Good ideas have a habit of not always working exactly as they should. We assembled a 0.040-over 350 small-block Chevy in the Jan. '14 issue that we called the Quick Draw .357. It was intended to showcase all the budget engine parts we had tested in the previous year. The parts bolted together easily enough, but the results were less than impressive. Our cam, head, and intake package ran into valve-control problems that limited the peak horsepower to a mediocre 403 hp at 5,400 rpm. We determined that the Lunati cam uses a rather aggressive lobe design that requires a little more valvespring than the Jegs heads were equipped to deliver.
It appeared this package of camshaft, heads, and intake was capable of much more than 400 hp. So we ordered new valvesprings and, in case you haven't already jumped ahead, we were rewarded with a solid 25hp increase to 430 hp. At this point, we decided that this induction package promised greater potential—all the engine needed was a little more lift and duration. So we added a Lunati hydraulic roller camshaft, lifters, stronger dual springs, and reused the Lunati roller rocker arms from the flat-tappet test. This also worked extremely well, pushing the peak horsepower up to 483 hp. And all this was still with a dual-plane Edelbrock intake and a 750-cfm Holley carburetor. The details are what make this story, so let's get started.
Spring Time for Horsepower
Lunati makes this conical valvespring that bolted right on the existing Jegs heads. The sp
As you can see from the accompanying graph (page 28), just a simple change to new valvesprings made a big difference in power. Note how in the original test, the horsepower peaked at barely 5,200 rpm and then plummeted when the springs could no longer control the valves. With duration numbers of 231/241 degrees at 0.050, we knew we had sufficient cam timing to support the peak horsepower at something close to 6,000 rpm. So we added a set of Lunati conical valvesprings to the Jegs 195cc heads. We've included a spec box of the valvespring seat and open loads. The stock springs that came with the Jegs heads are not weak springs, but it appears that the Lunati cam uses an aggressive lobe design that requires more spring pressure on the seat. It might seem that adding pressure at peak valve lift would be beneficial, but the onset of valve float actually occurs at the point when the valve hits the seat on the closing side. Generally, the valve will bounce at higher engine speeds if there is insufficient seat pressure. So in our case, we raised the seat pressure from 100 to 140 pounds (a 40 percent increase) and the engine responded to this simple swap.
If you study the Flat-Tappet Hydraulic Cam Test graph, you can see that above 4,000 rpm the stronger valvesprings added 10-plus lb-ft of torque, which usually means the horsepower will also improve. Note how the weaker springs allow the valves to bounce, which kills the power after 5,200 rpm. But with the better springs, the horsepower curve (HP2) continues to climb right up to 6,000 rpm before it tips over at 6,100 rpm. Peak power stabilized at 430 hp, which equates to a solid 25-plus-horsepower improvement at the reasonable 6,000-rpm peak.
It's also interesting on the graph how at 3,000 rpm we lost as much as 30-plus lb-ft of torque. We realized once we began writing this story that we used a different header in the second test compared to the headers used in the first test session. The original set of Hedman headers use a slightly smaller diameter and longer primary pipe, and this would explain the difference in torque. The other change (other than the valvesprings) was using a 750-cfm HP Holley carb instead of the 0-3310 750-cfm vacuum secondary carburetor we had used in the previous test, although the air/fuel ratio remained similar between the two tests.
This is what our small-block looked like after completing testing. While we made 403 hp, w
We knew from Jegs' flow numbers that these heads would respond with even more power if we added more valve lift. Adding a 1.6:1 roller rocker arm to increase valve lift on the flat-tappet cam only resulted in lost power, again due to a reoccurrence of valve-control issues. A better solution was to go to a hydraulic roller cam that could add more valve lift. We also decided that with our good crank, rods, and forged pistons, we had the quality parts to allow us to spin the engine a little higher to make more power. This idea comes from the basic premise that if you make the same torque at a higher engine speed, you will make more horsepower. The easiest way to do that is to add a camshaft with more intake and exhaust duration, and this should move the torque curve higher in the engine rpm band. Of course, this almost always also comes at the price of losing power at engine speeds below peak torque.
Bolt-On Hydraulic Roller Power
All we did was bolt in a hot-load hydraulic roller cam in our 357-inch small-block that re
The basic advantage of a hydraulic roller camshaft is that roller followers allow the cam designer to be more aggressive in creating additional valve lift. Without getting into all the differences between a flat tappet and a roller follower, a roller follower allows the cam designer to increase lifter velocity, which increases valve lift for the same amount of duration. We chose to not only add a hydraulic roller cam, but also increase the duration slightly to increase our peak horsepower potential. As you can see from the Cam Specs chart (page 32), we added 10 degrees of duration to both the intake and exhaust lobes, measured at 0.050 tappet lift. This means that the intake and exhaust valves will open slightly sooner and close later. One important key to improved top-end power is to close the intake valve later in the cycle. This increases time (in degrees of duration) for the intake port to fill the cylinder at high engine speeds. Changing to a hydraulic roller camshaft also requires adding a cam button to the end of the cam to prevent the cam from walking forward as engine speed increases. We didn't want to increase the overall cost by adding a custom aluminum front timing cover, so we re-used the original tin cover, using a plastic cam button to create the proper fore-aft 0.010-inch endplay.