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

We try some more aggressive cams and nitrous on our 5.3 LS guinea pig.

Richard Holdener Nov 15, 2011
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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.

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.

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.

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.

Power Numbers:

Base 281 Cam Rkrs&Oil NOS
2,600 N/A N/A N/A N/A N/A N/A N/A N/A
2,900 204 369 N/A N/A N/A N/A N/A N/A
3,200 232 381 224 369 229 376 228 374
3,500 263 395 250 375 255 383 256 384
3,800 297 410 285 394 295 407 294 406
4,100 337 432 318 407 328 421 364 466
4,400 361 431 354 423 365 436 488 582
4,700 387 432 388 434 399 446 524 586
5,000 413 434 412 433 425 446 547 575
5,300 436 432 439 435 451 447 574 569
5,600 444 416 460 431 473 444 600 563
5,900 445 396 472 420 486 432 608 541
6,300 439 366 479 399 493 411 616 514
6,600 N/A N/A 480 382 492 391 521 415
Max 445 434 480 435 492 447 616 586

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.


Comp Cams
Memphis, TN 38118
Total Engine Airflow
Tallmadge, OH 44278
Memphis, TN 38118
Bowling Green, KY 42101



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