Image courtesy of Driven Innovations

Basic Concept:

The basic concept of intake manifold design is matching the intake manifold to engine demands. Furthermore, intake manifolds can be used to boost volumetric efficiency at certain rpm ranges through resonance tuning where the intake pulses in the intake system (which originated from the pressure waves created by the action of opening and closing the intake valves) are optimized to create a supercharging effect to boost air ingestion in certain rpm. This supercharging effect is typically on the order of a 5 to 7% boost in power for a first reflection and a 3 to 4% boost for a second reflection.

Typically, factory intake manifolds are designed where the volumetric efficiency boost from the intake manifold design is optimized for peak torque rpm to make the car more responsive, whereas highly modified cars may produce better horsepower figures by redoing the intake manifold design to target the peak reflection rpm to be closer to red-line (or around 500 to 1000 rpm lower).

Another thing to think about with intake manifold design (besides matching the overall manifold dimensions to the peak demands of your engine) is weather or not your engine relies on a high tumble (high swirl) design or a low tumble (low swirl) design.

Swirling air in the intake system (and inside the combustion chamber) produces a better air to fuel mix and is proven to boost fuel economy and power delivery by some 5% as well. High swirl intake systems that use round or oval intake manifold runners for example give the air enough extra runner area to tumble and swirl inside the intake manifold runner improving the mixing of the mixture. This high tumble design may be detrimental to performance on engines with higher RPM ranges (such as motorcycle engines) that would benefit from more uniform or laminar flow that can deliver a higher volume of air to the cylinder at the higher rpm range, while taking a 5% hit to power in the lower rpm from the reduced tumble effect. In these engines you may see an intake manifold with square intake runner tubes or more oval-ed runners (than round) where the shape of the runner is narrower (at the same overall runner area) to prevent the air from having enough room to swirl and tumble as freely as it would like to in a runner with a circular cross section.

The article and discussion below go into more depth on some aspects of intake manifold design. Our horsepower calculator will help you with the mathematical aspect of your intake manifold design by giving you the ideal dimensions for

  • Throttle body sizing (single,dual, or individual throttle bodies)
  • Plenum volume
  • Intake runner length
  • Intake runner diameter

To give you peak resonance tuning near your engine’s red-line, adjusted for your peak flow demands, your power level, and your boost level (the charge density).

Example intake manifold Design:

A friend of mine is building an intake manifold for a naturally aspirated Toyota Celica. The car is equipped with a 2.2 liter 4 cylinder engine that generates around 135hp @ 5500 with 6200 rpm red-line. With a lot of modifications, the engine can achieve 180 crank hp @ 6200 rpm and turbocharged and supercharged versions of the same car have broken the 320 wheel hp mark @ 21psi of boost @ 6200rpm.

Now that we have our parameters defined:

Displacement: 2200 cc

Peak RPM: 6200 rpm

Target hp: 180 hp corresponding to about 270 CFM of flow

Number of runners: 4

Number of throttle bodies: 1

Plugging these parameters into my power calculator I get the following dimensions for the intake manifold that I would build. For comparison here, we have the dimensions chosen by Mr. Turrani for his application.

Parameter Power Calculator Mr. Turrani’s Manifold
Intake runner diameter 1.22” 1.75”
Intake runner length 13.4” 14.3”
Plenum volume 3 liters 1.4 liters
Throttle body bore 58 mm 60 mm

(Gen 3 3sgte stock throttle body)

In general, when doing research for the power calculator, I found that typically intake manifolds have a volume that is 50 to 80% the displacement of engine. Obviously proper intake manifold design is much more involved than that as dynamic fluid flow modeling shows us that sometimes very large yet appropriately designed intake manifold shapes can maintain peak velocity while still having a decently sized plenum volume to promote top end power.

im-9 im-2 im-3

im-6 im-7 im-1

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The compromise in plenum volume is as follows:

A larger volume leaves more available air to the engine within its reach, and so long as this air can be replenished in time through the intake system, then the engine never has to work hard to get intake air because there’s always enough of it sitting there in the larger plenum.

As the plenum volume gets smaller, it becomes easier for the engine to rapidly consume all of the air in the plenum and thus it would have to spend a lot of effort (after the initial draw of air) trying to suck air in all the way through the entire intake system to stay alive.

The problem with a larger plenum is that it hurts throttle response. Throttle response is very much affected by throttle pressure (or in other words how fast the engine can consume all the air in the plenum and create a significant amount of vacuum in the manifold to draw in fresh air). The smaller the plenum (or smaller the runners), the higher the gas velocity, the faster the pressure drop, the sooner the new air rushes in, the faster the throttle response.

This usually leads to an oddball design by most OEM’s of an over-sized plenum wit h a smaller throttle body and runners to try to boost gas velocity, or an undersized plenum (that will be consumed faster for better response) but with a larger throttle body that will not bottle neck the engine as it tries to pull in more air from the outside to stay alive at higher flow demands at higher rpm.

Either way, shifting peak power from 5500 to 6200 has a potential increase of 12% especially coupled with a properly designed exhaust manifold, appropriate camshafts, and a proper tune (all of which Mr. Turrani already has on his car).

As far as superchargers are concerned, intake manifolds have lower diameter requirements for the throttle body and the intake runners because the air is compressed. At the same time, runner length and resonance calculations are not much affected because air in the manifold travels at the speed of sound, and the speed of sound is not drastically affected by a slight increase in temperature and a boost in pressure.

