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...
UPGRADE #3: Edelbrock Victor Jr. intake manifold w/ Ultra Low Profile elbow
RESULT: 613.9 HP (+27.1), 537.3 FT-LB. (+8.4)
Time to say goodbye to the GM L76 composite intake in favor of an Edelbrock Victor Jr. LS Series Gen IV EFI Manifold, which looks nothing like a factory LS manifold and everything like a carb-style manifold. We knew this bad boy would pick up some serious top end, and to do the swap we needed the manifold itself, a fuel rail kit, an intake elbow, and fuel injectors. The latter was a necessity as the fuel rail kit is designed for taller-style Gen III injectors, so we had FAST send us a set of 60-lb/hr Precision-Flow Fuel Injectors (PN 30600-8, $488 at major retailers) to replace our existing FAST Gen IV-style injectors. They were the closest poundage we could come to our prior 50-lb/hr units in high-impedance form. According to the company, they're designed to "deliver the outstanding idle quality, unparalleled flow consistency and durability essential for high-performance EFI engine applications" and are engineered for excellent throttle response, resistance to clogging, and optimal fuel atomization.
For the initial test with this manifold, we opted to go with the intake elbow that would allow the manifold to most easily fit in a height-constrained engine bay such as that of an F-body, PN 3847. Even with the necessarily restrictive nature of this elbow (compared to Edelbrock's other offerings), we easily surpassed our project goal of 600 hp! There was a tradeoff, though, with power and torque losses below 5,000 rpm, which is not unexpected when swapping to a shorter-runner intake.
UPGRADE #4: Edelbrock Victor Jr. intake manifold w/ High-Flow elbow
RESULT: 625.0 HP (+11.1), 542.2 FT-LB. (+4.9)
To see just how much we were losing with the Ultra Low Profile elbow, we decided to pull the stops and go with the highest-flowing elbow Edelbrock makes, with the caveat that this is putting us out of the realm where stock hoods and cowls are a possibility to use in a fourth-gen F-body without significant modification. The difference was clear, with gains at nearly every point in the rpm range; we say "nearly every" because there was a loss of a fraction of a hp and ft-lb at 3,500 rpm, indicating that the High Flow Intake Elbow likely doesn't provide an advantage over the Ultra Low Profile Intake Elbow below this point, where the latter is able to keep up with the 408's airflow needs.
UPGRADE #5: Edelbrock Victor Jr. intake manifold w/4-bbl Throttle body
RESULT: 635.6 HP (+10.6), 553.2 FT-LB. (+11.0)
At the suggestion of RaceKrafters, and in the interest of leaving no stone unturned, we had to throw one last trick into the mix. Bob had a four-barrel throttle body hanging around that he decided to bolt atop the Edelbrock manifold and see what it could do without incoming air needing to make a 90-degree turn on the way in (though again, this is an unrealistic configuration for a stock F-body engine bay). The result gave us over 635 horsepower and allowed us to finally eclipse the average torque and horsepower achieved during the best runs with the L76 intake manifold, but not by much. Clearly, the L76 intake is best for a street application (at least for our particular engine), even in the totally unported form we've tested it in. This is also evident even from a cursory glance at the dyno graphs. But no matter what type of elbow or throttle body sits atop it, the Victor Jr. manifold is clearly king from 5,000 rpm up and, hence, the hot ticket for race applications.
As you can see, a bit more work gave us an 80 hp improvement above our Part 2 totals, proving once again that a never-give-up attitude can help wring max performance from any application. That's the kind of dedication you can always expect from GMHTP, and we trust you (the readers) will hold yourselves to the same level on your next engine build. Stay tuned for future issues, as we just might find a home for our 408 in a badass street-going project car.
Baseline = (L76 Intake, Small Cam, 1 3/4 Headers)
Run 1 = L76 Intake, Small Cam, 1 7/8 Headers
Run 2 = L76 Intake, Big Cam, 1 7/8 Headers
Run 3 = Edelbrock Intake, 3847 Elbow, Big Cam, 1 7/8 Headers
Run 4 = Edelbrock Intake, 3849 Elbow, Big Cam, 1 7/8 Headers
Run 5 = Edelbrock Intake, 4-Bbl Tb, Big Cam, 1 7/8 Headers