GM 350 Crate Engine Build IV - The Goodwrench Quest, Part IV

In the continuing saga of our Goodwrench budget 350, we’re now approaching serious horsepower. Because we know how you think, you’ve already skipped to the dyno charts, so it’s no surprise to tell you we’ve made 384 hp and an impressive 401 lb-ft of torque all below 6,000 rpm. But this information doesn’t do much good unless you know how we did it. Here’s the skinny.

As you may remember from the previous installment, we had rigged our Scoggin-Dickey Goodwrench 350 with a set of aluminum L-98 Corvette heads first used on TPI Corvettes. As it turned out, the stock, unported heads made the best overall power, while the ported versions made more peak horsepower but cost torque. Ultimately, a single-pattern camshaft with the same exhaust specs as intake would have worked best in that specific application. That would have improved torque and perhaps even pumped the horsepower.

But now we were ready to test a set of heads that not only have excellent flow numbers but are extremely affordable as well. Generally, rebuilding a set of stock iron heads will cost in the neighborhood of $400 to $500. The problem with this is that all you have to show for five big ones is a marginal set of stock heads. A better solution is the new Vortec iron factory cylinder head first used on the ’96 Vortec L31 Chevrolet pickup engines. This is an amazing 1.94-/1.50-inch valve head that not only outflows a stock cast-iron Bow Tie head but also is hugely affordable. Scoggin-Dickey will sell you a pair of these heads for $399 per set, complete with valves, springs, retainers, keepers, and rocker studs. That’s an awesome deal. But there’s more to the story.

The Plan

The Vortec heads use a different intake manifold pattern than a stock small-block Chevy cylinder head. While it is possible to drill the heads for a standard small-block six-bolt intake pattern, it’s also possible to ruin a head if drilled incorrectly. A better approach is to use one of the new intake manifolds from Edelbrock that are designed to fit the new Vortec heads. To this end, we used an Edelbrock Performer RPM intake and an out-of-the-box Holley 0-4779 750-cfm carburetor to complete the induction package. We also retained the Hooker 1-5/8-inch street headers and the 2-¼-inch Hooker-muffled exhaust system.

The Heads

So why are these production heads so good? Chevy borrowed the excellent intake and exhaust port design from the LT1 aluminum head and dropped it into the iron Vortec head, which includes a small but efficient 64cc combustion chamber and pressed-in 3/8-inch studs. The stock springs are a little weak for the kind of abuse we intended, so Todd McKenzie of McKenzie’s Cylinder Heads installed the appropriate Comp Cams valvesprings and new seals on the heads to allow the cam to do its stuff without suffering from valve float. While pushrod guideplates could have been installed, we already had a set of rail-style rocker arms from Comp, so we elected to stick with these and avoid the added cost of machining the heads for screw-in studs and guideplates.

While these heads offer excellent flow potential (see “Flow Chart” sidebar), power-crazed hot rodders always want to make them better. McKenzie’s previous experience with these heads revealed that overzealous pocket porting can hurt flow rather than improve it. So, to duplicate what a typical hot rodder might do, the heads were first tested stone stock on the dyno, then removed and just lightly dusted with the grinder to remove the lip just below the factory three-angle valve job to improve low-lift flow. Then Todd added a 30-degree back-cut to the exhaust valves that bumped up the flow below 0.250 inch of lift.

All this is conjecture, however. The real test is bolting the parts on and twisting the motor on Ken Duttweiler’s dyno to see what happens. The numbers don’t lie. These budget iron heads are probably the best per-dollar deal on the planet.

The Test

If you refer to the Power Chart that lists the dyno test results, we’re using Test 1 as the baseline, which was the best overall power curve from the previous installment’s test of the Corvette aluminum heads. At 355 hp at 5,400 and 402 lb-ft of torque at 3,500, this is an excellent street package. One point worth noting is that adding the stock iron Vortec heads on the Goodwrench 350 also increased the compression ratio. The Vortec heads come with a 64cc combustion chamber, while the Corvette heads use a smaller 58cc chamber. However, this 64cc chamber is still smaller than the stock Goodwrench 76cc iron castings. This means that the compression is down from the aluminum heads but is still higher than the stock iron heads.

Looking at the results from Test 2, you can see that the Vortec suffers slightly compared to the Corvette heads below 4,000 rpm, but this is an average power loss of barely 4 lb-ft of torque, which would be tough to detect in the car. The good news is that above 4,000 rpm, the heads cranked an average increase of 13 hp from 4,000 to 5,800 rpm with a maximum increase of 38 hp at 5,800. That’s power you can feel!

For Test 3, McKenzie put about a half-hour into a minor tune-up of the area directly below the valve seat in the intake and exhaust ports and machined a 30-degree back-cut on the stock exhaust valves to improve low-lift flow. Ed Taylor then bolted the heads back on the engine and tested it again, with no other changes. Again, the engine suffered in the torque comparison versus the Corvette heads below 4,000 rpm, but the total average loss was less than 2 lb-ft of torque. On the plus side, above 4,000 rpm the porting had a chance to strut its stuff. While peak horsepower jumped slightly to a best of 384 hp at 5,700, the interesting info was that the entire torque curve above 4,000 averaged an increase of over 20 lb-ft at each rpm point. Peak torque didn’t change significantly at 401 lb-ft at 3,600. From this test, you can imagine this motor would really pull between 4,000 and 6,000 rpm.

Conclusion

This put our under-$2,500 pedestrian cast piston Goodwrench 350 at a stout 384 hp at 5,700 rpm and only an agonizing 16 hp away from 400. This is so tantalizingly close that we’re not quitting yet. We think we can break the 400hp barrier with this engine without resorting to exotic aftermarket parts. The plan involves some minor tuning, perhaps a bigger set of headers, or different mufflers, but you’ll have to wait until the next installment to find out if we pulled it off.