from the editors of:
GM High Tech Performance
LOG IN / SIGN UP
GET THE MAGAZINE
tech & how to
engines & drivetrain
Chassis & Suspension
paint & body
Best of the Best
GM High Tech Performance
Mast Motorsports LS7 Crate Motor - Going Overbored
An Inside Look At Mast Motorsports' 669HP LS7 Crate Motor
Dec 7, 2010
View Full Gallery
View Full Article »
Mast Motorsports LS7 Crate Motor - Going Overbored
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.
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 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, 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 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 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 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 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 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 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.
The LS7 SS utilizes factory GM head castings to which Mast applies its own CNC machining to the ports and combustion chambers. Working in concert with Mast's valve job, the tweaks boost airflow from 370 to around 400 cfm.
The LS7 head castings are fitted with 2.20/1.625-inch Ferrea hollow-stem valves. While the intake valves retain the stock diameter, the exhaust valves are slightly larger than the stock 1.61-inch pieces to increase flow. Keeping them out of float are Mast dual nitride springs with 410 pounds of open pressure held in place by titanium retainers.
To achieve its goals of big power and good street manners, Mast uses a custom 246/260-at-0.050 hydraulic cam with 0.659/0.666-inch lift. It's ground on a wide 114-degree LSA to improve idle quality. The GMPP lifters, which were designed for the Cadillac CTS-VR factory race car, feature a ceramic check ball that enables them to operate at engine speeds up to 8,500 rpm.
Like the timing set, the Mast 427 uses factory LS7 rocker arms, as they're plenty capable of handling the motor's 7,250-rpm rev limit. Note how the intake rockers are offset to enable optimization of the port geometry.
A stock LS7 intake manifold comes standard on the Mast 427, but our test subject was equipped with a FAST LSXR intake instead. It's fed by a stock 90mm throttle body.
One huge advantage of Mast's standalone ECM is that it's compatible with the factory drive-by-wire throttle-body using a GM gas pedal that's provided with the crate engine package. Not only does this yield the ultimate in throttle actuation precision, it means that there's no throttle linkage to hook up, further simplifying the installation.
Mast dyno tests every crate motor that it ships. During the R&D process, Mast calibrates each specific combination at 160 different rpm and load points, from idle to redline. This involves painstakingly dialing in the air/fuel ratio using a modeled speed-density strategy that calculates airflow using throttle models, pressure models, and injector models. The M90 ECM also utilizes an octane-learn function that can adjust the spark maps automatically for octane ratings that range between 87 and 93.
The M90 ECM is durable enough to survive harsh marine applications, so it can be mounted just about anywhere in a car. For easy access, mounting it on the firewall is a viable option, eliminating the need to snake the harness through the interior.
It's one thing to brag about dyno numbers, but the true test of horsepower is at the dragstrip. Strapped inside a full-weight street/strip Malibu with a stock-style suspension, Mast's 650hp LS7 SS propelled this sled to deep 10-second e.t.'s at nearly 135 mph.
Mast Motorsports LS7 Crate Motor - Chevy High Performance Magazine
Check out Mast Motorsports LS7 Crate Motor. Read more only at www.chevyhiperformance.com, the official website for Chevy High Performance Magazine!
Supercharged LS7 Engine Build - We Blend the LS7's 7.0L Displacement with the LS9's Blower - GM High-Tech Magazine
In this tech article GM HIGH-TECH PERFORMANCE builds a supercharged LS7 engine which combines its 7.0L displacement with the LS9's blower to yield a 770HP street engine with a Jekyll-and-Hyde character - GM High-Tech Magazine
LS7 Stroker Build - The Power Of 7 - Super Chevy Magazine
LS7 Stroker Build - Upgrading the small-block takes some time and patience... As well a list of other things that makes it a monster. - Super Chevy Magazine
1978 Chevrolet Corvette Indy Pace Car with 13.6 Miles - Rare Finds
At first glance, Schoenleden thought the 1978 Chevrolet Corvette Indy Pace Car was "an absolute mess." "When I opened the door...I couldn't believe my eyes."
recent how to articles
How To Install Emissions-Legal Headers on a 2002 Corvette - Fresh Lungs
How To Choose the Right Tire for Your Classic Corvette - Tire Talk
Chevy Performance Tech Q&A - December 2014
How To Build a 1,200 HP LSX Engine - Pump Gas Powerhouse
Chevy Resto Tech - November 2014
subscribe to the magazine
Subscribe and Save 74% off the Cover Price!