De-Stoked LS7 - Short-Stroke Of Genius
Thomson Automotive's De-Stroked LS7 makes 615 HP and revs to 8,000 RPM.
From the March, 2012 issue of Vette
By Barry Kluczyk
Photography by Barry Kluczyk
First of all: Yes, the intake manifold shown in our lead photo is backwards. That's intentional, for reasons we'll go into in a moment. But the larger question should be, Why would anyone want to take a perfectly good, 427-cube LS7 and make it smaller?
1 The impetus for this project...
1 The impetus for this project was the need for a racing engine that delivered greater high-rpm performance and less low-rpm torque, enabling an 8,000-plus-rpm upper range and reducing tire spin out of corners.
After all, the LS7 was envisioned as a large-displacement sledgehammer for the racetrack. In a 3,200-pound Corvette Z06, it's rated at 505 horsepower and 470 lb-ft of torque, enabling some truly world-beating performance from a more or less stock vehicle. But for dedicated race cars that have been significantly lightened--even those only wearing a Corvette skin over a custom racing chassis--the parameters and optimal performance range for an engine change.
Such was the case of a racer who recently walked into Thomson Automotive, a cutting-edge LS-engine builder that is no stranger to this magazine. The racer was looking for an engine that would deliver higher-rpm power in a car that would weigh more than 1,000 pounds less than a Z06. Along with road-course capability, the car was also intended for flat-out, top-speed contests such as the Texas Mile and the Silver State Classic Challenge.
2 Thomson Automotive started...
2 Thomson Automotive started with an LS7 block (PN 19213580) from GM Performance Parts. It’s a 319-T5 aluminum casting with Siamesed bores and 4.125-in cylinder lines that are deck-plate-honed from the factory. The big-bore block was crucial in building the high-revving, over-square engine combination.
3 For the high crankshaft...
3 For the high crankshaft speed this engine will see, extra care was taken in the preparation of the block, including careful align-honing of the main bearing bores. This procedure helps ensure optimal main bearing alignment and truer bore diameters for smoother, more balanced high-rpm performance. That reduces wear the crank and bearings.
4 A signature block modification...
4 A signature block modification for Thomson is the addition of piston oil squirters (arrow), which are inserted at the bottom of the cylinders. As in the factory LSA and LS9 engines, the squirters drench the bottoms of the pistons with oil to reduce temperature and friction—both major concerns in an engine running to 8,000 rpm and higher.
"He wanted an engine that made around 600 horses and spun to 8,000 rpm or more, to provide long, sustained horsepower for high top-end speeds," says Brian Thomson, the shop's founder. "We've got an LS7 package that makes 630 naturally aspirated horsepower at 7,000 rpm and almost 600 lb-ft of torque, but that wasn't going to fit the bill for this project."
Indeed. Besides the stock- displacement LS7 not providing the desired rev range, the nearly 600 lb-ft of torque--coming on strong at low rpm--was more than the lightweight car needed, as it would make tire spin problematic. And while 600 hp could be coaxed out of the small-displacement LS3 engine, it couldn't be done as easily, and there was still the issue of revving it safely beyond 8,000 rpm.
"The great airflow of the LS7 heads is what we really wanted for high-rpm horsepower, but we just didn't need all the torque," says Thomson. "It became clear that de-stroking the LS7 would not only help trim some of the low-end torque, but help the engine spin to 8,000 rpm, too."
5 Nestled in the block is...
5 Nestled in the block is the linchpin of the engine’s over-square dimensions and high-rpm capability: A 3.625-in Callies Dragonslayer forged crank. Using it with the LS7 block’s 4.125-in bores takes the LS7’s displacement down to 388 ci. The short stroke keeps the piston speeds lower for reduced friction at high rpm.
6 The rest of the reciprocating...
6 The rest of the reciprocating assembly includes a set of domed Diamond pistons, which help push up the compression ratio to nearly 12.3:1. The stock LS7 compression ratio is 11.0:1, and the increase here is one contributor to the smaller-displacement engine’s ability to pump out more peak power.
7 The forged H-beam Oliver...
7 The forged H-beam Oliver rods measure 6.125-in long, which actually makes them a little longer than stock LS7 units. That’s an optimal trait for a high-rpm engine because it reduces the connecting-rod angle, which reduces friction and puts less stress on the thrust surface of the piston during combustion.
A few taps on the calculator brought the configuration into focus: By cutting the LS7's stroke from 4.000 to 3.625 inches and maintaining the stock 4.125-inch bores, the displacement would be 388 cubic inches (6.35 liters). It would also make this short-stroke LS7 an "over-square" engine, with a 1.14 bore/stroke ratio. A "square" engine has equal values for the bore and stroke dimensions, for a 1:1 ratio.
8 The stretch method is...
8 The stretch method is the most accurate way of fastening the connecting-rod bolts. That’s because the bolt acts very much like a spring, with different clamp loads as it stretches ever so slightly while tightening. Tightening to a traditional torque yield merely delivers the estimated twisting force required to achieve the designed clamp load, but the stretch method ensures the bolt is tightened to its optimal clamping strength.
