Now that we have a decent handle on the secret language of camshafts and perhaps an appreciation of what the valvetrain has to endure in a typical street performance engine, it’s time to put all this knowledge to work by choosing a camshaft for your next performance small-block. And it is here that many enthusiasts forget what they’ve learned and go straight to the biggest “lumpy” cam they can find.
Perhaps it is all this complex engineering that sucks the romance out of performance engines. But if you let your enthusiasm, or worse yet—your buddy’s Kentucky Windage choice of a big cam to color your decision, then you don’t need to read this chapter. But chances are that if you’ve plowed your way through most of this book to get to this point, you’re here because you want to learn. So let’s get started.
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The first thing we need to do when deciding on a camshaft for any small block is to honestly answer a few questions about how this cam is to be applied. Being honest here is critical. How is this engine to be used? Is it a mild street engine that is to be driven to school every day with an occasional jaunt down the quarter mile, or is this more of a hot street engine that only comes out on Saturday night and only to impress that snobby cruise crowd at the Boffo Burger? Car weight, auto or manual transmission, rear gear ratio, induction and exhaust system configuration, and a host of other factors all play a part in this concert that’s directed by the camshaft.
Once you have an idea of the basic cam specs, you’re not done yet. Budget comes into play here since we can go with a flat tappet hydraulic, or mechanical if you’re looking to do this on a budget. Or, you could spend a little more cash and step up to a roller. Hydraulic rollers are hot right now, but our money is on the mechanical roller if you’re looking to make power since we can squeeze a good spring on top of this application and make it work. But now we’re talking about an engine that probably spends most of its time in the garage and not on the street.
Most of the cam companies address this selection process by jumping right into rear gear ratio and cruise RPM. This is a good first step, but let’s take the time to find out why. It’s been our experience that guys building engines or even swapping cams tend to overlook a somewhat simple application question. To be honest, your typical street enthusiast wants it all. He wants a cam that makes “big power” and he’s willing to sacrifice drivability and fuel mileage to get it. The more enthusiastic yet less informed also want that big power, but they are unwilling to give up anything to get it. Those are the guys who should bolt on a centrifugal blower or nitrous, because they’re not going to get that kind of overall performance gain out of a camshaft.
The real question should be: Do you want to build your existing car around the engine, or would it be better to build the engine around the rest of the car? This is an essential question. The problem that occurs almost daily is that many enthusiasts get so excited about building power that they forget that the car is not configured to maximize the engine’s potential. Keep in mind here that we are talking about accelerating quickly in a quarter-mile, which is the most common application for a hot street engine.
The Saga of Ricky Racer
Let’s get a little deeper into this because it’s worth the time. To do this, allow us to introduce you to our pal Ricky Racer. Ricky has a 1968 Nova with a 355–ci small-block that he wants to make “faster.” He opens up his favorite cam catalog and instantly goes to the bottom of the page, looking for a “big cam.” He chooses a 300degree advertised duration flat tappet hydraulic cam from Friendly Cam Company and somehow manages to properly bolt it in the engine. Luckily, he changed valvesprings too, so at least the springs won’t immediately go into coil bind! Unfortunately, Ricky didn’t take into consideration that his Nova is still spinning a stock torque converter, TH350 trans, and a lame 8.2-inch ring gear diameter 10–bolt rear with 3.08 gears. He’s got a big dual plane intake and an even bigger 750–cfm Holley carb on the intake, but he’s only planned on a set of headers and an exhaust system. And for now he’s saddled with a set of cast-iron manifolds and a dual 2-inch exhaust with hideously restrictive mufflers. Must we go on?
Ricky soon discovers that his shootfrom-the-hip combination is a pig. Not only has it lost all its low-speed torque, but the cam also doesn’t “pull” at high RPM either. In his zest to make hero power, Ricky has managed to kill the engine’s mid-range torque potential with a long duration cam that includes an incredibly late-closing intake valve. This means all his low- and mid-range cylinder pressure has evaporated. Even better, by the time the engine speed has achieved an RPM where the cam starts working, the exhaust system has long since maxed out, choking any further power. So our pal Ricky is left with a slug that makes less power everywhere. Of course, he blames all this on Friendly Cam Company when all they did was sell him what he wanted!
