Intake Manifold design theories

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Intake Manifold design theories

Post by wildbama37 » Thu Sep 28, 2006 11:57 am

Ok i'm not too sure if this has been discussed, but I'd like for people to chime in on their thoughts/knowledge of designing an intake manifold.

I'm not a physicist at all and don't know much about air flow and all that stuff, but I do know that air takes the path of least resistance and that's what we're trying to accomplish in our turbo setups, right? Don't we want to maximize airflow abiltiy and velocity into the head without sacrificing power, etc? I am wanting to make/have made h a decent intake manifold sometime this fall and see what kind of difference it will actually make on the dyno...just to kinda explore the dynamics of having a different manifold as opposed to running the stock one.

Does anyone know what's best or what are the general rules for making a more efficient manifold than stock? Do we want to have a more rounded off plenum or a boxy style plenum? How do we know what is a good size volume for the plenum? Do we use the existing runners all the way to where they bolt to the stock plenum and just make a new plenum or do we want to cut the runners off somewhere in the middle so that the air travels in a pretty much straight shot towards the head instead of like oem and having to curve up and back down into the head? Would it be better to not use any part of the stock manifold at all and instead fabricate up a new head-manifold flange, make new runners and a whole new plenum? What is the ideal material to make the manifold out of...aluminum, steel, etc (i'm assuming aluminum)? Why do some people make manifolds with long runners and others have very short runners?

I'm just wanting to know what you guys think is best and why people choose certain designs of manifolds based on ease of construction or efficiency. If you can chip in to this thread, feel free to do so. I'm here to learn as much as possible about my car and others as well as everyone else. Go ahead and post what you guys do and if you can, include pictures to help illustrate certain designs. If anybody is very knowledgeable in this area, don't be bashful, contribute to this if you can...please.
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Post by seanc » Thu Sep 28, 2006 12:11 pm

with a forced induction set-up runner length is kinda of mute point, plenum size though is very important. you want your plenum to be have a larger internal volume than the engine. with the throttle butterfly before the plenum you will always have a lag between stabbing the throttle and engine response. this is where ITB's come in. you could use the helmholtz approach to match runner length to your desired power band.

on the other hand the big plus that i see is that a aftermarket plenum is alot easier to work on :)

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Post by fcvapor05 » Thu Sep 28, 2006 1:13 pm

Good topic to start IMO. Sorry Sean, but I disagree with the thinking that runner length is not important. If anything, runner length and flow irregularities in a manifold become magnified under boost. A longer runner, boosted or not, will give higher internal velocity and usually an increase in midrange torque and power at low RPMs. A short-runner manifold will give more top end power and overall flow, at the expense of midrange torque and low end power.

Usually, a manifold should be designed with the fewest number of directions changes as possible. In response to the question about the stock runners, the stock manifold design with one long curve is better than one that uses one or more sharp 90 degree bends in the same area, so take that into consideration when you design your own manifold.

One common misconeption to avoid- many people assume that the most aerodynamic shape for an interior edge is a sharp, razor-like finish. The truth is that air will flow best around a shape with a radius, usually the larger the better. If you dont believe me, look at the front of a 747 or the inside of a turbine housing and you will see what I mean.

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Post by Verik » Thu Sep 28, 2006 1:43 pm

i don't have much to add but I thought I'd share some pics of what I thought was a great job on not only ITB's but an Intake plenum for a KA-T as well.

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Post by wildbama37 » Thu Sep 28, 2006 2:32 pm

Verik wrote:i don't have much to add but I thought I'd share some pics of what I thought was a great job on not only ITB's but an Intake plenum for a KA-T as well.

