GM 350 Crate Engine Build III - Goodwrench Quest, Part III

Testing Aluminum L-98 Corvette Heads On GM’s Budget 350

Jeff Smith Nov 1, 1999 0 Comment(s)

Step By Step

This time we wanted to evaluate the performance advantage of a set of Corvette aluminum heads. These heads were originally used on L98 Corvette Tuned Port Injected (TPI) engines that preceded the LT1 engine. The heads are extremely lightweight with small ports and require a centerbolt valve cover configuration. If you choose to use roller rockers with these heads, there may be some interference with the centerbolt fixtures and some machining of the covers (especially the Corvette magnesium style) may be required.

The 58cc chamber on the L98 heads is substantially smaller than the typical 76cc iron cylinder heads, which drastically increases compression. Since our Goodwrench engine was seriously down on compression to begin with, the squeeze factor went from a measly 8.4:1 (using a 0.015-inch Fel-Pro head gasket) to a much more agreeable 10.1:1 with the smaller chamber.

The Edelbrock Performer RPM (left) is the big brother to the more stock-appearing Performer. Both are dual-plane designs, but the RPM uses a taller carburetor mounting flange with larger runners to flow more air while still creating excellent torque for street use. As you can see from our tests, even with stock Corvette heads, the intake and carb combination is worth as much as 25 lb-ft of torque at 3,500.

We chose Holley’s 4779 750-cfm double-pumper not only because it is an excellent street carburetor but also because it is one of the most popular carburetors on the market. Besides, we wanted to test the Goodwrench small-block with off-the-shelf carburetors that are less expensive than highly modified units.

This is the stock Vette chamber. McKenzie again performed a minimal amount of porting to duplicate what could be done both easily and inexpensively within an inch of the valve seat on both ports. As you can see from the numbers in the Flow Testing chart, the exhaust improved dramatically while the intake port did not respond as well to the pocket work.

All late-model engines now use self-aligning rocker arms, which are identified by the “rails” or ribs on the stock stamped rockers that are used to keep them centered over the valve tip since no guideplates are used. Comp Cams offers rail-style Magnum roller-tipped rocker arms that are a direct replacement. We tried increasing the rocker ratio to 1.6:1 in this configuration, with no improvement.

We used ARP head bolts with thread sealant to ensure a positive seal to the block as well as to prevent coolant leakage past the threads.

Because of the aluminum heads’ small 58cc chamber size, we used Fel-Pro 0.041-inch-thick composition gaskets to keep the compression within the 92-octane pump gas limits. Test 1 versus Test 2 is basically a comparison of the effect of compression, since the pocket-ported iron heads flow very similarly to the stock Corvette heads.

In the continuing saga of the Goodwrench Quest, we now embark on a journey into the exotic world of aluminum heads. In this installment, we’ll start with a set of affordable GM Corvette L-98 aluminum cylinder heads from GM Performance Parts to see what power these heads can deliver. Last time, we pocket-ported the iron heads while retaining the stock 1.94-/1.50-inch valve sizes. We also bolted in a much larger Comp Cams Xtreme Energy dual-pattern, flat-tappet hydraulic cam that, together with the Performer intake and Hooker headers, was worth a total of 336 hp at 5,300 rpm and torque of 377 lb-ft at 3,800. The original baseline with headers and the Performer intake was a meager 265 hp and 350 lb-ft of torque. We’ve already made great gains, but we’re not through yet.

The Corvette aluminum heads are a great bolt-on for mild street engines for a number of reasons. They are some of the least expensive aluminum heads on the market, with a new pair selling for roughly $800, complete. While the small 58cc combustion chamber size can be a compression problem for 350s, in this case the small chamber puts the squeeze right where you want it. With the 73cc iron heads, compression on the Goodwrench 350 was barely 8.4:1. Just bolting on the Corvette heads increased compression to 10.1:1, which should be worth some power yet will still tolerate 92-octane pump gas.

