orMichaelDelaney on Jan/04/04 said:nice tech tip reading on IM's for the more advanced:
remember the term "mass flow" here in the article means flow capacity or flow volume or the amount of air in cubic feet per minute (cfm) which was you normally only see from flowbench graphs. It does not account for reverse flow, wet flow with fuel or "flow quality" issues.
Quote: excerpt from Jim McFarland's Understanding IM design in circle track magazine tech articles While many articles have discussed how intake manifolds tune and how to select the appropriate manifold for your engine, few discuss the variation of air/fuel ratio from cylinder to cylinder. This is a critical factor in engine tuning because the mixture can only be leaned to the point where the leanest cylinder is at its operational limit. With individual runner (IR) induction systems and electronic fuel injection, the variation can be tuned to less than 0.5 of an air/fuel ratio.
Cornering g-forces can have significant effect on the mixture distribution.This can be seen when comparing dynamometer air/fuel ratio data to on-track data...For optimum power, three air/fuel ratio variations from cylinder-to-cylinder is not a desirable condition. When tested on the dynamometer, this manifold showed a 2.0-2.5 air/fuel ratio variation, thereby verifying the inherent differences between ratio spread on an engine dynamometer and on the track. As you might expect (on a lefthand turn only track), the data shows the impact of the g-forces making the right bank richer than the left. Obviously, this effect will be more pronounced on tracks with high cornering loads.
These can make or break a good set of cylinder heads ... or be easily flawed by improper use or modification. But regardless of which design or brand chosen or changes made, there are some essentials to keep in mind.
Intake manifolds don't flow in only one direction. There are times, depending upon engine speed and load, when pulses are directed back...Technically, this describes bi-directional, unsteady state flow. Despite how this is labeled, these "reverse flow" pulses are disruptive to airflow and air/fuel mixture quality (homogeneity). Either or both conditions can impair power. Therefore, there are only certain intake manifold features that can be evaluated on an airflow bench, although these include mapping of flow pressure profiles (pressure distributions) and specific velocity patterns.
It's also important to recognize intake manifold pressure conditions that encourage increases in unburnable combustion residue (principally exhaust gas). For example, conditions creating some level of manifold vacuum at wide-open throttle allow more contamination of fresh air/fuel charges than when near-zero vacuum exists when the influence of atmospheric pressure is greatest.
Overall, the problem of changing the direction of air and fuel flow from approximately vertical to the entry angles of intake manifold runners is crucial to delivering efficiently combustible mixture. Plus, air tends to respond more quickly to throttle changes than fuel. Air and fuel are also prone to separate...Surface finish in a manifold's interior can also play a role, trending toward rough instead of smooth surfaces in order to help create or maintain efficient atomization (of fuel).
Although it may be difficult to separate the need for mixture quality from net airflow, each must be considered vital to proper manifold function... An important ingredient is to become familiar with various air (flow)bench techniques that extend beyond mere mass flow measurements, to include pressure patterns and air flow quality.
As are many engine components, intake manifold selection (as rules permit) should include specific ranges of engine speed most frequently used (i.e. powerband). While peak power numbers may be impressive, or applicable in certain situations, torque production within an intended span of rpm is important to overall race car performance.
Rod length also plays a role. As piston speed around TDC is decreased (with rod length increased), it's helpful to use intake manifolds and inlet port sizes trending toward smaller section areas that aid flow velocity, independent of large piston displacements and high rpm. The rate of pressure drop across the inlet path (boosted by smaller runners) aids volumetric efficiency in the lower- and mid-rpm. In fact, it is worthwhile to consider an intake manifold's runners as extensions of intake ports, requiring that they are mutually compatible in airflow potential and uniformity of pressure distribution ... the latter is particularly important at the interface between manifold and head surfaces. Manifolds that neither reduce port flow nor (by themselves) exceed port flow can be considered "extensions" of a cylinder head....
Runner length tunes an intake manifold based upon pressure waves, or sound. The longer the runner, the lower the engine speed range in which the tuning will take place. To illustrate this concept, two computer models were prepared: a baseline model and a model with 1 inch of length added to all the runners (no other changes made).
