Modified Mustangs & Fords
Engine Building Part 3 - Power Principles, Heads, Cam, and Intake
Choosing the right Induction components
In this month's Power Principles installment, we conclude our series on engine building basics. In the Feb. '12 issue of MM&F, we stressed the importance of developing a clear, underlying goal as far as your engine needs are concerned. The second issue dealt with block basics and starting with the correct foundation for whatever engine project you have planned. In this issue, we will talk about what is probably the most important aspects of an engine build: the top end. The "top end" usually refers to everything above the short block, i.e. the heads, cam, and intake--basically everything that's needed to make horsepower. Our goal here is to offer some information to help you make better, more informed choices when it comes to picking the top end components. Let's get down to business.
The simple fact is that you really can't put a finger on which engine part is the "key" to making horsepower. You need a strong short-block to contain the horsepower. You need the correct camshaft to put the horsepower range where you need it to be. You need the intake to bring the air/fuel mixture in at the velocity/volume that you need it. Every part in an engine works together.
That being said, it's hard to say that the cylinder head isn't the glue that holds everything together. The ports bring the air/fuel mixture in, the valves allow it into the chamber, and then the chamber provides the venue for a complete combustion. Important role isn't it? When selecting the right head, you will need to keep your eyes on several factors: port volume, flow numbers, valvetrain allowances, header flange bolt patterns, and exhaust port locations.
Most heads are advertised in "sizes," which refers to the intake port volume. If you thumb through your favorite aftermarket parts supplier catalog or magazine advertisements, you'll see SBF heads listed in sizes such as 185cc, 205cc, 225cc, and so on. Those are the intake port volumes. This is a fairly critical variable in sizing up the correct cylinder head for your application. If you pick a head that's too small, you may actually create a bottleneck. If you pick a head that's too large, you could make the engine unfriendly at lower rpm.
An air/fuel charge has its own inertia and a larger volume of air/fuel would be harder to get moving than a smaller volume. In Part 1 of our Power Principles, we talked about how important it is for the engine to fit the application. The '70 Boss 302 Mustang is often an example in this. Here we have a car that's not really a lightweight, but combined with a small engine and enormous (comparatively speaking) 351C 4V heads. In street cruising mode, the engine wasn't really a performer (again, comparatively speaking), due to the fact that the engine needed high rpm to create the velocity necessary to move the larger volume of intake charge. If you fitted the car with a 4.56 gear and kept the engine screaming, then you had something that could take advantage of the larger head volume. So the underlying theme here is to choose the cylinder head for the application. Lots of guys tend to get overexcited with larger spec numbers, but it can often shoot you in the foot. Bigger is not always better.
Everyone always asks, "What do the heads flow?" That's a very important question when looking for the correct cylinder head for your engine, however, it's not the only thing to take into consideration. For one thing, a flow bench is like a dyno: they're all not going to show you the exact same numbers. Manufacturer flow benches may not show you the exact same numbers as your engine builder's flow bench. You also have to look closely at how the heads were flowed. A head may flow more air on a larger bore because the valves are not shrouded by the cylinder walls. Often, manufacturers will use a 4.125-inch bore when flowing aftermarket SBF heads, when in reality, they may get bolted to a 4.000- or a 4.030-inch bore. It makes a difference.
The valve lift also makes a huge difference. A cylinder head may flow 320 cfm, but at what amount of valve lift? If it's at 0.700-inch lift, then how are you going to take advantage of that with your 0.550-inch lift camshaft? One good rule for qualifying cylinder head data is that it's better to choose a smaller port volume head when flow rates are similar. For instance, if you have a 205cc head that flows 300 cfm at 0.600-inch lift and you have a 225cc head that flows 300 cfm at 0.600, then it may be a better decision to choose the smaller port volume. If you can flow the same rate with a smaller port, then it could be a good indicator that the velocity may be much higher. High velocity is a good thing. Remember our discussion on air/fuel charge inertia?
When looking for an aftermarket head, keep in mind that most aftermarket heads rely on stud-mount rocker arm setups (available in both 3/8 and 7/16 stud sizes, which need to be matched to the rocker arm trunion size). A lot of factory heads used pedestal (or rail style) rocker arm mounts. If your plan was to reuse some factory parts in your build, then you will need to make sure that everything is compatible. Also, it's a good idea to have your camshaft in mind when you're buying assembled heads. Flat tappet camshafts call for different valvespring pressures than roller cams. Keep an eye on the valve lift as well, as the springs that come with a specific cylinder head package may coil bind at the lift that you will use with your camshaft. Everything needs to work as a package.
