This small-block Chevrolet...
This small-block Chevrolet spec manifold from Edelbrock shows an intent to equalize runner lengths, as compared to a stock manifold, although some length variation lends itself to individual cylinder tuning at described in the story. Rocker-arm ratio and primary header pipe are two possible tuning tools for spec manifolds with greater runner length variations than this design.
While particularly beneficial...
While particularly beneficial when using two-plane spec manifolds, two-inch spacers (such as this Swirl-Torque design from TD Performance Products featuring mixture-enhancing slots) can aid cylinder-to-cylinder mixture quality and distribution during mid- and high-rpm operation. Power gains stem from increased carburetor-to-plenum floor height (on the upper plane), especially with the two-plane design, although similar improvement can be found with spacers on single-plane manifolds.
NOTE: Although somewhat exaggerated...
NOTE: Although somewhat exaggerated in this illustration, the effects of tuning short and long spec intake manifold runners tends to produce a wide, flat torque curve ... the effects of which are the result of two independent torque curves or a composite curve that broadens the effective range of an engine by way of optimizing the output of cylinders served by runners of different length dimensions. Other factors influence this type of engine output, as discussed in the accompanying story. This tuning process actually produces a family of torque curves, the net of which can extend the range of operation ... particularly beneficial in many circle track situations.
NOTE: Particularly in two-plane...
NOTE: Particularly in two-plane spec manifold designs, carburetor spacers where permitted can provide additional room for exiting air/fuel mixtures to lose energy and negotiate turns into upper plane runners. This aides mixture quality and, depending upon circumstances, benefits overall combustion efficiency and power. Impingement of mixtures on the plenum floor can further degrade cylinder-to-cylinder mixture distribution.
NOTE: This sample arrangement...
NOTE: This sample arrangement of intake path flow volume (cfm) is a theoretical representation of variations that may occur in a spec intake manifold ... perhaps somewhat exaggerated to illustrate a basis for companion parts selection (per the story).
In the first installment we dealt with the essentials of spec intake manifolds. The story was intended to establish a level of understanding that would enable more specific information to be helpful, optimizing engine performance in the face of limited intake manifold function.
Among suggestions previously made was the possibility of augmenting restrictions placed on manifold modifications by careful integration of other major engine components into the spec equation. This month the story concludes with more in-depth discussions, sprinkled with comments from two engine builders whose credentials are unquestioned.
spec Manifold Limitations
Generally, youll find that unless a spec intake manifold has been designed to address the problems of an otherwise stock version (or the subject is a racing manifold with certain deficiencies), there are some problems you need to consider. Of them, cylinder-to-cylinder air or air/fuel mixture distribution (or both) is common. Aside from the fact this issue is fundamental with the design of any intake manifold, some finished products are worse than others. Intake manifolds designed primarily to increase net airflow in deference to improved distribution are cases in point.
Stated another way, intake manifolds that deliver differing amounts of air or varying qualities of air or air/fuel mixtures tend to prevent all cylinders from developing equal or near-equal amounts of power. The subject of equalizing power among cylinders has been a previous Circle Track technical topic of discussion. Intake manifolds with poor cylinder-to-cylinder delivery of the working fluid create opportunities to compensate for this problem. Several of these opportunities will be addressed in later paragraphs.
The point here is that treating all cylinders on the assumption each will produce equal levels of power is flawed by spec intake manifolds that create distribution problems. Therefore, by identifying the problem cylinders and making adjustments that compensate for (or correct) these conditions, its possible to improve the overall power of an engine package. Keep that thought foremost in your mind as we dig into the details of how this can be done.
Begin With An Airflow Study
If possible, do an airflow study of the spec intake manifold, bolted onto the cylinder head(s) to be used. Flow each runner/port combination by itself, sealing off all other passages when you do. In this fashion, youll develop an idea about the airflow potential of each runner/port combination, leading to some indication of cylinder-to-cylinder air distribution. While this method does not contemplate certain dynamic conditions (so-called cross talk or pressure excursions) created in a running engine, it still provides workable indications.
Perform these tests throughout the range of valve lifts that accompany the camshaft youll be using. Pay particular attention to airflow at valve lifts of about 65 percent of net for the intake and 85 percent for the exhaust. Each of these two percentages represents piston positions at which critical intake and exhaust flow occurs during engine operation. These are also points of valve lift where piston position and port flow are important (during engine operation) for optimizing volumetric efficiency (intake stroke) and cylinder blow-down (exhaust stroke).
