Читать книгу How to Build New Hemi Performance on the Dyno - Richard Holdener - Страница 7
ОглавлениеWorking 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.
The early 5.7L heads flow the least, but factory 6.4L heads can flow as much as 340 cfm.
Another great thing about the Hemi engine family is potential interchangeability. Cylinder heads from an early 5.7L bolt physically onto the 6.1L, 6.2L (Hellcat), and 6.4L. There would be differences in chamber volume, valve sizing, and even valve length, but the heads bolt-on and can be used. Going the other way, the larger 6.1L heads required a different intake manifold (5.7L and 6.1L intake ports differ) as do the later 5.7L Eagle and 6.4L Apache heads. The Apache heads will also require something larger than the factory 5.7L bore of 3.917 inches because the 6.4L featured a bore size of 4.09 inches. The larger valves and revised valve position in the Apache heads (the reason they flow so well) require a minimum bore size of 4.060 inches (think 6.1L block). Unless you find a set of Apache or even 6.1L heads cheap (which is difficult), the best bet for the typical 5.7L Hemi owner is to install a cam and beehive valve springs and call it a day. If you want more, porting the stock 5.7L heads will yield flow numbers equal to the 6.1L and better (exhaust flow) than the 5.7L Eagle heads.
Test 1: Stock vs Ported 5.7L Heads on a Stock 5.7L Hemi
Many tests in this book were designed to illustrate what happens if you install the right part on the wrong application. Hemi owners are lucky in that the factory cylinder heads already offer tremendous airflow. In terms of head flow, stock Hemi heads flow better than fully ported Magnum heads. The impressive head flow is why it is so easy to make power with any of the modern Hemi combinations. As we will see in this chapter, the factory 5.7L heads can easily support more than 500 hp thanks to peak flow numbers nearing 270 cfm. Those numbers increase to more than 700 hp for the new 6.4L heads. Given the impressive flow rates of the stock heads, the question becomes will ported heads offer any power on an otherwise stock 5.7L Hemi application? Let’s spoil the surprise and say that ported heads should not be the first modification on your otherwise stock Hemi.
Even though the stock 5.7L Hemi heads flow well, porting them can offer even more power potential.
This test was run on a stock 5.7L Hemi pulled from a truck. The engine was equipped with a set of long-tube headers, a converted DBW throttle body, and FAST XFI management system. Everything else on the engine was left stock, including the camshaft, displacement, and compression ratio. This test was run to illustrate what happens when you increase the head flow on an engine that already has enough cylinder head. Run with the stock Hemi heads, the stock 5.7L truck engine produced 373 hp at 5,200 rpm and 412 ft-lbs of torque at 4,300 rpm. We then installed a set of fully ported 5.7L heads (flowing more than 300 cfm). While the additional head flow would support far more than 600 hp, run on this stock application, the ported heads improved the power output to only 381 hp and 418 ft-lbs of torque. Does this mean the ported heads don’t work? The answer is a resounding no because ported heads work very well on the right application that can utilize the extra flow.
Installation of ported heads on a wild combination can yield substantial power gains (see tests 3, 4, and 5 in this chapter), but don’t expect big things on an otherwise stock engine.
Stock vs Ported 5.7L Heads on a Stock 5.7L Hemi
Stock 5.7L Heads: 373 hp @ 5,200 rpm
Ported 5.7L Heads: 381 hp @ 5,400 rpm
Largest Gain: 12 hp @ 5,900 rpm
Replacing the stock 5.7L Hemi heads with ported versions netted only minor power gains, but not because the porting was performed incorrectly. The test engine simply could not utilize the extra airflow (up 35 cfm) on this otherwise stock 5.7L.
Stock vs Ported 5.7L Heads on a Stock 5.7L Hemi
Stock 5.7L Heads: 412 ft-lbs @ 4,300 rpm
Ported 5.7L Heads: 418 ft-lbs @ 4,300 rpm
Largest Gain: 12 ft-lbs @ 3,900 rpm
The torque curve tells the same story. The stock 5.7L Hemi simply couldn’t utilize the additional airflow. Since the stock 5.7L heads will support well more than 500 hp, they were more than enough for this stock truck engine. The head swap would be worth much more than 12 ft-lbs on the right application.
Test 2: Stock 5.7L vs TEA Heads on a Modified 5.7L Hemi
Having just tested ported heads on a stock engine, we decided the extra airflow offered by the heads deserved a better test engine. Starting with a MP crate engine, the 5.7L was augmented with a camshaft that could better take advantage of the flow potential of the cylinder heads. The Comp 273H-14 cam profile offered a .547/.550 lift split, a 224/228-degree duration split and 114-degree LSA. According to the Comp catalog, this cam was designed with a broad operating range from 2,000 rpm to 6,200 rpm. But on our application, especially with the ported heads, the 5.7L pulled hard all the way out to 6,800 rpm. The remainder of the test engine included an MP single-plane intake, FAST injectors, and Accufab 4-hole throttle body. The single-plane intake was part of the reason the engine wanted to rev; the intake was designed to enhance high-RPM power production, but as we saw in chapter 1, not without a drop in low-speed torque.
