Calculating ideal runner lengths and plenum volume

[whipped]

I think everyone's seen the effects of the 3.4 dohc short runner intake, and the trueleo intake on the 3.4 (that may not be a fair comparison due to the 2.8 intake being uber-restrictive on a 3.4), but I was curious how I would compute the ideal plenum volume and runner length, saying I wanted a peak HP around 6500rpm on a 4.6 northstar... I don't care about off-idle response , just want to maximize mid-top end hp.


Reason for bringing this up, I've been kinda thinking about going to the junkyard, finding some cheap 4 cyl intake with the correct port spacing, and dual-TB'ing-it-up.

I think it would just look good, and hopefully flow a lot better than stock.

x2

[Kohburn]

would that even fit in there?

[whipped]

who knows

I just grabbed a pic off ebay of something that looks like it might work. I think from the intake port to the decklid there's about 12" of vertical clearance, but I haven't measured. The stock intake is pretty fat, probably close to 10" tall.

[teamlseep13]

Then you have to sync the throttles which is a pain in the ass. Trust me I have done it too many times.

As for runner lengths and plenum volume....

L= ((EVCD x .25 x V x 2) / (RPM x RV)) - .5D

EVCD is the effective valve closed duration. Find the cam's intake duration in degrees. Then 720 - (CD - 20). 720 degrees is because of the 4 cycles of the engine. You subtract about 20 degrees off the advertised cam duration for a typical high rpm motor.

V is the velocity of the intake air, which is around 1300 FPS.

RV is the reflective value, the pressure wave which allows you to tune intake lengths like this. Normally we select the 2nd or 3rd pressure wave, so the value is 2 or 3.

D is diameter of the intake pipe.

Plenum size for a higher RPM motor should be about 30-40% of the total cylinder volume for that plenum.

And no I am not gonna do any of this math....thats for you;)

Good Luck

[donk_316]

I went through all this shit before when i was looking into the same ideas...

Seriously hit up google with stuff like "port volume calculations <calculator>" and "plenum design calc" and so on....lots of sites with calculators and theories.

[The Dark Side of Will]

Then you have to sync the throttles which is a pain in the ass. Trust me I have done it too many times.

As for runner lengths and plenum volume....

L= ((EVCD x .25 x V x 2) / (RPM x RV)) - .5D

EVCD is the effective valve closed duration. Find the cam's intake duration in degrees. Then 720 - (CD - 20). 720 degrees is because of the 4 cycles of the engine. You subtract about 20 degrees off the advertised cam duration for a typical high rpm motor.

V is the velocity of the intake air, which is around 1300 FPS.

RV is the reflective value, the pressure wave which allows you to tune intake lengths like this. Normally we select the 2nd or 3rd pressure wave, so the value is 2 or 3.

D is diameter of the intake pipe.

Plenum size for a higher RPM motor should be about 30-40% of the total cylinder volume for that plenum.

And no I am not gonna do any of this math....thats for you;)

Good Luck

What's your source for all this... particularly the comment on the 2nd or 3rd pressure wave?

[teamlseep13]

Fluid Dynamics classes I took, but also How to Modify and Tune the Small Block Chevy by David Vizard, along with a dozen other pretty credible sources on the matter of intake pulse tuning.

When the intake valve closes, it send a pressure wave back to the plenum, when there is is reflected back towards the intake valve. It can do this more than once and that is where the RV, reflective value comes into play into the equation.

To take advantage of the intake air's internia (the "weight" of the air not wanting to change directions) we tune the length of the runner so that when the intake valve is closing at the end of the next intake cycle, this charge of air gets rammed into the clyinder, giving a supercharging kind of effect, and adding power at this RPM.

The second reflevtive pulse is normally the strongest becuase it hanst been bouncing back and forth giving up its engery for a while, so we normally use that in intake manifold calulations.

In new motors, vairable intake length manifolds are used to broaden the "ram" effects usefullness by changing the intake lengths based on rpms.

