Читать книгу How to Build LS Gen IV Performance on the Dyno - Richard Holdener - Страница 8
ОглавлениеWorking 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.
Bolting on the right set of LS3 or LS7 cylinder heads can yield impressive power gains.
Factory LS3 and LS7 heads offer exceptional flow and power potential, but CNC-ported, aftermarket heads offer even more.
To understand the reason for this, you need to first understand the correlation between airflow and power potential. There is, of course, an equation to calculate the “potential” horsepower offered by cylinder heads using the airflow data. This formula is:
Using an LS3 (with 318 cfm) as an example, the formula indicates (.257 × 234 cfm × 8) that the stock heads will support 653 hp (though have made more than the formula suggests). Were you to upgrade the heads on a stock or mild LS3 (making less than 560 hp), the gains offered by the head swap might be minimal because the stock heads already flow more than enough to support the current power level. Test 1 in this chapter illustrates what happens when you add cylinder head flow to a mild combination.
Using a flow bench is one way to determine the power “potential” of a set of cylinder heads, but the only way to know for sure is to run them on a dyno.
LS3 and LS7 heads offered by the aftermarket 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. As in the example in Test 1, just because you have 800-hp head flow doesn’t mean your combination is in a position to take full advantage of the available flow. This is especially the case in LS7 applications, where aftermarket head flow can exceed 400 cfm (or more). It took a 495-inch super stroker (see Chapter 8) to tax the flow limits of the best LS7 heads, and the stock LS7 heads produced 773 hp.
Even the best heads were only up by 25 hp or so, the gains offered by ported LS7 heads would be even less. On a stock LS7, there may be no gain at all. 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 to use a .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.
For the ultimate in valve control and RPM potential, a dual-spring upgrade is the way to go on a high-performance LS3 or LS7.
The great thing about the LS engine family is the interchangeability. Cylinder heads from an LS7 physically bolt onto an original LS1, but the small bore size does not allow them to actually run without valve interference. This interchange allows the later LS3 heads to serve as inexpensive upgrades to the earlier cathedral-port engines. The most popular upgrade is to replace the stock 317 cathedral-port heads on a 6.0 truck (LQ4 or LQ9) or LS2 (243 castings) with the rectangular-port LS3 heads. The large valves in the LS3 heads require the 4.0-inch bore of the 6.0 (and do not work on smaller 4.8, 5.3, or 5.7 blocks), but the results are impressive.
The LS3 head upgrade also requires the corresponding offset (intake) rockers and intake manifold, but the stock LS3 heads offer an additional 70 cfm per runner. Tested on a 408 stroker, the LS3 head upgrade was worth almost 40 hp over the stock 317 truck heads. The smaller cathedral-port 317 heads offered more power up to 4,000 rpm, but the LS3 heads pulled away up to 6,500 rpm.
Test 1: Stock LS3 vs Chevy Performance CNC L92 on a Stock LS3
Several tests in this book were designed to illustrate what happens when you install the right part on the wrong application. Unlike factory cathedral-port applications, LS3 engines were blessed with high-flow cylinder heads. In terms of head flow, there was a substantial step up from the cathedral-port LS6/LS2 heads to the rectangular-port LS3 heads. That is why adding LS3 heads to a 6.0 is such a popular swap.
As you learn in this chapter, factory heads can support nearly 700 hp on the right application, but that doesn’t mean ported heads don’t offer any power. Just don’t expect the huge gains normally seen with cathedral-port head testing; the stock LS3 heads flow nearly 315 cfm. This test shows that, especially on a stock application, cylinder head flow was not the limiting factor in terms of performance.
