Email I sent one of the members, feel free to critique it.

The calculation for determining where to cut the intake manifold is based upon wave theory. When the valve opens it will create a negative pressure wave. This causes air to rush inwards and displace leaving a small amount of negative pressure in their wake which pulls the molecule behind it. This wave is more of a pulse due to the vacuum source not being constant, as the piston travels it does so in a non-linear acceleration curve. Once the valve shuts it will create a relative high pressure front. If you plot out the pressure you will have a sine curve. Apply this over time and you have a resonance model. When you achieve pressure front speeds that are in tune with the opening of the next valve you will achieve the desired effect, increased airflow. Read up on Helmholtz resonance, it will explain this further.

The volume of the plenum, the length of the runner, the area of the runner, piston speeds, valve timing, and alot of other aspects go into designing the intake manifold. Plenum volume should be designed first, once you have decided upon a camshaft, you need to plot it out in a 360 diagram of valve timing and overlap. This will give you the Delta P moments you need to determine optimal timing. After figuring the optimal timing for the pulse per a given RPM (where you want peak power, typically cam dictated) you can design the runners. The length of the runners is the easiest thing to modify. It allows you to run a gradual venturi flow and dictate the geometry of the flow inside the runner via radiusing. Lastly you will modify the area and geometry of the inlets. This is restricted in large part due to port design.

One of the many things people do not look at is the geometry of the internal flow. Once the air enters the intake manifold it will be traveling at a certain vector and velocity. The vacuum causes this vector to adjust towards the low pressure source. The flow at this point is fluous and the port radii will cause variations in the flow velocity and vector direction. Running the flow at too high of a velocity can cause turbulance as the flow grades against the wall of the port, this is typically an issue on high port heads with narrow radius turns. Likewise if you slow the flow too much with a sharp radius you will reduce the mass flow. The objective is to increase flow velocity as much as possible without exceeding about 0.98 Mach at the valve (lift, valve sizing, and volume dictate this number). Exceeding 1 Mach will create a turbulent flow and will decrease mass flow. Typically the KA will not have this issue (the valves are adequate), although you could possibly achieve it running a small lift cam.

Another mistake people commonly make is assuming that a forced induction car requires less intake manifold design. While this might be true in practice (due to N/a engines requiring additional engineering to make power), it doesn't make it correct in theory. In theory you simply are looking at delta P. Increasing manifold pressure just alters the atmosphere in which the engine runs. An N/a engine runs up to about 14.5 PSI of atmospheric pressure in the intake manifold, aka 1 BAR absolute. A turbo engine simply runs as if you could raise atmospheric pressure. The physical principles of the flow remain the same in relation to the application of laws [of physics].

All of this is worthless if you are running a sub-par exhaust manifold. Just something to remember.

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Edub1 wrote:Wow, all that. Well perhaps I spoke too soon. I must admit you have sparked my interest a little.

How much does one of these cost & do you have any data that might demonstrate these things?

Data? It's all in how volumetric efficiency works.

Here it is plotted out.

Assume an engine VE increase of 5% roughly, and a 0.15 - 0.20 BAR drop in pressure to maintain equal engine flow.



Notice how the green line (with intake manifold) is not only flowing the same amount of air, but the compressor is turning at a slower RPM. This in turn allows the turbine (typically) to be more efficient, reducing backpressure and increasing engine output further.

Reducing the shaft RPM at the turbo will cool the oil temps to a small degree and that will help to reduce coolant temps, as will removing the restrictive passages in the old intake manifold.

Nevermind the fact that it is a lighter manifold with less crap on it and you stand to make more power at the limit since you are at a wider point in the compressor map.

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Edub1 wrote:How much benefit would such a manifold be over stock?

Say a guy has a T3/T4, is he going from 400HP to 500HP at the same boost level?

What is the actual HP gain and how is it so much better than another few lbs? By the way, manifolds are specifically tuned to the motor based on tubal resonance and many other factors so you would probably have to do a lot of monkying before you get it right.

Like I said, if you want sick power that is a different matter. How much does a good aftermarket one cost?

Because it's engine efficiency.

With an intake manifold properly designed to make power you will see less exhaust backpressure, lower boost temps, greater turbo life, lower oil temps, lower coolant temps, and you can more accurately take advantage of a turbochargers mapping (maps are widest at lower pressures).

It's not about the power, it's about the efficiency and doing it right. You could make 600whp on a stock manifold thats been hogged out, but the powerband will not be ideal, and you will be pushing the turbocharger hard.

Feel free to critique

wow, well researched and informative...definately over the heads of some people, but you cant argue with the physics. i recently picked up a book from motorbooks called Tuning Engine Management Systems. It was written in conjunction with Bob Norwood and features pretty much every sick car that has been in Sort Compact car over the past few years. anyone who wants a simplified version of what Nismo Freak is saying should pick it up, its a great read, and has a TON of great info. their chapter on manifold design is pretty long, but it has real world formulas that any person could use to built and adequate manifold, even if your not a physics whiz...great info Freak, way to set the noobs straight

Added to the stickey, thanks Alan

I am so glad someone else sees an intake manifold as more than a ballon.

Great write up!

nissanfanatic wrote:I am so glad someone else sees an intake manifold as more than a ballon.

Great write up!

