Engine Dyno Tuning - Dyno-Might

Tuning our LS7-killer small-block for optimum performance

From the July, 2011 issue of Vette

By Dave Young

Photography by Auto Performance Engines, Dave Young

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.

Ignition Timing
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.