An over-square engine configuration is ideal for high-rpm horsepower--it's how Formula 1 engines rev to seemingly impossible plateaus of 13,000-plus rpm--because the shorter stroke minimizes the maximum piston speed. That reduces power-robbing friction and heat, while also reducing stress on the crankshaft. And because these traits favor high-rpm horsepower, an over-square engine typically makes its peak torque higher in the rpm band, too, which is just what this racing-engine project called for.
"The short-stroke concept is by no means a new one, but it's not typically associated with LS engines, which are known for making big power down low," says Thomson. "With the right camshaft and a few other key components, the 600-horsepower level shouldn't be a problem--even when taking nearly 40 cubic inches out of the displacement."
9 The cam grind—0.670/0.671-in...
9 The cam grind—0.670/0.671-in lift and 293/301-deg duration—complements the short-stroke combo’s power delivery. A wide, 115-deg LSA minimizes overlap and helps push peak power and torque higher in the rpm band. It’s also a solid-roller type, which is preferred for engines that will spend most of their time at high rpm and wide-open throttle.
10 Thomson installed a set...
10 Thomson installed a set of Jesel solid-roller lifters instead of the more commonly used hydraulic units. The latter type can be a drawback in engines like this one, as the hydraulic mechanism can become unstable at high engine speeds. Undesirable flex, deflection, and insufficient valve-spring control at high rpm can never be fully tamed, but there is always more-precise control with a solid lifter.
11 When it comes to racing...
11 When it comes to racing oil pumps, the multi-stage dry-sump systems from Dailey Engineering are becoming the go-to choice for many LS engine builders. The scavenge sections use two-lobe Roots-style rotors—just like a positive-displacement supercharger—for excellent volumetric capacity and greater crankcase vacuum. The five-stage system used on this engine includes four scavenge stages for the pan and one for pressure.
The dyno test would prove Thomson right on his performance prognostication, but more on that in a moment. For now, let's take a look at some of the supporting components in this unique combination:
The factory reciprocating parts were replaced with all-new, all-forged components, including Oliver 6.125-inch rods hung on a Callies Dragonslayer crankshaft. The rods are actually longer than the stock 6.067-inch LS7 units, which is a good thing in a high-rpm engine because it reduces the rod ratio.
A solid-roller Bullet camshaft was used, spec'd with 0.670/0.671-inch lift and 293/301-degrees duration with 1.7-ratio Jesel rockers. The lobe-separation angle (LSA) is pretty wide at 115 degrees.
The factory dry-sump oiling system was replaced with an exquisite Dailey Engineering five-stage racing dry-sump system.
12 The crank-driven oil-pump...
12 The crank-driven oil-pump system uses a billet 6061 T6 aluminum pan. The pump bolts directly to the pan, and the scavenge lines are internally machined into the pan itself; this eliminates the need for AN fittings and lines between the pump and pan. Be warned, though: These Dailey pump systems are works of art, and they’re not cheap. This one cost about $3,800.
13 The airflow capability...
13 The airflow capability of an out-of-the-box LS7 cylinder head is hard to beat, even with aftermarket ported units. The fist-sized, straight-through ports lead to big, 2.200/1.610-in valves held at a 12-degree angle (versus the 15-deg angle of most other LS engines). The intake ports measure 270cc, and the as-cast combustion chambers are 70cc.
14 From the factory, the...
14 From the factory, the LS7 head comes with titanium intake and sodium-filled exhaust valves. To optimize the lightweight advantage at high rpm, Thomson swapped the sodium valves for Del West titanium units.
The stock, as-cast LS7 were machined to accept the Jesel rockers for unflappable ultra-high-rpm performance, and the valves were swapped for Del West Engineering titanium parts on both sides.
A new FAST 102mm intake manifold for LS7 heads was used to maximize high-rpm airflow. It was used with FAST fuel rails and a stock LS7 90mm throttle body.
As for the reason the intake is turned around on the engine, it's to match the unique air-intake system the engine's owner developed for the car. The manifold bolts right up in either direction, so its installation in the reverse position was not an issue.
For an engine builder who is used to building bigger-inch LS engines that make boatloads of torque at low rpm, the dyno details of this short-stroke LS7 must have seemed almost alien, but they nonetheless hit the mark for the project. Here's the bottom line: 615 hp at a lofty 7,200 rpm and 506 lb-ft of torque at 5,800 rpm.
||HP @ RPM
||TQ @ RPM
|Thomson Automotive 6.35L LS7 V-8
||615 @ 7,200
||506 @ 5,800
|Lamborghini Gallado 5.2L V-10
||562 @ 8,000
||398 @ 6,500
|Ferrari 599 6.0L V-12
||612 @ 7,600
||448 @ 5,600
|Aston Martin V-12 Vantage 6.0L
||510 @ 6,500
||420 @ 5,7050
With the excellent airflow characteristics on the smaller-displacement engine, it was up to the camshaft to deliver the horsepower. It did, with the wide, 115-degree LSA working with the mild lift and duration specifications to optimize performance to the outer reaches of the rpm band. A wider LSA enables the intake valves to open and close later, and the exhaust valves to open earlier. That reduces overlap, fosters a broader, flatter torque curve, and enables the power to hang on at higher rpm.
