We all know the fact that air passage through a carburetor mixes air and fuel in some proportion for combustion. But what hasn’t been talked about is the fact that there are mixture conditions from carburetor to engine cylinder that can reduce both power and efficiency. Typically, the region where this can take place is called the intake manifold. This month we’ll discuss some of the features that separate good intake manifolds from those that might not be worth the time it takes to read about ‘em.

Essentially, there are two types: (1) single-air-cavity (single-plane) and (2) two-air-cavity (two-plane). Each has specific features to be discussed later in this story, but both are required to provide properly conditioned air/ fuel mixtures to an engine for which good efficiency is anticipated. First of all, you need to understand that air and fuel do not have the same weight (equal volumes of air and fuel do not weigh the same). As a combined mixture flowing through passages in a given intake manifold,

each will respond differently to changes in direction (as shown in Figure D). What usually happens is that fuel will not follow the same flow path as air, resulting in nonatomized fuel that passes into and out of the combustion chamber (unburned and reducing the amount of power produced by the engine).
So it becomes the responsibility of an engine’s intake manifold to (1) provide equal amounts of air (and air/fuel mixtures) to each cylinder and (2) prevent the separation of air and fuel between carburetor(s) and engine cylinders. And given the freedom of design space, there is the chance that intake manifold runner length and size can contribute to a given engine’s ability to produce torque. From the standpoint of design, this air cavity between carburetors) and engine cylinder head(s) can be reduced to a single or dual air space. If the manifold connects all of the engine’s cylinders into a common volume, the design is said to be a single-level (single-plane) manifold.

Separation of half the engine’s cylinders from the remaining number classifies an intake manifold as a two-cavity (two-level or two-plane) design. Such manifolds tend to increase the “fuel delivery signal” (amount of fuel flow) at low engine speeds and improve torque output below 4000 rpm.
But regardless of the types of intake manifolds you may be considering, there are certain features affecting the ability of each design to produce optimum engine performance. For example, in multiple-cylinder engines, there is a requirement for each cylinder to receive the same (or nearly the same) amount of air as any other. Unequal amounts of air to an engine’s cylinders means unequal amounts of power produced. And if the intake manifold being used is responsible for these inequalities, nothing short of a change in manifolds is going to solve the problem.
As a matter of fact, unequal amounts of cylinder-to-cylinder air flow (air distribution) is a common problem with “factory” intake manifolds.

A. Pressure “excursions” (variations in intake manifold mixture flow pressures) are shown here in a trace of pressure vs. time (or crankshaft angle) in only one runner of a single-plane manifold. Since the measuring pressure transducer was located in the No. 8 runner of the test manifold, reversion pulse pressure (or spikes) is the strongest in this manifold runner. Note that even after the intake cycle for the No. 8 cylinder is completed, there are pressure peaks (spikes) measured in a “non-flowing” runner. Also note that the strength of the reversion pulse is greater than the strongest “vacuum” or cylinder filling pulse . Reversion (intake manifold back-flow) is a significant problem in overall combustion efficiency. B. At the same engine speed, the smaller the carburetor throat, the higher the flow energy and the deeper the penetration in the plenum chamber of air/fuel mixtures, increasing the chance for mixture separation. As a rule of thumb, the smaller the carburetor the higher it should be from the floor of the intake manifold’s plenum chamber. This applies to both single-plane and two-plane manifold designs.

Regardless of how well fuel may be atomized as it passes out of the carburetor, some amount of flow velocity within the manifold can be used to help keep fuel suspended all the way to the intake ports of the cylinder head(s). Typical flow velocities (for passenger car engines) range from around 120-140 feet per second (fps) to nearly 320 fps, depending upon size of intake manifold, total piston displacement of the engine and rpm. And while it aids fuel suspension to keep flow velocities in the higher ranges, the problem of air/fuel separation is increased in proportion to mixture and speed and directional changes.
You might think of it this way: Air and fuel moving through the intake manifold have some amount of kinetic energy (flowing energy or pressure). Mathematically, kinetic energy is directly related to the mass (weight) and speed (velocity) of the air/fuel mixture as it flows through the manifold.

