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Stock vs Kenne Bell 102-MM Throttle Body (17psi) (Horsepower)
Stock 90-mm LS3 TB: 755 hp @ 6,300 rpm
KB 102-mm TB: 789 hp @ 6,400 rpm
Largest Gain: 34 hp 6,400 rpm
Running the throttle-body test at a higher boost level (17.3) resulted in a significant rise in power. Run at this elevated power level, the throttle-body upgrade was worth 34 hp and increased boost by 1 psi.
Test 8: Custom Dual-Plenum Adjustable-Runner Intake on a 468 Stroker
Long before the introduction of the FAST Adjustable LS3 or Edelbrock Cross-Ram intake, enthusiasts were tinkering with custom intake designs. I designed this adjustable intake for LS3-headed applications in 2008 to illustrate changes in the power curve. In addition to the dual-plenum design (with removable plenum connection), I was able to quickly adjust the runner lengths to optimize power production at different engine speeds.
The runner length acts as a tuning device to tailor the shape of the power curve. Longer runners optimize power production lower in the rev range than shorter runners. The downside to any given length is that there are trade-offs at the other end of the rev range. The additional low- and mid-range torque offered by longer runners is offset by a loss in high-RPM power. The opposite is true of short runners because they give up low- and mid-range torque for optimization at high RPM. The idea is to tune the combination for the desired use.
The test engine was a 468 stroker that was made possible by combining an LS6 block with Darton sleeves to allow for a 4.185-inch bore.
To test the custom intake, I needed an engine capable of using the massive flow capability of the 2.25-inch runners. In short, I needed something more than either a stock LS3 or LS7. Knowing this, I assembled a big-bore, LS stroker engine by combining a Darton-sleeved LS6 block with a Lunati stroker crank and K1 connecting rods. The Darton MID sleeve system provided the necessary room to allow me to bore the block out to 4.185 inches.
I then combined the big bore with a 4.25-inch Lunati forged-steel stroker crank. The result was a stroker displacing a massive 468 ci, or more than enough to properly test the merits of the custom intake system. The 468 also featured a static compression ratio of 12.25:1, a healthy 305LRR HR15 Comp cam (.624 lift, a 255/271 duration split, and 115 LSA), and Speedmaster CNC LS3 heads.
Run with the short (7.25-inch) runners, the 468 produced 723 hp and 620 ft-lbs of torque, but these numbers changed to 704 hp and 638 ft-lbs with 10.5-inch runners, then to 688 hp and 648 ft-lbs with the longest 16.5-inch runners tested. As length increased so did torque production, but the peak power fell off. Such is the trade-off inherent in runner length.
The sleeved block was then treated to a Lunati 4.25-inch stroker crank and K1 rods, along with a set of Wiseco forged pistons.
Custom Dual-Plenum Adjustable-Runner Intake on a 468 Stroker (Horsepower)
16.5-inch Runners: 688 hp @ 6,500 rpm
10.5-inch Runners: 704 hp @ 6,400 rpm
7.25-inch Runners: 723 hp @ 6,900 rpm
Largest Gain: 36 hp @ 6,800 rpm
As I saw with the FAST Adjustable LS3 intake, adjusting the runner length on this custom, dual-plenum intake on the 468 stroker had a similar effect on power production. Shorter runners push power production higher in the rev range; longer runners optimize power at lower engine speeds. The 16½-inch runners offered the most power up to 5,400 rpm, but lost out to the shorter 10½ and 7¼-inch runners thereafter.
Custom Dual-Plenum Adjustable-Runner Intake on a 468 Stroker (Torque)
16.5-inch Runners: 648 ft-lbs @ 5,100 rpm
10.5-inch Runners: 638 ft-lbs @ 5,400 rpm
7.25-inch Runners: 620 ft-lbs @ 5,400 rpm
Largest Gain: 62 ft-lbs @ 3,900 rpm
The 16½-inch runners were the clear winner in torque production up to the crossover point of 5,400 rpm. The torque gains were as high as 62 ft-lbs at 3,600 rpm over the shortest runner length. The 10⅕-inch runners offered more torque than the 7¼-inch runners all the way up to 6,500 rpm. Only above that did the short-runner combination excel.
Working with the intake and camshaft, the cylinder heads are part of the trio of performance components that dictate the power output of the engine. In the case of the LS3 and (especially) LS7, the factory heads offer exceptional airflow. Unlike cathedral-port heads (706, 317, 243, etc.), it is difficult to improve upon the power output of the already impressive factory heads.
I have seen power gains eclipsing 70 hp when upgrading cathedral-port heads (on a 408 stroker), but the gains were nearly half that (or less) when replacing factory LS3 heads (on a larger 468 stroker). The reason for this is not that the aftermarket doesn’t know how to produce a good LS3 head, but that the factory LS3 heads already flow enough to support such high power levels. A stock LS3 head flows near 318 cfm. This compares to the very best cathedral-port head (317 or 243) that flows 244 cfm. The difference between an LS3 and the best factory cathedral-port is more than 70 cfm.
I have exceeded 690 hp using stock LS3 castings on a 468 stroker, and the stock LS7 heads are even more impressive. Run on a 495-inch stroker, the stock LS7 heads produced 773 hp. The impressive head flow offered by the stock heads is both a blessing and a curse. On the plus side, they offer impressive power right out of the box, but just don’t expect huge power gains when upgrading the heads on your LS3 or LS7.