If the manifold has problems, find out where they are and which cylinder head intake ports and manifold runners are sources of reduced or quality-impaired flow and potential mixture-quality disturbance. This is basic and required information, without which youre operating in the dark regarding tuning one cylinder at a time. Tuning adjustments to be discussed later cannot be properly directed to specific cylinders without some identification of good and bad runner/port combinations.
Tuning One Cylinder At A Time
Once identification of good and bad intake paths is made (either by airflow study, spark-plug readings, air/fuel ratio measurement or a comparable method), companion parts or tuning methods can be employed to optimize individual cylinder output. An initial objective is to determine which engine components or functions can affect individual cylinder output. With this information in hand, you can begin making adjustments for improved performance of the entire engine package.
Following are some areas of typical investigation. For further validation of the importance placed on areas of potential power gains, two prominent engine builders were consulted on the spec manifold issue: Keith Dorton, owner of Automotive Specialties Racing Engines, and Dennis Wells, proprietor of Wells Racing Engines. Both have extensive engine building experience that includes optimizing power from spec intake manifolds.
We requested two sets of responses from these professionals. One was to give priority to a list of engine components or systems that can influence spec intake manifold performance. The other involved a list of four questions, each directed to dig further into the experience base these two builders have amassed.
Following is the list of components or systems proposed for placement in order of importance and tendency of an engine to be sensitive to change:
Air cleaner design This category includes size of filter element and design of cleaner assembly lid and base.
Carburetor spacer Here we referred to spacer use, height and design (open, four-hole or hole configuration).
Header system Included primary pipe steps, pipe size, collector dimensions and use of cross-over pipes.
Camshaft design Not excluded was the possibility of multiple intake/exhaust lobe profile specifications and positioning on the shaft.
Ignition timing Sensitivity of a spec manifold engine to variations in spark timing.
Carburetor Not restricted to size alone, this category includes booster design and other possible mixture distribution fixes.
Rocker-arm ratios In particular, the issue here is potential benefit of varying ratios among cylinders.
Spark plugs This dealt with heat ranges, indexing and depth of plug into the combustion chamber.
Here is the order suggested by Dennis Wells:
Air cleaner design
Here is the order suggested by Keith Dorton:
Air cleaner design
To expand on these topics, consider the following additional thoughts about some of the topics and priorities listed.
Air cleaner design While the amount of filter surface area and top configuration of a given air cleaner assembly can affect net airflow, the shape of an assemblys bottom is often more critical. Since air tends to take the shortest path in an induction system, the distance from the air cleaner base to the carburetor entry frequently represents this path.
Therefore, the manner in which air moves across the floor of the base and enters the carburetor can affect net airflow. Experimenting with base shapes often helps compensate for spec manifold runners otherwise experiencing flow problems. Its a way for helping a bad condition become somewhat better.
Header configuration Sizing primary pipes to match good and bad intake manifold runners can be an effective way of optimizing individual cylinder power. For example, a port deficient in flow may be tuned for a lower range of rpm by lengthening its companion primary header pipe. Ports flowing higher levels of air may be tuned to higher rpm by shortening pipe length. In a similar fashion, pipe diameter can also be used as a tuning tool (low rpm vs. high rpm).
Since header collectors tend to be torque-productive at engine speeds near or below peak torque rpm, general improvements in torque within this range can further support spec manifold runners limited to lower rpm. Generally, the greater the collector volume, the higher their output below peak torque rpm.
Rocker-arm ratio This can be a very effective way of aiding poor-flowing intake manifold runners. Slight increases in rocker ratio can boost volumetric efficiency, leading to improved net port flow for runners incapable of matching the flow provided by other and more efficient runners. In such cases, increasing the ratio on the intake side is often the most effective area in which to work.
Camshaft design Depending upon how friendly you may be with your favorite camshaft grinder, this is an area of potential benefit. The reduced airflow of certain spec manifolds runners can be addressed by increasing the specifications of intake lobes (cylinder by cylinder), increasing valve overlap periods for the same cylinders and (in some cases) incorporating higher lift values can help equalize or balance cylinder-to-cylinder airflow. Delaying the intake closing point is another area of sensitivity.
