Those of you who have been following Project C3 Triple-Ex have undoubtedly been anxiously awaiting our decision as to which powerplant would be placed between the framerails. In previous issues we outlined our planned engine build, which was instigated in part by Editor Heath's challenge to build an old-school, first-generation small-block Chevy to similar specs as the Z06 LS7, but with a higher output. While this goal might seem easily attainable, we knew that building a 505-horse (or higher) small-block that would run on pump gas presented several daunting obstacles. This month we'll outline how we elected to overcome these obstacles, and show you our final engine assembly and dyno results.
It's no secret that GM's LS series of V-8 powerplants offers superior design in several ways, particularly in terms of cylinder-head flow. Let's face it: The small-block Chevy has been around since the '50s, and engine technology has come a long way since then. By virtue of a better valve angle (15 degrees in the standard LS, 12 degrees for the LS7), along with larger ports and massive 2.200-inch titanium intake valves, the CNC-ported LS7 head is simply a better-engineered piece than most heads available for the old-school SBC. With that in mind, we knew our work was cut out for us when deciding which parts to use for our project engine.
Starting with the basics, we decided that since the LS7 is built with an aluminum block and heads, our small-block should meet the same criteria. After choosing a cast-aluminum block from Dart Machinery, we had Auto Performance Engines (APE) in nearby Auburndale, Florida, machine it to a final bore size of 4.125 inches, identical to the LS7. We then filled the block with a Scat 4-inch-stroke forged crankshaft, Scat 6-inch forged I-beam connecting rods from Summit Racing Equipment, and forged-aluminum dish pistons from Mahle.
While the LS7 has lighter titanium connecting rods and cast pistons, we felt the exotic rod material didn't justify its cost in our case. Forged pistons will also ensure our engine is durable, and they won't give any discernible power advantage over cast units. We chose not to add a dry-sump oil system to our engine, even though the LS7 does have this feature. Admittedly we're giving up some power here, but the simplicity of a wet-sump system from Stef's Fabrication Specialties will make final installation into our '71 Stingray somewhat easier.
To top this engine, Dart's aluminum Pro 1 CNC cylinder heads with 227cc intake runners were chosen for their proven performance and relatively reasonable cost. While we're sacrificing a fair amount of runner volume, flow, and valve size to the LS7 heads, we felt that the 23-degree Pro 1s would meet our needs while not requiring any exotic parts. Even better, these heads come from Dart fully assembled and ready to bolt on, eliminating the need for expensive cylinder-head machine work (although we did find it necessary to have APE angle mill them to achieve our required compression ratio of 11.1:1, very close to the LS7's 11:1 ratio).
We're giving up quite a bit of flow here, as Dart claims a 309-cfm intake flow (at 0.700-inch lift), compared with some 360 cfm for the LS7 heads, but achieving comparable flow would require fully ported race heads. Additionally, the best valve angle available for the SBC is 18 degrees, well short of the 12-degree LS7. As you're likely figuring out, we aren't making our job any easier, but if we can achieve our performance goals using less exotic 23-degree heads, it will keep costs down and more closely resemble an engine that the average enthusiast might build for his (or her) car.
To actuate the valvetrain, we chose a combination of a Comp camshaft, lifters, and pushrods; with Jesel shaft-mounted rocker arms and a Jesel adjustable beltdrive timing set. Knowing that accurate valve timing is one key to making consistent power, our Dart block features a raised cam location and 50mm cam journals. This allows the camshaft to be ground from a larger core, eliminating torsional flex. The raised cam location also allows the use of shorter pushrods, which are less prone to bend and flex under load, keeping the valvetrain more stable throughout the rpm range.
Additionally, we decided to take advantage of Jesel's shaft-mounted rocker arms, which are the industry standard in terms of valvetrain stability. Although we're giving up a fair amount in terms of intake runner volume, valve size, and valve material to the LS7 (which uses a titanium intake and sodium-filled exhaust valves), our combination of Comp and Jesel products will capitalize on the design of the SBC, allowing the use of items that aren't available to production LS engines.
Our Comp cam is a solid roller with 0.680 intake lift and 0.652 exhaust lift after lash, which we felt was an even trade-off given the LS7's superior intake-runner volume, flow, valve angle, and intake-valve size. We also chose the LS firing order, which swaps the No. 2 and 3, as well as the No. 4 and 7, cylinders in relation to the standard SBC order. Though it's difficult to find data to support that this swap makes more power, the Winston Cup (or is it "Sprint Cup" now?) NASCAR teams are currently using it, so we feel it must be worth something. It will also help our engine more closely mimic the design of the LS7.
