Because it's been a few months since the last installment of this story series appeared in GMHTP ("Finance-Friendly 408, Part 2," September 2009), a quick refresher is in order. In the August 2009 issue, we kicked off our low-buck build by stuffing a LQ4/LQ9 iron block chock-full of a 4-inch K1 Technologies forged crankshaft, 4340-steel K1 Technologies connecting rods, and 2618-alloy Wiseco pistons, yielding a stout short-block of the 408-cube (6.7L) ilk. In the second installment, we added a lumpy Comp LSR camshaft (247/263 @ 0.050, 0.624/0.624 lift), CNC-ported L92 heads from GMPP, a factory L76 intake manifold, and all the necessary accoutrements to complete our 11:1-compression, pump-gas-slurping, finance-friendly mill (see Part 2 for price breakdown).
Dyno pulls at RaceKrafters rewarded us with 554 horsepower, but we knew she had more in her, so it was decided a Part 3 was in order to finish the job right. Since the 408 was intended from day one for the engine bay of a street-going F-body, we'd need to ensure any and all additions were compatible with that ultimate goal. Let's go though our upgrades one-by-one and see how things panned out.
UPGRADE #1: Kooks Headers, 1 7/8-Inch
RESULT: 577.7 hp (+23.7), 522.9 ft-lb. (+14.4)
Up to this point, all testing had been performed expelling spent fumes through a generic set of long-tubes RaceKrafters uses whenever an LS is strapped to the dynamometer. But since they only feature 1 3/4-inch primaries, we figured a bump up in size would help unlock some ponies. Kooks Custom Headers sent us a set of its (PN 6506S) 1 7/8-inch Stainless Steel LS1 Headers which, being designed for 2001-2002 F-bodies, feature all the necessary emission fittings for those model years. They retail for $966.57 a pair, and while they probably don't offer the ultimate power of the company's step headers, they're a couple hundred bucks or so cheaper.
On the dyno, the Kooks headers got us more than halfway to our 600hp goal. In addition to the impressive peak gains, average power and torque were up as well, with only slight losses below 4,000 rpm.
UPGRADE #2: COMP LSR cam, 251/267, .624/.624
RESULT: 586.8 HP (+9.1), 528.9 FT-LB. (+6.0)
Though we already had a stout Comp LSR hydraulic roller camshaft in place in our 408, RaceKrafters advised that we step up to the next larger cam and see if it couldn't make even better use of our high-flow heads (though with an unported GM L76 intake still in place, miracles were not to be expected). The new cam, PN 54-474-11, specs out at 251/267 @ 0.050, 0.624/0.624-inch lift, 115 LSA, and can be had for about $396 from major retailers. That's identical lift to the last cam but with a bit more duration and a slightly wider LSA; it's advertised as being appropriate for rectangular-port LS engines 420 cubic inches and larger, but we figured our slight ci deficit wouldn't hurt us.
We did gain a bit in both peak power and torque, but the best news was that the increases were had at every rpm tested. We also recorded our highest average midrange (3,500-5,000 rpm) power and torque for this engine, a record that would stand through the end of the test. While we were left about 13 ponies short of 600, the cam helped set the stage for our next upgrade...