All the General's Heads. Author/s: Scott Parkhurst Issue: March, 2000
Testing the three best small-block Chevy heads from GMPP
General Motors really hit a bullseye with their series of "ZZ" smallblock Chevy engines. The fair prices, nationwide availability, and serious horsepower produced by these engines have made them the preferred choice of street rodders and racers alike. We decided to look at the cylinder heads offered by General Motors Performance Parts (GMPP) on the most popular of their small-blocks, the ZZ4 350.
The ZZ4 has a nice combination of features, including 355 hp and 405 lb-ft of torque. The retail price of just over $3,000 makes it an affordable, reliable, and available performance engine with a 12-month warranty. The engine arrives with an intake manifold, HEI distributor, and many of the little details crate engines normally ship without. Topping the ZZ4 is the aluminum L98 cylinder head that was available on the pre-LT1 Corvette. The aluminum angle-plug cylinder heads have screw in rocker studs, 1.94-inch intake and 1.50-inch exhaust valves and 58cc combustion chambers that yield a crisp and responsive 10:1 compression ratio. Also included are lightweight valvespring retainers that weigh half as much as the previous design. The valve seats are radiused and include valve stem seals to help keep the oil where it belongs. To complement the cylinder head design, GMPP incorporated a steel roller tappet camshaft with .474-inch intake and .510-inch exhaust valve lift.
We wondered how much more power we could get from swapping to GM's bottom-dollar Vortec cast-iron head, and also their new "Fast Bum" aluminum heads. We decided that back-to-back testing would be the most effective way to find out, so we got all the stuff we needed for the test and headed to the Westech Performance dyno to make some pulls.
We began with a baseline run on the ZZ4 as delivered, with the addition of GMPP plug wires, AC plugs, and a Demon 750-cfm carburetor. For the dyno testing, we also added a pair of Hooker headers (designed for a '69 Camaro), and Flowmaster threechamber mufflers. After setting timing at 35 degrees, we made a couple of pulls to check jetting, and were rewarded with as-advertised horsepower and torque numbers (355 hp, 405 lb-ft). Satisfied, we yanked the L98s, and hauled the iron Vortec heads to the dyno cell.
The Vortec cylinder head is the sleeper in the bunch. It was never a Corvette standard like the L98, nor is it being touted as a small-block head for the next millennium like the Fast Burn. It's been a standard on pickup trucks and SUVs for the last few years, gaining little recognition. As it turns out, the head was designed to flow very well so the truck engines could make good power for pulling heavy loads, trailers, and the like. The intake port was borrowed from the LT1, and the 1.94-inch intake and 1.50-inch exhaust valves allow the head to breathe. Combustion chambers on the Vortec heads are listed at 64 cc, but GM tells us most heads are actually around 61 cc. This bottom-dollar cylinder head ($420 per pair, complete with valves and springs) proved to be a capable performer, producing 376 hp at 5,600 rpm. The rest of the ZZ4 remained as-delivered. We used the same stamped-steel factory rocker arms that were used in the L98 aluminum head test. The only variance between these tests was the intake manif old design. The Vortec cylinder heads feature a simple, eight-bolt intake manifold design that has no access or clearance issues. We fell in love with this feature immediately. The L98 heads had the traditional 16-bolt pattern, so different intake manifolds were a necessity (Vortec cylinder heads that are double-drilled for both traditional and new-style intakes are available, but ours were not). We tested both dual- and single-plane designs on the Vortec heads, and found best power with the single-plane Edelbrock Super Victor for Vortec heads (PN 2913). Suprisingly, the dual-plane intake didn't have a huge torque advantage over the single-plane, although peak torque did occur at a lower rpm level.
Impressed by the performance of the Vortec heads, we anxiously looked to the Fast Burn aluminum heads. General Motors designed these heads to perform exceptionally well in low-compression street engines, as well as providing a good basis for a track-bound engine. In moving up to the Fast Burn heads, we also upgraded to 1.6:1-ratio roller rocker arms and the valve springs and retainers recommended by GMPP. The self-aligning rocker arms are necessary on the Fast Burn head, since it is not machined for guideplates or guideplate studs. GMPP offers self-aligning stamped 1.5:1 rocker arms, but we'd recommend stepping up to the 1.5:1 or 1.6:1 roller rockers. The GMPP "hot" cam we knew we'd be testing is designed for use with a 1.6:1 rocker, so that's what we used. The combustion chambers of these heads are a reasonable 62 cc, and compression remained low enough that we could run 92-octane pump gas for all our testing.
Stepping up to the Fast Burn heads (which are double-drilled to run either traditional or Vortec intake manifolds) and the 1.6:1-ratio roller rockers got us over 400 hp, but we wanted more. We removed the water pump and timing cover so the GM "hot" cam could be installed. The specs on the hot cam are .525 inch of lift (with the recommended 1.6:1 rocker) on both intake and exhaust, with intake duration at 210 degrees at .050 inches of lift, and exhaust duration at 220 degrees at the same .050 inches of lift. The lobe separation angle is 112 degrees.