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That is the design that I had in mind right there. I never thought of the ITB idea, but that seems like an awesome thing to do..just don't know what kind of TB's those are. The whole cutting the runners off half way is kind of what I was wanting to try and do, just wasn't sure what it would do for flow, etc.
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Post by seanc » Thu Sep 28, 2006 2:36 pm

alot of people use the GSXR throttle bodies off of a 600

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Post by Verik » Thu Sep 28, 2006 2:46 pm

Those ITB's are GSX 750 throttle bodies and velocity stacks. he's using a megasquirt to tune it. when his car was N/A he made 155 whp with exhaust and the itb's so they are quite effective.
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Post by neoamd » Thu Sep 28, 2006 3:16 pm

Verik wrote:Those ITB's are GSX 750 throttle bodies and velocity stacks. he's using a megasquirt to tune it. when his car was N/A he made 155 whp with exhaust and the itb's so they are quite effective.
hrmm I bet tappering the plemnum, would also aid in the issue of 4th cynlinder leaning out also. Nice setup though!

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Post by Verik » Thu Sep 28, 2006 3:28 pm

if you were running a stand alone like that though would couldnt you just adjust the fuel in the 4th cyl via the programming in order to counteract the leaning out? or can you only adjust the fuel curve for all 4 cylinders at once?
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Post by mattmartindrift » Thu Sep 28, 2006 6:26 pm

Verik wrote:Those ITB's are GSX 750 throttle bodies and velocity stacks. he's using a megasquirt to tune it. when his car was N/A he made 155 whp with exhaust and the itb's so they are quite effective.

Think about it...those TBs are wayyy too small. They're made to run a 750cc engine @ around 130hp tops....and you're trying to push 300+hp on a 2.4L

Do things add up????

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Post by nissanfanatic » Thu Sep 28, 2006 7:38 pm

^Would you say the KA throttle body is made to run a 2389cc engine at 155bhp tops so you can't make any more power on it..??
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Post by monooxide » Thu Sep 28, 2006 8:46 pm

nissanfanatic wrote:^Would you say the KA throttle body is made to run a 2389cc engine at 155bhp tops so you can't make any more power on it..??
Ouch, Very true.

Supra TB only capable of 327HP. Must be some magic.

Also, Most runners bottleneck down to that Inside Diameter anyhow so what does that matter?

My idea for my Intake Manifold is using the GroundZero(Xcessive) Manifold and welding Velocity Stacks on the individual runners to increase flow efficiency.

I figure since FLOW = HP, and using velocity stacks can yield a 17% increase in flow efficiency then that is where you need to focus is your flow efficiency and tuning and the "flow"er.
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Post by Redline » Thu Sep 28, 2006 9:33 pm

monooxide wrote: I figure since FLOW = HP, and using velocity stacks can yield a 17% increase in flow efficiency then that is where you need to focus is your flow efficiency and tuning and the "flow"er.
May i ask where you got this number from?


And i'm pretty sure the gsxr 600 and 750 tb's are the same...130hp sure, but at 14,000rpms, lol...itb's seem to work just fine for the RB26, good enough for me, lol...
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Post by wildbama37 » Thu Sep 28, 2006 10:22 pm

Redline wrote:
monooxide wrote: I figure since FLOW = HP, and using velocity stacks can yield a 17% increase in flow efficiency then that is where you need to focus is your flow efficiency and tuning and the "flow"er.
May i ask where you got this number from?


And i'm pretty sure the gsxr 600 and 750 tb's are the same...130hp sure, but at 14,000rpms, lol...itb's seem to work just fine for the RB26, good enough for me, lol...
I'm also curious about this. So correct me if i'm wrong, but velocity stacks are like little horn type things that are inside the plenum that lead to each runner? Never though of using that, but whatever works best. This thread keeps getting better and better..more ideas makes me happy heh
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Post by Verik » Fri Sep 29, 2006 8:42 am

btw they are TB's from the GSX 750 and are 1mm larger than those on the 600. These are 44mm where as the 600 runs 43mm TB's
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Post by mattmartindrift » Fri Sep 29, 2006 6:52 pm

what size are the RB throttle bodies. You don't want to create a bottle neck with the turbo system.

