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5.3L LS Small Block Build - Here Comes Modern Mouse!

If you liked our Danger Mouse and Major Mouse series of articles, you’re gonna love this one

Richard Holdener Oct 24, 2011
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If the past has shown us anything, it is that readers of Super Chevy relish their small-blocks. Not that we don't show equal love to their big-block brothers, but there is something about a Mouse motor thumping out serious power that really gets the blood pumping. Loyal readers will remember the Major Mouse and Danger Mouse series that covered all manner of small-block performance. Though the original continues to fill the engine bays of countless performance machines, we decided to take an in-depth performance look at the current Gen III (and Gen IV) small-blocks, better known by enthusiasts as the LS engine family.

Introduced to replace the original Mouse (Gen I) and LT1 (Gen II) engine families, the LS had tall shoes to fill, but came cocked, locked, and ready to rock. Ranging in displacements from 4.8 liters (293 inches) all the way up to 7.0 liters (427.5 inches), the impressive LS had plenty to offer, including cross-bolted blocks, high-flow cylinder heads, and even a factory windage tray. With so much to offer, we decided the current configuration deserved a series of its ownùenter Modern Mouse.

The decision to start the Modern Mouse series was easy, but what exactly to start with somewhat less so. The wide range of factory displacements, configurations, and production power outputs made it difficult to choose a starting point. Given the current economic situation, we decided that (at the very least) Modern Mouse should start by embracing affordability. Sure, the series will eventually take us beyond the realm of low-buck bolt-ons, but they should still be covered in earnest. The combination of affordability and availability (the two are obviously interrelated) led us straight to the 5.3L (LM7) truck motor. Sure, the 5.7L LS1 and LS6, 6.0L LS2, or even a modern 6.2L LS3 will offer more power, but that extra grunt comes with a hefty price tag. Even the larger 6.0L truck motor commands a premium over the smaller 5.3L. The 5.3L (324 cubic inches) has become the small-block engine of choice, especially for cost-conscious buildups. Though down on displacement in stock trim, the 5.3L truck mill is high on performance potential (we look forward to punching this one out to 383 cubic inches).

The Modern Mouse series will eventually take us through stroker cranks and boosted buildups, but for now we decided to concentrate on the basics. Knowing that many readers are looking at their 5.3L to serve as a daily driver, we decided to start Modern Mouse off with a few select performance upgrades, including heads, cam, and an intake. Before making any hardware changes, we took the software route by altering the factory tune. When all was said and done, we had a powerful, aluminum-headed small-block that would be perfect for any daily driver.

Before getting Modern Mouse on the dyno, we took the liberty of making a few changes. After getting our 5.3L from a local wrecking yard (complete running motor for $395), we tore it apart to verify that it was indeed a 5.3L. The 5.3L and smaller 4.8L truck motors are externally identical. The internal differences included a change in stroke, connecting-rod length, and use of flat-top pistons on the 4.8L (The H.O. version of the 5.3L also runs flat-top pistons). The casting numbers identify the 4.8L (121 for rods and 12553482 for crank), so make sure which engine you have.

After ensuring our 5.3L was indeed a 5.3L (a 4.8L will respond to the proposed mods as well, just with slightly less overall power and torque), we took the liberty of making a few changes. Knowing that the future held both boost and nitrous for Modern Mouse, we elected to replace the factory head gaskets and bolts with upgrades from Fel Pro and ARP. Not necessary on this normally aspirated application, the upgrades were simply done in anticipation of future abuse. After reassembly, the 5.3L was installed on the engine dyno using a set of 1-3/4-inch headers, a Meziere electric water pump, and the FAST XFI engine management system.

For the baseline test, we programmed the FAST to provide the factory timing and air/fuel curves (2003 truck application). Run with the mild factory tune, the 5.3L produced 343 hp and 372 lb-ft of torque. Letting our fingers do the walking over the keyboard, we tuned the XFI management system (more timing and less fuel) and dialed up the power output to 363 hp and 385 lb-ft of torque. To put these numbers into proper perspective, we ran replicas of the original 1970 LT-1 (370hp 350) and 1965 L76 (365hp 327) and neither of these muscle-car small-blocks eclipsed the power output of our stock 5.3L.

Satisfied with our baseline numbers, it was now time for bolt-ons. First on the list of performance upgrades was a camshaft. This should come as no surprise, as nothing wakes up an LS motor like the right cam profile. The reason the 5.3L (any LS, but especially the newer 6.2L LS3s) responds so well to cam changes is that the motors offer such impressive head flow right from the factory. Stock LS heads (including the 706 heads on our LM7 5.3L) flow 225 cfm, or enough to support over 450 hp right out of the box. It takes a fully ported fuelie head to eclipse those flow numbers. With plenty of airflow, all the stock LS motor needed was more aggressive cam timing. For part one, we elected to cater Modern Mouse toward the daily-driver end of the spectrum, but fear not, as even wilder cam timing is on the way. To start, we chose a mild Comp XR265HR grind that offered a 0.522/0.529 lift split, a 212/218 duration split, and a 114-degree lobe separation angle. The XR265HR cam was just a step or two above the stock cam, though the 5.3L offered the mildest production cam profile of the LS family. Power improved through the entire rev range with the XR265HR Comp cam, as the 5.3L belted out 385 hp and 412 lb-ft of torque. The cam improved the power output by as much as 30 hp at the top of the rev range.

Happy with the results of the cam upgrade, we focused our attention on the cylinder heads. Like the original Mouse, aftermarket cylinder head choices abound for the LS engine family. Rather than splurge on a set of new heads, we decided to take the less expensive route and send the factory heads out for porting. The stock heads were removed and shipped out to Total Engine Airflow (TEA) for the company's Stage 2 port job. The heads were treated to full CNC porting, larger intake valves, and a valve spring upgrade that provided sufficient spring pressure and clearance to safely run 0.650-lift cams and rev cleanly to 7,000 rpm.

The TEA 5.3L heads flowed nearly 300 cfm, or enough to support over 600 hp on the right application. They were obviously more than sufficient for the needs of our mild 5.3L, but we liked the fact that they had plenty of potential should we elect to increase cam lift and/or cubic inches (we plan on both). The head upgrade improved the power output to 424 hp and 423 lb-ft of torque but (best of all) did so while maintaining idle vacuum. The power gains offered by the cam swap resulted in a slight drop in idle vacuum (less than 2 inches), but the head swap suffered from no such trade-off.

The final performance upgrade included a new induction system from FAST in the form of the company's 102mm LSXR intake and throttle body. Possibly a tad on the large side for the mild 5.3L, we were still anxious to replace the truck intake to see what would happen. We tried a larger (aftermarket) throttle body on the stock truck intake during testing but saw no power gains at the lower power levels. The stock LS1, LS6, or LS2 intakes might be an affordable alternative to the FAST, but will not likely produce as much maximum power. We may look at the stock intakes in the future, but Modern Mouse was ready for action after receiving the FAST intake, throttle body, and billet fuel rail.

The induction upgrade improved the peak power numbers to 445 hp and 434 lb-ft of torque, but the FAST intake did suffer slightly lower in the rev range. After the upgrades, Modern Mouse was now officially a performance motor, and a good one at that. How do you argue with the combination of smooth idle, a broad torque curve, and 440 hp? Check back with us next month as we cam up, then put the squeeze on Modern Mouse.

Power Numbers


TEA head flow

lift in. ex.
.100 64 47
.200 145 113
.300 215 166
.400 254 222
.500 293 244
.600 310 256


Comp Cams
Memphis, TN 38118
Total Engine Airflow
Tallmadge, OH 44278
Spokane, WA 99223

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