RPM's because they have a higher resonant frequency, and the larger diameter is less restrictive to air flow.
...Choosing the right length and diameter of the intake runners is a trade off between high and low RPM performance.
Some more in-depth stuff.
1. / One Formula: David Vizard's Rule for IM Runner Length
The general rule is that you should begin with a runner length of 17.8 cm for a 10,000 rpm peak torque location, from the intake opening to the plenum chamber. You add 4.3 cm to the runner length for every 1000 rpm that you want the peak torque to occur before the 10,000 rpm.
So, for instance, if peak torque should occur at 4,000 rpm the total runner length should be 17.8 cm + (6 x 4.3 cm) = 43.6 cm.
Vizard also suggests that you can calculate the ideal runner diameter by the equation :
SQRT [ (target rpm for peak torque x Displacement x VE)/ 3330 ]
SQRT = square root
VE = Volumetric Efficiency in %
Displacement in Liters
SQRT [ (5800x 1.8 L x 0.95)/3330]
= 1.73 in. or 43.8 mm (1,73 x 25.4 mm/in.) is the ideal runner diameter.
2./ Another Formula to Calculate Runner Length for a Specific Peak Torque RPM: from Steve Magnante at Hot Rod magazine
N x L = 84,000
where N represents the desired engine rpm for peak torque and L is the length in inches from the opening of the runner tube to the valve head.
II B.) Helmholtz Resonator Calculations
Remember at the start of the article I mentioned that the dimensions of 3 parts of an IM can affect where peak torque can occur? Well here is another way we can calculate estimates for our IM dimensions for the peak torque location we want.
A Helmholtz resonator is an acoustic resonance chamber (as described by our plenum above) that modifies the acoustic frequency of a sound wave like a spring oscillating with a mass attached on the end.
where f = the rpm at which you get peak torque ( the natural frequency of pressure oscillations in the acoustic chamber ) , c = the speed of sound (= 340 m/sec.) , S = runner area, L = runner length, V = displacement per cylinder
A simplified version of this is using the Englemann formula for the above which also takes into account static CR of the engine:
RPM for peak torque =
642 x c x [ SQRT (S/[L x V] ) ] x [ SQRT { (CR-1)/ (CR+1) } ]
= 218,280 x [ SQRT (S/[L x V] ) ] x [ SQRT { (CR-1)/ (CR+1) } ]
For a more detailed explanation on the application of Hermann Ludwig Ferdinand von Helmholtz's acoustic resonator theory applied to intake systems, please check out:
http://enaf1.tripod.com/teche.html#helm
http://www.mecc.unipd.it/~cos/DINAMOTO/risuonatore/risuonatore.html
A Helmholtz resonator is used not only in an automotive induction sytem but also in the designing of exhausts to suppress sound and many other non-automotive designing that involves amplifying sound like in the music industry.
III. RAM INTAKE TUBE DIMENSIONS
What are the best intake tube dimensions for the IM that we have just designed for a particular peak torque rpm?
III a./ INSIDE DIAMETER (D) of a RAM INTAKE TUBE
First Method:
D in inches = SQRT [ ( Displacement x VE x Redline) / (V x 18.5) ]
Displacement = Total Displacement in Liters, VE = Volumetric Efficiency in %, V is the velocity of the air flow in the IM plenum for resonance (usually estimated at 180 ft/sec max.)
eg. SQRT [ (1.8 x 85 x 8500) / (180 x 18.5) ]
= SQRT [ (1,300,500)/ (3330) ]
= SQRT (391)
= 1.98 in.
Second Method:
Throttle Body Size is Determined by IM Plenum Size.
from the Dave Thompson of Thompson Engineering and Endyn: http://www.theoldone.com/archive/intake-manifold-design.htm
The plenum volume is critical on N/A engines, and a basic rule of thumb is: The smaller the plenum, the lower the rpm range, and bigger means higher rpm. The throttle body size and flow rate also affect the plenum size: Bigger TB, smaller plenum, small TB, larger plenum.
The best way to find out if your TB is too small for your IM plenum is to determine what the intake manifold absolute pressure (MAP) sensor is reading (in the plenum) when you are at full throttle ( or wide open throttle (WOT) ) while the car is accelerating using a datalogger. The MAP should be equal to, or close to, atmospheric pressure. If it isn't or there is a MAP drop at WOT, then your TB is still too small.
Once we have determined the optimal TB size for our IM, we can then determine the best intake inner diameter.
The ideal diameter for an intake is when the intake has 25% more cross-sectional area than the TB's bore cross-sectional area . Your TB diameter (overbored or not) dictates your intake diameter.
Remember that the area of a circle (your TB bore) is pi x radius squared and the diameter = 2 x radius. If you calculate your TB's area and then multiply it by 1.33, you will determine the intake's area. Then, use the area of the circle equation to determine the intake's radius.
Therefore, for example, with a 64mm (plate side bore) TB, the calculated "best" intake diameter is 2.8 in. ID.
III. b/ LENGTH OF RAM INTAKE TUBE
A suggested starting point for the length of a tube with peak torque at 6000 rpm is 13 in.
You add 1.7 in. for every 1000 rpm that you want to move the peak torque below 6000.
Or subtract 1.7 in. for every 1000 rpm you want to move the peak torque above 6000.
Calculator for Various Stuff:
http://www.rbracing-rsr.com/runnertorquecalc.html
Let the discussion begin!
II. CALCULATIONS
How do we calculate and design the IM dimensions so that the stacked columns of air waves arrive at a certain rpm ?
There are 2 ways to calculate the dimensions for an IM. Using:
1. Variable length runners formulas
or
2. A Helmholtz resonator method
II A.) Variable Length Runners Formulas
From the header tech article you have learned that longer tubes create peak torque at an earlier rpm. This is true whether you are looking at air flow in terms of a fluid or in terms of a sound wave.
