In the first installment working with our 402 big-block (9.5:1 compression), we swapped out our factory oval port, cast iron heads for a set of alloy heads from Trick Flow Specialties. Without changing anything else on the engine, going to the better flowing heads netted us a 30-plus horsepower increase.
Superior flow on its own gave great results, and with the capabilities the Trick Flow heads gave us, our cam choices for replacing the flat tappet unit in the engine increased dramatically. But while it’s tempting to throw the biggest cam your heads can take at the engine for a big horsepower number, that temptation leads to a very common and often committed mistake when it comes to cam selection. Bigger, or biggest, isn’t always better, and things like drivability, power band, idle quality, and other factors have to be considered before picking out a grind.
While a big peak horsepower number is flashy and useful in bragging competitions, it’s only one part of the equation when putting together a good engine for the street, or the strip for that matter. Whether you’re starting from a stop light or a Christmas tree, you need strong bottom end torque to get off the line and into the meat of your powerband. But it’s a balancing act, because when you change your cam profile to increase bottom end power, it comes at the cost of top end grunt. That’s where the application comes in.
If you’re putting together a motor for a street car that has little chance of every seeing time on a dragstrip, then a cam that gives you a beefy bottom end and lots of vacuum is what you want. If you’re engine’s gonna spend most of its time making quarter-mile sprints, a cam with more top end power that matches up with your trans and rear gear combo is the way to go.
For those building something with a dual street/strip personality, you’ll need to pick a cam that gives you the best possible peak horsepower, without sacrificing too much bottom end and driveability.
Since Summit Racing was our go to for the heads, we went back to Summit’s website to peruse the cam offerings and pick out one that best suited our needs, and what we wanted to show in this story, the benefits of going to a roller cam. Our flat tappet’s cam specs (Comp part no. 11-207-3) were 0.510/0.510 lift, 110-degree lobe separation angle, and duration at 0.050 of 224/224. Comp Cams roller part no. 11-408-8 on the Summit Racing website had the same lift and lobe separation angle, but had a duration at 0.050 206 on the intake and 212 on the exhaust. While this roller is a bit milder than the flat tappet cam with its shorter duration, this pattern should give us a significant increase in bottom end power. And since our subject car never goes anywhere near a dragstrip, that’s exactly what we want.
Some Roller Cam Basics
The most common myth with roller cams is they give you more power because of reduced friction. The fact is the frictional difference between a flat faced lifter on a cam lobe and a roller lifter on a cam lobe is minimal. Just like with the crankshaft and rod bearings, the oil film between the two surfaces, combined with the fast spinning motion of a flat tappet lifter, create nominal friction.
The true benefits of a roller cam are the less aggressive ramp rates (which reduce stress on the rest of the valvetrain), faster opening and closing of the valves (creating better cylinder pressure on the intake side, and better scavenging on the exhaust side), the area under the lift curve (the area of the cam lobe coming off the base circle and approaching the nose, or the flanks of the cam lobes) is greater on a roller cam. A roller cam will typically increase your midrange and upper rpm horsepower and torque because of the increased valve action.
Another benefit of roller cams is they don’t need the complicated break-in procedure flat tappet cams do, eliminating a problem that has become commonplace with flat tappets over the last few years.
Now that we’ve explained the basics, we’ll let the pictures and dyno results show the rest.
At first glance, a lot of people will mistakenly think we took a step backwards with our new cam. With everything else the same, including the 750 Demon carb, we lost 10 peak horsepower. But where the peak was down, we saw an increase of over 20 average horsepower and over 30 average lb-ft of torque across the entire bottom end. So, we traded in 10 peak horsepower for more than 20 extra bottom end horsepower, and 30 more pound/feet of torque through the bottom end and into the upper rpm range.
On the street, the drivability of our big-block went way up. The seat of the pants dyno told us off the start the 402 was much happier, pulling 20 inches of vacuum versus the 14 inches of vacuum the old cam pulled. Any time your engine starts pulling more vacuum, that means you’ve increased its efficiency.
For the next step, we’re gonna do some tuning and dialing in on the engine, including experimenting with some different carbs to see what the results are. We’re confident the horsepower lost with the cam swap can be regained through some basic tuning, without hurting drivability.