Check on the Ferrari Enzo's motor. Its intake runners are fully adjustable, controlled by the computer and servo's attached to the runners. It can vary length from 1.5" to 7" depending on the rpm of the motor, allowing the boost of this tuning to be felt over a large rpm range.
Its pretty fucking sick to watch the motor as it rev's, seeing the intake manifold move, like the engine is alive, along with the fact that is a 6.x L v12 making over 600hp.....yum

[whipped]

Find the cam's intake duration in degrees.

whoops, ran into a problem here. Where would I find this info for a 300hp northstar?

nope, found it. 266 degrees.

What's the "intake pipe diameter"? Runner diameter?

Something's not right here. oh wait, effective runner length = runner length+ any length from the manifold to the valve, correct? Then its probably closer to 15+"

no, I'm still getting crazy numbers like 8800rpm...

[The Dark Side of Will]

Everone's stuck on the intake valve closing when they should be worried about what happens when it OPENS.

When the intake valve opens, it sends a low pressure pulse up the intake runner. This pulse gets to the end of the intake runner at the plenum (or open air in the case of velocity stacks) and reflects back down the runner as a high pressure pulse.

This is the way wave mechanics works. Anyone who tries to tell you that a high pressure pulse hits an open pipe end and relfects as a high pressure pulse has no phucking clue.

Anyway, this high pressure pulse travels back down the runner and under ideal circumstances arrives at the intake valve just before it closes and gets that last little bit of air into the cylinder.

A good system of headers works exactly the same way and can synergistically interact with the intake tuning by strengthening the low pressure pulse caused by the intake valve opening.

When the exhaust valve opens, it sends a high pressure pulse down the header primary. This pulse hits the collector and reflects back up the primary as a low pressure pulse. This low pressure pulse arrives just as the exhaust valve is closing and scavenges or extracts the last bit of exhaust from the cylinder. Because both valves are open during the overlap period, the low pressure pulse from the headers can interact with and strengthen the low pressure pulse from the intake valve opening, which increases the effectiveness of the intake manifold tuning even more.

The catch is that he headers and the intake manifold have to be tuned to the same RPM.

The above is what I call "resonant tuning". You can see that it doesn't really involve pipe diameter.
Pipe diameter is chosen to achieve a certain desired flow velocity to impart a certain momentum to the air charge to help ram the air into the cylinders. By getting this flow velocity right and tuning pipe diameter to the same RPM as pipe length, you can make an engine VERY efficient at that RPM.

[The Dark Side of Will]

Check on the Ferrari Enzo's motor. Its intake runners are fully adjustable, controlled by the computer and servo's attached to the runners.

BMW's valvetronic V8's and V12's used in the 5, 6 and 7 series do the same thing with their intake manifolds.

I've been thinking that it would be really cool to do that with velocity stacks sticking through the hood. Rev the engine and they drop down...

[S8n]

Everone's stuck on the intake valve closing when they should be worried about what happens when it OPENS.

When the intake valve opens, it sends a low pressure pulse up the intake runner. This pulse gets to the end of the intake runner at the plenum (or open air in the case of velocity stacks) and reflects back down the runner as a high pressure pulse.

This is the way wave mechanics works. Anyone who tries to tell you that a high pressure pulse hits an open pipe end and relfects as a high pressure pulse has no phucking clue.

Anyway, this high pressure pulse travels back down the runner and under ideal circumstances arrives at the intake valve just before it closes and gets that last little bit of air into the cylinder.

A good system of headers works exactly the same way and can synergistically interact with the intake tuning by strengthening the low pressure pulse caused by the intake valve opening.

When the exhaust valve opens, it sends a high pressure pulse down the header primary. This pulse hits the collector and reflects back up the primary as a low pressure pulse. This low pressure pulse arrives just as the exhaust valve is closing and scavenges or extracts the last bit of exhaust from the cylinder. Because both valves are open during the overlap period, the low pressure pulse from the headers can interact with and strengthen the low pressure pulse from the intake valve opening, which increases the effectiveness of the intake manifold tuning even more.

The catch is that he headers and the intake manifold have to be tuned to the same RPM.

The above is what I call "resonant tuning". You can see that it doesn't really involve pipe diameter.
Pipe diameter is chosen to achieve a certain desired flow velocity to impart a certain momentum to the air charge to help ram the air into the cylinders. By getting this flow velocity right and tuning pipe diameter to the same RPM as pipe length, you can make an engine VERY efficient at that RPM.

