Blown Z: 388 LSX Rebuild Adds Boost, Lowers Compression Ratio
In a quest to reach 1,500-plus horsepower this season, the Blown Z team is stepping up to a larger ProCharger for its 388ci LSX engine. That prompted the customary winter refresh on the engine to include lowering the compression ratio with new JE pistons and also upgrading a few supporting components on the car.
The Blown Z project is based on a 2002 Camaro prepped for NMCA 275 Drag Radial competition. Other possible outings include the LSX Shootout and Outlaw 8.5 races. Last year the car posted a top quarter-mile time of 8.24 seconds at 168 mph. Chassis dyno tests revealed a best of 1,100 horsepower to the ground, but a pull at the end of the season witnessed a cylinder head lifting slightly. The engine was removed and sent to L&R Engines in Santa Fe Springs, California, for disassembly and inspection.
“All the critical parts looked normal,” notes L&R’s Derek Ranney of the teardown. “The bearings were in great shape, and all the clearances were normal.”
With no major problems to correct, the team could focus on ensuring that the engine will reap the benefits of switching from the ProCharger F-1R to the company’s new F-1X model. The boost rating is the same, but the larger F-1X spins faster and flows more air while remaining legal for the 275 Drag Radial class. Results of the last dyno pull indicated the tuneup was right on the edge last season, and obviously a larger supercharger would be less forgiving. So, the Blown Z crew opted to lower the compression ratio from 11.3:1 to around 9.7:1 on the rebuild. That required ordering new JE asymmetrical flat-top pistons, but all other engine components return to action — save for new bearings, gaskets, rings and valve springs.
As evident in the accompanying photos, the assembly process and engine installation into the Camaro were routine. The LSX block was hot-tanked, checked and honed to accept the new JE pistons and Total Seal rings. New Clevite bearings were used for the Lunati crank and GRP rods while the Comp Cams camshaft returned to its original roller bearings.
You need to spin the F-1X faster than the F-1R to make them effective. — Steve Morris, SME
The Trick Flow heads were cleaned and reassembled using Ferrea valves, Manley retainers, Comp springs and Jesel rockers. Additional long-block assembly included ARP hardware, SCE head gasket, Jesel belt drive, ATI damper, Fel-Pro gaskets and Dailey Engineering oil pan and dry sump pump. After the long block was dropped into the Camaro’s engine bay, it was fitted with a Wilson-modified Holley manifold, FAST injectors, MSD ignition, Fragola plumbing and Jet-Hot-coated Kooks headers before the new ProCharger was installed on the Chris Alston Chassisworks gear drive.
The F-1X is designed to work with naturally aspirated engines producing around 375 to 550 horsepower, with potential making more than 1,800 horsepower. ProCharger says the maximum airflow is 2,000 cfm, which is up from the 1,700 cfm rating on the F-1R. Dimensionally, the F-1X boasts a 5-inch inlet diameter, up from 4 inches on the F-1R. The 3.5-inch outlet is also a half-inch larger than the older model. The F-1X is rated to spin at 72,000 rpm, which is up from the F-1R’s 68,000 rpm rating.
Wants to spin faster
“I liken the F-1X to a 2-stroke motorcycle engine,” says Steve Morris of Steve Morris Engines, which specializes in a wide variety of ProCharger applications. “It really comes on like a light switch. You need to spin the F-1X faster than the F-1R to make them effective, but they’re definitely good for 300 to 400 more horsepower than the F-1R.”
Anytime you can use flat-top pistons, you’re going to have the best efficiency, period. — Nick DiBlasi, JE Pistons
The F-1X will deliver up to 38 psi of boost, features self-contained oiling and relies on an internal 5.4:1 step-up ratio. Morris says there is no downside to the F-1X, with the exception of misguided expectations.
“Some have a tendency to think that because they bolt an F-1X in place of an F-1R their car will instantly go faster,” adds Morris. “Typically, it doesn’t happen because they put it on spinning the same speed as the F-1R. They need to be spun fast and have good frontal air.”
Stepping up the blower speed
With the Chassisworks CDS gear-drive system, drive ratios are easily changed. The Blown Z team stepped up from a 1:1.65 ratio to a 1:1.70 ratio. Combined with the ProCharger’s internal 1:5.4 step-up ratio, the impeller rpm should increase 2,400 rpm at 8,500 engine rpm.
Since we ran between 20 to 22 pounds of boost for most of last year, the expectations of putting 24 to 28 psi into the manifold gave cause for rethinking the engine’s compression ratio.
