Here at VETTE, we've heard both sides of the engine-dyno argument. On one hand, a stout dyno sheet offers bragging rights to the owner, but on the other, you can't race a dynamometer, and they cost a lot of money to rent. We tend to agree somewhat with both sides, but see the engine dynamometer mainly as a very useful tool, especially when searching for peak power or trying a new engine combination.
Dyno time can be costly, but when compared with tuning an engine at the track, where you're at the mercy of the weather and traction conditions, it can also be less frustrating. Let's face it: Bending over freshly painted fenders, swapping spark plugs to a different heat range is no fun, and it risks damaging the painted surfaces around the engine. Track tuning also requires deadly consistency on the part of the car and the driver, meaning variables such as traction, shift points, and weather must be accounted for—not an easy task. On an engine dyno, there's never a traction issue, and it's far easier to duplicate or account for weather conditions and monitor engine functions in the controlled environment of a dyno cell.
Another reason for performing your initial break-in and tuning on a dynamometer is the ability to easily check for and correct any leaks. Nothing is worse than installing your new engine, finding a fluid or vacuum leak, and having to pull everything back out for repairs. Furthermore, since most coated exhaust headers aren't designed for the heat generated by an engine while seating the rings or breaking in the camshaft, the coating will often discolor or burn off during this process. Most dyno cells have multiple sets of headers for popular engine applications, allowing you to break in your engine with a seasoned set, or to try different header combinations to see what works best—without risking damage to your own pipes.
We recently dyno'd our 427ci small-block Chevy engine, dubbed the LS7 Killer, on the Superflow engine dynamometer at Auto Performance Engines here in central Florida. This engine certainly exceeded our expectations, making nearly 615 hp and 540 lb-ft of torque, and will make our '71 project car, C3 Triple-Ex, a fun ride. We didn't just dyno this engine to get power numbers, however; while on the dyno we made nearly 30 pulls to test everything from carb spacers to camshaft timing. This month we'll share what our dyno time taught us, and tell you what worked and didn't work on our stroked first-generation small-block Chevy.
Carburetors and Jetting
When tuning an engine, we usually start by jetting the carb for the proper air/fuel ratio. There's an old saying in engine tuning that you can be too rich a bunch of times, but only too lean once. With that in mind, we began this session by choosing an appropriately sized carburetor and making some partial pulls while watching the air/fuel meter. Most dyno facilities have an oxygen (or lambda) sensor in the dyno exhaust. This allows for real-time readings, so a pull can be aborted before a lean condition turns dangerous. Once on the rich side, we adjust the carb jetting up and down to find peak power within the safe range of 12.5 to 13.3 parts air for every one part fuel.
We tested two different Quickfuel carburetors on our small-block, the first being a 1,000-cfm Q-series four-barrel, and the second a 1,050-cfm QFX-4700 with a larger base flange. Both of these carbs had power valves to aid in street driving. While our engine ran well with both, it liked the volume of the 1,050, making the highest peak power and torque numbers. Using an Innovate Motorsports air/fuel meter, we achieved our best pull with No. 82 primary jets and 90 secondary jets. Moving to larger jets is the safer, richer direction to tune; smaller numbers mean less fuel and a leaner mixture. When tuning, remember that jets and ignition timing go hand-in-hand, so once a jet change is made, the timing should be adjusted and any changes noted.
While some engine tuners consider ignition timing to be a separate function from the air/fuel ratio of the engine, in reality the two must be considered together. If more fuel is added, the spark will likely need to ignite the mixture sooner to burn the fuel efficiently. Also, the amount of advance and when it comes in are both important considerations, especially when the engine has high compression like ours. We usually begin with a very conservative timing setting—somewhere around 28-30 degrees total advance—then listen very carefully during the first couple of dyno pulls for detonation (spark knock). If none is heard, or indicated by high EGT readings, we advance the ignition timing a degree at a time, searching for the setting that nets the best power without detonation.
Our engine liked 37 degrees total advance, and changing that by one degree up or down didn't make much difference. A total of 40 degrees of ignition timing saw a small power decrease, indicating our engine was suffering a little spot-detonation. We also tried different timing curves (which dictate how quickly the timing comes in) using the springs and bushings provided with our MSD distributor. Our 427 small-block responded well to the blue distributor bushing, which gave the second lowest amount of advance, along with the tightest springs so the timing curve would advance slowly. By using this setup, we netted our peak torque number without sacrificing top-end horsepower. Remember that when jetting, start big and work your way down. With timing, start small and work your way up to stay safe. Each engine is different, which is why an engine dyno is such a critical tool for safe and effective engine tuning.