15 The heads were machined...
15 The heads were machined to accept a Jesel pedestal-mount rocker-arm system. The 1.7-ratio rockers are the lightweight Pro-J2K models, and the entire system is designed to minimize flex and deflection at high rpm. They work with a set of PAC Racing Springs dual-coil springs that have an installed load of about 220 pounds and an open load of 625 pounds.
16 Rather than a stock LS7...
16 Rather than a stock LS7 intake, Thomson used a FAST LSXR 102mm piece. On an otherwise stock engine, it’s worth a few extra hp, but on this engine, the longer runners should provide a bigger boost as the revs climb. Thomson figured it would be worth upwards of 25 hp.
17 A stock LS7 90mm throttle...
17 A stock LS7 90mm throttle body was used with the FAST intake, although it could have accepted a 102mm unit. Using the stock unit probably gave the engine a bit more torque down low, but it also made it a lot easier when it came to tuning the engine with a GM E67 engine-control module.
A look at the accompanying dyno chart bears this out: The torque curve isn't so much a curve as it is a gently sloping arc. It takes nearly a 2,500-rpm sweep to go from 416 lb-ft at 3,500 to the peak 506 lb-ft at 5,800 rpm--and even after that, the torque doesn't drop off a cliff. It simply recedes at the same basic rate. In fact, the torque is essentially the same at 6,100 rpm as it is at 5,100 rpm. It's smooth, predictable, and controllable in a lightweight race car, especially with the peak torque not occurring until nearly 6,000 rpm.
And on top of it all, this shorter-stroke, smaller-displacement engine still made 35 lb-ft more than a stock LS7.
As for the power, it comfortably exceeded the 600-horse target and did up at the far end of the tachometer, just as designed. And like the torque values, the power came up smoothly and predictably. In fact, the wide LSA of the camshaft also gave this racing engine a decidedly docile idle quality. We could totally see this combination working in a street car, minus the racing-spec oiling system, making it more than a contender for some of Europe's high-winding exotics.
Just for kicks, we looked up the specs on some of those exotics, and here's how this engine compares with them:
If nothing else, this comparison validates the viability of the cam-in-block, overhead-valve LS architecture in producing high-rpm horsepower. It bettered the lot of those other engines, and did it with two or four fewer pistons. It's enough to make you chant "U-S-A! U-S-A!"
18 FAST also supplied the...
18 FAST also supplied the red-anodized aluminum fuel rails, while the injectors are stock LS7 squirters rated at 35 lbs/hr. They’re plenty sufficient to feed the engine at 8,000 rpm and beyond.
19 Apart from some accessories...
19 Apart from some accessories to be added on the dyno, such as the water pump and a set of LS7 exhaust manifolds, the basic engine assembly is complete—and looks admittedly weird with the reversed intake manifold. Additional details include an ATI damper and a set of tall valve covers to clear those tall Jesel rocker arms.
20 On the dyno, the short-stroke...
20 On the dyno, the short-stroke LS7 didn’t disappoint. It revved easily past 8,000 rpm and delivered its peak 615 hp at 7,200 rpm and 506 lb-ft of torque at 5,800 rpm. And despite the emphasis on high-rpm performance, the engine nonetheless made more torque than a stock LS7. Sure, it’s going in a race car, but it sounds like it would make a pretty good street engine as well. vette
"It really is a sweet, high-rpm engine," says Thomson. "We figured out that the customer's car will theoretically do about 240 mph with it--if he can find enough room to wind it out."
And does Thomson recommend a similar combination for those whose street-driving plans are a little less ambitious?
"With the extra weight in a production-based Corvette, you'll definitely notice the lack of low-end torque," he says. "But if you really like the feeling of ultra-high-rpm power, it works great, and the driveability is excellent."
We'll point out that while Thomson suggests the engine is low on torque, the stock 6.2-liter LS3 makes 423 peak lb-ft at 4,600 rpm and his 6.4-liter engine made 474 lb-ft at the same rpm level. It also makes 400 lb-ft by about 3,000 rpm, which is hardly chump change in our book. So, while it may not have the sheer grunt of one of Thomson's supercharged engines, it's hardly lacking for pound-feet. And you've never heard anything as melodious as an LS engine pushing beyond 8,000 rpm.
Defying convention has never been a worry for Thomson Automotive, and this high-winding, short-stroke LS7 is the perfect example, as it proves the exception to that old adage about there being no replacement for displacement.
Maybe it should be changed to, "There's no rpm like high rpm."