Monster Mouse is now in full swing. For those not familiar with our latest LS project engine, the 418-inch stroker LS3-based short-block came courtesy of Texas Speed & Performance. The impetus for Monster Mouse came from our original LS series Modern Mouse, or more accurately from the limitations imposed by the Modern Mouse series. Modern Mouse started out life as a 5.3L LM7, but was bored and stroked to 383 ci. Despite the displacement hike, Modern Mouse was still limited by a maximum bore size of 3.905 inches. Given the 4.065 bore employed by GM on the late-model LS3, mods to Modern Mouse could not utilize hardware designed for the big-bore LS3 motors, more specifically the rectangular-port (big bore) cylinder heads. Measured stock to stock, LS3 heads are a serious upgrade to a cathedral-port motor. Aftermarket versions are much more comparable, but the bigger bore motors tend to further improve head flow. Recognizing the popularity of the LS3, we decided the big-bore boys deserved their own series.
Back in part 1 (July 2012), we introduced the 418 stroker from Texas Speed by comparing it to a stock and modified Chevrolet Performance crate mill. Run on the engine dyno in stock trim, the LS3 crate motor produced 493 hp and 484 lb-ft of torque. After adding a cam and ported factory LS3 heads, power jumped to 569 hp and 522 lb-ft of torque. The cam was responsible for most of the power gains, since the LS3 was already sporting heads that flowed well. The final test in part 1 involved replacing the stock LS3 short block with the 418 stroker assembly provided by Texas Speed & Performance. Using the same heads, cam and intake, the stroker upped power output to 609 hp and 570 lb-ft of torque. The torque curve offered by the normally aspirated combination was impressive, bettering 550 lb-ft of torque from 4,200 to 5,700 rpm. Torque production exceeded 445 lb-ft as low as 3,000 rpm. Strokers offer not only significant peak power gains, but torque gains through the entire rev range. You will use the extra 50-60 lb-ft of torque at 3,500 rpm a lot more than the extra 50 hp at 6,500 rpm, but it is nice to have both.
As change is the only constant with any engine dyno series, we decided it was time for a change. Off came the ported stock heads, and on went a set of Black Label LS3 heads from Mast Motorsports. The 11-degree Mast LS3 heads were designed specifically for the 4.070-inch bore motor, offering full CNC porting, a 2.165/1.60-inch valve package, and 69cc combustion chambers. The chamber size on the CNC-ported Mast heads was combined with a set of Fel Pro MLS head gaskets (0.041 thick) to produce a static compression ratio of 11.4:1. This was less than ideal for the Kenne Bell blower we had in mind, but we wanted to maximize the power output of the NA combination before stepping up to the blower.
The Mast heads were combined with a new cam profile from Crane Cams. The Crane cam offered 0.600-inch lift, a 240/246-degree duration split, and 114-degree LSA. The combination was topped with a FAST intake and 102mm throttle body. Run with the Holley Dominator EFI system, the new combination produced 639 hp and 578 lb-ft of torque, with torque production exceeding 550 lb-ft from 4,300 to 5,900 rpm. This 418 stroker was starting to get serious, but we were just getting started. Having further improved the power output with Mast heads and Crane cam, it was time for some boost, but not before lowering the static compression ratio. Off came the Mast heads once again, and on went custom MLS head gaskets from Cometic. The head gaskets thickness increased from 0.041 to 0.065, reducing static compression by 6/10ths of a point. In truth, we could well have just run the motor with the elevated static compression, but we wanted to hedge our bets and make some pump-gas runs before cranking up the boost.