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 the 5.7L–6.4L Hemis, the factory heads offer exceptional airflow. Compared to the previous Dodge Magnum engines, the modern Hemis offer serious power gains due in no small part to the impressive cylinder heads. Case in point, even the early 5.7L Hemi heads, which flow as much as 285 cfm, are capable of supporting more than 570 hp on the right application. The larger 6.1L heads flow even more. Checking in with more than 320 cfm, the stock 6.1L heads will support nearly 650 hp on a serious Hemi. The 09-up 5.7L Eagle heads nearly match the flow potential of the 6.1L SRT8 heads, but are limited somewhat by exhaust flow (which more closely matches the early 5.7L heads). Of course, the latest 6.4L Apache heads flow even more, with peak numbers eclipsing 340 cfm, enough to support 680 hp on the naturally aspirated stroker. Much like the LS offerings by GM, all Hemis were blessed with an overabundance of cylinder head flow.
Bolting on the right set of Hemi heads can yield impressive power gains.
The factory Hemi heads offer exceptional power potential, but CNC-porting can unleash even more flow.
The impressive head flow offered by the stock Hemi 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 (especially on your 6.4L). To understand the reason for this, we need to first understand the correlation between airflow and power potential. There is, of course, a very generic equation used to calculate the “potential” horsepower offered by cylinder heads using the airflow data. This formula reads as follows: HP = .257 × airflow × number of cylinders. Using an early 5.7L Hemi (with 285 cfm) as an example, we see that the formula suggests (.257 × 285 cfm × 8) the stock heads can support 585 hp. Were you to upgrade the heads on a stock or mild 5.7L (making significantly less than 585 hp), the gains offered by the head swap might be less than desired, since the stock heads already flow more than enough to support the current power level.
Aftermarket ported 5.7L–6.4L heads are usually purchased based on flow numbers. The problem with purchasing cylinder heads based on airflow is that the airflow numbers represent only a potential power output. Like the example given previously, just because you have a 585-hp head, it doesn’t mean your combination is in a position to take full advantage of the available flow. This is especially the case in ported 6.4L Apache applications in which the ported head’s flow can exceed 375 cfm. It would take a serious super stroker to tax the flow limits of the best Apache heads. This is especially the case if the maximum flow rate given for the heads you plan to purchase exceed the lift of the cam you plan to run. Big flow at .700, .750, or .800 lift is useless if you plan on running a .550 or .600-lift cam. Besides, you should be more concerned with the mid-lift flow numbers because the valve spends much more of its time sweeping through the mid lift (opening and closing) than it does at peak lift. The upshot of this is that you shouldn’t expect huge gains from ported heads unless you have an engine that will take advantage of what ported heads have to offer.
