Without question, the LS7 is the top dog of the naturally aspirated world of LS engine horsepower. It’s also the most expensive–by a wide margin. As with most engines, the key to the LS7’s exceptional power profile is the air flow efficiency of its cylinder heads. The same can be said of the LS3. Although the flow characteristics of LS3 heads don’t rival those of their LS7 counterparts, they don’t miss by all that much, at significantly lower cost. LS3 cylinder blocks are also about half the price of their LS7 brethren, making an LS3 tough to beat on a horsepower per dollar basis.
At 376 ci (6.2L) an LS3 gives up 51 ci to an LS7. And since there truly is no substitute for cubic inches in the world of naturally aspirated engines, an LS3’s journey towards horsepower parity with an LS7 begins with a crankshaft that increases stroke from the stock dimension of 3.622 to 4.00 inches. Combined with a bore diameter of 4.070 inches (a .005-inch overbore) a 4.00-inch stroke crank brings LS3 displacement to 416 or 417 ci, depending on how you round-off the numbers.
Depending on the type of engine control module that will be calling the shots, an LS3 crankshaft must be fitted with either a 24- or 58-tooth reluctor. With first generation LS engines, commonly referenced as Gen 3 small-blocks, Captain Semiconductor directs his troop of sensors and controllers through an LS1 PCM (Powertrain Control Module) which issues orders to both the engine and (automatic) transmission, and expects to see pulses generated by a 24-tooth reluctor. Later model Gen 4 engines (LS2, LS3, LS7 and truck-based 4.8, 5.3, 6.0 and 6.2L engines use E40, E38 or E67 ECMs) and separate transmission controllers which Captain Semiconductor has configured to receive pulses from a 58-tooth reluctor. Some aftermarket engine controllers require the use of a reluctor with a specific tooth count, others can be programmed as required to accommodate the type of reluctor installed on the crank. Consequently, before tightening the main cap bolts, it’s advisable to verify that the crank reluctor has the number of teeth that Captain Semiconductor is expecting to see.
In selecting the crankshaft, pistons and rods for this particular engine, the (engine) owner worked with Brian Nutter of Wiseco Pistons. Since K1 is a sister company, Brian put together a rotating assembly kit consisting of a 4-inch stroke K1 forged crank (with a 24-tooth reluctor, required because the engine was slated for installation in a Corvette equipped with an LS1 PCM) K1 forged 6.125-inch connecting rods and Wiseco custom-forged pistons.
When building anything other than a stock or mildly enhanced LS3, high quality components are essential. According to engine builder Garry Grimes, “The power output of these LS engines is truly amazing. I work on them just about every day and I still wonder how they do it. We’ve built a naturally aspirated version that puts out over 1000 horsepower—well over 2 horsepower per cubic inch. That’s not bad for an ‘outdated’ pushrod engine.”
The engine shown here was destined for a street-driven Corvette and produced somewhat less than 1,000 hp. That was by design. Grimes states, “The customer for this particular engine had a few ideas of his own about some of the internal components. He selected a lot of the parts based on his prior experience and the level of performance he wanted to achieve. He was more concerned with having reasonable low-speed and mid-range torque than with top-end horsepower.”
Assembling an LS3 short-block is a straightforward affair providing all the machine work is properly done. Typically a bare LS3/L92 block (GM P/N 12623967) requires nothing more than standard prep work. Stock bore diameter is 4.065 inches, so it doesn’t take much machining to achieve a 4.070-inch finished bore. These blocks are supplied with powdered metal main caps. With some higher horsepower builds, it’s advisable to switch to steel caps, but that’s not considered a necessity at sub-600-hp levels.
For most street and street/strip engines, a production-style wet sump oiling system is entirely adequate. Oil pan choice is generally dependent on the vehicle into which the engine is to be installed and stroke dimension. GM offers a good variety of original equipment oil pans, but for some installations, a custom pan or pan modifications may be required.