C. Air flow velocity into an engine can be calculated in units of feet per second (ft./sec), resulting in the indicated graph. At standard conditions of temperature and pressure (60° F. and 14.7 psi), differences in pressure (based on inches of water) provide the indicated rates of air flow. If you have (or are planning construction of) an air flow bench, flow velocities in the range of 15-25 inches of water would relate very well to typical race engine operation. D. A basic problem in the movement of air and fuel into an engine is the prevention of air/fuel separation along the induction system path. Since fuel is much heavier than air (comparatively speaking), there is considerable difference in flowing (kinetic) energy between the two. Mixture flow around corners can cause separation of air and fuel, which leads to reduced power and overall engine efficiency.

A part of current exhaust emissions devices is the practice of putting exhaust gas back into the intake manifold during times when the engine is under load (or at times when combustion temperatures are higher than thought desirable). Exhaust gas recirculation (EGR) passages are cast into an intake manifold in such a way that allows direct communication between the air cavity (plenum) just below the carburetor and an exhaust gas source. Usually, there is a vacuum-operated valve that keeps recycled gas from flowing into the intake manifold all the time. But when exhaust gas is allowed into the intake manifold, some amount of air/ fuel mixture dilution results. This dilution reduces the amount of combustion heat that follows, thereby decreasing the amount of oxides of nitrogen (NOx) during operation of the EGR system. But for good combustion efficiency, it’s already been mentioned in a previous Shop Series that dilution of any sort can reduce both power and net efficiency.

This isn’t to suggest that you run out and plug your engine’s EGR system, but it brings us up to the point of examining any other sources of fresh air/ fuel mixture contamination. Perhaps the most important one is the dilution that takes place during the first few thousandths of intake valve opening. As you might expect, the more time there is for this dilution to pass into the intake manifold the more of a problem it is. At low rpm, there’s

more time. So the old problem of “reversion” (back-flow into the intake manifold) is also one of contamination, since it is exhaust gas that is coming out of the cylinder and into the intake manifold. Some new-car manufacturers have even exercised this idea in the design of camshafts so that some amount of “built-in” EGR could result from slight encouragement of reversion at the lower engine speeds. Curious, huh?
And finally, before we leave the subject of contamination, there’s one more source to ponder.

E. Two basic types of intake manifolds are: (1) two-level (two-plane) and (2) single-level (single-plane). Typically, two-plane designs provide two separate air cavities, while single-plane manifolds join all cylinders of the engine into one air cavity.

1. By combining the best features of both single-plane and two-plane manifold designs, this cutaway shows how each runner can be brought directly to the plenum chamber while maintaining conventional 180-degree firing order in a two-level configuration. Note very small cross-sectional area of each runner. High mixture velocities at low engine speed result from such a design. This results in more torque output. 2. As an example of how pressure varies in each runner of an intake manifold, this oscilloscope photograph of pressure variations can be compared to Figure A, inasmuch as the basic trace is the same as that produced by theoretical data. It’s also what’s happening inside your favorite intake manifold—like it or not.

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.

Suppose we review all this stuff for a minute. Thus far, we’ve examined the passing of air and fuel into an engine’s combustion chamber(s) and what takes place during combustion. There has also been some discussion about the different types of engines, all of which “burn” air/fuel mixtures for the production of horsepower. But up to this point we’ve only gotten to the stage where power has been developed, with no regard as to what happens to “spent” air/fuel mixtures and what influence this combustion residue may have on subsequent fresh air/ fuel mixtures.
For purposes of discussion, let’s break the subject of exhaust systems into two parts: (1) muffling devices and (2) exhaust manifolding ahead of the mufflers. Each of these categories will be discussed with some expansion of the exhaust manifolding part, especially since there is a variety of types of exhaust manifolds