Ricky has two options to remedy this mess. The first recommendation, if Ricky has the money, is to salvage his cam selection by adding a set of 1–5/8inch headers, a mandrel-bent exhaust system, and a pair of less restrictive mufflers like a set of Flowmasters. Next, he has to step up to a 12–bolt rear end (why invest money in a spindly 8.2-inch 10–bolt that will never live?) with a set of 3.73 or 4.10 rear gears and a good limited slip unit for traction. Of course, that means a set of sticky tires as well to hook all that torque multiplication. Speaking of torque multiplication, he also needs a 2,600- to 3,000-rpm stall speed converter, and at the very least some type of shift kit to complete the deal. He also needs a better ignition system, as well as roller rockers, and better heads. But let’s stop here for the sake of Ricky’s ego and his heavily ventilated wallet.
The second recommendation is much simpler and far less expensive, but it isn’t what Ricky wants to hear. This involves yanking the cam and putting it on the shelf. Instead, we choose a more conservative cam that works best with a 355–ci small-block with stock heads and a mild induction system. Ricky still needs to install a better exhaust system; in fact, he should have done that before he changed the cam. But now, a cam around 216/224 degrees at 0.050 with a lift of around 0.460/0.470 and a lobe separation of 110 to 112 degrees would be a good choice. This cam creates decent low-speed and mid-range torque to accelerate the car because it has a tall gear and a stock stall speed converter. It has a dual pattern to “cheat” the exhaust side since it’s restricted by the stock heads and iron exhaust manifolds. Even with headers this is still a good cam selection. The down side to this cam is that it doesn’t have that drive-in lumpy sound that everybody wants. But since the rest of the car is a far cry from a “fast” car (actually what Ricky wants is a quick car), it doesn’t make sense to build the car around the cam.
What we’ve accomplished here is to choose a cam that complements the existing car. At the least, Ricky shouldn’t start the buildup of his fast street machine with the cam. The smarter move would be to start on the exhaust system to get the most from what the existing engine is capable of producing. The problem with that, for many street rats, is that the exhaust isn’t nearly as sexy or appealing to his ego as a big, lumpy cam.
Perhaps that was a long way around to make this point, but it makes more sense to configure the camshaft around the existing car combination. Or, at least be willing to spend a lot of money to build the car around the engine. Either way works, and it again comes down to how much money you have to spend and what your ultimate goals are for the vehicle.
Another big dilemma for a street engine builder is that the entire concept of building a performance engine for the street is absolutely rife with compromises. First, the engine must perform well throughout an incredibly wide RPM band – from idle to 6,500 rpm or more. It must run on pump gas, it better have some type of low-speed torque for drivability, it must be able to have some type of longevity, oh and we don’t want to do any maintenance like setting valve lash either. And of course, we don’t want to spend $10,000 to build this engine either. In fact, our budget is less than $2,000. But it’d better make a bunch of power!
As we see it, four levels of street engines with an almost limitless number of engine variations fall somewhere in between these categories. At the most conservative end, we have the Computer Controlled engines, which are also emissions-controlled and limited in the amount of camshaft they can sustain, mainly because of idle quality concerns. Next, we have the Mild Street engines that should be constrained by an approach aimed at making as much mid-range torque as possible. Next we have Strong Street engines willing to sacrifice some idle quality and some low-speed torque in search of more power in the mid-range and the top end up to 6,000 rpm. Finally, we have the Big Power engines that are larger in displacement or are willing to spin engine speeds higher than 6,000, don’t care about idle quality, and have only a passing interest in midrange torque below 4,000 rpm.
While these descriptions are about the engines, we’re really talking about the cars. The Big Power guys have the money for high stall speed converters, deep gears, light cars, big tires, and the cash to make it all work. Much like the ubiquitous bell curve, the majority of the street engines actually fall into the two middle categories. With both of these two categories, the camshafts should still be considered conservative mainly because we are attempting to create the widest torque band possible. This is probably where you’re thinking: “Wait, what happened to horsepower? Where’s my big horsepower numbers that I read about in the magazines?”
These are good questions; so let’s start by delving a little deeper into Mild Street and Strong Street car acceleration. While we don’t want to downplay peak horsepower, it should not be the street-car enthusiast’s Holy Grail. We get plenty of opposition to this statement, but the truth is that torque accelerates the car while horsepower creates those big MPH trap speed numbers. The real battle on the torque-vs.-horsepower controversy that everyone argues about is really about separating torque from horsepower. The torque pushers argue that we should have as much torque as possible. The horsepower heroes all push the analogy that if torque were king, then we’d all be driving low-speed diesel engines.
