Intake Manifold Science

Most all intake manifolds for V-type engines (noncompetition variety) have a passage connecting a port (in each head) leading to an exhaust port. This allows exhaust gas to heat the lower portion of the manifold, usually just below the plenum chamber. But what you might not have considered is the fact that this isn’t just a heated passage. Let’s say that we have an engine with an intake manifold connecting the exhaust ports of cylinders number 4 and 5. Depending upon the firing order of the engine, the design of its particular camshaft, and how much exhaust gas from each cylinder actually gets into the crossover passage, these two cylinders could share an amount of contamination. Blocking either end of the passage does not remove the heat from beneath the plenum chamber, since one still feeds in exhaust gas. But when you do this and an additional 6-8 horsepower suddenly appears, you begin to wonder about the old adage that blocked heat means colder mixture, which means more power. Well, you might want to think about it, anyway.

On another subject, much has been said about the relationship between an intake manifold and long/ short connecting rod engines. It seems to be a pretty good rule of thumb to say that as connecting rod length increases, there is a reduction in piston velocity around top and bottom dead center piston positions. This decreases the rate at which intake manifold pressure drops in the early stages of the induction stroke, placing more responsibility on the intake manifold to produce good air/ fuel mixture flow rates. So long-rod engines like small manifold runners. On the other harvd, as rod lengths grow shorter (assuming they can grow at all), piston motion around TDC and BDC increases. This helps establish proper mixture flow rates (based more on piston speed than manifold size) and places less responsibility on the manifold. Conclusion? Short-rod engines like larger intake manifold passages. And even though you’ll occasionally find exceptions to this line of thought, it’s not too far off.
Now, by way of a little review, let’s go over a couple of points we’d like to emphasize. First, we placed intake manifolds into two basic categories: (1) single-air-cavity and (2) two-air-cavity. Even plenum-ram-type race manifolds fall into the single-air-cavity group.
Next is the fact that you’d like for both of these two types to have good cylinder-to-cylinder air distribution. Regardless of how fuel is metered, unequal amounts of power are going to result from unequal amounts of air distributed to different cylinders. Then it was mentioned that since both air and fuel have mass (weight), they have some amount of kinetic (or flowing) energy when they move through a manifold. Since they are of unequal weights (usually around 12-14 parts of air to one part of fuel) they will react differently to changes in direction when passing through the manifold. The result is often air/fuel separation, some loss of combustible fuel in the cylinder and raw fuel passed out the exhaust system (increased unburned hydrocarbons).
Other factors relative to intake manifolds concern various sources of dilution or entry of exhaust gas into fresh air/fuel mixtures. The results of this are lost engine power and efficiency .. . and fuel economy.
Also worthy of mention are some of the features of carburetor spacers and plenum dividers. And to do this, let’s return to our discussion about kinetic energy. As air and fuel pass from beneath a given carburetor, their “exit velocity” will depend largely upon engine speed, total piston displacement, size of carburetor and intake manifold, and camshaft design.

But for now, let’s assume that we have no option other than relocation of the carburetor relative to the floor of the plenum chamber. If, for example, we find that spacing the carburetor higher helps top-rpm power, then we’ve probably stopped some air/fuel separation on the plenum floor (like pointing a water hose into a bucket. . . and turning on the water). But raising the carburetor weakens the fuel metering signals, possibly requiring a larger jet. Lowering the carburetor usually requires smaller jets.
Four-hole spacers tend to get back mixture exit velocity lost with “open” spacers. Plenum dividers seem to operate in much the same way. That is, more divider means more exit velocity, stronger fuel delivery signals, and probably an enrichment change for best power.
Finally, there was deliberate omission here of any thoughts on intake manifold tuning. Next installment, when we get into exhaust systems, you’ll see some similarities between intake and exhaust systems. And when you do, there’ll be some bits stuffed in here and there to whet your appetite.

REVIEW QUESTIONS: True or False
1. A 180° intake manifold design is most efficient when an engine reaches a coolant temperature of 180° F.
2. Carburetor spacers tend to make an engine run “richer” (relative to air/fuel mixtures) than it would without such increases in carburetor height.
3. Long-rod engines tend to have more problems with reversion (combustion efficiency reductions resulting from fresh air/fuel mixture contamination) than short-rod engines.
4. The larger the manifold runner size (cross-sectional area), the stronger the fuel delivery signal.
5. Assuming no change in intake manifold configuration, the smaller the carburetor flow size the greater the fuel delivery signal.
6. Intake manifold design has little if any effect on an engine’s ability to produce torque in a specific range of rpm.
7. The relationship between connecting rod and crankshaft stroke lengths has no bearing on intake manifold design.
8. Since air and fuel are of particular difference in weight (mass), both can be expected to flow through a given intake manifold with identical energy characteristics.
9. The most important function of an intake manifold is to eliminate air distribution inequalities among an engine’s cylinders.
10. Intake manifold selection has little to do with an anticipated range of engine rpm.
11. The lower an engine’s rpm range, the more difficult it is to reduce the influence of reversion (exhaust gas contamination of fresh air/fuel charges).
12. Two-plane manifolds have both (1) good low-rpm torque characteristics and (2) weak fuel delivery signals at the carburetor.
13. Carburetor size has nothing to do with how quickly you read this particular Shop Series.

This entry was posted on Wednesday, April 14th, 2010 at 7:19 PM and is filed under Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.