In short, each intake runners flow limitations can be addressed by tailoring intake and exhaust lobe specifications (including displacement angle) to match the flow deficiencies. The result, at least in this writers experience, can be a finished camshaft that sports a variety of intake and exhaust lobes, frequently positioned at differing displacement angles along the shaft. You might want to give it some thought.
Ignition systems Where allowed, the use of programmable ignition systems is providing individual cylinder spark timing ... or at least a formatting of spark maps that are more applicable to optimizing single-cylinder tuning than systems that dont. As this technology continues to evolve, racers will likely have access to increased ability for addressing the specific spark requirements of cylinders that vary in volumetric efficiency, one from another ... for whatever the reason.
Summary of the engine builder interview The following four questions were asked of each builder. Their responses are noted accordingly.
Question #1: Based on the list of components and systems listed, which do you feel can be the most influential for adjusting for spec manifold performance, although not necessarily the easiest change?
Dorton: From what weve seen, adjusting valve timing (primarily through the selection of rocker ratios) and establishing individual cylinder spark timing appear to be the two most effective areas for optimizing overall power.
Wells: Header configuration seems to have the most effect on power output when trying to adjust for low-performing cylinders. By matching primary pipe length to variations in individual manifold runner airflow, weve found its possible to boost torque in an rpm range where the cylinder seems to perform best. Of course, primary pipe diameter is also a tool that helps move torque around in the rpm range
Question #2: Some pretty good results have been produced tailoring camshaft lobes and displacement angels to specific cylinders. This seems to work for spec manifolds with differing length of runners or airflow variations runner to runner. Any thoughts on this?
Dorton: Im sure there are benefits to be derived by this method. However, my opinion is that it requires an extensive amount of testing to arrive at the best combinations of lobe specifications.
Wells: This approach has shown to be a good way to help balance out the differences that may occur in spec intake manifold runner lengths. In other words, the longer the runner the lower in rpm it tends to make torque. The shorter the runner, the higher the rpm where it tends to make torque. Camshaft lobes can be designed to address these conditions.
Question #3: Pertaining to the above question, there have been header systems sized and built to directly match spec intake manifold runner lengths. The results tended to produce engines with flatter torque curves than those using headers that didnt incorporate this feature. What can you say about this approach?
Dorton: We have used many different header primary designs, including step, tri-Y and expansion chambers, with sometimes huge differences in power. Although Im not sure about the effects of these designs on individual cylinder power, I expect there are gains to be made by experimentation.
Wells: I think this will work well but only so long as you keep the car geared to take advantage of the additional torque thats available when coming out of the corners. By this I mean that if youve built additional torque into the lower or mid-rpm range and dont gear the car to run in this range, your efforts will be lost.
Question #4: Depending upon the airflow limitations of a given spec intake manifold, do you think its best to determine the rpm range where the manifold works best and then select engine parts around this range. If so, how would you go about finding out what this rpm range is without running the engine?
Dorton: Things such as carburetor size limitations and camshaft restrictions will determine the engines rpm range. In many cases, rules limit what can be done to an engine, thereby causing airflow limits. Even in these cases, optimizing individual cylinder airflow, within the range of restricted rpm, is a worthwhile path to take.
Wells: This is a good approach. It can save a lot of time and reduce mistakes made in choosing engine parts. What I have done, and do, is use one of the engine simulation programs currently available. A little personal computer time spent with one of these programs can really be worthwhile. Specific power levels predicted may not conform to actual test data, but you can certainly see trends and eliminate many of the variables that can lead to buying the wrong parts.
Some Concluding Thoughts
Single-cylinder tuning is becoming more common in the building of contemporary race engines. Even the OEMs have gotten into this act, targeting increased fuel economy and emissions reduction in the process while aided by the use of on-board electronic fuel and spark management systems. However, even in the absence of electronics, there are mechanical ways of accomplishing similar individual cylinder power objectives.
But as suggested at the outset of this story, its important to identify which runners in a spec intake manifold may be flow deficient, of unequal length or both. And when you can, do the airflow math. Even armed with this information, it can be a task to deal with problems these conditions create. Without such information, however, the job becomes more difficult.
For the best results, analysis becomes a combination of several data inputs, some derived before an engine is run and some afterwards. Most important, recognize the fact an engine is a system of individual cylinders, each of which must be optimized (within the range of most frequent engine operation), if a maximum power package is to be assembled. In this particular case, the whole is the sum of its parts.
To view Outtakes On Intakes, Part I, Click Here.