Print

Email

Pro Stock Engine Technology

Page 2 of 3


We spoke to Scooter Brothers at Comp Cams who gave us a very interesting look at the evolution of Pro Stock camshafts and valvetrain designs (see sidebar in Figure 2). According to Brothers, Comp was one of the companies that helped develop the Spintron that has led to huge improvements in valvetrain performance. Early on, the lightest parts and the highest spring pressures were thought to be the best – now the stiffest and correctly designed parts are recognized as better.

As the stiffness of the system evolved, the valve springs changed. Not only did the quality of springs greatly increase, but the understanding of springs as a part of the system began to emerge. When the cam, pushrod and rocker arm were bending and flexing under the extreme 1,300-lb.-plus spring pressures, the system acted like a “pole vault.” When the valve opened the system flexed, and when the lifter went over the nose of the cam, it slammed back to normal, releasing all of that energy right back into the spring. The old solution was to add spring, but that turned out to be exactly the wrong direction. Today’s Pro Stock (in fact, most race) engines use the stiffest components available up to the valve, and the valve, retainer, locks, and springs are the lightest possible. These same theories and designs are used today in almost every racing application and are now creeping into production engines.

Dan Jesel founder of Jesel Valvetrain Innovation has been a driving force behind many of the gains in Pro Stock valvetrain technology. He has advocated large diameter billet cams for many years for both increased rigidity and the fact that you can run less rocker ratio due to the availability of more “lobe lift.” He recommends 1.85-1.90 rocker ratios that make the valvetrain easier to control. Jesel also believes that you can grind a smoother cam profile on a large diameter cam that is also much easier on the valvetrain at high rpm.

Many teams run Jesel’s keyway style roller lifters that are extremely light because they eliminate the weight of the traditional tie-bar or extended body necessary in a dog-bone style lifter. They are aligned to the cam lobe with a slotted bronze bushing that a tab on the lifter keeps it from rotating. Jesel recommends his 1.095˝ lifters for Pro Stock because they increase the axle and roller bearing length, spreading out the incredible load they are under. Also, the bigger lifter body allows the use of a larger .940˝ roller.

We were surprised to hear that the majority of Jesel’s Pro Stock customers are still using aluminum body rocker arms. Jesel offers steel Pro Stock rockers, but there seems to be no advantage in this application. Dan Jesel pointed out that one of the biggest gains in valvetrain performance came from “tying the rocker stand down to the head.” When he invented his “box-style” stands, racers saw improvement in all aspects of the valvetrain, including spring life.

Pro Stock pushrods look more like telephone poles than pushrods. Not only does the raised cam make them shorter, but the 1/2˝-9/16˝ diameter .200˝-wall thickness makes them virtually impossible to flex ensuring total advertised cam timing at the valve. And in case you’re wondering, Comp Cams puts the timing in the neighborhood of 276-300 degrees duration at .050˝ lift, 1.100˝-1.300˝ of lift and lobe centers around 120 degrees.

Joe Squires from Bryant Racing took us through the crankshaft specs for a typical 500 cid Pro Stock crank. They start out with a huge block of Timken 4330M double-melt steel and machine it into a work of art. Typical strokes are 3.575˝-3.600˝, and the main bearing journals are 2.500˝ in diameter (accepts readily available 409 Chevy bearings), and 1.888˝ rod journals. Every trick is used (rifle drilling, scalloped flywheel flange, etc.) to reduce weight to around 51 pounds, compared to a stock BBC crank weight of 70 pounds.

Each engine builder has a different preference for counterweight shape, (knife edging or an aero contour), not a problem as every crank is a custom order. All operations are done in house at Bryant including up to 4-weeks of heat-treating, cryo and nitride hardening processes and REM super finishing. Typical lead-time is 12 weeks to build a crank from start to finish.

Nick Ferri  is a noted Pro Stock engine builder who has built engines for various racers including the dominant Mike Edwards. Now he is starting a Pro Stock engine program at Jegs Performance with Roy Simmons. They will supply Jeg Coughlin Jr. engines along with leasing engines to other teams. We asked Ferri  about a basic short block assembly. He said that most engine builders use a short stroke 3.500˝ x 4.750˝ or longer stroke 3.600˝ x 4.700˝ combination. An interesting fact is that the rod/stroke ratio is in 1.66 – 1.75 range, and Ferri prefers the 1.66 ratio or even lower ratio if he could physically do it. For many years the ideal rod/stroke ratio was thought to be a 1.9 allowing the piston to “dwell” at TDC for a longer period. Not the case in NHRA Pro Stock.

Another interesting point is that a few years ago NHRA mandated minimum weights for valves, pistons and other reciprocating parts, to keep the ultra-exotic F1 materials out of Pro Stock. Ferri said that he doesn’t come close to minimum piston weight, and has gone for rigidity and stability over light weight. An informal survey between Kurt Johnson, Roy Johnson and Ferri “ball-parked” a typical compression ratio at 14.5:1 – 15.5:1.