Close, but not quite. I've been factory trained by BMW and have spoken to many engineers (the one's that speak english) about it. The pressure wave occurs when the intake valve CLOSES. Before the air in the runer slows down,the air traveling at a high velocity slams into the closed valve and "bounces" back up the intake. In order to take advantage of this pulse, there has to be something in the intake to bounce it back towards the intake valve, most of the time it's a 80-90 degree bend in the intake. The optimal timing is when the pulse that is bounced back towards the intake valve ad reaches there when the valve starts to OPEN. This pulse going into the cylinder creates a sort of "siphon" to draw more air into the cylinder. Some of the BMW engines (mainly the S54 M3 motor) actually have more than a 100% volumetric efficiency at certain RPMs because of this.
Ok, now to practical. Don't do math. An engineers view point and real world application are usually two different things (at least in the auto industry) Trial and error will get you your best results. For high RPM and hopefully HP appliactions, you need a large cross section runner that is short. I'm talking 6-8inches here, max. Cut and weld it back together until you find optimal results. You can go longer and try for the 2nd harmoic power pulse, but it's not worth it. The cross section should have the largest area right off of the primary. Then it should, if need be taper down to the area of the intake port on the head. Smooth transition here. Have fun!

[Indy]

Close, but not quite. I've been factory trained by BMW and have spoken to many engineers (the one's that speak english) about it. The pressure wave occurs when the intake valve CLOSES. Before the air in the runer slows down,the air traveling at a high velocity slams into the closed valve and "bounces" back up the intake. In order to take advantage of this pulse, there has to be something in the intake to bounce it back towards the intake valve, most of the time it's a 80-90 degree bend in the intake. The optimal timing is when the pulse that is bounced back towards the intake valve ad reaches there when the valve starts to OPEN. This pulse going into the cylinder creates a sort of "siphon" to draw more air into the cylinder. Some of the BMW engines (mainly the S54 M3 motor) actually have more than a 100% volumetric efficiency at certain RPMs because of this.
Ok, now to practical. Don't do math. An engineers view point and real world application are usually two different things (at least in the auto industry) Trial and error will get you your best results. For high RPM and hopefully HP appliactions, you need a large cross section runner that is short. I'm talking 6-8inches here, max. Cut and weld it back together until you find optimal results. You can go longer and try for the 2nd harmoic power pulse, but it's not worth it. The cross section should have the largest area right off of the primary. Then it should, if need be taper down to the area of the intake port on the head. Smooth transition here. Have fun!

I don't think Will is trying to deny the fact of a pulse occurring on valve closure, just adding a piece of info that isn't as intuitive as thinking about a pressure wave when the valve closes. Even in the absence of an opposing wall, as is the case with velocity stacks, the low pressure wave will cause an opposing high pressure wave to travel back down the runner, the low pressure pulse being caused by equalization of the pressure differential between the cylinder and runner...

[teamlseep13]

I love how smart everyone is here, thank god.
Will you are right about the pressure wave when the intake vales opens, but at least how I learned it, the most important one is when the valve closes. Not to dis-credit anything you said either.

Anyway, I also think that doing the math aint worth it.
Unless you are designing a motor from the ground up, its not worth it in power gains. Just play around and look at similar applications in different motors.

Those BMW engines are something else. No throttles, variable intake manifolds, variable cam phasing, lift and duration.
Give me the new v10 in the M5 and M6 and I would find a way to shoehorn it in to the back of a fiero:)

[The Dark Side of Will]

Close, but not quite. I've been factory trained by BMW and have spoken to many engineers (the one's that speak english) about it. The pressure wave occurs when the intake valve CLOSES. Before the air in the runer slows down,the air traveling at a high velocity slams into the closed valve and "bounces" back up the intake.

No argument. I believe that happens.

In order to take advantage of this pulse, there has to be something in the intake to bounce it back towards the intake valve, most of the time it's a 80-90 degree bend in the intake.

I'm not quite buying that a bend in the pipe can create a strong enough reflection to do any good, expecially without unduly compromising flow. Also, the valve is closed longer than it's open, so any runner length that tries to do anything while the valve is closed will have to be pretty long.

