Words And Photos: Richard Holdener
Ignition timing is a critical element in any internal combustion engine. Because of the changing loads and engine speeds associated with any performance engine, not only must the ignition timing be right for optimal performance, the proverbial “right” time varies considerably. Tossing forced induction into the mix only makes things more difficult, as boost considerably narrows the (safe) tuning window. Miss by a degree or two on a normally aspirated motor and you might be lucky enough to just experience a drop in power. Run excessive timing in a boosted application and say good bye to a piston or two. While the potential repercussions might temp builders of supercharged and turbocharged motor to err on the conservative side, there is considerable power to be had with sufficient ignition timing. To illustrate this point, we altered the total timing on three different LS engine combinations with as many different forms of forced induction. Check out what happened on both the turbo and supercharged 4.8L motors.
All three combinations actually shared the same short block that consisted of a stock 4.8L block, crank and rods teamed with a set of forged JE pistons. On the turbo motor, the short block was teamed with a set of TFS Gen X 205 heads, a Crane 224 cam and Holley Hi Ram intake. The turbo kit consisted of a single 76mm Precision turbo fed by a pair of DNA tubular headers and custom Y-pipe. Boost control came via a pair of Turbo Smart waste gates armed with 7-psi springs. Despite the low boost level, we employed a CX Racing air-to-water intercooler.
The combination of the low boost, intercooler and use of race fuel allowed us to safely run the timing test without fear of detonation. Tuning (both air/fuel and timing) was via a Holley Dominator EFI system, but no changes were made to the air/fuel ratio during this timing test. Check out the supplied graph for a complete rundown of the power gains, but know that increasing the ignition timing from 18-24 degrees netted an increase of 36 horsepower and 21 lb-ft of torque. The additional timing improved power through the entire rev range, though (as expected) the gains were more significant in the higher rev ranges.
The Twin-Screw Blower
The twin-screw blower shared the same 4.8L short block with forged pistons but the similarities ended there. Topping the short block was a set of a stock 706 heads with a COMP Cams 26918 valve spring upgrade. The spring upgrade was used later in testing when we added a wilder cam, but this ignition test was run with a stock 5.3L (LM7) cam. Unlike the turbo motor, boost was supplied to the nearly stock LR4 by a 2.9L Whipple supercharger. The ATI crank and blower pulley combination combined with the stock heads and cam to produce a peak of nearly 19 psi! Suspecting elevated boost levels, we started out with very conservative timing values.
As with the turbo testing, the supercharged motor also featured an air-to-water intercooler and race fuel to eliminate any chance of detonation. Starting at 15 degrees of total timing resulted in peak numbers of 462 hp and 453 lb-ft of torque. This was interesting since the normally aspirated 4.8L produced nearly 340 hp, meaning 19 psi of boost only managed to add 120 hp (a pathetic 6.3 hp per pound). Obviously the combination needed more timing, as each successive jump added big chunks of power. Running 23 degrees eventually resulted in peak numbers of 576 hp and 503 lb-ft of torque. The additional eight degrees of timing improved the power output of the supercharged 4.8L by 114 hp and 50 lb-ft of torque!
The Centrifugal Supercharger
Once again we relied on the 4.8L truck motor but in slightly different guise this time around. Still present on the 4.8L were the stock block, crank and rods attached to the forged JE pistons. Also present were the TFS Gen X 205 heads, ATI Super Damper and stock truck intake, but the cam used on this test was a Stage 1 truck cam from BTR. Providing positive pressure to the tune of 13 psi was a Vortech V3 supercharger.
Rather than go through the various timing levels, we jumped right from 18 degrees to 23 degrees. Using race fuel and the proper air/fuel ratio provided the necessary safety margin. Run with 18 degrees, the Vortech supercharged 4.8L produced 569 hp and 498 lb-ft of torque. Note from the curve we only ran the motor to 6,000 rpm with 18 degrees but stepped up to 6,300 rpm for the 23-degree run. The additional 5 degrees of ignition timing (from 18-23 degrees) resulted in a jump of over 50 hp at 6,000 rpm with the boost and relative power gains increasing with engine speed.
You will notice that each successive step up in ignition timing increased the power output of the turbocharged 4.8L. Starting at 18 degrees, the turbo 4.8L produced 539 hp and 502 lb-ft of torque, but stepping up to 20 degrees increase the numbers to 551 hp and 507 lb-ft of torque. Jumping to 22 degrees resulted in 558 hp and 517 lb-ft of torque while 24 degrees brought 575 hp and 523 lb-ft of torque. The changes in timing were made with no change to the waste gate setting. All testing was performed with 114-octane race fuel to eliminate any chance of detonation, though the use of an intercooler and a peak boost pressure under 6 psi kept testing plenty safe. Timing adds power, just make sure you don’t get greedy.
We see a similar trend on the supercharged 4.8L. Starting at just 15 degrees, the supercharged motor produced 462 hp and 453 lb-ft of torque, but stepping up to 19 degrees improved the power output significantly to 523 hp and 489 lb-ft of torque. The next jump to 21 degrees brought 548 hp and 501 lb-ft of torque, while 23 degrees pushed the peak numbers to 576 hp and 503 lb-ft of torque. It is important to note that the supercharged 4.8L combination differed from the turbo version, as the supercharged motor was sporting stock heads and cam. The boost was also much higher on the supercharged combination (19 psi vs less than 6 psi on the turbo).
Like the rest of these timing tests, the change in timing occurred during the initial tuning of the motor. We always start out with conservative timing values and dial in the air/fuel mixture before increasing the timing. On the 4.8L equipped with the Vortech V3 supercharger, the increase in ignition timing from 18-23 degrees resulted in a jump in peak power from 569 hp to 667 hp, but part of that difference came from the increased engine speed (from 6,000-6,300 rpm), Measured at 5,900 rpm, the increased ignition timing improved the power output by 54 hp with gains increasing with engine speed. This test was run with at 13 psi using race fuel with no intercooler. The 4.8L was equipped with the same TFS Gen X 205 heads used on the turbo motor but the cam was a stage 1 blower grind from LJMS.
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