By choosing the length and diameter of the runners, an intake manifold can be "tuned" for optimum performance at a certain RPM range.
Longer, narrower runners favor lower RPM's because they have a lower resonant frequency, and the smaller diameter helps increase the air velocity.
Shorter, wider runners favor higher RPM's because they have a higher resonant frequency, and the larger diameter is less restrictive to air flow.
...Choosing the right length and diameter of the intake runners is a trade off between high and low RPM performance.
Some more in-depth stuff.
1. / One Formula: David Vizard's Rule for IM Runner Length
The general rule is that you should begin with a runner length of 17.8 cm for a 10,000 rpm peak torque location, from the intake opening to the plenum chamber. You add 4.3 cm to the runner length for every 1000 rpm that you want the peak torque to occur before the 10,000 rpm.
So, for instance, if peak torque should occur at 4,000 rpm the total runner length should be 17.8 cm + (6 x 4.3 cm) = 43.6 cm.
Vizard also suggests that you can calculate the ideal runner diameter by the equation :
SQRT [ (target rpm for peak torque x Displacement x VE)/ 3330 ]
SQRT = square root
VE = Volumetric Efficiency in %
Displacement in Liters
SQRT [ (5800x 1.8 L x 0.95)/3330]
= 1.73 in. or 43.8 mm (1,73 x 25.4 mm/in.) is the ideal runner diameter.
2./ Another Formula to Calculate Runner Length for a Specific Peak Torque RPM: from Steve Magnante at Hot Rod magazine
N x L = 84,000
where N represents the desired engine rpm for peak torque and L is the length in inches from the opening of the runner tube to the valve head.
II B.) Helmholtz Resonator Calculations
Remember at the start of the article I mentioned that the dimensions of 3 parts of an IM can affect where peak torque can occur? Well here is another way we can calculate estimates for our IM dimensions for the peak torque location we want.
A Helmholtz resonator is an acoustic resonance chamber (as described by our plenum above) that modifies the acoustic frequency of a sound wave like a spring oscillating with a mass attached on the end.
where f = the rpm at which you get peak torque ( the natural frequency of pressure oscillations in the acoustic chamber ) , c = the speed of sound (= 340 m/sec.) , S = runner area, L = runner length, V = displacement per cylinder
A simplified version of this is using the Englemann formula for the above which also takes into account static CR of the engine:
RPM for peak torque =
642 x c x [ SQRT (S/[L x V] ) ] x [ SQRT { (CR-1)/ (CR+1) } ]
= 218,280 x [ SQRT (S/[L x V] ) ] x [ SQRT { (CR-1)/ (CR+1) } ]
For a more detailed explanation on the application of Hermann Ludwig Ferdinand von Helmholtz's acoustic resonator theory applied to intake systems, please check out:
http://enaf1.tripod.com/teche.html#helm
http://www.mecc.unipd.it/~cos/DINAMOTO/risuonatore/risuonatore.html
A Helmholtz resonator is used not only in an automotive induction sytem but also in the designing of exhausts to suppress sound and many other non-automotive designing that involves amplifying sound like in the music industry.
III. RAM INTAKE TUBE DIMENSIONS
What are the best intake tube dimensions for the IM that we have just designed for a particular peak torque rpm?
III a./ INSIDE DIAMETER (D) of a RAM INTAKE TUBE
First Method:
D in inches = SQRT [ ( Displacement x VE x Redline) / (V x 18.5) ]
Displacement = Total Displacement in Liters, VE = Volumetric Efficiency in %, V is the velocity of the air flow in the IM plenum for resonance (usually estimated at 180 ft/sec max.)
eg. SQRT [ (1.8 x 85 x 8500) / (180 x 18.5) ]
= SQRT [ (1,300,500)/ (3330) ]
= SQRT (391)
= 1.98 in.
Second Method:
Throttle Body Size is Determined by IM Plenum Size.
from the Dave Thompson of Thompson Engineering and Endyn: http://www.theoldone.com/archive/intake-manifold-design.htm
The plenum volume is critical on N/A engines, and a basic rule of thumb is: The smaller the plenum, the lower the rpm range, and bigger means higher rpm. The throttle body size and flow rate also affect the plenum size: Bigger TB, smaller plenum, small TB, larger plenum.
The best way to find out if your TB is too small for your IM plenum is to determine what the intake manifold absolute pressure (MAP) sensor is reading (in the plenum) when you are at full throttle ( or wide open throttle (WOT) ) while the car is accelerating using a datalogger. The MAP should be equal to, or close to, atmospheric pressure. If it isn't or there is a MAP drop at WOT, then your TB is still too small.
Once we have determined the optimal TB size for our IM, we can then determine the best intake inner diameter.
The ideal diameter for an intake is when the intake has 25% more cross-sectional area than the TB's bore cross-sectional area . Your TB diameter (overbored or not) dictates your intake diameter.
Remember that the area of a circle (your TB bore) is pi x radius squared and the diameter = 2 x radius. If you calculate your TB's area and then multiply it by 1.33, you will determine the intake's area. Then, use the area of the circle equation to determine the intake's radius.
Therefore, for example, with a 64mm (plate side bore) TB, the calculated "best" intake diameter is 2.8 in. ID.
III. b/ LENGTH OF RAM INTAKE TUBE
A suggested starting point for the length of a tube with peak torque at 6000 rpm is 13 in.
You add 1.7 in. for every 1000 rpm that you want to move the peak torque below 6000.
Or subtract 1.7 in. for every 1000 rpm you want to move the peak torque above 6000.
Calculator for Various Stuff:
http://www.rbracing-rsr.com/runnertorquecalc.html
Let the discussion begin!