The thing with which I was disagreeing above is the idea that a high pressure pulse can reflect at the plenum and come back down the pike as a high pressure pulse. Wave mechanics dictates that this will NOT happen. In order to get a high pressure reflection from an open pipe, you need to start with a low pressure pulse.

The optimal timing is when the pulse that is bounced back towards the intake valve ad reaches there when the valve starts to OPEN. This pulse going into the cylinder creates a sort of "siphon" to draw more air into the cylinder. Some of the BMW engines (mainly the S54 M3 motor) actually have more than a 100% volumetric efficiency at certain RPMs because of this.

Again.. I'm not buying it... as the intake valve opens, there's plenty of vacuum signal as the piston is just starting down the cylinder. The intake system needs help as the intake valve is closing, when there's very little vacuum signal and momentum may have even overpacked the cylinder a bit and mixture is trying to get back up the pipe. That's when a high pressure pulse will do the most good.

[The Dark Side of Will]

I love how smart everyone is here, thank god.
Will you are right about the pressure wave when the intake vales opens, but at least how I learned it, the most important one is when the valve closes. Not to dis-credit anything you said either.

Anyway, I also think that doing the math aint worth it.
Unless you are designing a motor from the ground up, its not worth it in power gains. Just play around and look at similar applications in different motors.

Ok, now to practical. Don't do math. An engineers view point and real world application are usually two different things (at least in the auto industry) Trial and error will get you your best results. For high RPM and hopefully HP appliactions, you need a large cross section runner that is short. I'm talking 6-8inches here, max. Cut and weld it back together until you find optimal results. You can go longer and try for the 2nd harmoic power pulse, but it's not worth it. The cross section should have the largest area right off of the primary. Then it should, if need be taper down to the area of the intake port on the head. Smooth transition here. Have fun!

I'm going to have to disagree with both of you on the math. Math is absolutely necessary. However, so is testing. You define the initial configuration via theory, then refine via testing. Just like Newton's method... it will never converge if your initial guess is uneducated.

Take the classic intake tuning example of a 350 TPI:
Intake duration: 202 degrees (presumably at 0.050)
Exhaust duration: 206 degrees
Lobe sep: 115
I think we can guesstimate with reasonble accuracy that this cam has approximately 250 degrees of total duration.
This engine makes peak torque approximately 3,000 RPM. At 3,000, each revolution takes 20 milliseconds. Of that, the intake valve is open for ~14ms (250/360*20).

According to my handy pocket reference, the speed of sound in dry air at 100F is 1160 fps. Thus sound travels, in 14 ms, 16 feet. which would be a runner length of 8 feet. Which means that I'm all wet. Of course it also means everyone else in this thread is all wet, as well.

Now if the pulse speed is significantly less than the speed of sound, that could work.

Unfortunately 1300 fps won't work because that's sonic flow and needs to be avoided.

[S8n]

Close, but not quite. I've been factory trained by BMW and have spoken to many engineers (the one's that speak english) about it. The pressure wave occurs when the intake valve CLOSES. Before the air in the runer slows down,the air traveling at a high velocity slams into the closed valve and "bounces" back up the intake.

No argument. I believe that happens.

In order to take advantage of this pulse, there has to be something in the intake to bounce it back towards the intake valve, most of the time it's a 80-90 degree bend in the intake.

I'm not quite buying that a bend in the pipe can create a strong enough reflection to do any good, expecially without unduly compromising flow. Also, the valve is closed longer than it's open, so any runner length that tries to do anything while the valve is closed will have to be pretty long.

The thing with which I was disagreeing above is the idea that a high pressure pulse can reflect at the plenum and come back down the pike as a high pressure pulse. Wave mechanics dictates that this will NOT happen. In order to get a high pressure reflection from an open pipe, you need to start with a low pressure pulse.

The optimal timing is when the pulse that is bounced back towards the intake valve ad reaches there when the valve starts to OPEN. This pulse going into the cylinder creates a sort of "siphon" to draw more air into the cylinder. Some of the BMW engines (mainly the S54 M3 motor) actually have more than a 100% volumetric efficiency at certain RPMs because of this.

