Words and Photos By: Richard Holdener
One of the most common questions we get while dyno testing our typical 5.3L LM7 LS motor is how can a stock 5.3L make 355 hp? This is a fair question, given the fact that it was rated at just 285 hp (275 for the early versions). Do we not know what we are doing? Is it a case of happy dyno numbers? Actually, the answer comes from a simple difference in test methods. The factory power rating comes from a specific test procedure, where the motor is run with every component as installed in the vehicle. This includes everything from the air intake system out through the complete exhaust. This also includes all of the accessories, and that the motor is run at the normal operating temperature using the factory calibration (meaning conservative tune). When we pluck some unsuspecting 5.3L from the junkyard, we normally run it with long-tube headers feeding collector extensions, an open throttle body (no air intakes system) and a Meziere electric water pump (meaning no accessories). We also optimize the air/fuel and timing to maximize power and run the motor colder than the factory.
The difference in test procedures means little if we are performing a direct back-to-back test, as the critical point in any test is that you change just 1 component. As long as the test procedure is consistent for each component, the difference in power offered by that component is accurate. Testing on a higher horsepower combination might yield slightly greater gains, but we are still just looking for a delta change. That said, we still wondered just how much each of the subsystems was worth on the dyno. How much power is hiding in the air intake system? Are the factory exhaust manifolds really that restrictive? Just how much power does the fan consume? To find out, we set up one of our junkyard 5.3L LM7s and proceeded to test the individual subsystems one at a time. We started with the motor run in our normal configuration, meaning long-tube headers, open throttle body and Meziere electric water pump. We then added stock manifolds, a complete air intake assembly and full accessories, then finished off by increasing the water temperature and adjusting the tune. How much was each change worth? Check out the graphs and see for yourself!
Graph 1: 5.3L Headers vs Stock Exhaust Manifolds
The first test involved running the junkyard 5.3L LM7 in our usual manner with long-tube headers, open throttle body and no accessories (just a Meziere electric water pump). Run in this manner, the stock LM7 produced 355 hp and 381 lb-ft of torque. After replacing the headers with stock exhaust manifolds, the power dropped to 345 hp and 374 lb-ft of torque. The stock manifolds were run with open 2.5-inch exhaust extensions, so a complete exhaust (with cats) would hurt power even more.
Graph 2: 5.3L Factory Air Intake System
After testing the stock exhaust manifolds, we installed the factory air intake assembly on the open throttle body. The factory air intake assembly consisted of the airbox and filter, MAF and rubber inlet tube. Obviously, the intake system represented an inlet restriction, as the power dropped to 335 hp and 367 lb-ft of torque (a drop of 10 hp and 7 lb-ft).
Graph 3: 5.3L Full Accessories (Minus the fan)
Having installed the factory manifolds and air intake, we turned our attention to the accessories. LS applications typically run a water pump, power steering pump, alternator and A/C. For truck applications, they also employ a clutch-driven cooling fan, but that was left off for this first test. Run with all the accessories (minus the fan), the power output of the 5.3L dropped from 335 hp and 367 lb-ft of torque to 324 hp and 358 lb-ft (a drop of 11 hp and 9 lb-ft).
Graph 4: 5.3L Full Accessories with Fan
Since we ran the accessories without the fan on the last run, we decided we had to see just how much power the fan was robbing when it was activated. The clutch that engages the fan is thermostatically controlled, meaning it is activated when hot. We simply heated the clutch mechanism with a torch to get it hot and engage the fan. Having to spin the fan cost power, as the new output of the 5.3L dropped to 318 hp and 352 lb-ft of torque (a loss of 7 hp and 6 lb-ft).
Graph 5: 5.3L Hot vs Cold Water Temp
We typically run test the motors at or near 140 degrees of water temperature but most LS applications run the motor much warmer (including during factory testing). To illustrate the change in power offered by water temperature, we adjusted the thermostat on the dyno (the engine one is removed for testing) to increase the temperature from 140 degrees to 185 degrees. Normally we see a decrease in power from the change in temperature (especially in the lower rev ranges), but on this application we saw a slight increase in power to 322 hp and 355 lb-ft. We duplicated the test but found the same thing happened. Possibly the clutch in the fan was not fully locked, but each combination repeated again and again (we are still scratching our heads on this one).
Graph 6: 5.3L Factory Tune
We know from experience that the factory air/fuel and timing values are purposely conservative. They have to make the combination safe for people who insist on running cheap gas and lugging the motor up a steep grade in top gear. This safety margin is why there is usually power to be had from hand-held, flash programmers. Knowing this, we dialed back the optimized air/fuel and timing values we run on each combination for this last test. Without knowing the exact air/fuel and timing that the factory combination would run, we simply pulled 4 degrees of timing out and richened the mixture from 12.8:1 down to 12.0:1. The conservative tune dropped the power output to 313 hp and 347 lb-ft of torque.
Graph 7: 5.3L Start to Finish
This graph illustrates the difference in power between the way we normally run a motor on the dyno and after adding the stock manifolds, air intake and accessories. This also shows the change in temperature and conservative factory-style tune. The difference between the two is significant (40+ hp), and shows just how much power is hiding in the various factory systems. Of course, it is difficult to eliminate these on anything but an all-out race car, but it is nice to know where you could spend your time looking for extra power.