The Test

For the sake of continuity, Test 1 in our dyno charts is the best power combination from last time, with the pocket-ported stock iron heads combined with the Edelbrock Performer intake and the Q-jet. This became the new baseline. Test 2 added only the stock aluminum Corvette heads with everything else remaining the same. Given that the airflow figures between the stock aluminum heads and the pocket-ported iron heads are so similar, this gives us a chance to see what gains can be had from the increased compression. With this done, we decided that Test 3 would evaluate the induction system change to an Edelbrock Performer RPM intake and a Holley 750-cfm carburetor. This gave us a chance to see if the manifold would add a little more power. Finally, Test 4 looks at whether any advantage would be gained by pocket porting the Vette heads.

With the Goodwrench 350 back on Ken Duttweiler’s dyno, Test 2 started with the stock Corvette aluminum heads. As soon as the engine fired up and settled into an idle, it was obvious that we had increased compression because the engine now had a decided lope to the idle that had not been noticeable with the iron heads. Once warmed up, CHP’s intrepid dyno man Ed Taylor yanked the handle and made a series of pulls to determine timing, uncovering best power numbers of 348 hp at 5,500 rpm with max torque coming in at 380 lb-ft at 3,600 rpm. This included the use of the Performer intake and Q-jet along with the 1-5/8-inch Hooker headers and a pair of 2-¼-inch mufflers. Power didn’t really increase over the ported iron heads, even with the 1.7:1 bump in compression.

Test 3 left everything the same except the induction system. Ed added the Performer RPM intake and Holley 750-cfm carburetor, with the intention that we would mainly be evaluating the effect of the intake manifold and carb on the engine. Since this Goodwrench engine was nowhere near the maximum airflow limit of the Q-jet, the reason for the change was mainly because most enthusiasts would have converted over to a Holley carburetor. The Holley was used with out-of-the-box jetting, and timing remained at 36 degrees total lead. From the power chart you can see that the intake produced a significant gain in torque virtually through the entire powerband, with a torque gain of an amazing 25 lb-ft at 3,500 rpm where peak toque occurred. This improvement is probably evenly split between the better manifold and the Holley’s more stable fuel curve. Peak horsepower increased slightly from 348 to 355 at almost the same rpm.

Now it was time to see if some pocket-porting work on the aluminum heads would pay off with a power dividend. The airflow numbers hinted that the power should improve, especially on the higher rpm side since the Vette heads gained much more exhaust flow than intake. The dyno flogging in Test 4 pointed in that direction, but at a cost. Note that between 3,100 and 3,900 the torque dropped off compared to Test 3. This is probably due to the over-efficient exhaust port combined with the cam’s longer duration exhaust lobe. For this particular application, we would have been better served to install a single pattern cam with a wider lobe separation angle to maximize the flow numbers. This is why we always stress looking at the entire power curve rather than just the peak numbers. While Test 4’s peak torque was almost identical at 401 lb-ft and horsepower improved slightly with 361 hp at 5,500 over the stock heads at 355 hp, the loss of torque in the midrange would result in minimal (if any) improvement on the dragstrip.

If we go back to the Test 1 numbers of 366 hp and 377 lb-ft of torque, it’s clear that the best overall power curve in this series of tests lies with Test 3 using the stock Vette heads, a Holley 750 carburetor, an Edelbrock Performer RPM, Hooker 1-5/8-inch headers, and a pair of 2-¼-inch turbo-style mufflers. The torque curve is amazingly strong, with over 390 lb-ft of torque from 3,100 to 3,900, and is never less than 350 lb-ft from 2,500 to 5,200 rpm. This combination also produced over 100 hp and 50 lb-ft of torque more than the original baseline using a stock aluminum intake, a Q-jet, and cast-iron exhaust manifolds. But we have barely scratched the surface of what can be done to this otherwise stock short-block 350 Chevy. In fact, before you rush right out to buy a set of GM Performance Parts aluminum Vette heads, you might want to wait until next time to see what kind of power we can pull out of a set of those excellent GMPP Vortec iron heads.

We won’t spoil the surprise by spilling the numbers here, but if ultimate power is what you’re seeking from a mild-mannered 350 Chevy, then you’ll certainly want to read the next episode of the Goodwrench Quest. There’s power to be had with these heads. The best news is that these Vortec heads are extremely affordable at less than $400 for a complete pair from Scoggin-Dickey and other GMPP dealers! So if a powerful small-block for less bucks makes your mouth water, stay tuned for the next Goodwrench Quest.

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