Again.. I'm not buying it... as the intake valve opens, there's plenty of vacuum signal as the piston is just starting down the cylinder. The intake system needs help as the intake valve is closing, when there's very little vacuum signal and momentum may have even overpacked the cylinder a bit and mixture is trying to get back up the pipe. That's when a high pressure pulse will do the most good.

First off, never use the term "vacuum" to describe how a cylinder fills. I've had an engineer bitch me out cause I did once. A cylinder fills with air by atmospheric pressure pushing it in. Here is things in simple terms for the people following along: Imagine your front door being the intake valve, and a straight line of people trying to get in. The door opens and people cram through it. The door closes and the first person in line bouces off of it. He bounces into the person behind him and so on. Let's say the last person in line has a wall behind him (the 90 degree turn in the intake) When the bounce reaches him, he bounces off of the wall and back into the person in front of him. The bounce continues back up until it reaches the first guy. Ideally, you would want the door to start opening when the bounce reaches him to give him a push forward. Make sense?

You have to remember that air is stopped before the intake valve opens. Only after the valve starts to open does pressure starts to force air into the cylinder. The air reaches a certain max velocity and then slows down when the pressures start to equalize. But momentum keeps air going forward. That's why the intake valve doesn't close until after BDC, air is still going in.

Every inline 6 intake manifold from BMW has a 90 degree bend in it since the early '90s, Ill see if I can take a picture of it tomorrow. The bend is about 8 inches from the cylinder head. The new V8 (N62) has an infinitely variable intake runner length to optimize cylinder filling. They increased power by about 20 HP across the band.

You brought up math, and I don't do math. Everything in your equation looks alright, so I don't know what to say about it. I don't know the velocity of the pressure waves or how many. I'll look through my books and see what I can find.
All things that I state here are things I've been taught by people in the business, and I can see the results of the engineering that went into the systems. I'm keeping an open mind beacuse I know there are things that happen in the intake that I don't know about. Does any of this make more sense?

[S8n]

Now if the pulse speed is significantly less than the speed of sound, that could work.

I think so. I would think that the speed of the pulse would be a factor of the speed of the intake valve right beofre it touches the valve seat. But it is called "Resonance tuning" in the business, so sound has a big part of it somehwere.

[The Dark Side of Will]

First off, never use the term "vacuum" to describe how a cylinder fills. I've had an engineer bitch me out cause I did once. A cylinder fills with air by atmospheric pressure pushing it in. Here is things in simple terms for the people following along: Imagine your front door being the intake valve, and a straight line of people trying to get in. The door opens and people cram through it. The door closes and the first person in line bouces off of it. He bounces into the person behind him and so on. Let's say the last person in line has a wall behind him (the 90 degree turn in the intake) When the bounce reaches him, he bounces off of the wall and back into the person in front of him. The bounce continues back up until it reaches the first guy. Ideally, you would want the door to start opening when the bounce reaches him to give him a push forward. Make sense?

You have to remember that air is stopped before the intake valve opens. Only after the valve starts to open does pressure starts to force air into the cylinder. The air reaches a certain max velocity and then slows down when the pressures start to equalize. But momentum keeps air going forward. That's why the intake valve doesn't close until after BDC, air is still going in.

Every inline 6 intake manifold from BMW has a 90 degree bend in it since the early '90s, Ill see if I can take a picture of it tomorrow. The bend is about 8 inches from the cylinder head. The new V8 (N62) has an infinitely variable intake runner length to optimize cylinder filling. They increased power by about 20 HP across the band.

You brought up math, and I don't do math. Everything in your equation looks alright, so I don't know what to say about it. I don't know the velocity of the pressure waves or how many. I'll look through my books and see what I can find.
All things that I state here are things I've been taught by people in the business, and I can see the results of the engineering that went into the systems. I'm keeping an open mind beacuse I know there are things that happen in the intake that I don't know about. Does any of this make more sense?

Vacuum means "depression of air pressure below ambient" and I'll use the term as long as it fits the situation. Ask that same engineer what operates power brakes... The answer is vacuum. Tell him a physicist told you so. Engineers hate that.

Anyway, because the piston is beginning its downward stroke (accelerating) as the intake valve is opening, the pressure depression across the port increases as the intake valve is opening. A high pressure pulse arriving during this time is superfluous and basically irrelevant.

