ATX-Iroc

Can some one explain what effect plenum volume has. Does it have to correlate with engine displacement. And then runner length, i know long tpi runners don't breathe up top but would a larger diameter runner allow an engine to breathe at high rpm's. I'm about to go to school for metal fabrication and would like to attempt a sheet metal intake, any input on behavior factors associated with intake's would be great, equations would also be greatly appreciated

racing geek

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I'm also interested in the information he is asking about. Please post the tech.

rhit_rs

iTrader: (1)

I'll recommend a good book and give you some of the basics I leaned in the IC engines class I took last year. The book: Internal Combustion Engines by Heywood. It's pretty much the book for IC engine theory and operation.

One word of caution on intake fabrication is that it makes a large difference from a design stand point on whether the intake is only flowing air (port fi), or air and fuel together (carb or tbi).

I like to think of the plenum as the volume of air that is just kinda waiting to be pulled in one direction or another. Air in the plenum is moving at relatively low speeds (compared to the air in the runners), and is being tugged different directions depending on which cylinder is on the intake stroke. From the experimenting I've done with an engine modeling program called Wave by Ricardo, in the case of a single plane manifold only flowing air, the bigger the better. I think there are some general rules of thumb on minimum size for a given displacement, but I don't know them off the top of my head. I'll check the Heywood book and see. My understanding on why larger plenums typically produce more power is that they reduce the effects that each cylinder has on the other. For a 90 deg V8, at any given time, two different cylinders are on the intake stroke. That means that the air in the plenum is being pulled two different directions at the same time. This inhibits flow to each of the cylinders, reducing volumetric efficiency. With a larger plenum, the effects that the two cylinders that are simultaneously drawing air is reduced, leading to better cylinder filling and consequently more power.

There are a couple different considerations when it comes to optimizing runner design. First you have the length to consider. Generally, given the same diameter, longer runners will produce more low rpm power and shorter runners help high rpm power. But why is this? The Helmholtz resonator theory as applied to standing waves is the key to understanding it. Basically, the air in each intake runner behaves like a mass-spring system and resonates back and forth in the runner as the intake valve creates pressure waves by opening and closing. If the timing is just right, a pressure wave is traveling down the runner just as the valve opens, providing a ram air effect for the cylinder. Longer runner have lower resonant frequencies, meaning they ram air effect occurs at lower speeds, while shorter runner have very high frequencies which create the effect at higher engine speeds. This is assuming the diameters of the long and short runners are the same. An example of this effect used to be found on F1 cars. They had variable length runners that changed position based on engine speed to allow for the resonance effects over a broader rpm range. I think they've since outlawed this practice - most likely to cut cost and keep power down.

Next parameter to tackle is the cross sectional area (diameter) of the runners. Large areas lend themselves to high rpm performance (racing single planes) while smaller areas are better down low (tpi). The important factor here is the air velocity. At all engine speeds, the runner with a smaller diameter is going to be flowing air at a higher speed than its larger diameter counterpart. The higher the velocity, the more momentum the air has. The air's momentum allows it to continue to flow into the cylinder even as the piston comes up on the compression stroke. That is why camshafts are ground to leave the intake valve open past bottom dead center. At low rpm, the smaller runner with it's higher velocity allows the cylinder to fill better than the slow moving air of the large runner, hence tpi engines with their long skinny runners make great low end torque. As the rpm's increase, the small runners become more of a restriction than a benefit. At 6000 rpm, the velocity through the small runners is so high, that the amount of work required to pull the air into the cylinder is quite high and the air at the top of the runner which used to be able to make it to the cylinder before the valve closed no longer can make it in time. The large runner allows for good velocities at high rpm with significantly less pumping losses to help more air get crammed into the cylinder. The diameter of the runner also has an effect of the rpms at which the resonance phenomenon occurs.

Hopefully this give you a little insight into the fluid dynamics occurring in the intake. I'll dig up my Heywood book and see what equations I can find for you and what other thoughts he has on the matter.

jwscab

some general rules of thumb, which are usually based in some calculations at some point or another:

-try and keep the runners as equalized as possible
-a slight taper on the runners helps create a venturi effect to help smooth and raise the air velocity.(crossfire intake is an extreme example of this.) this will also guarantee that the runner is not the restricitive portion of the system(provided there is not a large number of sharp bends)
-plenum volume is usually 1/2 to full engine displacement, which is usually tied to engine rpm and cylinder flow. high flowing and hi rpm engine operation usually dictate more plenum volume. too large a plenum makes a soggy throttle response.
-it's also helpful to try an spread apart adjacent cylinders, especially when they are in successive firing order, to minimize 'starving' a cylinder from the previous cylinder.
-try and keep the volume of the plenum as equalized as possible between cylinders, and place the throttle body central to this shared plenum. a good example is the mini-ram, where the throttle opening at the front end tends to create lean front cylinders, as the air has a tough time entering then turning quickly downward. the shape of the plenum is good(equal 'view' to each cyinder, but the throttle body should really be smack in the middle of the top of the manifold.

then, fabricating something that meets the most of those parameters is the tough part.

rocko350

take this with a grain of salt. this is a very rough calculation.


intake manifold runner length

Simplified it is 84,000 divided by RPM target equals runner length ( including half of the intake port runner in the head to the valve head) intake valve size does matter. too big a valve, needs a smaller cross section for a targeted rpm. Where space is a concern for runner length, a tapered runner can be used(i cannot give more acurate data at this time)

plenum volume is estimated to be 1.5 engine displacement, including half of runner volume. the space after the last runner, should be 1.5 times the size of the last openeing on a "run" . the tb opening should be tapered 7 degrees to meet the plenum walls from the edges of the tb bore. HTH