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5.3L LS Small Block Build - LS Modern Mouse: Part 3

We go from 5.3 to 383 with our no-longer-little LS engine.

Richard Holdener Dec 20, 2011
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In the previous two installments of Modern Mouse, our latest Super Chevy dyno mule, we subjected the little 5.3L LS to a whole slew of aftermarket components, including ported cylinder heads, a variety of performance camshafts, and even a dose of nitrous oxide. What started out as a humble stock 5.3L producing 343 hp and 372 lb-ft of torque eventually produced 493 hp and 447 lb-ft of torque. With the giggle gas, the numbers jumped to 616 hp and 587 lb-ft, but even without the nitrous, the 5.3L was one healthy small-block.


Readers with a few muscle car moments under their belt will realize that any traditional 327 small-block that produced nearly 500 hp was considered a race motor. This little 5.3L (324 cubic inches) did so while retaining more than acceptable levels of idle quality, driveability, and even emissions output (had we elected to install it in a vehicle with catalytic converters). The point is that the modern LS is every bit the worthy successor to the original Mouse and more than deserving of a dedicated series.

After a few bench-racing sessions and weighing the options, we decided to go with a hike in displacement. Sure, there was more power to be had while retaining the stock short-block, but so too were there inherent trade-offs. Additional power could come from wilder cam timing, but that would simultaneously increase effective engine speed and decrease driveability. Don't get us wrong, 7,000-plus-rpm small-blocks are cool, but they do tend to be somewhat less reliable, fuel efficient, and fun to drive on a daily basis.

The other option was to add boost, a decision we always like, but opted against until we exercised a few more normally aspirated combinations. Boost will come in time, but for now, it's all about the cubes.

The benefit of cubic inches is that we can substantially increase the power output without a decrease in driveability. Extra displacement actually tames the cam timing, so a cam profile that would produce peak power at 7,000 rpm on a 5.3L will drop substantially on a larger 383. The displacement also improves idle quality while offering mega torque gains through the entire rev range. It is these power gains lower in the rev range that make a stroker motor so much fun. Stab the throttle and instead of waiting for the motor to come on the cam, you are rewarded with an instantaneous surge of torque.

A little (simple) math helps to better explain the benefits of displacement. If you build a 324-inch motor that offers 1 horsepower per cubic inch, the result is a 324-hp motor. If that same specific output were applied to a 383 cubic inch motor, we have an increase to 383 horsepower, a gain of nearly 60. Back in part two of Modern Mouse, our 324 (5.3L) produced 493 hp, which equates to 1.521 hp per cubic inch. If we apply that specific output to a 383 stroker, we wind up with 582 hp. Such can be the benefit of displacement.

Simple math aside, the reality is that we were not looking to match the specific output of the modified 5.3L from part 2. Rather, we looked to eclipse the maximum power output while improving both average power production and driveability. What we wanted was a healthy street/strip motor that could double as a daily driver. On the back burner was also the ability to readily accept the boost we had planned in the future. To that end, we disassembled the 5.3L and took it to our good friends over at L&R Automotive for some machining.

The stock bore was punched out to 3.902 inches to accept a new stroker assembly. The rotating kit included a 4.0-inch, forged steel stroker crank and 6.0-inch rods from Procomp Electronics combined with forged pistons from Probe Racing. Each forged slug featured a 21cc dish which combined with the 64cc chambers on the GenX 215 heads from Trick Flow Specialties produced a static compression ratio of 9.6:1. Though a tad on the low side for a dedicated all-motor application, the combination was ideal for the boost we had planned. It is possible to mill the heads down to 60 cc, which would bring the static much closer to 10.0:1. Flat-top pistons (with valve reliefs) are available as well, which we used in a second 383 build--but more on that later.

Looking to improve both the idle quality and driveability, we opted to install a milder cam profile in the 383 stroker. Fear not, as we had a wilder cam profile waiting to unleash serious power, but, we relied on the extra displacement offered by the stroker to offset the mild cam timing. The TrackMax cam for our stroker came from Trick Flow Specialties and featured 0.575 lift, a 220/224-degree duration split (@ 0.050), and a 114-degree lobe separation angle. The cam was installed with a new oil pump and timing chain from Sealed Power, but we retained the factory hydraulic roller lifters.


Additional features employed on the 383 short-block included a set of Total Seal piston rings, a Moroso oil pan, pick-up and windage tray, and a fresh set of cylinder heads. Instead of reusing the TEA-ported 5.3L heads on Modern Mouse, we decided the stroker deserved something new. The larger bore allowed us to run a set of Gen X 215 heads from Trick Flow Specialties. As the name implied, the Gen X 215 heads featured 215cc intake ports, full CNC porting, and a 2.04/1.575 stainless steel valve package. The Gen X 215 heads were ideally suited to our 3.902 bore and offered serious flow numbers (over 320 cfm) to properly feed the added displacement.

Once again we relied on a FAST LSXR intake and matching 102mm throttle body, a set of 1-3/4-inch long-tube headers and stock rockers. The motor was run with Holley engine management system and a set of 42-psi injectors. All testing was performed on 91-octane pump gas. After tuning by Westech's Ernie Mena, the 5.3L stroker (383) pumped out 510 hp at 5,700 rpm and 507 lb-ft of torque at 4,700 rpm. This compares to 493 hp and 447 lb-ft of torque offered by the modified 5.3L.

Despite the milder cam profile, the 383 came through by bettering both the peak power and torque outputs (achieved 600 rpm lower in the rev range to boot). Not only was the stroker up by 17 hp (measured peak to peak), but the additional displacement made itself known by thumping out an additional 60 lb-ft of torque. Lower in the rev range, the differences were even greater. Toss in the fact that the stroker offered an additional 2 inches of idle vacuum and you have the makings of the perfect street stoker--one that still looked like a 5.3L.

Power Numbers:

Mod 5.3LMild 383Wild 383

A Wilder Mouse

Happy as we were with the results of the mild 383 stroker LS, we thought we would offer up an alternative to Modern Mouse. For those adventurous few who exist on a diet of Axe body spray and Red Bull, and are willing to put up with diminished idle quality and fuel economy, we assembled Modern Mouse's evil twin, Mayhem Mouse. It is obvious from the results that these twins are anything but identical, as Mayhem Mouse thumped out nearly 600 horsepower and an additional 40 lb-ft of torque. Though the twins shared the same displacement of 383 cubic inches, Mayhem Mouse was assembled with a set of forged flat-top pistons to increase the static compression by nearly two full points. Further improvements included much wilder cam timing (0.624 lift, 239/247 duration, and 114 LSA), 1.72:1 roller rockers, and 1-7/8-inch headers from American Racing Headers. One additional change was to substitute the Gen X 215 heads for a set of Stage 2.5 LS6 (243 castings) from Total Engine Airflow. We suspect the combination would work just as well with the Gen X 215 heads with equal chamber volumes, but Mayhem Mouse was sporting the CNC-ported factory LS6 heads for this test. The combination belted out 595 hp at 6,300 rpm and 547 lb-ft at 4,700 rpm. For you mathletes, that represented 1.553 hp per cubic inch, bettering the specific output (1.521) of the modified 5.3L in part two. - Richard Holdener



Trick Flow Specialties
Tallmadge, OH 44278
Total Engine Airflow
Tallmadge, OH 44278
Probe Racing
Torrance, CA 90505
Procomp Electronics
Ontario, CA 91761



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