These days, opting to build a Mouse motor over an LS small-block probably means that familiarity and practicality both stepped into the boxing ring, and the former won via TKO in the 12th round. With all due respect to the legendary Gen I small-block Chevy, from a horsepower-per-dollar standpoint, it just can’t hang with its LS counterpart. Pricier parts and fear factor used to be the biggest strikes against the LS, but that’s no longer the case. Yank a Vortec 6.0L long-block out of junkyard, swap in a mild hydraulic cam, and you’re looking at an easy 500 hp for well under $2,500. With a plethora of carbureted intake manifolds and MSD’s trick LS ignition controller, you can chuck all that EFI—and the fear that comes along with it—right in the trash. While the Gen I small-block Chevy is the most prolific engine ever conceived in terms of sheer production volume and its impact on amateur and professional racing, each successive iteration of the Gen III/IV design reminds enthusiasts why the General jumped ship to a new platform in the first place.
The proof is in the horsepower. To illustrate the point and test some of the hottest new LS components on the market, we hooked a fresh GM Performance Parts LS3 crate engine to Mast Motorsports’ SuperFlow 902 dyno for a proper WOT christening. While the concept of bolting on a bigger cam, heads, and intake manifold on a crate motor is so cliché in the walk of magazine stories, the results this time around are anything but ordinary. Simply installing a set of Mast’s new 12-degree LS3 cylinder head castings along with a granny-certified 225/231-at-0.050 hydraulic cam netted a gain of 99 hp. That’s not too shabby at all for a naturally aspirated 376 that already puts out 469 hp on stock trim. Throw some more cubic inches and cam into the mix, and the horsepower potential is truly ridiculous.
The Test Specimen
While the 638hp LS9 and the 505hp LS7 have it handily covered in the power department, the 430hp LS3 used on our dyno session represents the best bang-for-the-buck package in the GMPP crate engine lineup. Currently serving duty in the fifth-gen Camaro SS and the base C6 Corvette, at $6,700 the crate LS3 costs less than half of the price of an LS7. Granted, that’s not exactly pocket change, but it buys a whole lot of motor. Essentially an L92 with a lower profile intake manifold, minus the variable valve timing, the LS3 boasts the same 4.065-inch-bore aluminum block, rectangle-port raised-runner cylinder heads, and 376ci displacement as its Escalade-derived forbear. The combination of a respectable cube’s tally and high-flow heads yield loads of untapped potential, evidenced by the fact that the LS3 produces 430 hp, despite a dinky 204/211-at-0.050 factory cam. Furthermore, the factory’s 430hp rating is quite conservative, as it’s derived using far more stringent SAE testing procedures and correction factor. Under the more conventional Standard Temperature and Pressure standards used by most engine shops, the crate LS3 actually produced 469 hp and 465 lb-ft on Mast Motorsports’ dyno.
The phenomenal airflow capabilities of the Gen III/IV cylinder heads, and GM’s need to keep its cam specs conservative for streetability and emissions purposes, means that tremendous performance gains can be had by simply upgrading to a larger camshaft. Late-model enthusiasts realized this about two days after the first LS1s hit the showroom, but the odds are stacked in favor of hot rodders even more with the LS3. That’s because the 376ci LS3 uses the same camshaft as in the 346ci ’01 LS6, albeit with a smidgen more intake lift. Furthermore, the LS3’s rectangle-port cylinder heads flow roughly 60 cfm more than the cathedral-port LS6 castings (260 versus 320). Considering that its factory cam was originally designed for an engine that’s 30ci smaller and heads that flow 60 fewer cfm, it’s not surprising that the LS3’s torque curve plummets rapidly after 4,700 rpm. To remedy the situation, Mast installed one of its 225/231-at-0.050 hydraulic cams in our test subject, which features 0.602/0.607-inch lift and a 115-degree lobe-separation angle.
Call the specs girlie if you must, but it’s tough to find fault with a 64hp gain we witnessed on the dyno, bringing the grand total to 533. That’s right, folks, 533 hp from an otherwise-stock LS3 with nothing more than a cam swap. Examining the dyno sheet reveals that the Mast cam effectively moved peak torque up to 480 lb-ft at 5,400 rpm compared to the stocker’s 465 lb-ft at 4,700 rpm. While the Mast cam only yielded an additional 15 lb-ft, moving the torque peak higher up on the powerband boosted peak power big time. The cam did give up some grunt below 3,000 rpm, but after that it produced 400 lb-ft of torque or more all the way up to 6,200 rpm. Mast attributes this remarkably broad powerband, in part, to the cam’s wide 115-degree LSA. “The stock LS3 heads flow so much air that you don’t need a ton of duration to make lots of power,” Horace Mast, of Mast Motorsports, explains. “This cam was designed for a stock displacement engine, and our goal is to increase power while retaining smooth idle quality and OE driveability. We also offer a slightly larger 230/237-at-0.050 cam that makes an extra 8-10 hp.”
Without question, the biggest aces up the LS3’s cylinder sleeves are its outstanding factory heads. These 260cc castings borrow their basic architecture from the LS7 heads, utilizing raised rectangular intake ports and monster 2.165/1.590-inch valves. With 320-330 cfm of flow right out of the box, they can easily support 600 hp. As great as the stock LS3 heads may be, Mast says that their brand-new castings blow them away by 40-50 cfm on the flowbench. To see if those bold claims actually held true on the dyno, we had to test them out.
Mast’s all-new CNC-ported castings sell for $3,600 fully assembled, and feature 256/89cc ports, 2.165/1.600-inch valves, and 69cc combustion chambers. Those critical dimensions are similar to stock, but Mast says that flattening out the valve angle from 15 to 12 degrees and revising the port geometry gives its heads a substantial increase in flow. Despite repositioning the valves, the heads are fully compatible with stock pistons. On a 4.070-inch bore fixture, Mast advertises 351 cfm at 0.600-inch lift, and 370 cfm at 0.700-inch lift on the flow bench. Once bolted to the crate LS3, the Mast heads lived up to the hype. Output jumped to 568 hp and 507 lb-ft for gains of 35 and 27 numbers, respectively, over the stock hardware. Last we checked, 507 lb-ft of torque from a 376ci motor—which works out to 1.34 lb-ft per cubic inch—is stout by any measure, a testament to how efficiently the Mast-modified LS3 fills its cylinders with air.
Mast Medium-Bore LS3 Heads
More Inches, Please
Although a 99hp gain with a cylinder head and cam swap is mighty impressive, unless you’re willing to turn 7,500-plus rpm, a 376ci short-block is far too small to take advantage of 370 cfm of airflow. To demonstrate the potential of the Mast heads, given more cubes and cam, we bolted them up to a 416ci stroker short-block and matched it up with a 242/260-at-0.050 cam ground with 0.615/0.642-inch lift and a 114-degree LSA. Essentially a stroked LS3, it’s comprised of a 6.2L block bored 0.005-inch over, a Callies 4.000-inch forged crank and rods, and 11.2:1 Mahle pistons. The enlarged short-block did bump up compression a half-point over the stock LS3, but it’s safe to assume that the majority of the gains were attributable to the extra cubes and more aggressive cam. The final dyno tally registered 630 hp and 555 lb-ft of torque for an increase of 62 and 48 numbers, respectively. Just imagine what this puppy would do with another 20 degrees of cam duration.
|GMPP LS3 crate engine||19201992||$6,675|
|Mast cam and valvesprings||963-102||$680|
|Mast cylinder heads||510-202||$3,675|