Since both camshafts are hydraulic roller designs, we were able to use the same lifters and pushrods. This was a very simple swap, and we were back running again within 30 minutes. Much of the credit for this efficiency lies with General Motors' reusable Vortec intake gaskets, which we feel is one of the best ideas to come out of Detroit in a long time. Sure, most people won't be swapping intakes manifolds very often, but it's cool to know that if and when you decide to, the intake gaskets are still good for a few more miles. The pair we used had been swapped over 20 times, and was no worse for wear. Certainly, a dyno shop like Westech doesn't leave gaskets like this on any engine for long, so they'd see the true benefit of a reusable gasket design, but we feel engineering like this is long overdue.
The idle and vacuum qualities of the hot cam were excellent, allowing a nice 850 rpm while pulling at least 12 inches. Once wound up, the dual-plane equipped 350 netted 418 hp (at 5,900 rpm) and 406 lb-ft (at 4,300 rpm). A quick swap to the single-plane Vortec intake gave us our best numbers, peaking horsepower at 432 (at 6,100 rpm) and grunting 412 lb-ft (at 4,700 rpm).
The cylinder heads we tested were untouched, and went directly from the box to the block. The Speed Demon 750-cfm mechanical secondary carburetor was also out-of-the-box. Our jetting requirements varied with the heads, as did timing. Typically, Demon carbs like a bit more jet than traditional Holleys, and ours was no exception. While the L98 and Vortec heads liked an 82 primary jet, the Fast Burn heads found best power with an 84. Secondary sizes were 92 jets on the L98 and Vortec heads, and 91 on the Fast Burns.
Tuning was tested at various points in all tests, and we found the Vortec heads liked 32 degrees of total advance, while the Fast Burn heads were happiest with 35 degrees. The L98 heads also preferred 35 degrees of total ignition advance.
So, what have we learned from all this? We've concluded that the ZZ4 performs as advertised, and is very receptive to well-chosen bolt-ons. We learned that GM has some killer iron heads that'd be a great upgrade for anyone on a budget. Finally, we've found that GM's new Fast Burn cylinder head makes great power when teamed with the rest of the package GM has designed, but even if you can't get the cam and the single plane all at once, you'll still see some benefit from simply bolting these heads on. With no porting or modifications at all, these out-of-the-box parts worked great, and their reasonable price and nationwide availability make them a definite consideration for anyone looking for a killer small-block Chevy street engine.
Our three contenders were, from left to right, the L98 aluminum head, the castiron Vortee (L31) head, and GM's new Fast Bum aluminum head.
The intake ports of the three heads show some basic design differences. The L98 head has a good port opening, but has pushrod clearances that hamper its performance. The Vortec head (center) uses a different intake port design, credited for much of the power increases. The Fast Burn head has a CNC-finished intake port that helps airflow.
The basic D-shape port of the L98 head is a solid design that could benefit from a bit larger valve or some simple porting. The Vortec head uses gentle curves and proper sizing to gain efficiency, while the Fast Burn head goes with a slightly larger D-shape to take advantage of its larger valve. We know the angle is slightly different, but look how much more of the exhaust valve you can see in the Fast Burn head. This "line-of-sight" design limits how much exhaust gases have to turn while exiting the chamber, and there's power in that.
The combustion chambers are comparable in size, but vary slightly in design. The L98 head relies on a simple half-moon shaped chamber with the spark plug aimed in the general direction of the exhaust valve. The Vortec head gains more of a kidney bean shape, increasing quench, and has had some attention paid to the swirl characteristics of the incoming charge. You'll also note that the exhaust valve is less shrouded to aid in its ability to herd hot gases out of the cylinder. The Fast Burn head borrows much from the Vortec, and with a slightly larger valve, and angled plug, and excellent efficiency characteristics. It excels as a street-friendly aluminum head.
The dual-plane intake we used (top) carries GM PN 12464340, and provided solid performance numbers on both the Vortec and Fast Burn heads. Incorporating the simple eight-bolt Vortec design, intake swaps could not have been much easier. The single-plane we tested was an Edelbrock Super Victor for the Vortec head (PN 2913), and it provided the fattest horsepower numbers without sacrificing torque. GMPP offers a very similar single plane called the Eliminator (GM PN 12496822), and we'd expect similar results from it.
The single timing chain performed well in our dyno tests, but if we were going to slide this bent-eight into our daily driver, we'd upgrade here.
GMs hydraulic roller lifter design is simple and effective. We used the same lifters on the two different camshafts we tested, and the simplicity of the design combined with good quality makes this a part we like. The retainers cannot be reinstalled incorrectly.
The new small-blocks also use a cam plate to keep the roller cam from walking forward in the block. This replaces the more traditional use of a cam button, and this simple plate has got to be much cheaper to manufacture. The camshaft has to be machined for this, so if you get a new cam for a ZZ engine, make sure the cam grinder knows about it.
Here are the intake gaskets we mentioned in the text. These outstanding units have seen upwards of 20 intakes, and are still sealing well. One set should last the life of your engine.
COPYRIGHT 2000 McMullen Argus Publishing, Inc
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