LS3 Small-Block Build - Monster Mouse, Part 1

Like the cathedral-port LS engines, the big-bore motors respond just as well to bolt-ons.

With the success of our Modern Mouse series, we decided it was time to recognize the big-bore crowd. Loyal readers of Super Chevy will remember the Modern Mouse series started with a stock 5.3L LM7 that we transformed into a 383 stroker. Along the way, we tested all manner of performance upgrades on the cathedral-port-headed small-block, ranging from simple intake and cam swaps, to strokers and forced induction.

The one limitation of Modern Mouse was use of the 5.3L block. Though we were able to bore and stroke it to 383 inches, the block limited bore size to 3.905 inches. It was possible to step up to the larger 6.0L truck block (LQ4 or LQ9), but even those are limited to 4.030-inch bores. The current LS3 motors (including L92, LSA, LS9 etc.) come factory-equipped with a 4.065-inch bore, so we decided the big-bore guys deserved their own series. After all, a great many performance products were designed specifically for the big-bore LS3 motors. Hence our new series, entitled Monster Mouse.

When GM introduced the LS3/L92 family, the two major changes were an increase in bore size and rectangular-port heads. The change from the LS1-style cathedral-port heads to the new rectangular-ports netted impressive flow gains. How big were the flow gains offered by the new LS3/L92 heads? In a recent flow test against a set of cathedral-port 317 truck heads (that shares the intake port with the 243 LS6), the as-cast rectangular-port LS3 heads offered gains as high as 70 cfm. Just bolting these stock LS3 heads on in place of the cathedral-port heads was worth 40 hp. The flow numbers indicate that a set of stock rectangular-port LS3/L92 heads will support 650 hp. In fact, dyno testing a set of stock LS3 heads on a wild 468 stroker netted almost 700 hp.

The big thing to remember here is that just because the heads offer the potential, doesn't mean they will make 700 hp on your mild LS3. While an extra 40 hp from an LS3 head upgrade might sound enticing, know that they were designed for big-bore motors and will not bolt onto your small-bore 4.8L-5.7L and will only just fit on the 4.0-inch bore 6.0L truck and LS2 combinations. The head swap would naturally require the matching rec-port LS3 intake manifold and offset intake rocker arms.

Given the use of the rectangular-port heads, Monster Mouse required a suitable big-bore-block. Lucky for us, the LS experts at Texas Speed came through with not just an LS3 block, but a complete LS3-based stroker sporting no less than 418 ci. We were excited about testing the new Texas Speed stroker, but needed something to compare it to. As luck would have it, we had access to a stock LS3 crate motor secured through Summit Racing. With the stock engine in hand, we decided that the ideal first test for Monster Mouse would be to run the stock LS3 motor, then modify it with ported heads and a cam, then install the same components on the Texas Speed stroker. This would provide readers with plenty of information on both stock and stroker displacement LS3 applications. We have plenty of future testing in store for the stroker, but for now, let's get this party started.

First up was the stock motor. The LS3 crate piece is essentially a Corvette milled pulled from the assembly line. Displacing 376 ci and rated by the factory at 430 hp and 424 lb-ft of torque, our test method would unearth significantly more power from the impressive small-block. Tested on the engine dyno at Westech, the LS3 was equipped with an electric water pump, long-tube headers and run devoid of any accessories. Also on the missing components list were the induction system ahead of the throttle body, the entire exhaust system and any accessories. Rather than the conservative factory tune, we also optimized the air/fuel and timing using a FAST XFI/XIM management system.