Words and Photos: Richard Holdener
When we run tests on intakes, camshafts or cylinder heads, one of the questions we often get is, do you tune each combination? The obvious answer is yes, as tuning each combination to maximize the power output is the only way to know the true difference between them. This becomes even more critical when the differences between components are minimal (say 2-5 hp). When it comes to tuning any combination, there are essentially two major components, they include air/fuel and timing. In terms of the air-fuel mixture, which is basically the amount of fuel supplied in relation to the amount of airflow, the mixture can be too rich (meaning too much fuel) or too lean (meaning too little). Like the air/fuel, the amount of total ignition timing can be either excessive (too much) or insufficient (meaning too little). If the optimized amount of timing and fuel is ideal, the question then becomes, what happens if it is less than optimum?
To illustrate what happens when the tune is less than ideal, we set up a test using a 383-inch LS stroker. Based on a 5.3L LM7, the motor included forged internals from Speedmaster, K1 and Wiseco, along with power producers from COMP Cams (the cam), TFS (heads) and FAST (the intake). The motor was not built with this specific test in mind, but rather as a test bed for things like induction systems, nitrous and even boost (though with good gas).
To test the effect of air/fuel mixture and timing, we dialed in both the air/fuel mixture supplied by the FAST 75-pound injectors and timing curve using a FAST XFI/XIM management system. The 383 test motor was run with long-tube headers, a Meziere electric water pump and FAST fuel rails. After dialing in the air/fuel mixture to 12.8: and the total timing to 29 degrees, the 383 stroker eventually produced peak numbers of 569 hp at 6,200 rpm and 532 lb-ft of torque at 4,900 rpm. The next step was to alter the air/fuel mixture and timing curves individually to see how much power they were worth.
The first step was to change the air-fuel mixture, from roughly 12.8:1 down to 12.3:1. The richer mixture had a minimal effect on power production, as the peak numbers dropped by a scant 1 hp and 2 lb-ft of torque to 568 hp and 530 lb-ft of torque. We then increased the fuel supply even more using the XFI management system and FAST injectors, to produce a mixture near 11.8:1. This resulted in a drop in peak power to 563 hp and 527 lb-ft of torque, a drop of 6 hp and 5 lb-ft of torque.
The final test was to drop the air/fuel ratio down to 11.3:1, a mixture usually associated with a boosted combination. This produced a drop in power to 556 hp (14 hp) and 522 lb-ft of torque (10 lb-ft). Though most tuners and engine builders would scoff at running a mixture this rich on a normally aspirated combination, the power loss was not excessive. Obviously, the motor would consume considerably more fuel, and problems might arise if running catalytic converters, but it does illustrate that changes in power are not excessive even with extreme changes in the air/fuel mixture. You will note that we did not run the mixture leaner than 12.8:1, as it did not pick up power and (excessively) lean mixtures tend to hurt the motor!
After illustrating the changes in the air/fuel mixture, we adjusted the XFI back to the 12.8:1 tune then started lowering the total ignition timing. Since excessive timing (like lean mixtures) can cause detonation, we only lowered the timing, though we have run more than 29 degrees on this combination and found no power gains (and eventually power losses) with excessive timing. It is important to note that the timing was not locked at a fixed 29 degrees through the entire rpm range. It started at 24 degrees at 3,000 rpm then rose 1 degree for each 500 rpm (meaning 24 degrees at 3,000, 25 degrees at 3,500 and 26 degrees at 4,000) then was a flat 29 degrees from 4,500-6,500 rpm.
After dropping the total timing curve by 2 degrees everywhere, to a maximum of 27 degrees, the power dropped by 2 hp but there was no change in peak torque. Dropping it another 2 degrees to a maximum of 25 degrees, showed a similar trend, with peaks of 563 hp (a loss of 6 hp) and 527 lb-ft of torque (loss of 5 lb-ft). The final change of 3 degrees (to a total of 22 degrees) resulted in a significant drop to 544 hp and 523 lb-ft of torque. As expected, the reduced timing had much more of an effect on power production higher in the rev range. These tests show that, even if you miss the tuning mark by a full air/fuel point and 3-4 degrees of timing, your combination will still make good power, but it will be even better if you have it dialed in!
Here are the dyno results of our 383 stroker run with an optimized tune. An optimized tune includes the proper air/fuel and timing curves to maximize power production through the entire rev range. An optimized tune means that any changes in the tune to add timing or lean out the mixture will result in either no change, or a drop in power.
After optimizing the tune, we decided to demonstrate the changes in power offered by altering the air/fuel mixture. Since leaning out the mixture can cause detonation and damage the motor, we decided to only show changes in power caused by a rich mixture. Using the FAST XFI management system, we added fuel for each run and altered the air/fuel mixture by roughly .5 points for each step. Check out graph 3 to see how each of these changes in air/fuel mixture altered the power output of the 383 test motor.
These results represent the power curves generated by each of the air/fuel curves in graph 2. As you can see, the optimized air/fuel (near 12.8:1) resulted in peak power numbers of 569 hp and 532 lb-ft of torque. Dropping down .5 points in air/fuel (to 12.3:1) had almost no change in power (1 hp and 2 lb-ft), while the next .5 drop (to 11.8:1) lost just 5 hp and 5 lb-ft. Adding even more fuel to produce a mixture of 11.3:1 resulted in peak numbers of 556 hp and 522 lb-ft of torque, a loss of 13 hp and 10 lb-ft. Even missing the air/fuel mixture by 1.5 points (from 12.8-11.3:1) was worth a total of just 13 hp. The point here is don’t expect any gains from minor changes in air/fuel and don’t expect huge gains from even major changes in the mixture.
The other critical element in any optimized tune is the ignition timing. The ideal timing value for will vary with both combination and engine speed. Obviously a supercharged or turbocharged motor will tolerate different timing values than a normally aspirated motor. The same can be said of engine speed, as the ignition timing needs increase with engine speed. For this test, we started out with a total of 29 degrees of total timing then dropped timing in 2-degree increments to 27 degrees then to 25 degrees. For the final test, we dropped timing by 3 degrees to 22 degrees total. As you can see, dropping from 29-27 degrees had almost no change in power (2 hp and 0 lb-ft). Dropping down to 25 degrees saw a loss of 6 hp and 3 lb-ft of torque. The final drop of 3 degrees to 22 degrees total resulted in the biggest change in power, especially at the top of the rev range where timing is most critical. Running just 22 degrees of total timing, the stroker LS lost 25 hp (a peak of 544 hp) though just 9 lb-ft of torque.