5.3L LS Small Block Build - LS Modern Mouse: Part 2

Though we tested a new cam, rockers, and an NOS nitrous kit in this episode of Modern Mouse, we decided to give our readers a sneak peak at what we have planned for future installments. Modern Mouse will be subjected to strokers, intakes, and even boost.

Though we tested a new cam, rockers, and an NOS nitrous kit in this episode of Modern Mous

Loyal readers of Super Chevy (you know who you are) will no doubt remember our first adventure with Modern Mouse last month, but here is a recap for those of you just joining the party. Back in part one, we plucked a 5.3L LM7 truck motor from a local wrecking yard and propped it up on the dyno. As with our previous SBC rodent series (Major Mouse, Danger Mouse), our plan was to subject Modern Mouse to all manner of modifications, ranging from the mild to wild, even venturing off into the bizarre.

To get the party started, we simply installed the motor and managed a few simple bolt-ons, namely ported stock heads, a cam, and an intake. The combination of TEA CNC 5.3L heads, a mild Comp hydraulic roller cam, and the Fast LSXR intake netted us 445 hp and 443 lb-ft of torque (100 hp and 70 lb-ft over the baseline runs).

These were good numbers considering the mild cam profile (212/218 @ 0.050). Those of you wondering why we didnít start with a larger 5.7L, 6.0L, or even 6.2L should know that the 5.3L is more readily available and considerably less expensive than any of its big brothers. Affordability is a critical element, but fear not: increased displacement is definitely on the to-do list.

After a review of the hard parts currently employed on Modern Mouse, we came to one simple conclusion—the thing needs more camshaft. Back in part one, we chose the conservative route and installed a cam that offered sizable power gains without negatively affecting things like idle quality and (likely) fuel mileage. That gains were present through the entire rev range was a good sign, but the 5.3L was now sporting heads and an intake manifold that could support 600 hp. While we knew 600 hp was out of the question in normally aspirated trim, we also knew that we could get a lot closer to 500 hp with a simple cam change. If not the cam alone, then certainly with a few other tricks we had up our sleeve. The icing on the performance cake came in the form of a plate nitrous system from NOS. The wet system offered plenty of power, but in deference to our stock short-block we installed what might be considered conservative jetting. As luck would have it, we also had the perfect displacement test, having run a 4.8L for sister-pub Hot Rod magazine with identical components.

1. The 5.3L short block was still sporting all of the factory internals. Donít be too quick to discount the strength of the factory stuff; our sister book Hot Rod ran some testing on a stock 4.8L short block and managed to exceed 1,200 hp using a pair of turbos.

1. The 5.3L short block was still sporting all of the factory internals. Donít be too qui

Part two of Modern Mouse begins where part one left off, with our 5.3L sporting CNC-ported stock heads from Total Engine Airflow, a Comp XR265HR cam and a FAST LSXR intake and throttle body. Also present were the American Racing headers, a Meziere electric water pump and a FAST XFI/XIM management system. Equipped as such, Modern Mouse managed peak numbers of 445 hp at 5,900 rpm and 434 lb-ft at 5,000 rpm. With plenty of head and intake flow, we decided to further improve the peak power numbers of 5.3L, even if the additional power came at the expense of low-speed torque production. If the 5.3L were to see some strip action, the additional power would certainly come in handy. If you are all about torque and driveability, stick with the smaller XR265HR grind, but for this installment, we replaced the mild stick with a wilder XR281HR.

The new cam offered a 0.571/0.573 lift split, a 228/230 duration split at 0.050, and a 112-degree lobe separation angle (LSA). In one fell swoop, the cam swap increased both lift and duration while tightening the LSA. The result of the cam swap in our 5.3L was immediate and impressive, with the peak power output jumping to 480 hp. Peak torque was up by just 1 lb-ft, but the cam swap netted power gains from 4,700 rpm on up, with a loss in torque below that point. That’s the tradeoff.

5. The stock 5.3L cam was replaced by a mild XR265HR grind from Comp Cams. The XR265HR cam offered 0.522/0.529 lift split, a 212/218 duration split, and a 14-degree lobe separation angle. The mild street cam offered impressive power gains through the entire rev range, but we knew there was more power to be had.