(headers) that relate to different types of engine performance.
In order to understand more easily what follows, suppose we introduce some terms and definitions that will crop up in the story. There’s even a chance you’ll see one or more of ‘em in the question and answer section this month. Well, at least they could be in the question section. The first term is backpressure. Since we know exhaust gas is trying to leave a cylinder and reach the outside world, any resistance to this movement might be called backpressure. Restricted exhaust systems (for example) could cause unwanted backpressure. But there are times when this might be helpful. More on this in a minute.
Blowdown is a term that has nothing to do with the three little pigs. Suppose we relate this to how efficiently a cylinder is cleaned of exhaust gas before fresh air and fuel enter. More technically, blowdown includes the amount of work required to be performed by the piston to rid a cylinder of exhaust gas. You didn’t know the piston did any of this, right? Well, think of it this way. If the exhaust valve was opened exactly at bottom dead center piston position, the piston is going to be working against higher cylinder pressure during the remainder of the exhaust cycle than if the valve had been opened sooner. However, if the exhaust valve is unseated too early, some usable cylinder pressure is going to be blown right out the exhaust system—and some power loss will result. So one of the critical factors in obtaining good cylinder blowdown is proper exhaust valve opening points (relative to piston position). A good compromise is to open the exhaust valve somewhere in the 40°-70° before bottom dead center piston position (at the end of the power stroke). You might check out all this verbal footwork by examination of the illustration. Pictures sure help, don’t they?

A. Three designs are common to mufflers. Note how each tends to diminish the strength (amplitude) of the exhaust pulse or pressure disturbances as passage through the muffler is accomplished. Noise reduction and low backpressure characterize most good muffler designs. B. The relationship between exhaust gas flow velocity, engine rpm and torque production is critical with respect to where resonant (or “critical”) flow velocity is produced. Almost irrespective of engine size or parts mix, when this critical flow velocity is achieved, an “exhaust system torque peak” is produced.

C. Valve overlap period begins when the intake valve starts to open and ends when the exhaust valve closes. Pressure conditions in the cylinder during this time can be “seen” by both intake and exhaust systems. D. Changing the length of a section of primary exhaust pipe tends to “rock” the torque curve about that rpm point already established by the i.d. of the pipe. Amounts of torque produced above and below the peak change, but the peak point does not.

E. The addition of a second “volume” (collector) at the end of a primary pipe (or pipes) tends to increase the amount of torque produced below the so-called high-rpm torque peak. F. A change in primary pipe i.d. tends to shift the rpm point at which peak torque is produced. Note that the basic shape of the curve remains unchanged, while the shift upward in rpm (based on an increase in i.d. size) takes place.

Typical of this is the four-into-one (tubes into collector) system for which there is no regard for the engine’s firing order. Four tubes on one bank of cylinders (V8 engine) lead into a collector, and four tubes on the other bank lead into their respective collector. And if nothing else, this allows the passage of exhaust gas pulses into separate tubes without the attending disturbances of adjacent cylinders dumping into the same passageway. Experience has shown that the pressure excursions of a slug of exhaust gas are most efficient (beneficial to cylinder evacuation) if allowed to pass to the atmosphere as undisturbed as possible. But that’s ideal. Actually, dumping a cylinder’s exhaust tube into a collector “shows” the remaining three tubes some amount of pressure disturbance, regardless of how neat we’d like all this to be. But it’s a start.
The next approach is to tie header tubes to collectors on the basis of the engine’s firing order. For example, let’s say we’re looking at a firing order of 1-8-4-3-6-5-7-2. (Obviously not a V6 unless it’s overworked.) In this case, exhaust tubes for cylinders Nos. 1, 4, 6 and 7 would dump into one collector and the remaining Nos. 8, 3, 5 and 2 into another. Since 180° of crankshaft rotation would take place between each pipe in the same collector, this could be termed a set of 180° exhaust headers. Turn up the volume on the next Grand National NASCAR race you hear and it’ll sound like everybody’s running 6-cylinder engines. But they aren’t.
Other methods of tying cylinders together are used (or have been experimentally tested), including Tri-Y and 90° separation (see illustrations). The reasons and results from each of these various methods, including some variations in how pipes are joined in a collector, remain somewhat controversial and related to where a given engine is required to operate (in terms of rpm). But the fact remains that fresh air and fuel are what we are attempting to provide for an engine. Combustion residue can displace fresh air/fuel mixtures to the point that combustion heat (power) is reduced, causing a reduction in power and a foot that goes deeper into the throttle.
Mufflers are exactly that. But when you consider the rpm levels at which the engines of today are being operated (well under 4000 rpm), there is little chance that backpressure from a restrictive muffler will be much of a problem.