Further, we wanted our engine to make its peak torque and horsepower at approximately the same rpm as the LS7, rather than over- revving it to meet our goals. The LS7 has a 7,000-rpm redline, and we chose our cam to make peak power well under this limit, along with peak torque right at 4,800 rpm—just like the LS7. To feed our small-block, we chose a Dart single-plane aluminum intake matched with a Quickfuel Technologies Q-series 950-cfm carburetor. Remember that this is an "old-school" build, and our car is already set up for a carbureted fuel system, further simplifying final installation. Despite what you may have heard, quality carburetors are very reliable, and a top-notch piece like the one we chose from Quickfuel won't give up much in terms of driveability and throttle response. Even better, it'll actually make more peak power than would fuel injection.
With the pieces for our LS7-killer in place and the necessary machine work accomplished, we decided to assemble our engine in the clean room at APE, rather than performing the task in our shop and then transporting it for dyno testing and tuning. While we were hoping to out-power the LS7 by a significant amount, the closer we got to finishing our engine, the more nervous we became about making the necessary 505 horses, let alone topping it substantially.
An engine dynamometer is like a lie-detector machine, and it's very difficult to cheat. Further, APE's dyno is known to be a little stingy, and owner Kevin Willis won't fudge on correction factors simply to get a big torque or horsepower number. We either had our combination right or we didn't, and we were about to find out.
With the engine bolted up to APE's Superflow 901 dyno, we filled the fuel cell with 93-octane gasoline from the station up the road and installed a smallish 750-cfm shop carb for the initial startup. Using a known carburetor is generally a good practice on a fresh engine, eliminating any surprises during break-in. We also took the necessary precaution of priming the oil system. This step not only ensures adequate oil pressure, it also fills the oil galleys before the distributor is installed and the engine fired for the first time.
No matter how much engine-building experience you have, there's always a little doubt until you hear your latest combination run. We knew we had done our homework, though, and our LS7-killer fired on the first hit of the starter. Since our Comp roller camshaft didn't require any special break-in procedure, we tuned the idle circuitry of our shop carb and checked for leaks, then ran the engine between 2,000 and 3,000 rpm, applying a varying load with the dyno servo to help seat the piston rings. With that procedure accomplished, it was time for the true test of our research: making a dyno pull to see how much power our old-school small-block would really make.
We decided to make a conservative pull on our first attempt, using the 750 carb, setting our ignition timing at 30 degrees total advance, and only revving the engine to 5,500 rpm. In general, loading the engine for the first time should expose any weaknesses, so simply making it through this pull indicated that ours was healthy and ready for some real testing. Glancing at the numbers from the conservative first pull, we noticed that we were already making nearly 500 lb-ft of torque at 4,800 rpm and more than 500 horses at 5,500 rpm—and we hadn't even performed any tuning or installed our Quickfuel 950 yet. Realizing that our engine clearly had the potential to blow away the LS7, we got busy tuning.
After installing our Quickfuel 950 and adding 2 degrees of ignition timing, we made another pull up to 6,500 rpm, netting 519 lb-ft of torque at 4,700 and 573 hp at 6,500—handily topping the LS7 in both measures. Further, our oxygen sensor and exhaust temperature data indicated our mixture was still very rich and the engine needed more ignition timing. By adjusting timing, jetting in our Quickfuel Q-series carburetor, and eventually switching to a Quickfuel 1,050-cfm QFX 4700 carb, we gained power on each successive pull, adding to our already huge output advantage.
Our not-so-small small-block eventually made a best pull of 537.7 lb-ft at 5,000 rpm and 613.3 hp at 6,500 rpm. Even better, it had a very broad torque curve, making more than 500 lb-ft from 4,400 all the way up to 6,300 rpm. Horsepower topped 600 from 6,200 all the way to 6,700 rpm, but it didn't fall off dramatically from there, only dropping to 596.0 at 7,000.
In summation, we're very happy with the performance of our 427-cube SBC, especially in relation to the LS7 we were tasked with targeting. This powerhouse should be a blast in C3 Triple-Ex, ensuring rapid acceleration and even shaving some 200 pounds from the nose. Driveability should be great as well, especially considering we've already installed a Tremec five-speed and will have the ability to pull into any gas station to fill the tank with premium unleaded.
If you're considering repowering your Stingray, you might want to consider a traditional small-block Chevy. Ours topped the LS7's performance by more than 108 horses, and it won't require any special engine mounts, headers, or fuel-system parts to bolt right into our '71 edition. Look for future articles detailing our experimentation with cam timing, carb spac-ers, valve lash, and tuning while on the dyno, and be sure to log onto www.vetteweb.com to let us know what you think of our engine build.