Your example of people is ok, if you're talking about an intake runner with diameter of the same order as the size of the air molecules in it. Consider an intake runner is millions of air molecules wide, and you'll see that it's not that simple.

A bend sufficiently restrictive to cause a significant reflection would be an absolute flow killer.

I've seen the diagrams of BMW's variable intake manifold, and it really doesn't look to me like it has such a bend. The runner is continuously curved, of course, but there aren't any sharp bends. Sharp bends are extremely bad from a perspective of flow.

The point I was trying to make with that arithmetic was that I'm right... however, failing that, what I did show was that in order to take advantage a resonant effect with a period as long as the duration of the camshaft would require an inlet runner several feet long. To do the same with a resonance with period as long as the intake valve is closed would require a runner significantly longer, since the intake valve is closed significantly longer than it is open.

However, my arithmetic made the assumption that the pulse speed is the same as the speed of sound. This may not be a warranted assumption.

[S8n]

Vacuum means "depression of air pressure below ambient" and I'll use the term as long as it fits the situation. Ask that same engineer what operates power brakes... The answer is vacuum. Tell him a physicist told you so. Engineers hate that.

Anyway, because the piston is beginning its downward stroke (accelerating) as the intake valve is opening, the pressure depression across the port increases as the intake valve is opening. A high pressure pulse arriving during this time is superfluous and basically irrelevant.

Your example of people is ok, if you're talking about an intake runner with diameter of the same order as the size of the air molecules in it. Consider an intake runner is millions of air molecules wide, and you'll see that it's not that simple.

A bend sufficiently restrictive to cause a significant reflection would be an absolute flow killer.

I've seen the diagrams of BMW's variable intake manifold, and it really doesn't look to me like it has such a bend. The runner is continuously curved, of course, but there aren't any sharp bends. Sharp bends are extremely bad from a perspective of flow.

The point I was trying to make with that arithmetic was that I'm right... however, failing that, what I did show was that in order to take advantage a resonant effect with a period as long as the duration of the camshaft would require an inlet runner several feet long. To do the same with a resonance with period as long as the intake valve is closed would require a runner significantly longer, since the intake valve is closed significantly longer than it is open.

However, my arithmetic made the assumption that the pulse speed is the same as the speed of sound. This may not be a warranted assumption.

BMW intakes have a 90 degree bend. I see them dozens of times a day on the car and off the car. I'll find a diagram somewhere. The intake can flow alot more than the head can, so a restriction like a bend is no biggie.

[S8n]

Found my N62 book. Here is it word for word from BMW (Copyrigth 2001 BMW of North America LLC, E65 Complete Vehicle, Part 2, N62 Engine, Page 8-9)
"To ensure that there is good airflow to the engine cylinders, the intake pressure in fron tof the intake valve should ideally be high. This means that good airflow (high gas molecule density) in front of the intake valve is necessary.
This is only possible if the intake valve is closed and the mass inertia causes the intake air to flow in front of the clsed intake valve. THe air is comrpessed, the pressure and the air flow increase.
As soon as the intake valve is opened, the pressureized intake air flows into the cylinder, expands and draws the air molecules which follow into the cylinder. The suction waves form in the intake pipe (moving at sonic speed) in the opposite direction to the intake air.
These suction waves are reflected in the intake manifold and create pressure waves which then move once more at sonic speed in the direction of the intake valve. The intake pipe is at optimum length when the pressure waves are at the intake valve shortly before it closed. The increase in pressure in front of the intake valve results in increased air flow to the cylinders once more. This process is described as recharge effect.
The opening angle of the intake valve remains unchanged as the engine speed increases. The opening time however, is reduced proportionately (with conventional, non-Valvetronic engines).
Since the suction waves and pressure waves expand at sonic speed, the suction path length must be adapted depending on the engine speed to ensure that the tip of the pressure wave reached the intake valve before it closes."

This brings light to things (or confuses things more). I was wrong a few things, like when the wave starts to reach the valve. By the sounds of it, looks the wave hits the intake valve for the same time spand that the intake valve is open (suction wave created). Sorry for the mistakes, I'll do more research next time.