Nice writeup. Very informative.............

nice write up there, i knew something about manifold design but that explains it better.

on another though(sort of off-topic), most of the ka engines throw a rod on cyl #3, could it be that the stock intake manifold design have something to do with this?, maybe its pushing more air into cyl #3, i know this could be detected by way of detonation, but like in the rb engines(believe mostly rb26), some intake manifolds cause pistons #5 & 6 to run leaner.

I think it is characteristic of linear four cyls, and it is more of a heat issue. But that is only my understanding.

I'm flattered that you chose to use my quotes. In fairness you might want to mention that I made these statements in the context of practicality and cost effectiveness. I never suggested that no design improvements were possible.

Your own data suggests that the real world benefit of such an endeavor is marginal at best. That assumes you can even achieve results as good as those posted.

If you just want to design a manifold for interest sake or because that's what floats your boat, I'll be the first to offer words of encouragement.

But I think most people reading these boards are looking for advise on mods that are going to offer a little more return on their investment.

So, if you guys are just into the hobby end of it, fine. All I'm saying is that answers to questions ought to follow the context in which they are asked.

For your typical KA-T guy, an improved manifold offers little in practical performance gains for the enormous effort of designing and building one.

Now I'll leave you guys in peace to design your manifold. Just quote me in context please.

^lol, I suppose if it has anything to do with you being wrong, its out of context...???

nissanfanatic wrote:^lol, I suppose if it has anything to do with you being wrong, its out of context...???

Keep the drama to another thread please.

Chezedik wrote:I think it is characteristic of linear four cyls, and it is more of a heat issue. But that is only my understanding.

It's a characteristic of most engines, and remember that a lean condition is when too much airflow is being directed to a cylinder in comparison to the injector's specific fuel distribution. So if you starve a cylinder of air, it will typically run rich unless the ECU trims for this (you can tune individual cylinders with a Motec). This is why Top Fuel and other highly tuned engines run EGT sensors on each cylinder.

You will find that the longer inline engines like the RB's, Porsche big 4's, and BMW's inline 6's tend to have intricate intake manifold designs to avoid running cylinders with a large variance. You will notice higher trajectories in the rear tapers to aid in accelerating rearward airflow. Moving to a larger plenum would help to prevent individual cylinder starvation as well.

Heat also plays a role, but we are talking about rod bearings in this instance rather than headgasket issues. The oiling system is less proprietary in this instance.

As far as the KA having issues. I've noticed cylinder #2 tends to be where most failures occur. They tend to be due to rod bearing failure, most commonly associated with oil starvation, and not due to a detonation or pre-ignition situation.

Alan, if you don't mind, your original post is being "elevated" to Article status. If you'd like to make any additions or edits, go for it.

Great work. You never cease to make my head hurt.

But isn't our instance in #3 an instance of heat? Doesn't oil act as a cooler, and #3 is very hard to cool based on it's placement. By the way AZ, make it an article. I dare you.

Chezedik wrote:But isn't our instance in #3 an instance of heat? Doesn't oil act as a cooler, and #3 is very hard to cool based on it's placement. By the way AZ, make it an article. I dare you.

This has nothing to do with an intake manifold.

Has anyone given any thought to the purpose of the secondary butterfly valves with relation to elimination of destructive interfearance and/or standing waves.

An engineer buddy of mine is interested in this stuff and while on that subject he pointed out that automotive engineers are not paid to waste money by including unnecessay expenses in their designs. So, he reasons that those are probably there for that purpose.

I'll email this topic to him. Perhaps if he has time, he will join the discussion.

From my understanding of the topic as the secondary butterflies are concerned, I believe that they do serve a purpose... and that is to help to increase low-end power and help idling. Now I do understand that this doesn't play into the manifold design topic that the thread is about, but you did ask the purpose for them - and to my understanding that is the purpose for them

and on another note: I think that you might be underestimating the importance of a well-designed intake manifold. I mean look at the benefits of a well-designed exhaust manifold and the gains from them... Now I do know that they are completely different issues and serve different purposes. But as others have stated before keeping an engine cooler helps out and allowing for the air to more efficiently get into the cylinders with the least bit of interference possible will no doubt help...

(Oh and feel free to correct me if anything I said is not correct, as I did say this is all "to my understanding and knowledge" - which may or may not be correct)

But as you did state the concern of time vs. gain would be a large issue. The amount of time required to design a manifold of this caliber would be intense and some trail and error would probably be present. I personally am all for designing something similar as I find my car to be a hobby and it would be something to fool around with and of course something that I would enjoy... But I am one that does not currently have the money and time to devote to the cause - maybe someday though...

In closing, this thread has been a great read and the discussion is great (as always)!
Modified by Arrow at 3:47 PM 2/11/2006

Edub1 wrote:Has anyone given any thought to the purpose of the secondary butterfly valves with relation to elimination of destructive interfearance and/or standing waves.

An engineer buddy of mine is interested in this stuff and while on that subject he pointed out that automotive engineers are not paid to waste money by including unnecessay expenses in their designs. So, he reasons that those are probably there for that purpose.

I'll email this topic to him. Perhaps if he has time, he will join the discussion.

Devices like the secondary throttle plates are there to accelerate flow in lower RPM situations. Many motorcycles have these plates as well.

As far as an application of them in relation to the production of power. They are nothing more than a source of turbulant flow. True variable systems would alter the length or the diameter of the manifold without introducing a physical part in the airflow on the top end. To get an idea of something like this think of a sliding throttle plate rather than a rotating one.