“The theory is, if you can build more boost you don’t need the compression ratio,” explains Morris. “Lowering the compression ratio really makes the tuning window broader. That means, you can miss the tune with too much timing or a little too lean, and it doesn’t hurt the motor. As you pour on more compression ratio, it gets much more sensitive to the tuneup.”
An engine’s stated compression ratio generally refers to the “static” compression ratio; that is, a simple math calculation that compares the volume of air in the cylinder when the piston is at bottom dead center (BDC) and then again at top dead center (TDC). Plugging all the numbers from Blown Z’s rebuild, the calculator revealed a final 9.7:1 compression ratio. A much more complicated concept is “dynamic” compression ratio. That considers other engine thermodynamic factors, including but not limited to atmospheric conditions, boost and intake valve timing. A high-performance engine usually has an aggressive camshaft that keeps the intake valve open past BDC on the start of the compression stroke. But if the valve is open, then no air is being compressed by the piston. With boosted engines, the intake cam timing doesn’t have to be as aggressive as with a naturally aspirated engine.
How much compression?
Overall, the goal is to manage the pressure inside the cylinder that results from boost and the piston compressing the air. If the pressure is too high, harmful detonation can occur, especially if the fuel’s octane rating isn’t high enough for the application. Blown Z uses VP Racing Fuel C16 blend, which is rated at 116 octane and designed for boosted engines or naturally aspirated with up to 17:1 compression ratio.
“If I have a limited blower, like the F-1R, I’ll put more compression in,” says Morris. “If I’m unlimited on blower and large cubic inches, we’ll stick around 9:1 or 10:1. Also, we’ll be a little lower on blow-through engines than with EFI, because we can control the tune much better between cylinders [with EFI].”
The pistons in last year’s engine’s had a dome design to squeeze the air-fuel into the 63cc combustion chambers. With a change in compression strategy, the Blown Z team opted for JE’s new asymmetrical flat-top pistons with small valve-relief pockets. These pistons will not only lower the compression ratio, but reduce internal friction and contribute to more efficient combustion.
“Anytime you can use flat-top pistons, you’re going to have the best efficiency, period,” explains JE engineer Nick DiBlasi. “Because you’re allowing the combustion chamber chamber to do all the work. You’re not hiding cavities anywhere or going over a dome area where it’s going to get under the quench.”
Less friction with asymmetrical pistons
The asymmetrical piston design takes advantage of the V8’s configuration that inherently places different loads on each side of the piston, thereby allowing for different-sized skirts.
“Only the major thrust side of the piston actually takes significant load,” says DiBlasi. “There’s no point in having both sides the same size. The minor thrust side is virtually along for the ride and sees no load.”With a smaller skirt, the piston weight is reduced. Another advantage of the asymmetrical design is a smaller pin can be used, allowing engineers to stiffen up the overall strength.
“So, you have less friction on one side, a lighter part that’s very center balanced and stronger in the end,” adds DiBlasi. “If it’s lighter, you bring the rotating mass down and it’s a lot easier on bearings.
Other features of JE piston include both vertical and lateral gas ports to help maintain combustion-ring pressure and a tough moly-style skirt coating that reduces friction and allows for a slightly tighter piston-to-wall clearance.
“We do the gas ports in conjunction with each other because sometimes the top ports can get clogged,” says DiBlasi.With the rebuilt engine in place, a few other upgrades were added to help with consistency and performance. The Blown Z team took advantage of the CDS accessory drive options to mount a belt-driven Aeromotive fuel pump. Last year’s car had a small radiator offset to the right to allow for an air inlet on the left side of the grille opening. With the radiator gone, Blown Z’s team reshaped the inlet and mounted it directly in the center of the grille opening. Engine cooling is now achieved with a trunk-mounted Chiseled Performance cooling system with a Rule in-take pump providing coolant-line pressure through the Fragola plumbing. The 3.5-inch Vibrant intercooler tubing that supports the ProCharger blowoff valve routes from the supercharger to the intercooler and back to the intake manifold needed no changes for the new season.
Controlling the engine’s fuel and spark, respectively, will be a FAST XFI 2.0 and MSD Power Grid. The lower compression will help the team build a strong baseline and develop more consistent tuning strategies as the season evolves and track conditions change. During the first test outing, the car ran an early lifting 8.05 @ 154 while posting a strong 60-foot time of 1.26 seconds. After the run the team discovered a loose converter. Repairs have been made, and the team is expecting to crack the 8-second barrier next time out and have set a season goal of 7.70 at over 180 mph.