Gen IV LS Cylinder Head Shootout! Choose the Best for Power

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.

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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.

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:

HP = .257 x airflow x number of cylinders

Using an LS3 (with 318 cfm) as an example, the formula indicates (.257 x 234 cfm x 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.

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.

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. 

 

 

 

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.

 

 

 

Test 3: Chevy Performance vs Brodix vs LPE on an LS7 495 Stroker

Even more so than with LS3 head testing, comparing LS7 heads (especially improved versions) requires a serious test engine. Even at 427 ci, the stock 7.0 isn’t enough engine to tax the flow capacity of the stock LS7 heads, to say nothing of aftermarket heads that flow near 400 cfm. To properly test the merits of ported LS7 heads, I built a serious test mule that offered increased displacement, compression, and cam timing. Only then could I show what ported LS7 heads were really capable of.

Starting with an RHS tall-deck block, I bored out the engine to 4.185 inches then added a big stroker crank. Filling the stroker-friendly RHS block were equally stout components from Lunati, Wiseco, and K1. Lunati supplied a massive 4.5-inch billet stroker crank, which was combined with a set of 6.30-inch forged K1 rods and 4.185-inch Wiseco forged, flat-top pistons. The bore and stroke combined to produce a flow-taxing displacement of 495 ci.

The displacement and flat-top pistons combined with the 70-cc combustion chambers to produce a static compression ratio near 13.5:1. Sealing the beast was a set of Cometic MLS head gaskets secured by ARP heads studs. Additional components employed on the RHS stroker included a Moroso pan, pickup, and windage tray; a custom timing chain from Comp Cams designed specifically for the tall-deck block (1 extra link per side); and the largest off-the-shelf hydraulic roller cam available in the Comp Cams catalog. The 309LRR HR15 offered a .660 lift (with 1.8 rockers), 259/275-degree duration split, and 115-degree LSA. Feeding the beast was a Mast Motorsports (high-rise) single-plane intake designed to accept the Holley 1050 Ultra Dominator carb. Run with the stock LS7 heads, the 495 produced 773 hp and 704 ft-lbs of torque. Adding a set of ported LS7 heads from Lingenfelter Performance Engineering increased the power output to 793 hp and 719 ft-lbs of torque, while Brodix LS7 heads produced 799 hp and 714 ft-lbs of torque.

 

 

 

Test 4: Effect of Chamber Volume: TEA vs Speedmaster on an LS3

For the first three tests in this chapter, I have concentrated primarily on airflow, but the power offered by a cylinder head swap is a function of other variables. As mentioned in Test 2 on the AFR versus LS3 heads, a change in combustion chamber volume can alter the power output as well. To illustrate this, I selected two pairs of heads that offered similar flow rates but had a significant difference in chamber volume that substantially altered the static compression ratio. 

Before I continue, it is important to know that an increase in static compression ratio (from 9.0:1 to 10.0:1, for instance) will increase the power output of any engine. The general rule is that the power increases 3 to 4 percent for each full point of compression.

 

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It is also important to note that this rule is a guideline that is most accurate in what I call the normal range of compression ratios (8.0:1 to 13.0:1). Changes in ratios above and/or below this range have less of an effect on power. Given that the change in compression ratio came from increased chamber volume, the change in chamber shape (irrespective of size) might also have an effect on power production, but that requires a much more difficult test to prove.

This test on chamber volume (and static compression) was run on a 416 LS3 stroker. The stroker was built using a Speedmaster crank and rods combined with a set of JE flat-top pistons. Similar to many of the tests, I tried to lower the static compression to safely apply boost to the combination.

The Speedmaster crank, rods, and JE pistons were given a precision balance job and then assembled using Total Seal rings; Sealed Power bearings; and Fel Pro gaskets, oil pump, and timing chain. The stroker also featured a Texas Speed cam (.614/.621 lift split, 231/239 duration split, and 113 LSA) using Comp hydraulic roller lifters.

Run with the (big-chamber) Speedmaster LS3 heads, the stroker produced 576 hp and 551 ft-lbs of torque. After installation of the (smaller-chamber) Chevy Performance heads (with nearly identical flow numbers), the power output improved to 608 hp and 570 ft-lbs of torque. The change in compression ratio of 1.3 points improved the power output by 5.5 percent.

 

 

 

Test 5: Stock LS3 vs TEA vs Speedmaster LS3 on a 468 Stroker

I ran several ported LS3 heads on the 468 stroker engine. The problem is that I couldn’t show all of the results in one test, so I elected to show this one.

As a recap, the 468 stroker used a Darton-sleeved LS6 block with forged internals from Lunati and JE. Aiding in power production was a healthy static compression ratio of 12.0:1. To allow the stroker to maximize power, I installed an off-the-shelf Comp cam (.624 lift, 255/271 duration split, 112 LSA) along with a set of Comp short-travel lifters. Milodon supplied a pan and remote oil filter to work with the modified windage tray (to clear the stroker). ARP and Fel Pro secured each of the three heads tested. Once again, the two ported LS3 heads were compared to a pair of ported versions, in this case from Total Engine Airflow (TEA) and GM Performance.