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Post by GlacierFreeze » Fri Sep 29, 2006 7:38 pm

How many throttle bodies do bikes run? I'm guessing 1 (unless they're ITB, I don't know about bikes). Don't see how it would be a bottleneck on a car if you're running 1 per cylinder (unless the bikes are ITB as I said).

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Post by Redline » Fri Sep 29, 2006 7:38 pm

mattmartindrift wrote:what size are the RB throttle bodies. You don't want to create a bottle neck with the turbo system.
Than the important question is what is the inner diameter of the stock intake runners? Probaly less than 43mm....i'm too lazy to do the math, but wouldn't 4x43mm tb's flow more than say a 90mm? I dunno if it even matters since the piping is probaly 90mm(3") anyway....

Here's a thought though, since most recomend the plenium being x2 the liters of the engine what happens when you run itb's? I mean the plenium wouldn't really matter since its before the tb's and the volume of the few inches of runners after the tb's certianly wouldn't double 2.4L....so how does this effect lag? throttle response?
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Post by Verik » Fri Sep 29, 2006 9:10 pm

GlacierFreeze wrote:How many throttle bodies do bikes run? I'm guessing 1 (unless they're ITB, I don't know about bikes). Don't see how it would be a bottleneck on a car if you're running 1 per cylinder (unless the bikes are ITB as I said).
Bikes are ITB's. Lets see total volume of 4x 44mm ITB's = 72.38 cm^2

Stock KA TB = 63.62 cm^2

Pretty basic stuff lol. ITB's flow more.
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Post by wildbama37 » Sat Sep 30, 2006 8:51 am

how do you wire something like that up to the AEM EMS since each one has it's own tps sensor? another question, how do you have to hook up the throttle cables so that each one opens at the same time without one being partially opened and the other being 100%?
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Post by flip240 » Sat Sep 30, 2006 10:24 am

on the rb26, and most other inline 6 ITB setups i've seen, they use the classic single cable which pulls a pulley attached to a long rod which is connected to all 6 TBs.. the one pictured appears to operate the same way, with one tps, but the cable isn't connected yet
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Post by Burke » Sat Sep 30, 2006 12:07 pm

search "helmholtz tuning" for runner length and size and for the plenum size. you want to minize restrictions and using radiused curves are a must. in fact having the velocity stacks protrude upward in the plenum is better. you also need to optimize the shape of the plenum to ensure equal distribution to all cylinders.
there are alot of things to consider when designing an intake manifold and many aftermarket companies dont take them all into account.
for my master's thesis i designed and built an intake manifold for a turbocharged honda :)

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Post by nissanfanatic » Sat Sep 30, 2006 1:37 pm

If I used that setup on my car, I would be nowhere near maxxing those throttle bodies. At the current boost level, velocity would be at 196ft/sec. 300ft/sec is considered maxxing...or at least considered "time to upgrade"

http://not2fast.com/gasflow/velocity.shtml
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Post by JGSturbo » Sat Sep 30, 2006 1:52 pm

My thoughts about intake design (of which we have done for quite a few applications over the years including quite a few full chassis 3TC powered Starlets)...

1- Plenum volume
The bigger the better within reason, when that intake valve opens you want a large volume available to charge that cylinder (like an air cannon or potato gun :wink: ). Stacks and/or tapered horn style runners will help, not sure about 17% . My SOHC SQT has a 4" dia x ~14" long plenum and it idles just like stock and has very good throttle response.
ITBs are nice in the fact that you can use any size of plenum you want without any effect except better cylinder filling. :D
2- plenum shape
I've always been a fan of the round plenum with a domed rear cover. Any shape with sharp corners seems a little counter-productive as the corners will harbor pockets of more stagnant air. A cylinder with a dome has no flat areas or corners. The tapered plenum has been picking up popularity but IMO it reduces volume over #4 and also negates the equalization of the plenum. Again just MO but still ALOT of truly high HP cars still use the large domed cylinder type plenum. I have yet to see any true test of which if either is actually better...