5. The stock 5.3L cam was replaced by a mild XR265HR grind from Comp Cams. The XR265HR ca

Typical of a more aggressive cam profile, the idle vacuum dropped by over 3 inches, but Modern Mouse now sounded like one serious small-block, especially at full song. We’d love to stab this thing in a Camaro or Chevelle and start grabbing gears at 7,000 rpm! If that doesn’t bring a smile to your face, you must be a closet Ford owner.

Though our cam swap netted some impressive numbers, we decided to keep going with a set of roller rockers and synthetic oil. Unfortunately for us, both changes were made at the same time, so you can judge for yourself just how much each might be worth. We can say they are both worthwhile upgrades and every motor deserves synthetic oil.

After the oil change to Luca 5W-20 synthetic, we also swapped out the stock rockers for a set of 1.8-ratio rollers from Comp Cams. The revised rocker ratio pushed the lift of our new XR281HR cam over 0.600 lift, but our spring package from Total Engine Airflow was more than adequate to handle the extra lift. A quick check on piston-to-valve indicated we didnít have much more room for additional duration, but the combination was ready to rock. The two changes improved the power output of the 5.3L to 492 hp and 447 lb-ft of torque. The additional rocker ratio improved power across the board.

6. Before swapping the cam, we decided to upgrade the oiling system with a Moroso pan, pick-up, and windage tray. Designed for an F-body application, the combination is often used when swapping an LS into a early chassis.

6. Before swapping the cam, we decided to upgrade the oiling system with a Moroso pan, pi

The final test of the day involved nitrous oxide. NOS supplied a kit (part number 05169NOS) designed specifically for the LS2 (larger throttle body; it also has one for the LS1). The wet EFI kit included everything needed to increase the power output of the motor by as much as 200 hp. We chose slightly more conservative jetting to supply an additional 125 hp. We also added a can of octane booster to the premium unleaded (91 octane) pump gas we had been using for all of the testing. Using the Fast XFI management system, we also dialed out four degrees of timing to minimize the chance of detonation (remember those cast pistons). Westech had a bottle heater on hand to ensure over 900 psi of bottle pressure.

Given the high-mileage short-block, we were a little nervous before hitting the button, but Modern Mouse responded like a real trooper. After minor changes to the air/fuel mixture (factory jetting from NOS is a tad on the rich side for safety), the 5.3L belted out 616 hp and 586 lb-ft of torque on the juice. That’s enough power to push a street machine into the 10s, trapping more than a few Mustangs in the process.

16. As luck would have it, Modern Mouse was equipped with the very same heads, cam, and intake manifold as a 4.8L run for our sister book Hot Rod. The comparison is interesting, as the smaller 4.8L made peak power higher in the rev range and the larger 5.3L offered more average power through the entire rev range. Identically equipped, bigger really is better.

16. As luck would have it, Modern Mouse was equipped with the very same heads, cam, and intake manifold as a 4.8L run for our sister book Hot Rod. The comparison is interesting, as the smaller 4.8L made peak power higher in the rev range and the larger 5.3L offered more average power through the entire rev range. Identically equipped, bigger really is better.

Power Numbers:

Effect of Displacement: 4.8L vs. 5.3L

The graph illustrates the difference in power production offered by the 4.8L and 5.3L. Both motors were equipped with stock short blocks, sporting the same XR281HR cam, TEA CNC-ported stock heads and Fast LSXR intake. Both combinations were also equipped with the very same 1-3/4-inch long-tube headers, Fast XFI management system, and Meziere electric water pump. The 4.8L produces 449 hp and 378 lb-ft of torque to the 480 hp and 435 lb-ft of torque offered by the larger 5.3L. It is also important to note that the smaller 4.8l produced peak power at 7,000 rpm and peak torque at 5,800 rpm. This compares to 6,600 rpm and 5,300 rpm for the larger 5.3L. Displacement has a way of taming the cam profile. Were we to run this combination on a 6.0L or a 408 stroker, the motor would make peak power even lower than the 5.3L. The 4.8L was no slouch, but just check out how much extra average power is available in the lower rev ranges. There really is no replacement for displacement.