However, exhaust manifolding that is too small to provide adequate low-rpm flow rates can hurt blowdown efficiency. And then we’re right back to the air/fuel mixture dilution problem.
But once you’ve moved away from over-the-highway engine packages and are legitimately into the 5000-plus-rpm range, the rule of thumb seems to be “where would you like the engine to run in terms of rpm?” Exhaust pipe i.d. will fix the rpm. at which peak torque is produced, pipe length will “rock” the torque curve about peak (remember, longer pipe adds to low-rpm and subtracts from high-rpm torque, shorter pipe does just the opposite). Collectors are low-rpm torque boosters, and by joining collectors with a section of pipe, you’ll add even more torque to the lower rpm ranges. Tuning an exhaust system to the specific needs of a particular vehicle? What determines pipe and collector dimensions?

G. In a conventional 180-degree exhaust system (V8-type engine), cylinders are joined on the basis of 180 degrees of crankshaft rotation. H. One deviation from the 180-degree header design joins cylinders separated by only 90 degrees of crankshaft rotation. Based on the theory that more “idle” time in a given pipe increases exhaust system efficiency (delay the introduction of secondary pulses for as long as possible), this approach may only send your local tube bender to the chuckle bin. I. Tri-Y headers have long since been used on small-displacement, automatic-transmission Super Stock- or Stock-type engines. Low- and mid-rpm torque output is improved, often over that provided by 180-degree and conventional four-into-one header designs.

REVIEW QUESTIONS: True or False
1. Valve overlap periods are typically long for low-rpm engines and short for high-rpm engines.
2. Blowdown is a term applied to how fast a given exhaust system can overflow its muffler.
3. Exhaust gas residue in an engine’s cylinders can be compared to EGR in an exhaust emissions controlled engine.
4. If an exhaust valve is unseated too quickly (relative to pjston position), unwanted exhaust gas will pass back into the intake manifold.
5. If an intake valve opens before its corresponding exhaust valve has seated (at the end of the exhaust cycle), the time both valves are unseated is called the valve overlap period.
6. “Critical” exhaust gas flow velocities are found at all engine speeds other than the rpm at which peak torque is produced.
7. Resonant conditions in an exhaust system take place when backpressure is at its highest level.
8. Question No. 7 was unclear, so the answer to No. 8 is doubtful.
9. Increasing the i.d. of an exhaust system’s header pipe will cause an increase in the amount of torque produced above the engine’s torque peak.
10. Roughly one-third of the heat energy produced by an engine passes out into the exhaust system.
11. If you want to increase low-rpm torque output, you decrease the length of the exhaust system’s collector(s).
12. As engine rpm increases above the torque peak, the relationship between intake pressure and exhaust backpressure favors intake pressure.
13. A set of headers of the 180° design do not operate at maximum efficiency until the engine has stabilized at this temperature.
14. Volumetric efficiency is affected by the amount of combustion residue left in the cylinder following the blowdown period.
15. With any luck at all, the questions next month will be a little easier.