We've been toying with turbos and turbo cams for a few years now, and the more we think we know, the more we discover how much more there is to discover. Last year, we ran a theoretical piece on camshafts and their effect on horsepower ("The Truth About Camshafts and Turbochargers," June '11) and came away with a gem from noted engine builder Kenny Duttweiler: He told us to treat a properly sized, high-efficiency turbo motor like a naturally aspirated engine. In terms of turbo cam selection, this translates to choosing one based on engine size, head flow, and desired powerband.
To test the sage's advice, we set up a series of experiments using a 420-inch small-block and a pair of 76mm twin turbo. This built-for-boost stroker combination is none other than the Two-Headed Animal we built in the Aug. '11 issue ("Building the Two-Headed Animal"). It features a Dart block, K1 crank and rods, and JE forged slugs. Topping the beast was a set of CNC-ported 233 Brodix heads and a matching single-plane intake. We also installed a Jesel shaft-rocker system, a Milodon pan and oil pump, and a Holley 950 Ultra HP carburetor. The idea was to compare three different cam profiles on the twin turbo stroker, ranging from mild to wild, then perform the same test after adding the turbo system. Simple enough, right?
Comp Cams provided three custom single-pattern hydraulic roller cam, wherein the intake and exhaust lobes shared the same lift and duration figures. The first (mild) turbo cam offered 0.578 lift, 210 degrees of duration at 0.050 and a 112-degree lobe-separation angle (LSA). Turbo Cam number two stepped things up to 0.594 lift, 230 degrees of duration and 113 LSA, while the wildest cam (turbo cam number 3) provided 0.605 lift, 248 degrees of duration and a wider 114-degree LSA. Though the lift differed by just 0.027, the significant change in duration would shift the power production much higher in the rev range, especially when combined with our free-flowing heads and single-plane intake.
First up was cam number one. The mildest grind of the bunch illustrated what happens when you under-cam a normally aspirated twin turbo stroker motor. The minimal duration produced peak power at just 5,300 rpm, offering peak numbers of 467 hp and 495 lb-ft of torque at 4,400 rpm. Torque production exceeded 475 lb-ft from 3,500 rpm to 5,100 rpm, making for a healthy but rather narrow 1,000-rpm powerband (peak torque to peak horsepower) from this 8.5:1 compression engine.
Cam number two was next, and the increased duration both upped the power and increased the effective engine speed. The peak power output of 508 hp came at 5,800 rpm (an increase of 500 rpm from the small cam), while the peak torque of 505 lb-ft came at 4,700 rpm (up by 300 rpm), but the powerband remained the same around 1,000 rpm. The torque curve (exceeding 475 lb-ft) increased in size from 3,800 rpm to 5,500 rpm, an effective shift of 300 to 400 rpm more than the smaller cam.
This trend continued with the wildest of the three cams, which produced 534 hp at 6,200 rpm and a slightly lower 501 lb-ft at 4,800 rpm. This cam had the widest 1,400-rpm powerband (which is still somewhat narrow), but it lost 4 lb-ft of torque while shifting peak horsepower by 400 rpm while peak torque rpm remained constant. As is typically the case with cam swaps, eventually peak torque is unchanged (or lowered slightly) and the horsepower gains at higher engine speeds are accompanied by losses lower in the rev range. Basically, you are trading low-speed torque for top-end power. The intended application should dictate the best possible power curve, but for most street applications, low- and mid-range torque make a car much more fun to drive than one with an engine built for peak power.
Before installing the turbos, we decided to test all three of our cams naturally aspirated
For the NA testing, we relied on a slick, new 950 Holley Ultra HP.
After the first run, we pulled the BHJ damper to allow access to the three-piece front cov
This cam button was used to properly locate each retro-fit hydraulic roller cam and stop i
Cover removal provided access to the timing chain. Torx bolts were used to secure the timi
The multi-index sprocket allowed us to advance or retard the cam, but it remained in the s