[http://www.circletrack.com/circletrack/139_0302_class_03_z.jpg]Figure 3[/URL] shows the power and torque curves of the engine with the two manifolds. Clearly, the addition of 1 inch of runner length increases peak torque and moves the power peak down 200 rpm. Peak torque increased 2.0 lb-ft but peak power was down 7.9 bhp (brake horsepower). At 7,600 rpm, the power of the longer manifold is 8.3 bhp less than the baseline. (While these specific quantities may not be significant, their direction validates the theory behind the change.)
Figure 4 shows the pressure vs. crank angle at the outlet of the intake manifold (cylinder head juncture) at 7,600 rpm (power peak for the baseline). An increase in pressure here means that the charge density is higher and a better cylinder fill will occur (higher volumetric efficiency). The baseline manifold is doing a better job from shortly after the valve opens until just before it closes. It is interesting to note that both cases had volumetric efficiencies above 100 percent. The pressure vs. crank angle data shows how this can occur. When the intake valve closes, there is significant pressure above ambient (pressure) in the port that provides a mild supercharging effect. (Note: one bar is 14.7 psi or the equivalent of atmospheric pressure or one atmosphere.)
Overall, filling the cylinder is the goal. More mass flow into the cylinder means more power. Figure 5 shows mass flow vs. crank angle, at the intake valve at 7,600 rpm. The greater the area under the mass flow vs. crank angle curve, the more chemical energy is available for power production. The shorter runner manifold traps more mass in the cylinder at this engine speed than does the longer manifold and, therefore, makes more torque, and power.
Figure 6 shows the pressure vs. crank angle at the outlet of the intake manifold at 6,200 rpm (torque peak). The longer runner is doing a better job from peak lift until the intake valve closes. Again, there is pressure above ambien(atmosphere) at the close of the intake valve, in both cases.
Further to the issue of cylinder filling efficiency and manifold runner length, Figure 7 shows mass flow vs. crank angle at the intake valve at 6,200 rpm. The longer manifold runner traps more mass in the cylinder at this engine speed than does the baseline.
Runner taper Taper is the relationship between the size of the runner's inlet opening and the size of the same runner's exit. The effect of increasing taper (the opening is larger than the exit) is to foreshorten the runner. The greater the taper the greater the foreshortening effect. Taper is especially useful when it is difficult to shorten the runner length.
In the previous intake model, the 1-inch longer runners were modified by adding a significant amount of taper to all of the runners to see if the power could be recovered. The bhp improved by 11.6 over the longer runner alone and was up 3.3 over the baseline. Again, reviewing Figure 4, the improvement is from mid-lift on the opening thru mid-lift on the closing. Figure 5 shows again where pressure change affects the filling of the cylinder: by increasing mass flow.
Plenum Volume : The plenum controls the pressure interaction (so-called "cross talk") or communication among the cylinders. A large plenum can decrease communication while reducing the volume can increase the interaction. The geometry of the plenum can influence the reflection of the sound waves. Generally, more plenum volume will make more peak power but hurts throttle response and may negatively impact peak torque.
To show the effect that plenum volume can have, the long runner case (computer model) with taper was modified to have significantly more plenum volume. Power at 7,600 increased by 0.9 bhp, again not representing a significant gain but verifying the concept. Figure 8 shows the pressure vs. crank angle for this large plenum manifold. The pressure at the time of intake valve opening is less than in the other cases, due to the larger plenum volume.
The plenum acts as an accumulator and holds more mass at the time the valve is opened, allowing for a longer blow-down period than the other manifolds. (Note: This effect is also useful at lower engine speeds on "restricted intake" engines.) Figure 9 shows mass flow vs. crank angle for this case. The larger plenum delivers its benefit early in the valve event and just after peak mass flow, when compared to the tapered and longer runner. The mass flow vs. crank angle graph shows the longer period of blow-down as provided by the increased plenum volume.
although the thrust of the article was directed at top mounted carbureted V8 engines with isolated runner IM's, the concepts are very extrapolatable to our isolated runner plenum designs. There's a lot of "gems" or "pearls" in that article taken from a lot of practical applied racing experience derived from more complex theory.