The single-plane intake offered reasonable performance, but there are plenty of better intake systems available for your Hemi.
The individual-runner 8-stack system from Speedmaster looked amazing and offered plenty of performance.
MP Single-Plane vs Speedmaster Downdraft IR Intake for a Mild 5.7L Hemi
MP Single-Plane Intake: 430 hp @ 6,500 rpm
Speedmaster 8-Stack Intake: 462 hp @ 6,400 rpm
Largest Gains: 31 hp @ 6,400 rpm
The individual-runner intake offered impressive power gains through the entire rev range. Thanks to plenty of airflow and optimized runner length, the Speedmaster IR system improved the power output by as much as 31 hp.
MP Single-Plane vs Speedmaster Downdraft IR Intake for a Mild 5.7L Hemi
MP Single-Plane Intake: 391 ft-lbs @ 5,200 rpm
Speedmaster 8-Stack Intake: 418 ft-lbs @ 5,300 rpm
Largest Gains: 25 ft-lbs @ 5,200 rpm
The torque curve was even more telling because the long(er)-runner IR induction system improved torque output from top to bottom. Extra top-end power is always good, but more torque through the rev range will be even more useful.
Test 8: Effect of Runner Length of a Speedmaster Downdraft IR for a Mild 5.7L Hemi
Perhaps the best illustration of the effect of runner length in this chapter comes from this impromptu test on the Speedmaster downdraft system. With no adjustable intake system available from the aftermarket, the author whipped up something to work on the downdraft system offered by Speedmaster. Using a combination of slip-fit aluminum tubing, hose clamps, and duct tape, we were able to quickly adjust the runner lengths to optimize power production at different engine speeds on this 5.7L Hemi. 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 the longer runners is offset by a potential loss in high-RPM power. The opposite is true of short runners; they give up low and mid-range torque for optimization at high RPM. The idea is to tune the combination for the desired usage. Although the debate usually involves the generic terms long versus short runners, the best runner length is the one that provides the most average power production in the desired RPM range.
The downdraft IR intake from Speedmaster offered plenty of power over a standard single plane (see Test 7), but additional runner length can dramatically alter the power curve.
Having just run the test on the Speedmaster downdraft system (with the single plane), we decided to investigate the situation further by increasing the runners in the IR manifold. Run with the IR manifold as delivered by Speedmaster, the 5.7L produced 462 hp at 6,400 rpm and 418 ft-lbs of torque at 5,300 rpm. After extending the runner lengths by 4 inches, the peak numbers changed to 459 hp at 6,500 rpm and 432 ft-lbs of torque at 4,800 rpm. Note that the longer runners offered more torque but slightly less peak power than the shorter standard length. This trend continued after we increased the runner length another 2.75 inches (6.75 inches total). Run at this length, the peak numbers changed to 459 hp at 6,000 rpm and 443 ft-lbs of torque at 5,000 rpm. The longest runner length improved torque production by as much as 38 ft-lbs and dropped peak power by just 3 hp.
The author installed sliding runners inside the stock tubes to increase runner length. Additional runner length offered substantial gains in torque below 6,000 rpm.
Effect of Runner Length: Speedmaster Downdraft IR for a Mild 5.7l Hemi
Stock Runner Length: 462 hp @ 6,400 rpm
Plus 4 inches: 459 hp @ 6,500 rpm
Plus 6.75 inches: 459 hp @ 6,000 rpm
Largest Gains: 34 hp @ 4,900 rpm
Looking at the numbers, you might be tempted to pick the intake combination that offered the highest peak power. Unfortunately, man does not live by peak power alone. Each increase in runner length lowered the peak power number slightly but offered more power through most of the curve.
Effect of Runner Length: Speedmaster Downdraft IR for a Mild 5.7l Hemi
Stock Runner Length: 418 ft-lbs @ 5,000 rpm
Plus 4 inches: 432 ft-lbs @ 5,300 rpm
Plus 6.75 inches: 443 ft-lbs @ 5,000 rpm
Largest Gains: 38 ft-lbs @ 4,300 rpm
The results of this test on the Speedmaster downdraft system demonstrated the torque gains offered by changes in runner length. Increasing runner length on the downdraft intake dramatically enhanced torque production.
Working with the intake and camshaft, cylinder heads are one part of the trio of performance components that dictate the power output of the engine. In the case of