3- runner length / diameter / etc.
Smaller/shorter or longer /bigger? All have their particular res. frequency (rpm). We're all limited by the engine bay in terms of length and one can easily get lost in all the calculations based on assumptions about what is correct. I like to use a tube that can be formed to fit nicely on the shape of the stock intake port. Stacks are cool, we actually made a die to form stacks out of the tubes inside the plenum. The form makes a curved stack that sits nicely on the side of the plenum tube. If one want to get crazy, build runners that are the shape of the stock ports with an 8-10deg tapper and decreasing radius-ed inlets, basically making a horn shaped intake runner.
This is another huger undertaking that may gain a fraction :? .

Basically you can spend a TON of time trying to "remake the wheel" Or build a basic intake and enjoy the gains over the ultra long runner stocker.
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Post by wildbama37 » Sat Sep 30, 2006 2:25 pm

Not to sound dumb about this, but what is the actual ADVANTAGE of running ITB's? Does it have a real advantage over just running the single TB and if so, what is it? Another thing..if you are running ITB's then there's no need to have a single TB at the front of the plenum, right? you just have an open plenum with the intercooler pipe hooked right up and nothing stopping the flow until the ITB's, right?
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Post by Verik » Sat Sep 30, 2006 3:55 pm

wildbama37 wrote:Not to sound dumb about this, but what is the actual ADVANTAGE of running ITB's? Does it have a real advantage over just running the single TB and if so, what is it? Another thing..if you are running ITB's then there's no need to have a single TB at the front of the plenum, right? you just have an open plenum with the intercooler pipe hooked right up and nothing stopping the flow until the ITB's, right?
Throttle response and no restriction to flow.
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Post by Burke » Sat Sep 30, 2006 6:01 pm

JGSturbo wrote:
2- plenum shape
I've always been a fan of the round plenum with a domed rear cover. Any shape with sharp corners seems a little counter-productive as the corners will harbor pockets of more stagnant air. A cylinder with a dome has no flat areas or corners. The tapered plenum has been picking up popularity but IMO it reduces volume over #4 and also negates the equalization of the plenum. Again just MO but still ALOT of truly high HP cars still use the large domed cylinder type plenum. I have yet to see any true test of which if either is actually better...
you have some good points and seem to know you stuff but here you have some misconceptions. A tapered dome would be ideal. What happens on a side feed plenum is that the air rushing in from the throttle body does not want to change direction and flies past the other cylinders and smashes into the back of plenum right at #4, creating a more dense pocket in the plenum. This leads to #4 getting a greater mass flow rate or air, essentially creating a leaner condition in that cylinder.
That is why you have a tapered plenum. You create a greater restriction for that cylinder, evening out the cylinder distribution.
What would be ideal is a plenum so large that the runners act like there is no plenum and open to atmosphere. This is why you have the runners protrude up into the plenum so that are no restriction on the entrance from the side of the plenum. But in a cramped engine compartment this is quite difficult.
With the age of Computational Fluid Dynamics (CFD) one can create many variations of plenum shape and decide upon the optimal shape relatively quickly, which is why you are seeing what you are with tapered plenums.

I hope this helps. If you have any questions I spent a year studying this stuff to design mine for my thesis.

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Post by turbonola » Sun Oct 01, 2006 2:49 am

Verik wrote:Those ITB's are GSX 750 throttle bodies and velocity stacks. he's using a megasquirt to tune it. when his car was N/A he made 155 whp with exhaust and the itb's so they are quite effective.

kinda off topic but i thought he used haltec to tune? and also i love his setup because or the short intercooler pipes.
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Post by veilside180sx » Sun Oct 01, 2006 8:57 am

He used a Megasquirt at first and swapped it in favor of the Haltech afterwards.
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Post by Al_s14 » Sun Oct 01, 2006 3:51 pm

Does anybody know where he got ITB Flanges to weld to stock runners? or were they jsut sawed off extra set of itbs?
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