I'm an engine junkie. I pore over dyno tests, study cylinder head flow numbers, and speculate on camshaft selections. Yeah, geek. But, in all that research it occurred to me that I could take the GM Performance Parts 350/290hp crate engine from my '68 M-715 and upgrade it with a few select components to shoot for a 400hp/400lb-ft recipe that anybody could duplicate with no machine shop and only basic mechanical skills. Why so focused on the ubiquitous small-block Chevy in a Jeep magazine? Well, it's still the engine most commonly swapped into Jeeps both old and new and there are literally thousands of Jeeps running around already sporting small-block Chevy power.
After baselining the engine...
After baselining the engine as it ran in the truck, the crew at Westech Performance upgraded the GMPP 290hp engine with the parts you see here. Highlights include a Lunati Voodoo 268 camshaft setup, Lunati self-aligning Voodoo roller rockers, a Weiand Stealth Air Strike intake manifold, and bunch of stuff from GM Performance Parts including the assembled Vortec heads, heavy-duty pushrods, circle track valve covers, and other goodies you'll read about next month. Note the green packages in the foreground. We used E3 spark plugs for all of our dyno testing.
The GM Performance Parts PN 12499529 350/290hp engine used in this test is based on a GM Goodwrench long-block with a sturdy 4-bolt main bottom end and production-spec cast aluminum dished pistons. It's indicative of any '70s-'80s GM V-8 small-block you may find at your local auto recycler's, although the GMPP 290hp does benefit from a better Nodular iron crankshaft and stronger powdered metal connecting rods. Despite its rated 8.5:1 compression ratio, in reality the production-spec 76cc open chamber cylinder heads and a piston that lives about 0.038-inch in the hole deliver 8.0:1-8.25:1. It's great for use with 87-octane fuel, but doesn't scream high-performance. Regardless, if you can't afford to duplicate our efforts using a factory-fresh GM Performance short block, you can substitute any decent $100 junkyard short block with the same results.
Note the deep, open chamber...
Note the deep, open chamber design of the factory 76cc cylinder heads. With lazy, low-velocity intake ports full of restrictions and a cavernous combustion chamber these heads only just get the job done. In the past, builders would spend hours hogging, milling, and porting just to make the same power you can get out of an uncut set of Vortec heads.
Check out the sidebar "Head Specs" for the full rundown, but in a nutshell, when GM took the LT1 cylinder head and tweaked it for use in its '96-'00 5.7L-equipped trucks and SUVs, it created one of the best off-the-shelf iron cylinder heads ever made for the small-block Chevy. The 906 and 062 Vortec heads (referred to by the last 3 digits of their casting numbers) feature raised intake ports, high-quench 64cc combustion chambers, and 1.94-inch intake and 1.50-inch exhaust valves. When compared with some aftermarket heads, the valve size and rated 170cc port volume may seem small, but when you look at the flow numbers, especially in the crucial mid-lift values of 0.300-0.400-inch lift, you realize how much velocity the port shape and size generate. This allows the Vortecs to deliver as much or more airflow as many expensive aftermarket heads at levels below 0.500-inch valve lift where all the greasy, gooey torque lives. The only bummers with the Vortecs are the factory springs are good to only 0.475-inch valve lift before coil bind and valve control with high-lift cams becomes an issue, and they require a Vortec-specific intake manifold. If you're lucky, you can find Vortec heads in the junkyard for about $150 or so if you scrounge. However, you can still buy brand new, fully-assembled Vortec heads through GM Performance Parts for less than $600/pair. That's a screamin' deal. GMPP also has all the necessary gaskets, fasteners, and other parts required for the Vortec conversion, but more on that next time.