The first order of business was to run the 468 with the stock heads to establish a baseline. Equipped with stock LS3 heads, the stroker produced 692 hp at 6,500 rpm and 625 ft-lbs of torque at 4,900 rpm. Next up was a set of heads from TEA, which applied its Stage 2 porting to a set of factory castings. The porting improved the power output from 692 hp and 625 ft-lbs of torque to 719 hp and 631 ft-lbs.

The gains offered by the TEA were most prevalent at high RPM, but there were gains through the entire range. The Speedmaster LS3 heads showed similar gains, but offered slightly more peak power. Equipped with the Speedmaster LS3 heads, the 468 produced 730 hp, but peak torque dropped slightly to 629 ft-lbs.

This testing tells me that the stock LS3 heads are plenty powerful and that a number of different aftermarket heads can offer substantial power gains.

 

 

 

Test 6: Chevy Performance vs TS vs SDPC on an LS7 495 Stroker

The testing performed on the LS7 heads paralleled the testing on the LS3 heads in that I ran several different heads during the dyno comparison. After all, why take the time to set everything up multiple times when you have the test engine ready and all you have to do is swap the heads? In truth, there is much more to this type of testing than simple head swaps.

Every head must be first mocked up to measure for proper pushrod length. The last thing you want to do during testing is search around for pushrods when you have an engine on the dyno. Proper preparation allows you to maximize available (and expensive) dyno timing. This test compares the Chevy Performance CNC LS7 heads (basically stock) and two sets from Texas Speed (TS) and Scoggin Dickey Performance Center (SDPC).

Once again, I used the 495-inch stroker with an RHS tall-deck block, Lunati billet crank K1 forged rods, and Wiseco pistons. The 309LRR HR15 featured a .660-inch lift (with 1.8 rockers), 259/275-degree duration split, and 115-degree LSA. The induction system included a Mast Motorsports (high-rise) single-plane intake designed to accept the Holley 1050 Ultra Dominator carb.

First run with the GM Performance CNC LS7 heads, the 495 stroker produced 773 hp at 6,300 rpm and 704 ft-lbs of torque at 4,700 rpm. After installation of the Texas Speed LS7 heads, the power output jumped to 796 hp and 725 ft-lbs of torque. Credit a peak flow rating of 403 cfm offered by the TS heads for the big power gains. The SDPC LS7 heads offered 389 cfm, which allowed them to produce 802 hp, but torque fell slightly (compared to the TS heads) to 717 ft-lbs.

 

 

 

Test 7: Stock LS3 vs TFS Gen X 255 on a Modifi ed LS3

One of the critical components of any LS combination is the cylinder heads. As luck (or design) would have it, the LS3 was blessed with plenty of head flow right from the factory, but that doesn’t mean the stock heads can’t be improved on. Case in point, the Trick Flow Gen X 255 LS3 heads. According to Trick Flow, the Gen X heads combined a peak airflow of 380 cfm with an intake port volume that measured just 255 cc. That is where the Gen X 255 heads get their name. The port volume is important because it’s easy to make big flow with big ports. The key to a successful cylinder head is to maximize flow while minimizing port volume. To put the TFS numbers into perspective, the stock LS3 heads checked in with just 315 cfm from 260-cc ports. The TFS heads offered significantly more flow and less port volume, a true indication of a solid design. The Trick Flow heads also offered a spring package that allowed me to run a healthy camshaft; after all, why upgrade heads on an otherwise stock engine? The TFS heads also featured a spring package that offered 150 pounds of seat pressure and 400 pounds of open pressure.

For a test engine, I once again relied on the LS3 crate engine from Gandrud Chevrolet. Prior to the test, the LS3 was upgraded with a hot Crane hydraulic roller cam. The Crane cam offered a .600-inch lift (intake and exhaust), 232/240-degree duration split (at .050), and 113-degree LSA. This cam was nearing the limit of available piston-to-valve clearance offered by the stock flat-top pistons, or more specifically the lack of valve reliefs. With proper valve reliefs, additional power would certainly be available with more aggressive cam timing, especially given the flow rate of the TFS heads.

The LS3 was run with a Holley HP management system and FAST injectors. With stock heads, the modified LS3 combination produced 552 hp at 6,400 rpm and 513 ft-lbs of torque at 5,000 rpm. After installation of the TFS Gen X 255 heads, the power numbers jumped to 571 hp at 6,500 rpm and 525 ft-lbs of torque at 5,200 rpm. As with other tests conducted in this chapter, the greater the power output of the test engine, the greater the gains with a head swap.

 

 

 

 

Written by Richard Holdener and republished with permission of CarTech Inc

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