Even more than proper air/fuel ratio, ignition timing is THE critical element in any internal combustion engine, particularly when you add boost to the equation.
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. Toss forced induction into the mix, and things get more difficult.
Compared to naturally aspirated combos, the safe tuning window is much narrower, meaning much less room for error! 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 you can quickly say goodbye to an expensive forged piston or two.
The potential repercussions often tempt builders of supercharged and turbocharged motors to err on the conservative side, it should be pointed out, there is also considerable power to be had with sufficient ignition timing. The benefits and perils of timing my friends, are what create the tuning conundrum. 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 with timing adjustment on both the turbo and supercharged 4.8L motors.
Timing sure adds power on a boosted motor—but things can get ugly just as quickly. (Image/Richard Holdener)
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 Turbosmart wastegates armed with seven 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 to 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 higher in the rev ranges.
Whipple Twin-Screw 4.8L: 15 to 23 Degrees
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—we suspect a gauge error!
Given the 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, which is why we suspected gauge error).
Obviously, the Whipple 4.8L was in need of 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! Timing matters!
Centrifugal Supercharged 4.8L: 18 to 23 Degrees
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 GenX 205 heads, ATI Super Damper, and stock truck intake, but the cam used on this test was a Stage 1 blower cam from Brian Tooley Racing.
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 just 18 degrees of timing, 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 five degrees of ignition timing (from 18 to 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.
Graph 1: Effect of Timing – Turbo 4.8L
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 increased 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 wastegate 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 six psi kept testing plenty safe. Timing adds power, just make sure you don’t get too greedy. (Dyno Chart/Richard Holdener)
Graph 2: Effect of Timing – Whipple Supercharged 4.8L
We saw a similar trend on the Whipple 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—indicated 19 psi vs less than six psi on the turbo. (Dyno Chart/Richard Holdener)
Graph 3: Effect of Timing – Vortech Supercharged 4.8L
Like the rest of these timing tests on the 4.8L, 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 to 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 to 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 13 psi of boost 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 Brian Tooley Racing. (Dyno Chart/Richard Holdener)In preparation for boost, the 4.8L short block received JE forged pistons, Fel-Pro head gaskets, and ARP head studs. (Image/Richard Holdener)To enhance power both NA and boosted, we replaced the stock cam with a healthy COMP Cams profile that offered 0.590 lift, a 224/232 degree duration split, and 115 degree LSA. (Image/Richard Holdener)The 4.8L also received a set of Trick Flow GenX 205 heads. Previous testing showed that these CNC-ported TFS heads were worth an easy 30 horsepower over the factory heads. (Image/Richard Holdener)The induction system for the turbo test mule included a Holley Hi-Ram intake, fuel rails, and 83 pound injectors (Image/Richard Holdener)Boost was supplied by a single turbo system that included DNA headers feeding a custom Y-pipe. Designed to accept a T4 turbo, the Y-pipe also featured provisions for a pair of Turbosmart wastegates with seven psi springs. (Image/Richard Holdener)Tuning came from this Holley Dominator EFI system. Changing the ignition timing for this test required a few simple key strokes. (Image/Richard Holdener)Tested on the turbo 4.8L, increasing the timing from 18 to 24 degrees resulted in a gain of 36 horsepower, from 539 hp to 575 hp. (Image/Richard Holdener)The timing test run on the supercharged combination featured the same 4.8L short block equipped with stock heads and cam. (Image/Richard Holdener)The 2.9L Whipple supercharger featured a dedicated (short-runner) lower intake manifold. (Image/Richard Holdener)The short-runner, open-plenum design offered sufficient room to house an air-to-water intercooler. (Image/Richard Holdener)The supplied fuel rails required the short (LS3-style) injectors. FAST supplied these 75 pounders to ensure our supercharged 4.8L had sufficient fuel. (Image/Richard Holdener)The 2.9L Whipple was fed by a FAST 102mm throttle body and supplied adapter. (Image/Richard Holdener)Boost supplied to the motor was determined by the size of blower and ATI crank pulley. We ran a 4.0 inch blower and 7.5 inch ATI Super Damper. (Image/Richard Holdener)Run on the dyno at various timing levels, the Whipple supercharged combo was even more sensitive to changes in ignition timing. Stepping up from a low of 15 degrees to 23 degrees resulted in an extra 114 horsepower—from 462 to 576 hp. (Image/Richard Holdener)The 4.8L used for the Vortech supercharged timing test was configured with TFS GenX 205 heads and a BTR Stage 1 blower cam. Note the (pinned) ATI Super Damper once again employed to drive the Vortech supercharger. (Image/Richard Holdener)The Vortech V3 was configured with a 3.80 inch blower pulley and the 7.5 inch ATI damper to produce a peak of 13 psi of boost. (Image/Richard Holdener)Unlike the turbo and Whipple tests, the Vortech was run without an intercooler. Feeding boost from the V3 supercharger to the Accufab throttle body and stock truck intake was a simple discharge tube equipped with a Vortech blow-off valve. (Image/Richard Holdener)Altering the timing from 18 to 23 degrees on the Vortech-supercharged 4.8L showed improvements of over 50 hp. (Image/Richard Holdener)
Richard Holdener is a technical editor with over 25 years of hands-on experience in the automotive industry. He's authored several books on performance engine building and written numerous articles for publications like Hot Rod, Car Craft, Super Chevy, Power & Performance, GM High Tech, and many others.
Comments