Blown Big-Block Chevy Engine Build - Shock And Awe

The valvetrain components are used along with COMP's Xtreme Energy hydraulic roller cam.

Valvetrain
Alex wanted a ruthlessly fast street machine, not an all-out racer. That meant selecting a hydraulic cam that was spec'd to deliver good idle, with a touch of mean race-engine lope. The cam spec developed by Andy uses COMP's popular Xtreme Energy hydraulic roller profiles. For a street cam, the intake (PN 3370) has a seemingly huge 303 degrees of off-the-seat duration. The exhaust profile (PN 3371) has 309 degrees, and at .050-inch lift, the cam lobes deliver 248 and 254 degrees duration, respectively.

Unlike a solid roller, these COMP Xtreme profiles have a relatively gentle initial opening, so compared to a solid roller with the same off-the-seat duration, they act a lot shorter. Once these profiles have taken up all the valvetrain preload, they get with the program. Another key factor for a big-block Chevy cam is having a respectably high valve lift. With the .380-inch lobe lift from these profiles, a valve lift of .646 inch is generated with the 1.7 ratio PRW stainless roller rockers used. Also working toward streetability is the fact that a blower-spec cam should, along with slightly less advance, have a wider lobe separation angle than the equivalent optimally spec'd cam for a normally aspirated engine. Too much overlap means the boosted intake charge will not only scavenge the combustion chamber, but go right on out the exhaust port. The wide 114-degree LCA chosen by Andy has a significant influence toward making the cam more civilized for the street. With the 114-degree LCA, the overlap is a very streetable 78 degrees. To give you small-block guys a reference point, it would be like running a hydraulic flat-tappet cam of 284 off-the-seat degrees in a 350. It's big, but far from outlandish.

The business of transmitting the cam motion to the valves starts with a set of COMP's plus .300-inch-long Pro Magnum hydraulic rollers. The extra lifter length here is needed to clear the Dart block's taller lifter bores. The lifter motion reaches the rockers via a set of Manley 3/8-inch diameter pushrods. The rockers are PRW's ultra-stiff stainless items, which unlike aluminum rockers, have a virtually infinite fatigue life. This makes them good for the street where millions of cycles are involved.

Springs for a hydraulic valvetrain can be critical. Too little seat pressure and seat bounce can rob an unbelievable amount of power. Too much, and the hydraulic internals of the lifter can collapse. Andy's choice of spring, a COMP 930, proved to be right on the money. With 153 lbs on the seat, and 382 lbs over the nose, the valvetrain ran flawlessly to the desired 6,600 rpm.

These PRW stainless roller rockers are very stiff, yet light enough to turn the rpm you would expect from an aluminum rocker.

A blower intake for a big-block is a big hunk of aluminum, but the open area under the blower mounting position allows for easy access for any port matching operations that might be needed.

Take a look at all those air bleeds and such. These blower Holleys had just about every circuit recalibrated for the unique needs of a positive-displacement blower.

Ignition
Last item on the build is the ignition system. Andy used an MSD Pro Billet distributor with fixed timing. This might just have the scent of something a little too race-orientated, but the fact of the matter is that often a blown engine with a long-period cam has an advance curve requirement that is nearly a straight line. This engine falls into this category. The key issue to address is for the distributor to deliver a really healthy spark (not to mention physically clear the blower, which is in close proximity to the distributor).

Dyno Time
After a suitable break-in time, the 509 was given a nut-and-bolt service. This entails a check for any nuts/bolts that may have unseated after the break-in heat cycle. With that done, it was time to feed in a 50/50 mix of 114 octane with pump gas, and get acquainted with this engine's personality. It didn't take much in the way of running to establish this was the real deal. If Holley's claim that the carb calibration was a dead match for the blower, we'd soon find out. Those Holley's weren't close-they were right on. The only carb adjustment required was to set the idle adjustment screws and idle speed.

Blower speed is dictated by the ratio of the top to bottom pulley size. With a crank pulley of 61 teeth and a blower pulley of 54 teeth, the 8-71 is being overdriven by 13 percent.

Because the carb calibration was on the money, this left only timing optimization. Andy started the tests at 28 degrees, as this was considered a really safe advance for an engine of this compression and boost, which registered a peak of 9.6 psi.

A run at 30 degrees netted more of everything, so 32 degrees was tried, and this proved a little better yet. Thirty-two degrees is quite a lot of timing for a blower motor, but when the very conservative compression ratio and boost are factored in, that number falls into place nicely. The bottom line here is that this engine could well deal with a blower overdrive that delivers as much as 15 lbs of boost.

With the carbs right on, the only calibration required was to optimize the ignition timing.

With the ignition timing set, this 509 was almost a contradiction in terms. It ran like a super-loud 850hp Swiss watch. Everyone was impressed with the way the Holley carbs performed. Andy commented that it would be hard to imagine that fuel injection would be any better than these blower carbs. The steady transition from small-part throttle cruise to a relatively wide-throttle position was smooth, and free of the lean stumble sometimes seen in non-blower-specific carb setups. As for throttle response, this was like watching a stick of dynamite go off at a safe distance. It made no difference how fast the throttle was opened, the engine kept up. The response was, in fact, so fast that the torque reaction looked like it would cause the engine to leap off the dyno stand. There were smiles all around, and especially on engine owner Alex's face. Is almost 850 hp going to be fast enough? Alex says: "Speed is relative. I just need to be faster than my relatives!"