You'll also need to look at the exhaust flange pattern and location. Some heads are only drilled for certain flange patterns (some may not be factory).
When selecting a head, you will also see that there are different chamber sizes available, again measured in cc's. The combustion chamber is a very important component of your compression ratio. This of course needs to be taken into account when choosing a head, as you want to match the combustion chamber size to the rest of your block/piston combination. This will play a role in whether or not you can run 87-octane pump gas or if you have to mix some race gas in your tank. Most manufacturers will offer a few different chamber sizes so that you can take creative license, no matter if you're using a flat top piston or a dished piston.
A lot of the aftermarket cylinder heads have some very large sized intake valves in comparison to factory heads. Usually, aftermarket performance pistons have valve reliefs suitable for these valve sizes, but if you are upgrading an engine and you're using the stock pistons, this could be a temporary show stopper. Pistons designed to work with 1.94-inch intake valves could cause problems with intake valves that are 2.05- to 2.08-inch in diameter.
We could talk all day long about camshaft selection and still just scratch the surface. There's no way we could discuss every possibility for a particular engine, so our aim here is just to hit the major points and try to illustrate what to look for when planning a particular engine build.
When you look at camshaft specs, you'll see terminology such as duration, LSA, and lobe lift. There are many specifications that are used to describe a camshaft's personality, but these are the most common. Just like all of the other engine components that we've described in the past few articles, camshaft selection is extremely important, and a cam must be chosen to fit the application. The cam is often deemed the "brain" of the engine and rightly so. Its job is to open and close the valves at just the right time, so it essentially controls the engine's personality. Choosing the wrong cam can really put a damper on your day, but picking the right one can make you exercise your face muscles in a good way. So let's go over some terminology and see how it applies to your engine plans.
First and foremost, we have different types of camshafts: hydraulic flat tappet, solid flat tappet, hydraulic roller, and solid roller. Your application should greatly dictate the type of camshaft you need to run, but here are a few things to consider:
Hydraulic Flat Tappet Pros: Inexpensive. Low maintenance once broken in.
Hydraulic Flat Tappet Cons: Camshaft break-in can be tricky and there is always a chance of break-in failure. Rpm limits should be kept in the 5,500-6,000 rpm range, as valve float is a prevalent problem with hydraulic camshafts.
Solid Flat Tappet Pros: Inexpensive. Little more maintenance is required as valve lash can change. Can handle high revs.
Solid Flat Tappet Cons: Same issue with break-in as the hydraulic flat tappets.
Hydraulic Roller Pros: More expensive than the flat tappet camshafts. Expect to pay about $125 more for a cam/lifter set that uses factory-style lifters. If you're going to use link bar lifters, then you can add a few hundred bucks more to that figure. A hydraulic roller cam is the least painful of all cams. No cam break-in, no maintenance. Shove it in the motor, adjust the preload, and call it a day.
Hydraulic Roller Cons: Rpm limited. In a Ford engine with larger valves, 6,000-6,500 seems to be the upper rpm limit, depending on the spring/retainer package that you're using. Anderson Ford Motorsport does offer hydraulic roller cam and spring packages that are purported to have rpm ranges of 7,700 or more, but this isn't the norm. Typically hydraulic roller cams don't like to rev, especially with the larger valves (2.05- to 2.300-inch) that we see in SBF-BBF engines.
Solid Roller Pros: Lots of horsepower potential due to aggressive lobe designs. Also, lots of rpm potential with the correct valvespring package.
Solid Roller Cons: Can be pretty expensive, especially when you're looking at some good pressure-fed lifters. Pricing can approach $1,000 for the cam and a quality set of lifters alone, not to mention expensive valvesprings if you're planning on radical lobes and a super high rpm range. Solid rollers require a little more maintenance than the rest of the cams. Valve lash needs to be checked periodically, like the flat tappet cams. Also, valvespring pressures need to be tested in different intervals, due to the fact that a more aggressive lobe design will weaken the spring steel. In addition to those items, solid roller lifters lead a rough life. The bearings in the lifter wheel take an extreme amount of abuse because of valve lash coupled with high spring pressures. It's a good idea to R&R them as well.
