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Corvette Cylinder Heads - One Sweet LS Upgrade

An RHS Cylinder-Head Topping That Delivers

Cam Benty Apr 9, 2007
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After performing the first four stages of our LS upgrade program, it was back to the dyno for Stage V and the installation of RHS's Pro Elite cylinder heads.

We're back. And this time it's getting serious. Last month, you witnessed the first four stages of a muscle-building regimen for our trusty LS1 engine. As noted at the time, the modifications we made will work on any LS-series powerplant except for the unique LS7.

The concept behind the project was to create a fully engineered and tested series of incremental horsepower upgrades for the LS-engine family. Each step was a small bite of performance, making it possible to add a little or a lot of power depending on one's performance goals and budget limitations.

What We've Done So Far
To recap our previous efforts, we began our test with an engine-dyno check of a stock LS1 engine, performed in the Comp Cams R&D facility in Memphis. After determining that our engine cranked out 375 hp and 370 lb-ft of torque in unmodified form, we added a FAST-FLASH Power Programmer tune, a set of valvetrain-enhancing Hi-Tech pushrods, and a set of ZEX performance spark plugs. The result? An additional 11 hp and 12 lb-ft.

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Because of our previous engine upgrades, we selected a head with a 225cc intake runner. RHS also offers a 210cc version that's a great bolt-on for any stock or lightly modded LS engine.

Stage II saw the addition of Pro Magnum 1.75:1 roller rocker arms, Hi-Tech rocker-arm studs, and pushrod guideplates. But the biggest power-improving modification involved the installation of Comp beehive valvesprings and appropriate steel retainers. (All of these modifications were completed on top of the changes made in Stage I.) This time, the dyno showed 18 more hp and 16 extra lb-ft.

For Stage III, we added a Comp XE-R camshaft (PN 54-444-11). While other Comp cams were also tested, the test with this particular grind netted the most impressive numbers-56 hp and 36 lb-ft. Finally, for Stage IV, we bolted on a FAST LSX intake manifold, a 90mm throttle body, and a fuel-rail set (PN 54023). The final tally at the end of the four-stage test? A whopping 64 hp and 40 lb-ft.

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The RHS heads bolt right up to any LS1, LS2, or LS6 engine. They even retain all of the accessory holes for street operation.

How Big Should I Go?
To make really big horsepower, an engine requires increased air and fuel flow. While the factory LS engine is a vast improvement over the small-blocks of yore, its still begs for enhancement. Enter Racing Head Service (RHS), with its Pro Elite fully assembled cylinder heads. (Bare assemblies are also available.)

When it comes to LS-appropriate RHS products, there are two choices relative to the volume of the intake runners: 210 and 255 cc. So what is the difference? Well, the obvious difference is the amount of air provided to the engine. Depending on your engine setup, one version will likely perform better than the other.

According to RHS Tech Rep David McCarver, "If you are just bolting a cylinder head to your engine or have done mild modifications, the 210cc head is the right choice. In an engine such as this, you can actually push too much air into the cylinder. Regardless, you will probably see as much as 50 more horsepower from such a swap. It is important to remember, however, that you will need to change the pushrod length here as well, as these heads have a much thicker deck height, requiring longer pushrods."

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One of the biggest power secrets of the RHS heads is the valve angle. While hard to spot with the eye, the angle has been rolled back 4 degrees-a major performance enhancer.

In fact, the RHS heads have a 0.800-inch-thicker deck, making them suitable for high-power and supercharged applications where the stock head structure can be less than adequate. With the RHS head, it is recommended that you not only step up to a 31/48-inch-diameter pushrod when possible, but that you also utilize a Comp pushrod-length checker (PN 7703-1) to make sure you've selected the correct part. Considering that the stock-length pushrods are 7.400 inches long, the new heads will require somewhere between 8.050- and 8.150-inch units.

"The 225cc heads are for high-horsepower, nitrous-equipped, and boosted engines with superchargers or turbochargers," says McCarver. "If you put the 225cc heads on a stock engine, you will increase the horsepower, but it might make the engine a bit lazy for street driving. While both heads feature CNC machining for consistent performance-and the added benefit of the 4-degree rollback on the valve angle for better port configuration-the 225cc head is definitely the better head for high horsepower and torque, especially when the engine has been . . . stroked for larger displacement."

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All of the changes made in the first four stages were retained, save for the Comp roller rockers. That includes these FAST fuel rails...

As noted earlier, the 210cc (PN 54210-02) and 225cc (PN 54225-02) heads can be purchased bare or assembled. The assembled heads feature three valvespring configurations depending on the requirements of the engine builder. For applications where 0.600-inch-lift-or-less camshaft profiles are used, the Comp beehive springs (PN 26918) can be used. For engines running up to 0.650-inch lift, the Comp dual springs (PN 26921) should be specified. For extreme applications where a solid-roller camshaft is used, the ultra-performance Comp No. 943 spring (which will handle up to 0.750-inch lift) is recommended. (Note that this is a solid-roller-only spring. Using these springs with a hydraulic roller cam will cause engine damage.)

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...and the Comp XE-R camshaft.

Our Stage V test involved the installation of the RHS 225cc cylinder heads equipped with beehive valvesprings. The 225s and beehives were recommended based on our previous modifications, specifically because of the enhanced airflow from the FAST LSX intake and the significantly hotter-than-stock Comp cam.

In addition to the switch to longer pushrods, we had to revert to the stock rocker-arm configuration. Currently, the RHS heads are not compatible with the Comp Pro Magnum roller rockers without modification, so the factory rockers were installed along with the appropriate Comp rocker stands (PN 54201). New stock-style head gaskets were used, along with fresh head bolts. (LS engines use "torque to yield" bolts, which cannot be reused.) All of the components used in Stages I through IV (save for the Comp rockers) were reinstalled, including the FAST intake and 90mm throttle body.

Based on the results of our previous test, we were hoping for big things. We weren't disappointed. When the dyno finished its calculations, the engine with the RHS 225cc heads increased power a full 108 hp and 68 lb-ft over stock.

It goes without saying that we were pleased with the results of our testing. The five upgrade stages resulted in plenty of horsepower and street compatibility that can be duplicated on any LS1, LS2, or LS6 engine.

Parts List
Stage V Package Components
* RHS Pro Elite fully assembled cylinder heads*
* PN 54210-02 (210cc intake runner)
* PN 54225-02 (225cc intake runner, used in testing)

*Both heads in this configuration use Comp PN 26918 beehive valvesprings and appropriate hardware.

Additional Parts
* Comp Cams PN 7703-1 pushrod length checker, PN 54201 rocker stands (for factory LS rocker arms)
* Extended-length pushrods for compatibility with RHS heads


Comp Cams
Memphis, TN 38118
Memphis, TN 38118



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