Once the mixture and ignition timing of the engine are satisfactory, and a baseline is established, the engine can be tested to find the effect of carburetor spacers. Carb spacers are manufactured by many different companies, in all types of configurations. And while we can say that generally a carb spacer of some type will improve power, the type of spacer needed greatly depends on the engine itself. Since a dyno corrects for outside factors, this is a great place to test multiple spacers and see the results.
We knew the long stroke of our small-block would like more plenum volume, and experience tells us that large-displacement engines like carb (or throttle body) spacers. With the 1,000-cfm carb, we first installed a 1-inch open spacer, gaining nearly 10 hp from our baseline pull with a marginal increase in torque. Installing a 2-inch open spacer netted even better results, improving power by 14.4 for a best pull of 609.6 hp with the 1,000-cfm carb. After testing the open spacers, we installed first a 1.5-inch, then a 2-inch tapered spacer. While each of the tapered spacers made 3-5 more horses than our baseline, our combination clearly preferred the open versions. In fact, we thought about stacking the open spacers to learn our point of diminishing returns, but ultimately discounted the idea, since hood clearance would become an issue in the real world.
We love being able to make precise adjustments and see a result on the dyno, and camshaft timing is another area where this is possible. And while we can't add variable cam timing to our engine like some modern engines have, our Jesel beltdrive timing system means that resetting the cam timing only takes several minutes. Better still, it can be accomplished without invading the crankcase of the engine.
Though confident we'd chosen a good cam with the help of the engineers at Comp, we wanted the option of moving the power and torque curves up or down the rpm range by adjusting timing. This meant calculating piston-to-valve clearance for multiple cam settings, and cutting the valve reliefs to provide adequate clearance. This would allow us to really fine-tune the combination, and find the best average torque and power numbers.
We began by running our engine with the camshaft installed at our baseline of 108 degrees intake centerline, and calculated the average torque and hp between 4,000 and 7,000 rpm. Then, without changing the tuning (other than correcting ignition timing for the camshaft movement), we advanced and retarded the cam timing two degrees in each direction—to 106 degrees and 110 degrees, respectively—and recalculated our averages.
We discovered that although the engine made slightly more peak power with the cam at 108 degrees, the average hp and torque were some 1-2 percent higher at 106 degrees, meaning the engine would make more power throughout the rpm range at that setting. Additionally, by advancing cam timing to 106 degrees, the peak torque and hp occurred at a lower rpm, which is better for street driving and all-around durability.
Most camshaft manufacturers recommend a certain valve lash for their solid-lifter cams, indicating the best all-around setting for these components. Engine variables such as aluminum construction, pushrod flex, and rocker design can all make a difference in the initial valve-lash setting, as well as in how well the engine retains its lash. So while it's important to properly set the valve lash in an engine and check it periodically, valve lash can be used to tune the engine as well.
Since a solid-lifter cam requires clearance between the rocker arm and valve stem, adding to or subtracting from that clearance can add to or subtract from lift and duration. We thought our cam was matched pretty well to the rest of our combination, but we decided to experiment with valve lash just to see.
By tightening the intake lash by 0.008-inch, we effectively increased the valve lift as well as the duration. A pull on the dyno indicated a loss of power, however, meaning our cam is plenty big for this combination. By loosening the intake then exhaust lash we lost power as well, winding up with the valve lash set at 0.018-inch intake and 0.020-inch exhaust, just as Comp recommends. This is a good example of how a change you might think would help can actually take power away from the engine. It's also an excellent example of why it almost always pays to follow to the manufacturer's recommendations.
While there's no perfect recipe of fuel, air, ignition, and camshaft timing that works for every engine, the engine dynamometer makes tuning an efficient operation that can be accomplished in a controlled environment. The feeling of being able to drop the engine into your Corvette knowing it will run its best from the outset easily offsets the cost of dyno rental. All told, we netted an additional 20 lb-ft of torque and nearly 40 hp by dyno tuning our engine, meaning more time having fun in the car and less time searching for the proper combination.
Be sure to watch future issues of VETTE, as we install this potent small-block in project C3 Triple-Ex. And be sure to visit www.vetteweb.com to check out how this project has taken shape.