Here's the high-quench combustion...
Here's the high-quench combustion chamber of the Vortec head. The chamber size and shape promotes flame propagation for a more complete burn, better resistance to detonation, and simply more power.
To complement the airflow characteristics of the Vortec heads, keeping in mind that the flow is pretty played out by 0.500-inch lift, a Lunati Voodoo camshaft with an advertised duration of 268 intake/ 276 exhaust got the nod. The duration runs 227/233 @ 0.050 and lift comes in at 0.489/0.0504 with 1.5:1 ratio rockers. While the Vortecs feature great intake ports, the exhaust ports flow only marginally better than the 76cc iron heads they're replacing, so the longer duration and higher lift of the exhaust lobe on the Voodoo camshaft will help expel the exhaust gasses. The Voodoo camshaft also features the most aggressive flat-tappet profiles on the market, keeping the lifter on the seat almost as long as a hydraulic roller before snapping the valve open. Furthermore, the Voodoo lobes are asymmetrical. Although they really open the valve with authority, the closing ramp is ground with a smoother profile to allow the valve to close more softly for prolonged valvetrain life.
Since our Wieand Action Plus intake wouldn't work with the new heads and since we were upping the powerband and moving more air, we ordered a Weiand Stealth Air Strike high rise, dual plane intake with the Vortec pattern. The Air Strike features isolated runners that won't soak up engine heat and allow for a cooler, denser intake charge.
Like most production Chevy...
Like most production Chevy small-block engines of the '70s-'80s, the GMPP 290hp engine uses standard cast, dished pistons that only offer up 8.0:1-8.25:1 compression with the standard 76cc iron heads of the era. When used with Fel Pro PN 1094 steel shim head gaskets, the smaller 64cc combustion chambers of the Vortec heads up the compression to roughly 9.15:1.
With the heads, intake, and camshaft sorted, that only left the valvespring issue to deal with. And that's where I hit a wall. Standard high-performance valve springs require machining of the Vortec's valve pocket and valve guide. It's no secret that you can skip the machine shop by installing LS-style beehive valve springs over the uncut 0.900-inch spring seat of the Vortec heads. In an effort to avoid costly aftermarket springs, I first tried a set of factory LS6 springs. On paper they should have worked, but the LS6 springs went into coil bind at about 1.30-1.34-inch, which is earlier than spec'd, so that was out. Then, Lunati sent a set of its new 74881LUN springs and its 75787LUN retainers that delivered a seat pressure of 100lbs at an installed height of 1.750-inch. These springs worked great up until about 5,300rpm, at which point the aggressive Voodoo profile began manhandling them. Some Lunati 74818LUN springs should fix the instability issues, but we won't know until the engine takes another ride on the Westech Performance dyno. Tune in next time for the complete engine specs, part numbers, prices, and dyno results.
The Vortec heads feature standard...
The Vortec heads feature standard exhaust ports, so unlike fancy D-port heads, any small-block Chevy exhaust manifold or header will work. The intake ports are raised, having a taller ceiling for improved flow and higher velocity. The Vortecs use self-aligning rocker arms, so you can't reuse your old ones unless you have the heads machined for screw-in studs and run pushrod guide plates. Since the Vortec's pressed-in studs don't tend to pull out at higher lifts like those of earlier heads, converting to the screw-in studs so you can run standard rockers is financially a wash unless you've already got expensive roller rockers.
It's tough to see, but the...
It's tough to see, but the factory Vortec heads feature a complicated 5-angle valve job that some machine shops can't duplicate. If you can't score a set in the boneyard that doesn't need refurbishing, it's usually better in the long-run to simply buy a new set of heads from GM Performance Parts since often times a standard valve job will actually hurt flow.
Here's where the snag hit....
Here's where the snag hit. On bottom is the Vortec head upgraded with factory LS6 valve springs. The LS6 springs hit coil bind at the installed height of 1.75-inches, so the Lunati PN 74881s on top were installed. Although the 74881s allowed full lift with no bind, their spring rate just wasn't enough to handle the gnarly Voodoo camshaft. Tune in next time for the fix and the full dyno report.