On the top side, on a dollar per horsepower basis, it’s tough to beat GM’s LS3 cylinder heads, which are available bare (P/N 12629051) assembled (12629064) or CNC-ported and assembled (88958758). Bare castings and fully assembled heads are also available from a number of aftermarket companies. Grimes has also found that it’s tough to beat the stock .051-inch thick multi-layer head gaskets.
Conversely, it’s pretty easy to beat the stock GM head bolts. Like most of the original equipment bolts used on LS series engines, the head bolts are of the “torque to yield” persuasion. Although this design is advantageous for high-volume engine production, it provides neither confidence nor convenience in high performance use. Once a torque-to-yield bolt has reached its yield point, it can’t “unyield” and yield again without losing a considerable amount of strength. With larger diameter bolts (like head bolts) it also requires three men and a lot of beer to tighten firmly enough to reach the yield point. Consequently, Grimes advises using high strength conventional bolts from companies like ARP.
After the heads are bolted to the block and fears of yielding head bolts have been eliminated, an intake manifold is the next component to be installed. Like all LS series engines, the LS3 is fitted with a composite (plastic) intake manifold. The stock version does a surprisingly efficient job of routing air to the heads. A high performance manifold from FAST does an even better one. FAST manifolds also offer the ability to install 92mm or 102mm throttle bodies which obviously provide high air flow capacity than a stock 90mm throttle body.
Another vital component in any performance-oriented bill of materials is the camshaft. In this instance, the engine’s owner had specified a Crower hydraulic roller with intake and exhaust duration (at .050-inch lift) of 236 and 248 degrees respectively for intake and exhaust. The cam was ground with 114 degrees of lobe separation.
Whether you’re interested in a race-only or street/strip engine, as the accompanying dyno chart shows, a properly built LS3 is tough to beat.
RPM Torque Horse
Ft. Lbs. Power
3600 444.3 301.7
3700 437.9 308.5
3800 438.5 317.3
3900 441.2 327.7
4000 447.7 341.0
4100 454.9 355.2
4200 464.2 371.3
4300 474.3 388.3
4400 490.5 414.3
4500 491.7 421.3
4600 503.8 436.8
4700 518.2 463.8
4800 525.8 480.6
4900 527.7 492.3
5000 528.4 503.1
5100 529.3 514.0
5200 528.7 523.4
5300 527.1 531.9
5400 521.7 536.4
5500 517.1 541.5
5600 517.6 551.8
5700 512.0 555.6
5800 520.2 574.4
5900 528.0 593.1
6000 521.9 596.2
6100 512.8 595.6
6200 504.5 595.6
6300 504.5 605.1
6400 499.0 608.1
6500 506.3 626.6
6600 503.1 632.2
6700 498.0 635.4
The dyno tells all and what it says about LS3 engines is that they produce a lot of horsepower. The 417-ci street/strip engine produced 635 hp.
New LS3 bare blocks are reasonably priced and available through GM Performance parts dealers. Alternative sources are used engines and 6.2L iron blocks which are used in late-model GM trucks and SUVs.
Although the stock LS3 intake manifold provides excellent air flow characteristics, FAST’s composite manifold is the preferred model for maximum power. FAST manifolds are available with either 92mm or 102mm throttle body openings.
All LS series engines are fitted with crankshaft-mounted reluctor rings to provide positional data to the engine controller. Gen 3 engines use 24-tooth reluctors as shown. Gen 4 engines are fitted with 58-tooth reluctors.
LS3 heads are distinguished by their rectangular intake posts and “D”-shaped exhaust ports. Their flow capacity is considerably greater than that of “cathedral port” LS1/LS2 heads. However, they can only be used on blocks having a bore diameter of 4.00 inches or larger.
There’s an almost limitless number of cam profiles available for LS series engines. Hydraulic roller cams are most popular for street/strip applications and with the right lifters and valve springs, 7,000 rpm is achievable.
Text and Photos by Dave Emanuel