cam idle tuning - Page 2

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If you're one of the lucky ones who experienced the first result, congrats, your work is just about finished.  At this point you can sit happy where you're at or you can determine whether or not the idle speed can come down.  If you think it can, lower the target idle speed by 50 or 100 RPM (depending on your hunch) and then go through Steps 1-4 again, this time reducing the airflow minimum values by a small margin to make up for the reduced engine speed and considering the thought that you may want to lower idle timing advance a touch as well. If the idle becomes unstable, try raising the idle timing advance if necessary. If idle is still unstable, raise the target idle speed to the lowest possible point where idle stability exists and repeat steps 1-4.

If you experienced the second result, you probably have too much Base Running Airflow added (Airflow Final Minimum table).  If reducing these values fixes the issue but does so at the expense of a steady idle speed, then return these values back to where you had them set and modify the Throttle Follower torque values. When you look at this table, you'll notice that engine speeds higher than your target idle have negative values while speeds lower than your target idle have positive torque values. The ECM uses this table as a reference to add or subtract torque depending on whether the engine speed is below or above the target idle respectively. If the engine speed hangs after you goose the throttle, you can decrease the negative values (make them "more negative") to reduce the airflow adder and thus allow the engine to decelerate quicker and thus not hang at an elevated speed.  Conversely, if the engine overshoots the target idle (as would be the case in the third and fourth scenario), you can make the above-target-idle values less negative (raise the values toward a positive number) to slow the rate of engine speed deceleration.

In scenarios 3 and 4, you may need to add more Base Running Airflow at the critical engine speeds you're experiencing issues with and use the Throttle Follower tables to trim for the desired result.  Note that with a long duration / high-lift cam, you may have so much air flow in the Airflow Final Minimum table that you may find that you may need to reduce all of the Throttle Follower values to prevent hunting.  If this seems to be the case, you have another step you can take to sort out the last bit of your idle woes and that is to go on to Step 5.


In the Adaptive Idle section, specifically the PID's, you will find a "Proportional" Airflow correction table. This table maps out the rate of airflow addition or subtraction as a function of engine speed error above or below the target engine idle speed. If the engine decelerates too quickly, you can slow the rate of deceleration by increasing the negative values (make them less negative by moving the values closer to zero). On the other hand, if the engine speed hangs, you can decrease these values (make them more negative). The preceding applies to the situations where the engine speed in question is above the target idle speed. When you're below the target idle speed, you're dealing with the positive values on the other side of the table. If the engine overshoots the target idle speed, increasing these values adds more air and allows the engine to stabilize and recover proper idle if the speed is too low. Make small, incremental changes when adjusting this table as having values that are excessive will almost guarantee a hunting idle condition.


When tuning the engine for optimum idle speed, remember that Base Running Airflow sets the engine up for a stable idle, Throttle Follower adjusts for goosing the throttle and the Adaptive Proportional Airflow table trims the Throttle Follower for accurate function. Of course, all of this relies on the air/fuel ratio being calibrated properly and the idle ignition timing being sorted out. Spending some time fully mapping out all of these variables can result in an engine that runs at a nice slow, steady idle speed that's easy to drive and sounds incredible with a nice camshaft. There's nothing more embarrasing than having your engine stall whenever you tip-in and -out of the throttle when driving your hot-rod, so take the time to get these tables right. And if you need professional help to get your idle sorted out, remember and be respectful of the fact that time is money and it does take some time to sort through the idle calibration.  It's an iterative process that'll reward you with a fantastic engine to drive.