Intake Plenum Volume

[LGAFF]

Been reading about intake development; generally most builders indicate that the plenum volume needs to be 20% greater than the CI displacement of the motor, I roughed out our plenum at 17.5 X 5.5 X 3, or approximately 290 CI(please let me know if this sounds correct).....I am assuming that runner volume is not included in this number.....so our Plenums are in fact small in comparison to the motor CI, espcially considering the flow capabilties of a 4 Valve motor...????

Interested in your thoughts, and or setting me straight on principles or my calculations..

[LGAFF]

I guess some other school of thought is 50%....of CI



V-8's sharing one large plenum do not work well as far as resonation-effect tuning, but the optimal tuning size would be 40 to 50 percent of total cylinder displacement. For a four-cylinder, 50 to 60 percent of displacement works well. For three cylinders or a six with twin plenums, each plenum should be 65 to 80 percent of the displacement of the three cylinders being fed. For engines operating closer to 7,000 to 7,500 rpm, reduce plenum volume by 10 to 15 percent."

[Torchred96]

I don't know anything about what your saying, but if you want to find out the volume, why not just fill one of your spares with water and measure that.

Maybe if you included the ih and runners you might get closer to that optimal number? Or at least, if you come close to the 20% over the cubic inches, you may find out that the runners are included and lotus took that number into account in their design.

Anyway, bump for some expert opinions

[LGAFF]

If you include the runners up to the sinus its about 370-380ci....

[XfireZ51]

Given that we have 441s supported by that plenum, I'd say the calc is wrong.
Shouldn't the metric be how many CFM it flows v what's needed by motor?

[LGAFF]

Keep the purpose of the plenum in mind. It's supposed to simulate atmospheric conditions. The larger the plenum, therefore, the better job it does in simulating the constant atmospheric conditions. The smaller the plenum, the more adjacent cylinders are going to affect the resonance you're seeking in a "tuned" manifold.

Vizard and others indicate 120% of CI.....


With Kevins car you have to ask what has been done to the plenum...is it siamesed? Also, for reasons unknown to me....some say that plenum size can be up to 20% smaller on an auto trans car vs the 5 spd.....

The plenum is really utilized to build up as much air density as possible...essentially you are compressing as much air into the box as you can....Vizard even shows examples in his book of plenums that flow more CFM, but produce less power as they are inefficient.

[XfireZ51]

But Lee, isn't it true that the plenum is supplying an air charge to 2 cylinders at any point in time? Granted you need to take into account the inertia of the air.
Secondly, the throttle blades are a necessary restriction for maintaining a certain velocity and therefore responsiveness. I guess I am questioning the idea that the plenum must have a volume capacity at any one time that is equivalent to the total displacement of the motor. Maybe I am reading incorrectly what you're saying.

[XfireZ51]

Originally Posted by Paul Workman

I'm fascinated by the physics involved. So, in lieu of any expertise in flow dynamics, I'll just sit in the front row and...... Maybe flyin Ryan will step in and share some of his experiences? (I wish someone would turn over a "hole" card!)


Just to add a log to the fire, I noticed too that the new Ford Coyote motor uses a split plenum.
P.

The C5 and C6Rs have used split plenums for some
time. The Ford SHOs did also.

[LGAFF]

Basically the plenum is an air storage tank, and you are trying to shove(suck actually) as much air into it you can, creating great density. The larger the plenum the less harmonics disturb the air, and again the more air density is allowed and can be placed in the plenum.

Look at you reverse air filter, and the C5 aftermarket air intakes.....they utilize the area in the radiator shroud which is an air of high air density.....your car rams air molocules into the area of the shorud for cooling....as more air building and is sucked in the denser the air(more oxygen)

Same principles apply to a ram air system, you are cramming more air, thus denser air into the intake

Look at Viper, Ferrari, LAmbo...they run dual setups to avoid harmonic disruptions and the plenum volume is actually pretty large.

[LGAFF]

CFM or a flow bench does not measure how a engine works in the real world, in an engine the airflow receives disruptions from the pumping action of the engine....so the question of the plenum is not just a flow issue but efficiency/quality from a harmonics aspect.


Again, can't say these are my thoughts, and my orignal intention is to see what others have read and what they know.....I am by NO means proclaiming to be correct or an expert. i just have some ideas that I am going to try for next year.....gotta keep up the competition(new cars)