Those qualifications should give you a warm fuzzy feeling for which direction you need to head. Again, be honest with yourself, and make the engine fit the application. Yes, you'll be saying that in your sleep before too long. "Honey, you woke me up last night making speed shifting noises and saying something about making the engine fit the application. Oh, and by the way, you slept with a connecting rod under your pillow." Yes, it happens to all of us.
Let's go over some camshaft terminology and discussion now:
Duration: Camshaft duration is the amount of time (in degrees of rotation) that the cam holds the valve open. You'll commonly see durations listed as "advertised" or "at 0.050-inch valve lift." Essentially, when you see a number in front of the "duration," it's telling you how long the valve is open if you start at that number, open the valve fully, then bring it back to the position you started with. For instance, a camshaft with a 230-degree duration at 0.050-inch means that if you started with the valve 0.050-inch off the seat, then it would take 230 degrees to fully open it, then close it back to where it's 0.050-inch off the seat. Advertised durations are rated at different values, depending on the type of camshaft and the manufacturer. We typically see them rated at 0.006-inch and 0.020-inch. If you look in a lobe catalog, you'll also see durations rated at 0.200-inch.
So what do these numbers mean to you? If you look at the 0.050-inch duration, the 0.200-inch duration, and the advertised duration, you can get a very good handle on what the camshaft is going to do. For example, the 0.050-inch duration will give you a very good idea on where the engine’s horsepower peak will be (this also depends on head and intake flow, so you have to be careful here). A 347 with the typical 185cc AFR head package will peak at around 5,500 rpm with a 218-224-degree duration at 0.050-inch hydraulic roller camshaft. If you add 12 more degrees at 0.050-inch (i.e. 236 degrees at 0.050-inch), the horsepower peak can be moved up to around 6,500. That horsepower peak will give you clues as to how the rest of the horsepower and torque curves will look. If you have a very high horsepower peak, then typically the torque curve will follow suit, and both curves will have a sharper shape. Keep in mind here that if you shift the curves to the right, then you take away power from the left side of the curve. This means that you have high rpm power, but off idle and mid range power can suffer. Remember our discussion involving our 5,000-pound Thunderbird with the 302?
Advertised durations will help you with calculations for dynamic compression ratios and it will also clue you in on how aggressive the lobe is. Very simplistically speaking, if there's a smaller difference between the 0.050-inch duration and the advertised duration, then the lobe is more aggressive. For instance, Comp Cams has three popular hydraulic roller camshaft families: Magnum, Xtreme Energy, and XFI. The lobe's aggression increases with each family change. Let's take a quick peak here:
Comp Cams Magnum 280H: 280-degree advertised duration, 224 degrees at 0.050-inch=56 degrees
Comp Cams Xtreme Energy 276XE: 276-degree advertised duration, 224 degrees at 0.050-inch=52 degrees
Comp Cams XFI 274XFI: 274-degree advertised duration, 224 degrees at 0.050-inch=51 degrees
You can see that each lobe design has its own intensity, which means that it has a certain degree of aggressiveness. Looking at this a little closer, we know that advertised duration is measured at a very small amount of valve lift. The 0.050-inch duration is measured at 0.050-inch of valve lift. If you have a smaller amount of degrees between these two, this means that the valve is being jerked open, held open longer, and then effectively slammed shut. So if we look at our three cams up above, the cam with the Magnum lobes should be easier on the valve train than the XFI lobes. Of course, as with anything else, there are always exceptions to the rules, but this is a very good way of measuring how hard your particular camshaft is going to be on the valves, springs, retainers, and more. If you have an aggressive cam with inadequate valvesprings, then you can introduce valve float and valve bounce into the equation, which is a recipe for premature wear or engine failure. This is especially the case with roller cams, as the lifters and valvetrain components are much heavier.
We mentioned the duration at 0.200-inch and when you look at this duration in accord with the others, it will allow you to "map" out the valve action. Jerking the valves open, holding them open for a long period of time, then quickly closing them can equate to increased horsepower, but you need to be aware of the dangers too.
Now that we can look at the camshaft durations and gather more information about where the peaks will be and how radical the valve action will be, we can also look at the cam's LSA. The LSA is the lobe separation angle. This is basically the distance between the intake lobe centerline and the exhaust lobe centerline. You'll often hear this referred to as the cam's "lobe center." Most street cams are usually in the 110- to 114-degree LSA range and race cams can often dip down into the 106- to 108-degree range.
