Given the choice, would you rather have a stock or modified LS engine? The obvious answer is, of course, a modified one, or this would be a very short article indeed. Compared to the previous generation small block, the entire LS family is plenty powerful, but some are definitely better than others. A stock 5.3L truck motor is one heck of a little workhorse, and even better when turbocharged, but in stock trim, it can't hold a candle to the all-aluminum LS3. Blessed not only with the best factory intake and second best (only to the LS7) factory heads, the LS3 is tough to beat. Oh sure, cams go a long way, but it takes a serious set of cylinders heads to improve upon the stock (rectangle port) units. Given that the stock LS3 heads flow more than 315 cfm and are capable of supporting over 650 hp on the right application, improving performance takes not only a good set of aftermarket heads, but also one heck of a test engine.

Before covering the dyno thrash, we should clear up a common misconception. Be it bolt on or boost, the success of any modification is determined by its compatibility. Though we have all but bestowed it with mystical properties, the honest truth is that the LS engine is nothing more than a simple air pump. The power output of the pump is simply a function of the amount of air it can process. The more air through the engine, the more power out. Getting an engine to process the air effectively requires all the components working in harmony, meaning they must be designed to operate effectively in the same rpm range. The installation of high-flow cylinder heads on an otherwise stock engine (even one as good as the LS3) will have predictable, and less than optimal, results. The same can be said of any single component: a cam profile designed to make peak power at 7,500 rpm will be of limited value when combined with stock heads and a long-runner intake. The heads must support the desired power production in the rpm range dictated by the cam and intake design.

Though many wilder induction systems are offered for the LS3, we stuck with the very efficient factory intake for this test. The stock induction system was combined with a wilder cam profile and a set of AFR's new LS3 heads. Though late to the LS3 party, this test illustrates that these castings were well worth the wait. To ensure their LS3 heads surpassed the performance potential of the already impressive stock hardware, the AFR LS3 heads offered a number of desirable features. Naturally the AFR heads offered more flow, with peak numbers of 384 cfm on the intake and 255 cfm on the exhaust, but more flow was just a part of the equation. We liked the fact that the extra flow came with no change in port volume. More flow from a bigger port is easy, but adding nearly 70 cfm while maintaining the factory port volume was indeed impressive. The AFR LS3 heads also featured 95-cc exhaust port, 69-cc combustion chambers and a 2.165/1.60 valve package. The 0.750-inch deck thickness should make the heads plenty popular with power adder folks. The 450-pound springs and titanium retainers allowed us to easily run the 0.624-lift cam we had planned for the test engine.

For this test, AFR's LS3 heads were combined with a healthy cam profile designed to help extract power from the heads, and we chose Comp's 54-469-11 cam. Designed for a rec-port LS application, like the LS3, the cam is ground with 231 / 247 degrees of duration and 0.617 / 0.624-inch lift on a 113-degree lobe separation angle. This put the cam right near the limit of the available piston-to-valve clearance offered by the factory LS3 pistons.

To illustrate the power gains offered by new AFR heads and Comp cam, we first ran the aluminum 6.2L in stock trim on the engine dyno. For this test, the engine was configured with a Holley HP management system, a set of Hooker long tube headers and FAST manual throttle body. Equipped in this manner, the otherwise stock LS3 produced 495 hp at 5,800 rpm and 491 lb-ft at 4,700 rpm. Next, we replaced the stock parts with the components from AFR and Comp Cams, and before long, we were up and running again. After dialing in the air/fuel and timing with the new combination, the modified LS3 produced an impressive 591 hp at 6,600 rpm, and 532 lb-ft at 5,300 rpm. The AFR heads and Comp cam improved the power output of the crate motor by nearly 100 hp! Every bit as important was the fact that the impressive top-end power gains offered by the new head and cam combo came with no penalty in low-speed torque production. The combo out-performed the stock LS3 heads and cam even below 3,500 rpm. They may be late to the LS3 party, but it looks like the AFR heads are more than worth the wait.

LS3 Crate Upgrade-Stock vs AFR LS3 Heads/Cam

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The right head and cam package went a long way on the LS3 crate engine supplied by Gandrud Chevrolet. Run in stock trim with headers, no accessories and precise tuning using a Holley HP management system, the stock LS3 produced 495 hp at 5,800 rpm and 491 lb-ft of torque at 4,700 rpm. After installation of the new AFR LS3 heads and Comp 469 cam, the power numbers jumped to an amazing 591 hp and 532 lb-ft of torque. Torque production improved through the entire rev range, always a good sign, and remember, the AFR heads flowed enough to support over 750 hp in normally aspirated trim (on the right application).

See all 10 photos Supplied by Gandrud Chevrolet, the LS3 crate motor was a factory fresh, all-aluminum 6.2L. Note we replaced the stock DBW throttle body with a mechanical unit from Fast.

See all 10 photos The LS3 was run with Holley's HP engine management system with the manual throttle body, long-tube headers and Meziere electric water pump. In this configuration, the stock LS3 produced 495 hp and 491 lb-ft of torque.

See all 10 photos After establishing the baseline, out came the factory LS3 cam and in went our upgrade from Comp Cams. The 54-469-11 cam offers 231/247-degrees duration, 0.621 / 0.624-inch lift and a 113-degree lsa.

See all 10 photos Off came the stock LS3 heads to make way for something with a lot more power potential.Both heads were run with 0.053-inch thick Fel Pro MLS head gaskets and ARP head studs.

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See all 10 photos New from AFR, the LS3 heads featured -inch thick decks and a 450-pound dual-spring package to allow for high-lift and plenty of rpm potential. The 260-cc intake ports flowed 384 cfm, enough to support over 760 hp on the right application. Peak exhaust flow from the 95-cc port was equally impressive, checking in at 255 cfm.

See all 10 photos The 69-cc combustion chambers housed a 2.165/1.60 stainless valve combination. According to AFR, the heads were also compatible with 4.0-inch bore sizes, meaning they would be every bit at home on a 6.0L truck build as they would on an LS3.

See all 10 photos After removing the stock heads from our crate motor, we installed the AFR LS3 heads. The heads were bolted in place using the ARP hardware, then augmented with the factory (offset intake) rockers and hardened pushrods employed on the stock heads. On the dyno, this crate motor kicked out 591 hp at 6,600 rpm and 532 lb-ft of torque at 5,300 rpm. The cam and head added nearly 100 hp (peak to peak), and gains exceeded that number higher in the rev range.