How do you build a better mousetrap? The question was one asked of the Chevy engineers when it came time to upgrade the legendary small-block. The result of their hard work ultimately (after the LT1) was the Gen III and Gen IV engine family-more commonly called the LS small-block.
Original small-block owners might drool over the all-aluminum construction of many of the modern LS motors (iron block and head versions were also produced), but a large portion of the power production offered by the modern Mouse can be attributed to the impressive cylinder heads. Even the original LS1 cathedral-port heads offered flow rates that dwarfed those of the original small-block heads. It would take a fully ported set of original big-valve Fuelie heads to equal the flow of even stock cathedral-port heads. Ported versions of the LS1 heads pushed intake flow figures near 300 cfm. Never mind the small-block-these were factory (rectangular-port) big-block head flow numbers!
Not content to rest on their laurels, the GM engineers stepped things up even further when they introduced the LS3. Like the performance big-blocks of yesteryear, the new LS3 heads offered rectangular ports that flowed serious numbers, more even than ported cathedral-port heads. Flowing over 310 cfm right out of the box, the performance potential can be likened to installing big-block heads on your small-block.
As impressive as the LS3 heads were, GM engineers had something even wilder up their sleeve when the introduced the LS7. Designed to power the special all-aluminum 7.0-liter in the ZO6 Corvette, the LS7 was yet another evolutionary step up the performance ladder. Best of all, the LS7 heads bolted right onto any LS motor, though the valve size and spacing required use on a larger bore block. The valve position also required LS7-specific rockers, but the upside was that the LS7 rockers offered an increase in ratio from the 1.7:1 to 1.8:1.
To achieve the additional flow offered by the LS7 heads, the port location was raised relative to the LS3, meaning the LS7 heads also required a dedicated intake manifold. The differences between the LS3 and LS7 were similar to those of the original LS1 and the LS3. The question now is are changes required to install LS7 heads on an LS3 worth the effort? For some, just having the ability to brag about having LS7 components makes the swap worthwhile, but what about the power? Are the LS7 heads really the bee's knees, the cat's meow, or the schizzle, my nizzle?
Before running what amounts to a simple head test, we had to assemble a suitable test engine. Since we planned to run a set of ported LS7 heads (go big or go home), we needed something more than a stock LS3. Recognizing the need for a big-bore motor, we stepped up in a big way and built a stroker LS combination by combining a Darton-sleeved LS6 block with a Lunati stroker crank and K1 connecting rods. The Darton MID sleeve system allowed us to bore the motor to a massive 4.185 inches. When combined with the 4.25-inch Lunati steel crank, the result was a finished displacement of 468 cubic inches-more than enough to properly test the merits of even ported LS7 heads.
Using the 70cc chambers offered by both the LS3 and LS7 heads, the 468 featured a static compression ratio of 12.25:1. Comp Cams supplied a suitable hydraulic roller cam for our high-compression stroker. The 305LRR HR15 cam featured 0.624-inch lift (both intake and exhaust), a 255/271 duration (at 0.050), and a 115-degree lobe separation angle. Comp Cams also supplied the lifters, double-roller timing chain, and hardened pushrods for the application.
To establish a baseline, we equipped the stroker with a set of as-cast LS3 heads from GM Performance Parts. The only upgrade to the stock heads was to replace the factory valve springs with 26920 springs from Comp Cams. The spring upgrade was necessary to provide sufficient spring pressure and coil-bind clearance for use with the high-lift Comp cam. The LS3 heads were run with a FAST LSXR intake, 102mm throttle body and 75-psi injectors. We also relied on the FAST XFI/XIM management system to dial in the air/fuel and timing curves.
Running through a set of 1-7/8-inch headers from American Racing, the LS3-headed 468 stroker produced 660 hp at 6,400 rpm and 628 lb-ft at 5,100 rpm. The long-runner LSXR intake offered an impressive torque curve, with torque production exceeding 600 lb-ft from 4,100 rpm to 5,600 rpm. The production (as-cast) LS3 heads are capable of supporting over 650 hp-big numbers from stock heads. Now it was time to test the LS7 combination.
Rather than run a set of stock LS7 heads, we decided to take Texas Speed up on its offer to run its new 285cc, CNC-ported LS7 castings. A sizable step up from the already impressive stock castings, the CNC-ported LS7 heads from Texas Speed featured 285cc intake ports, 70cc combustion chambers, and a 2.25/1.61 valve combination. Texas Speed also set up the heads with a valve-spring package to allow for our 0.660-lift cam (the 1.8-ratio rockers on the LS7 heads increased the lift).
The intake ports of the 12-degree LS7 heads from Texas Speed flowed over 400 cfm, while exhaust-ports flow exceeded 245 cfm. The flow relationship between the intake and exhaust (just over 60 percent) is why we selected a cam profile that greatly favored the exhaust side. The flow numbers suggest that these Texas Speed LS7 heads can support well over 800 hp on the right combination, making them more than sufficient for our 468 stroker.
Off came the LS3 heads and on went the Texas Speed CNC LS7 heads. For both the LS3 and LS7 heads, we relied on a set of MLS Cometic head gaskets and ARP head studs. The LS7 heads were combined with a set of stock LS7 rockers and the FAST LSXR designed for the LS7 port configuration. After dialing in the air/fuel and timing curves to match the LS3 combination, we were rewarded with peak numbers of 737 hp and 667 lb-ft of torque. Not only did the LS7 head swap increase peak power, but the combination broadened the torque curve. Torque production with the LS7 heads exceeded 600 lb-ft from 4,100 to 6,400 rpm, a gain of 800 rpm over the LS3 heads. The ported LS7 heads offered impressive power, but we figured we were just scratching the surface of what they had to offer so we decided to take things to the next level with an intake swap.
This meant replacing the EFI intake was a single-plane carbureted intake from Mast Motorsports. Significantly more elaborate than your typical off-the-shelf carbureted intake, the single-plane from Mast Motorsports featured a two-piece design that facilitated porting. The CNC-ported LS7 intake (available for LS1 and LS3 applications as well) was also flanged to accept a 4500 Dominator carbutetor (4150 flanges also available). We selected the intake that was also plumbed for EFI use but ran it in carbureted form.
The Mast intake was topped off by a Holley Ultra Dominator 1050 carb that provided more than enough flow to feed our hungry stroker. After minor jetting, we were rewarded with peak numbers of 768 hp and 658 lb-ft. Compared to the long-runner FAST intake, the short-runner, single-plane manifold was up on peak power but down on peak torque. Hood clearance would obviously be an issue, but for an all-out drag race motor, the Mast intake has much to offer.
After the intake swap, you'd think we had enough, but we had one more ace up our sleeve. Sitting off in a corner at Westech looking lonely and unwanted was an NOS Cheater plate nitrous system that (lo and behold) was flanged for a Dominator.
It took all of five seconds to decide that what our LS7-headed stroker needed more than anything was a 200hp shot of nitrous. When it comes to power, nothing is easier than adding nitrous, right? After a quick hook-up, we warmed the bottle using the Westech bottle heater (a thermostatically controlled water tank). While our nitrous bottle pressure inched its way over the magic 900-psi mark, we installed jetting to provide an additional 200 hp. After retarding the timing by 6 degrees, we purged the system and let 'er rip! When the dust had settled, the dyno indicated a peak power output of 1,066 hp and 950 lb-ft. The spike was impressive, but the most of the pull numbers were just south of 1,000 hp. There was more power available from the NOS nitrous kit, as was there with additional tuning, but at this point, we were satisfied with the Power of 7-LS7!