Our counterpoint is that for street engines, we want both. We want an overall power curve with as much area under the curve as we can get. But we’re less willing to compromise torque for the sake of peak horsepower. This is because a street car with less-than-ideal drag strip gearing spends most of its time in the mid-range RPM band trying to accelerate a heavy car in between 4,000 and 5,000 rpm in first, second, and third gears (assuming a typical threespeed automatic). The key factor that drives this point home is that very few street cars cross the finish line at the drag strip at or above their peak horsepower RPM point. A Mild Street engine usually attains peak horsepower at around 5,500 rpm. Yet this same engine in a typical Chevelle or Camaro runs through the traps in third gear at barely 5,000— an easy 500 to perhaps as much as 1,000 rpm shy of its peak horsepower point! This is because most of these cars use 3.08:1 to 3.55:1 gears and relatively tall tires. So now, why would we want to place more emphasis on making more peak horsepower when it’s obvious we can only make use of this peak power in first and second gears? Hmmm…
On the other hand, if we decide to emphasize torque between 4,000 and 5,000 rpm, even if it means sacrificing some peak horsepower to do so, we can now accelerate our mild or strong street car much faster through first, second, and third gears because of this additional torque. So let’s choose a camshaft that improves mid-range torque while perhaps not hurting top-end power. Let’s also keep in mind that we don’t want to spin the engine too slowly since RPM is horsepower. But if we choose a cam with too much duration, we begin to lose mid-range torque. That’s because a longer duration cam merely shifts the engine’s torque curve higher in the engine RPM range. Can you begin to see how building a street engine is a giant set of compromises?
In an incredibly long equatorial route of circular logic, this brings us back to why the cam companies start their cam recommendations with asking what weight, rear gear ratio, and transmission equipment is present in the car. Their focus is to determine a highwaycruise RPM and then select a camshaft based on that simplified number. This has its advantages, but unfortunately most cam company recommendations end up being conservative ones that most enthusiasts don’t want to hear.
Take a look at the cam chart that we created. It makes some very general cam recommendations based on our four types of street small-block Chevy engines that take into account all the things we’ve mentioned up to this point. Now, the problem with generalized charts is that they’re just like universal parts—they don’t universally fit anything really well. But if you’re looking for some ballpark areas to work around, then this chart helps. You may notice that many of the cam specs overlap from one category to the next. This is because a slightly more muscular mild street engine could also be a conservative strong street small-block. The idea here is to create some general starting points without getting bogged down into incredibly long and boring descriptions of hundreds of individual engine combinations.
One key to understanding the effect of cam timing on an engine is to not necessarily think in terms of duration, but rather in terms of when the intake valve closes in the intake cycle. A longer duration intake lobe opens the valve sooner in the cycle and closes it later. While the sooner-opening intake tends to increase overlap, it is the later-closing intake valve that has the most telling effect on the power curve. Think about what effect this later closing intake has on lower engine speeds. As the piston begins to move upward in the intakeclosing portion of the four-stroke cycle, and as the valve remains open, the piston at some point begins to push fresh air and fuel out of the cylinder because the valve is still open. The cylinder cannot begin to build pressure until the intake valve closes. The net result is lower cylinder pressure at lower engine speeds, which means reduced power and a sluggish engine (see “Cranking Compression” sidebar).
The advantage of the later closing intake is that the inertial forces and higher inlet air speed of the inlet system at higher engine speeds continue to fill the cylinder with that later closing intake valve. This works because the air and fuel are rushing in at a high enough speed to cram more air and fuel into the cylinder even when the cylinder is at or above 100 percent volumetric efficiency. This is also due to the fact that the high engine speed does not allow the air and fuel time to bleed back into the intake port. The net result is higher cylinder pressure and more horsepower at higher engine speeds.
wer point. A higher engine speed does not help fill the cylinder any better, and more duration only hurts the volumetric efficiency of the existing engine combination.
The point of this discussion is to drive home the idea that additional duration means a later closing intake and higher engine speeds that may not be safe for your engine combination, so additional duration is not always the answer. Each engine combination has its own particular amount of duration and lift that makes the best power. The trick is to build that exact combination, and that’s what engine builders have been striving after for over 100 years of building performance engines.
By remaining conservative with duration figures, this improves the engine’s overall torque curve, since a long duration cam with a later closing intake means that the engine is sacrificing cylinder pressure all the way through the RPM curve in order to create the ideal conditions to make peak horsepower. For street engines that spend a lot of time in the midrange RPM, making more power—up to a point—is always beneficial. Of course, there comes a point with perhaps a big cubic inch engine where you make enough torque that you can’t hook the tires to the track and the car just spins the tires. Then you can consider closing the intake valve later to help make more peak horsepower while not hurting acceleration since you eliminate tire spin as well. All this can get complex, but if you think it through, it all does make sense.
Written by Graham Hansen and Posted with Permission of CarTechBooks