One thing to note is that with something like a roots or screw style supercharger, engine vacuum is not alone responsible for throttle response. As the air is being both sucked in (by the piston stroke) and shoved in (by the supercharger rotation) it becomes easier and faster to fill a larger volume plenum manifold. This allows for an over-sized manifold for higher rpm volumetric efficiency while relying on the screw supercharger to take care of the gas velocity, and throttle response.

Additional Considerations to intake manifold design

from an email between me and one of our customers

Here is the most successful plenum shape for a 4 cylinder with a side feed throttle body

There are a few things to notice in the design here (and why everyone uses almost the same design , with the limits of their pricing / fabrication costs).

1- When air flows in a pipe, not all of the air flows at the same velocity. There’s this thing called a velocity profile… the air rubbing against the pipe wall has fairly a lot of friction working against it, so it moves at a slower velocity. The air running in the center of the pipe , is rubbing against other air molecules, so it has little friction, and is able to move at a higher velocity.

The math is fairly complex, but what this amounts to, is that the air near the wall approaches 0 velocity, or in other words more than 80% of the air is traveling in the inner core of the pipe and the remaining 20% tails off towards the boundary.

What this means design wise is 2 things:

1- You never want your runner to meet the plenum at the floor … because theoretically near the floor, the air velocity approaches 0. So as you can see in the image, the ‘trumpets’ are raised off of the plenum floor.

2- You never want your plenum to end abruptly at the end of the last runner. Same thing … if there’s a wall right there, air velocity at the wall will be near 0, your runner will not be using it’s whole diameter to suck in air as near the wall there will be almost no air flow… so they usually use a rounded end cap or they space an inch or two off the end of the plenum to move the wall away from the runner.

Going back to the velocity profile… if you’re looking in from the throttle body and the ‘trumpets’ are slightly raised over the floor of the throttle body, then you’re in the right area… because not only is the floor of the plenum important , but the air coming in from the throttle body is coming in ‘elevated’ off the floor of the throttle body… so depending on where you put your TB with reference to the floor of your plenum, you want the runner to be raised off the floor of the TB as well

So if you’re looking into your throttle body (or the plenums round opening since the TB for you is on the supercharger side) you want to see the top of your trumpets about 1/8 to 1/4 way up the throttle bore … and NOT flush with its floor.

You can see that this is true in the image I’m linking to for the AMS manifold.

So that was consideration #1 velocity profile

Consideration #2: air mass….

Your engine consumes the air…

At the throttle body 100% of the air mass is flowing (say 375 cfm of air for 250hp)

After runner #1 , a quarter of the air has gone into the engine… that leaves you with 280 cfm of air (or 75%)

After runner #2 , another quarter is consumed leaving you with 50% or 187cfm

After runner #3, another quarter is consumed leaving you with 25% or 93cfm of air

Now if you look at the air velocity

Say you have a 4″ diameter tube as your plenum or 12.56 square inches….

your air velocity will drop as you go down the tube because you have less CFM flow divided by the same area tube

Here are the velocities:

#1 = 375 cfm * 1728 / 12.56 = 51,592 inches per minute or 23 meters per second

#2 = 23 * 3/4 = 18 meters per second

#3 = 23 * 1/2 = 12 meters per second

#4 = 23 * 1/4 = 6 meters per second

So what happens is the air slows down as it approaches cylinder 4

Now what this does in a traditional style intake (same diameter) is that cylinder #1 runs the leanest (gets the most air) and cylinder #4 runs the richest (gets the least air) with all 4 cylinders running exactly the same injector duty cycle (unless you have a good ECU that can do individual cylinder fuel trimming).

Now racers that know this build a cheap equal diameter manifold, and then just make sure that they tune based on the air fuel readings of cylinder #1 … if cylinder #1 is safe , then cylinder #4 will be rich, some power is wasted but there is no chance of blowing things up… if by mistake you tune to an oxygen sensor in cylinder #4… and you make it perfect… then #1 will run lean and you may lose th motor …

So what the smarter people do (or the people who have the ability to fabricate a slightly more complex shape plenum) is to taper the plenum towards cylinder #4 at almost exactly that ratio of 100%, 75%,50%,25% going from runner 1 down to runner 4…

These are ratios of area … so if you work it back to ratios of diameters you get this

100% diameter @ runner #1

86% diameter @ runner #2

70% diameter @ runner #3

50% diameter @ runner #4

Here is a great illustration showing how two similarly sized manifolds can perform radically differently with altered design. The first manifold is a standard 90* bends everywhere design. The other was optimized using computer modelling for best mass flow rate balance between all 8 runners (4 cylinders)

You can clearly see the turbulence in the top left corner of the traditional 90* designed manifold (left image) whereas this is reduced by sloping the roof of the plenum to reduce that dead space area forcing the air into the runner and away from the top walls.

Last part of the question is do you do a round or square plenum body… I believe this doesn’t really matter and is more down to what the manufacturer thinks is easier to fabricate (cutting a 4″ pipe or bending aluminum sheeting) … require different types of tooling, jigs and setups…

So if you build your manifold with those 3 considerations (raise off the floor, stay away from the wall, decreasing diameter profile 100%,86%,70%,50% … ) you will have a very high quality manifold

Note: Our new software, the virtual engine dyno, does 2nd order simulations for manifold resonance tuning as well as pressure drop and velocity calculations for your intake pipe, throttle body, and manifold runners. Combined, the flow velocity, pressure drop, and resonance tuning allows you to create a realistic dyno of different manifold designs before building your custom manifold

Click here to design the ideal manifold for your engine package



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