Mast Motorsports LS7 Crate Motor - Going Overbored
An Inside Look At Mast Motorsports' 669HP LS7 Crate Motor
From the January, 2011 issue of Chevy High Performance
By Stephen Kim
Photography by Stephen Kim
What We Did
Follow the build process of a 650hp LS7 crate motor at Mast Motorsports
Way more power than a stock LS7 for not much more money
As legend has it, the 638hp supercharged LS9 is the baddest production small-block ever built. While that's certainly true from a strictly empirical standpoint, for traditionalists raised on massive cubic inches and pressure-differential induced airflow, the LS9's reliance on an external breathing apparatus somewhat diminishes its mystique. Despite the fact that the 505hp LS7 comes up more than 100 hp short of its huffed-and-puffed stablemate, it's an even more impressive specimen of race-bred engineering. Highlights include an enormous 4.125-inch bore, a 4.000-inch steel crank, titanium rods, 11:1 compression, billet steel main caps, dry-sump oiling system, 211/230-at-0.050 cam with nearly 0.600 inches of lift, and CNC-ported 12-degree cylinder heads that flow 370 cfm. Available from GMPP for about $14,000, it's hardly surprising that tons of hot rodders have plunked crate LS7s into their project cars. Considering all the tricks the LS7 has up its press-fit iron sleeves, when Mast Motorsports informed us that it had substantially improved upon GM's original design at a similar price point, we felt compelled to take a closer look at its 650hp LS7 SS crate engine package.
The LS7 SS starts with a brand-new...
The LS7 SS starts with a brand-new GM block that's then zero decked, align-honed, and finished honed to 4.125 inches. Even with a fresh block, these machining procedures optimize quench and ring seal, and eliminate potential crank binding. Afterwards, the outside of the block is deburred.
Wise engine builders know that change just for the sake of change isn't always prudent, so shop owner Horace Mast takes an engineer's approach of building upon the stock LS7's strengths rather than haphazardly redesigning the most powerful naturally aspirated GM small-block ever built from the ground up. To that end, Mast sticks with the factory LS7 block and 4.125x4.000-inch bore/stroke dimensions, but upgrades the rotating assembly with a Callies/Compstar 4340 steel crank and rods, and forged 11.7:1 Mahle pistons. The factory LS7 head castings are improved as well, with a proprietary CNC port job that increases airflow to close to 400 cfm. Likewise, the LS7 SS's 246/260-at-0.050 hydraulic cam is quite a bit larger than stock, but not outrageous considering the 427 cubic inches of displacement, and slight bump in compression. With a factory LS7 intake manifold, the Mast 427 produces 650 hp and 560 lb-ft of torque. Upgrade to an optional FAST intake and the hp and torque figures jump to 669 and 583, respectively. Since factory LS7 engines are rated using the latest SAE testing standards, their 505 hp actually translates to roughly 540 hp under the STP correction factor used by most engine shops. Even so, Mast's combo still kicks out over 100 hp more than a stock LS7 for $16,995, and includes an 18-month/unlimited-mile warranty. That's not exactly pocket change in this day and age, but quickly adding up the costs of the individual components ($3,000 block, $3,000 heads, $2,500 rotating assembly) used in Mast's LS7 SS puts its value into perspective.
While the performance figures speak for themselves, what really sets Mast's LS7 SS apart is the comprehensiveness of the overall package. In addition to the motor, the combo includes a calibrated Mast standalone ECM, a complete wiring harness, a drive-by-wire throttle body, an accelerator pedal, a GM starter, a balancer, a water pump, an air filter, coil packs, spark plugs, and plug wires. Upon dropping a Mast crate motor between the framerails, all you have to do is add fuel and hook up three wires before firing it up. For muscle car enthusiasts thinking about jumping on the LS-swap bandwagon, it doesn't get much easier than this.
For all it does well, one...
For all it does well, one of the stock LS7's few quirks is its hypereutectic pistons. Mast steps it up a notch with forged Mahle slugs and matches them up with Callies/Compstar 6.125-inch steel H-beam rods. The pistons feature Mahle's Grafal coating on the skirt surfaces, which reduces friction and increases scuffing resistance. It also allows for running a tighter piston-to-wall clearance for reduced blow-by and piston slap. With -3cc valve reliefs, the pistons yield an 11.7:1 compression ratio.
Like the cylinder heads, the...
Like the cylinder heads, the LS7 block is based upon the C5-R engine design that powered the factory Corvette race team to multiple American Le Mans Series championships. It's cast from rugged 319-T5 aluminum, and features doweled billet steel main caps. In conjunction with ARP studs, the caps firmly hold the Callies/Compstar 4340 forged crank in position.
Sticking with a proven performer,...
Sticking with a proven performer, the Mast 427 retains the factory timing set. Considering that a production LS7 revs freely to 7,000 rpm and is built to last over 100,000 miles, the stock timing set was deemed plenty durable for the Mast 427, which keeps valvespring pressures reasonable and produces peak power at 6,600 rpm.
The factory LS7's dry-sump...
The factory LS7's dry-sump oil system makes swapping the motor into an older car a challenge. For drop-in simplicity, Mast ditches it in favor of a more traditional wet-sump setup. This involves replacing the stock LS7 oil pump-which integrates both a supply and scavenging pump into one assembly-with a Melling unit.
A staple of every Mast stroker...
A staple of every Mast stroker motor is the company's custom windage tray. They fit most LS oil pans, and are designed to prevent splash-back and optimize oil return to the sump. Eliminating one more potential headache from building a stroker combo, the tray clears up to a 4.250-inch stroke crank and thick connecting rods, and accommodates multiple pickup tubes. Each tray is treated with a black oxide finish to resist corrosion, and is compatible with the factory windage tray nuts.
Completing the wet-sump conversion...
Completing the wet-sump conversion is a choice of oil pans off of either an LS1 F-body, an LS3 Corvette, a 2010 Camaro, or a Colorado pickup. This allows customers to tailor the location of the sump to the packaging needs of their chassis. Mast plans on releasing its own cast oil pan in the near future.
The crank and rods ride on...
The crank and rods ride on tri-metal ACL bearings designed to provide an excellent balance of strength, anti-wear, anti-seizure, and embeddability properties. Furthermore, the aluminum, tin, and silicon alloy offers excellent load-carrying characteristics as well.
In order to accommodate the...
In order to accommodate the LS7's large dry-sump oil pump, the factory timing cover is much bulkier than a standard Gen IV unit. Bolting on a wet-sump requires swapping it out for a timing cover off of an LS2.
Cramming a large 4.125-inch...
Cramming a large 4.125-inch bore inside a block with 4.400-inch bore centers doesn't leave much space between the cylinder walls. To ensure effective cylinder seal, Cometic multi-layer steel gaskets are used on the Mast 427. The zero-decked block and 0.040-inch gaskets tighten up quench and bump up the compression.