This test was run on the LS3 crate engine from Gandrud Chevrolet in near-stock trim. The engine was equipped with a set of long-tube headers, manual FAST throttle body, and Holley HP 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 LS3 heads, the stock LS3 produced 493 hp at 5,700 rpm and 484 ft-lbs of torque at 4,800 rpm. I then installed a set of CNC-ported L92 heads from GM Performance (supplied by Gandrud) that flowed nearly 350 cfm (up from 315 cfm). While the additional head flow could support well over 700 hp, run on this stock application, the ported heads improved the power output to only 503 hp and 497 ft-lbs of torque. Does this mean the CNC-ported heads don’t work? Hardly. See the gains offered in Test 3.
The test engine was an LS3 crate engine supplied by Gandrud Chevrolet. It was run in stock trim with no changes to the cam, compression, or induction system.
For dyno use, I installed a set of long-tube headers, FAST throttle body, and XFI management system. The crate engine produced 493 hp and 484 ft-lbs of torque with the stock heads.
Stock LS3 vs Chevy Performance CNC L92 on a Stock LS3 (Horsepower)
Stock LS3 Heads: 493 hp @ 5,700 rpm
Chevy Performance CNC L92 Heads: 503 hp @ 5,900 rpm
Largest Gain: 13 hp @ 6,400 rpm
Replacing the stock LS3 heads with a set of CNC-ported L92 from Chevy Performance netted only minor gains on this otherwise stock LS3 crate engine. The CNC-ported heads flowed considerably more than the stocks, but the mild stock engine simply didn’t need any more flow to support the existing power level. Test 3 illustrates the amount of power that the extra airflow can be worth on the right application.
Stock LS3 vs Chevy Performance CNC L92 on a Stock LS3 (Torque)
Stock LS3 Heads: 484 ft-lbs @ 4,800 rpm
Chevy Performance CNC L92 Heads: 497 ft-lbs @ 4,600 rpm
Largest Gain: 16 ft-lbs @ 4,400 rpm
The torque curve tells the same story because the mild LS3 simply couldn’t use the additional airflow. Because the stock LS3 heads support well over 600 hp, they were more than enough for this stock crate engine. The head swap would be worth much more than 16 ft-lbs on the right application.
Test 2: Stock LS3 vs AFR 245 on a 408 Stroker
Since both the AFR 245 cathedral-port and factory LS3 heads used for this test flowed so well, I made sure I had a solid test engine for the comparison. Starting with a 6.0 block, the 408 was assembled using components from Speedmaster and Wiseco. The stroker assembly included a 4.0-inch forged-steel crank combined with 6.125-inch 4340 forged steel connecting rods and 10-cc dished pistons.
The forged pistons from Wiseco featured valve reliefs to allow for the hydraulic roller (PN 289LRR HR14) cam (.624 lift, a 239/255 duration split, and 114 LSA) from Comp Cams. Designed for a rectangular-port head application (which may not have favored the cathedral-port heads), the cam was combined with a set of standard-travel lifters and hardened pushrods from Comp Cams.
The .030-over 408 stroker short-block also featured a new timing chain and oil pump from Speed Pro, a set of ARP head studs and MLS head gaskets from Fel Pro, and a Moroso oil pan and windage tray.
Comparing the rectangular-port LS3 and the cathedral-port AFR heads also required an intake swap (to match the respective head ports). To keep the test as even as possible, I selected FAST LSXR intakes for both applications. Both were also run with the same 102-mm Big Mouth throttle body. As always, both heads were run with the same air/fuel ratio and timing values.
Equipped with the stock LS3 heads, the 408 produced 581 hp and 543 ft-lbs of torque. After installation of the AFR 245 heads, the peak numbers jumped to 604 hp and 665 ft-lbs of torque. It must be pointed out that the head swap also included a change in static compression ratio because the chamber volume on the two heads differed by 5 cc (64 cc vs 69 cc). This meant that in addition to the increased airflow offered by the AFR heads (349 cfm vs 316 cfm), they also increased the static compression ratio by .5 points. With the exception of a short, 250-rpm range (from 4,100 to 4,250 rpm), the AFR heads improved the power output from 3,000 to 6,700 rpm. Having more peak power is good, but having extra power everywhere is even better.
