Intake Manifold Science

How the mixture gets from the carburetor to the combustion chambers may be more significant than you imagined.

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,
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Most stock engines are fitted with a single carburetor that delivers fuel based on “average” intake manifold flow signals, so it stands to reason that some cylinders will receive more or less fuel than others (depending upon how correct the intake manifold design may be). And since most factory intake manifolds are of the two-plane (two-cavity) design, problems of unequal air distribution have historically been associated with this type of configuration.
In order to avoid some of the problems brought about by sudden pressure variations inside a given manifold, it has been found beneficial to join each of a manifold’s runners into a common volume (or plenum). This design (single-plane or single-cavity) typically has better cylinder-to-cylinder air distribution characteristics than a two-plane, but there is usually some loss of low-rpm flow velocity (and subsequent reduction in carburetor fuel flow signals). Through the combination of the better features of each type, it is possible to provide flow rates that are quite high (as if the engine were being operated at very high rpm) but with cylinder-to-cylinder air distribution approaching the characteristics of a single-plane design. Read the rest of this entry »

Any increase in velocity will also increase kinetic energy of air and fuel, making it difficult for both to navigate changes in direction from carburetor to combustion chamber. So what we have is sort of a seesaw effect. Fuel can be more easily suspended in air if both are moving rapidly, but it’s difficult to keep air and fuel mixed when it comes time to change direction. In recent years, considerable research has provided ways to compensate for many of the problems associated with high velocity air/fuel separation. And as engine sizes continue to decrease, you’ll likely see more changes in intake manifold design, to maintain high flow rates and low air/ fuel separation levels.

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
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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.
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In goes the good air, out goes the bad—or maybe it’s just the opposite. now, it’s exhaust system fundamentals

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
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Valve overlap. (Stay with us. This all gets back to the exhaust system in a couple of paragraphs.) For optimum engine performance, it has long since been found that intake and exhaust valves should not be opened and/or closed exactly at top and bottom dead center. Gas flow inertia (resistance to movement and changes in flow speed) is low at low rpm and high at higher engine speeds. For this reason, valve overlap periods are typically short for low-rpm and longer for high-rpm operation. Specifically, overlap is the period of time (measured in crankshaft degrees of rotation) at the end of the exhaust stroke and beginning of the intake stroke when both valves are still off their respective seats. Note the illustration showing this period. Exhaust systems, almost regardless of design, are capable of “seeing” overlap periods and are affected by the amount and timing of the overlap period.
Exhaust gas dilution. Let’s call this
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There’s a little chart included that typifies this engine speed vs. gas flow velocity. And a few extra minutes spent in understanding this relationship would be time well spent. You’re gonna hear about it again and again. Because it works.
And finally, we’d like to refer to a term first introduced in an earlier Shop Series: volumetric efficiency. Hopefully (if you passed the test that month), you’ll recall that this is a measure of cylinder filling efficiency. And since exhaust system efficiency (and operating pressures) can have an effect on volumetric efficiency, we thought a little discussion of how this happens might be helpful.
First, we know that exhaust gas residue left after blowdown is hotter than the incoming air/fuel charge. This results in a slight loss of volumetric efficiency on the order of 5-8% at wide-open-throttle operation. The assumption here is that the intake and exhaust gas pressures are about equal or that the relationship (ratio) of the two is about 1.0 (unity). But as the throttle is closed slightly, this ratio is changed in favor of greater intake pressure than exhaust, resulting in some gains in volumetric
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The addition of a No. 2 tomato juice can on the end of the single exhaust pipe increases the amount of torque produced below 3200 rpm, leading us to believe that such a “collector” affects low-rpm torque more than high-rpm torque. And it does.
Decreasing the length of the single exhaust pipe does not change the rpm point at which peak torque is produced, but we do notice that there is less torque below 3200 and more above this rpm. Lengthening the pipe (still no change in i.d.) seems to increase below-3200-rpm torque at the sacrifice of torque above this point.
And then we decide that an experimental increase in exhaust pipe i.d. should show the same results. But it doesn’t. Increasing the i.d. of the pipe simply raised the rpm point at which peak torque was produced, and decreasing the i.d. lowered peak torque rpm. So we concluded that pipe dimensional changes affect the shape of an engine’s torque curve, much like you’d expect a seesaw to react. Pipe i.d. fixes the point at which peak torque is developed. This becomes the pivotal point for the seesaw. Changing the length of the pipe merely “rocks” the torque about this peak point—just like a seesaw. Maybe you’ll want to refer to the illustration showing these relationships. If nothing else, you’ll see what we saw.
Now for the real world. The engine is multi-cylinder, and the exhaust pipes must join in some fashion. For example, let’s say the design is a V8 and each bank of four cylinders must join into a common volume before passing into the remainder of the system. Stock production exhaust manifolds usually incorporate very short passages leading into a “log” or manifold that connects to a head pipe leading back to the muffler. At low rpm, such stock exhaust manifolds exhibit low flow velocities, resulting in some amount of blowdown inefficiency and lost volumetric efficiency. As engine rpm increases, there is a tendency to cause the intake-to-exhaust pressure ratio to favor higher exhaust backpressure. This also hinders efficient cylinder blowdown and reduces volumetric efficiency. And it would seem that just about anything that diminishes volumetric efficiency also whacks fuel economy and net torque production. And it probably isn’t too good.
One of the first steps away from stock exhaust manifolding is some sort of individual tube (or header) design.

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.