Increasing or widening the LSA can provide more engine vacuum, better idle quality, and a broader powerband. You'll often see a lot of EFI cams with higher LSA's because it tends to "smooth" out the idle. A cam that hits hard will play with an EFI engine's knock sensor and that's usually not a desired result. Nitrous and forced induction cams will often have wider lobe centers as well. A tighter LSA will often give a more "radical" idle because it has more overlap, which is the amount of degrees that both intake and exhaust valves are open at the same time. It will also affect the engine's manners so that engine vacuum is lessened, idle qualities are a little less clean, and the powerband is often in a more narrowed range, instead of producing a broad curve.
One thing that needs to be mentioned is that engine displacement can, for lack of a better term, "dumb" down a camshaft's specs. For instance, a 302 with a 248-degree (at 0.050-inch) duration camshaft will be a very "high strung" engine with very peaky curves and very poor street manners. However, this cam would be right at home for a street application in say an engine that displaces 529 cubic inches. The more cylinder volume that an engine has, the more duration it needs to effectively fill those cylinders. Camshaft selection is often one of this author's most favorite parts of an engine build. Whoever said that the cam was the engine's brain, we would wholeheartedly agree with that. The cam dictates the powerband, it dictates the manners, and it dictates "the sound." All extremely important aspects of a well-thought out engine build.
The intake manifold is extremely important, as it's the door for the air/fuel mixture to enter the cylinder heads. You'll see various designs of intakes, mostly centering around "dual-plane" and "single-plane" intake designs. A dual plane intake has a divider in the plenum (under the carb or throttle body) that provides two plenum areas for the engine to draw from. A single plane's plenum is open--just one common area for all of the runners to draw from.
Generally speaking, single-plane intakes are suited more for higher-rpm engines or race applications. Most offer very large plenum volumes and short runners that can be easily ported and shaped to flow more air. The only issue with large runner volumes is that it tends to give the same effect as a cylinder head with large port volumes. It takes momentum to get the mixture moving and sometimes it requires more rpm to get that momentum. Dual-plane intakes are suited more for street applications, as they tend to offer more power in the lower/mid range powerbands due to their longer runner lengths. You'll also often see a better equalization of the fuel mixture to the cylinders, as the intake is divided instead of every cylinder having to pull from a large, shared, open plenum. This is more of a concern in carbureted applications where the fuel is not injected individually into each intake runner.
In the same fashion, a large engine can “dumb down” an intake manifold. Larger engines need much more volume so that they can be fed effectively. For instance, an Edelbrock Victor Jr. intake on a 302 can offer sluggish street manners, but it would be a perfect choice for a Windsor in the 427- to 445ci size.
Carburetor spacers can be used to modify the intake manifold's plenum design. Adding an inch or two of space between the carb and the intake can allow the fuel mixture to pick up some extra velocity as it enters the cylinder head. It can also add plenum volume to an intake manifold that may be a little undersized in that area. There are open spacers, four-hole spacers, and combination spacers. It's hard to put an exact description of what each spacer would do in a certain situation, but very generally speaking, an open spacer will add horsepower while a four-hole spacer will add torque. However, engines don't always understand this rule of thumb, and you can often be surprised at the result of adding each spacer, tapering a spacer, turning a spacer upside down, and so on.
What's our motto? Yep, build the engine for the application. As long as you keep that in the front of your mind, you will do an excellent job in the design of your new engine. If you're more adventurous, you will learn a great deal if you hand-pick each component. Choosing the cylinder heads to match the engine's desired manners, picking an off-the-shelf cam (or even ordering a custom cam made to your specs) to make your engine perform the way you want, and then topping the engine off with the correct intake manifold will give you a great feeling of accomplishment and increased knowledge.
If you're not one of the adventurous types and you'd rather use a company that puts together its own cylinder head/cam/intake packages, then there's absolutely nothing wrong with that. You can still decide what the desired result will be, then carefully look at the packages that are available from manufacturers such as Edelbrock, Trickflow, AFR, and so on to obtain that desired result. Plus, there's no rule that says that you have to keep those same components; you can always do a cam swap, intake swap, and more, to see how different items will make your engine behave.
The bottom line, however, is that if you start off with a good foundation (your engine block), good machine work, and select the parts that make the engine fit the application, then you will not be disappointed.