So you decided to put a nice big lumpy cam in your LS3 or LS7 engine and now find you're having some trouble getting the idle tuning squared away. No problem.  You're in the right place.  What we're going to do here is show you how to get the engine idle right if you've already managed to take care of the air/fuel

IDLE TUNING - GM LSx

cam idle tuning


 

STEP 1.

If you have the air/fuel ratio set properly and your engine can idle somewhat decently, even if it's by holding the throttle pedal down off closed-throttle to keep the engine from stalling, you can begin by setting a ball-park ignition advance value.  For mild cams, an idle ignition timing advance of approximately 16-18 degrees will do. For medium cams, try within the range of 18-22 degrees of advance.  Wild, lumpy cams may require 24-30 degrees depending on the compression ratio, cylinder head port sizes, etc. The optimum advance can only truly be found using a dyno, but another "cheater" method is to watch manifold vacuum levels. A damped vacuum gauge is best, but the MAP reading will do if a proper needle-type analog gauge isn't available. The goal is to run enough timing advance that you obtain the highest manifold vacuum level at the lowest engine speed possible.  So for example, if you have to pedal the throttle to keep the engine running at 750 RPM and you have 16 degrees Base Idle Advance with 70 kPa manifold pressure and you're holding the throttle at 22%, changing the Base Idle Advance to 19 degrees may result in 68 kPa manifold pressure with you only holding 20% throttle to maintain the same 750 RPM idle speed.  You can keep sneaking the idle timing advance forward while monitoring the above parameters to see where you reach a point of diminishing returns. Too much advance may lead to tip-in throttle knock (detonation) and unstable idle speeds. Be careful when adding timing advance at idle and only add a little at a time. If you try adding timing advance and nothing changes, stick with the lower value.


STEP 2.

The next step is to select your initial target idle speed.  Your engine's optimum idle speed is the RPM that allows the lowest possible engine speed while allowing a stable idle.  We'll define "stable" as +/- 50 RPM from the target idle speed you set.  It's not easy to get a big cam to idle smoothly at 650 RPM (but it may be possible), so you want to go for something easy first.  For example, if you have a cam that's between stock size and 224 degrees intake duration, try an initial target idle speed of 800 RPM.  If the cam is between 224-236 degrees duration (again, on the intake side) try a target of 900 RPM and for cams bigger than those listed above, set your target engine idle speed to 975 RPM.  These will be a good, easy starting point depending on your engine setup. Note that these idle speeds are for the engine idling at normal operating temperature.  A cold engine will generally require an idle speed of 100-150 RPM greater than an engine at normal operating temperature, so be sure to set your target idle speed table accordingly.  You'll come back and refine these values later, once you've gone through the steps below.


STEP 3.

Set the Base Running Airflow - Airflow Final Minimum values.  As your engine has electronic throttle control (ETC), think of this parameter as the idle set screw on a carburetor, only this offers you far more control.  For example, on the '08 Corvette Z06 with an E38 ECU, this table offers minimum airflow control for each gear as well as Park and Neutral and for engine speeds from 250 RPM through 8,000 RPM.  Most E38 and E67 calibration files are similar this way.  Since we're tuning for idle only, don't worry about anything above 1,200 RPM for now. The stock values in the table are usually around 8.0 gms/sec for the LS7 and between 6-7 gms/sec for 5.7, 6.0L and 6.2L engines.  For a mild cam with a stock-weight flywheel, increase these values by 2-3 gms/sec to begin with.  A wild cam with a light-weight flywheel may require an additional 4-6 gms/sec over the stock values.  The trick here is to increase these values gradually until the engine can maintain the target idle speed you've selected without forcing the ECM to add or subtract ignition timing, and of course without requiring you to hold the throttle pedal off idle.  If the commanded ignition timing is 16 degrees (or whatever you chose in Step 1) and you see that the ECM is idling with 20-22 degrees of idle timing (or several degrees more than your commanded value), you need more air flow, and thus a higher value in the Airflow Minimum table. Keep adding airflow through this table until you see that the ECM starts reducing ignition timing advance relative to commanded idle timing in order for it to maintain the target idle speed.  The goal is to arrive at the values that allow the engine to idle smoothly at the target idle speed without the driver pedaling the throttle to maintain idle speed. Be careful not to add too much air through this table, otherwise the ECU will always pull ignition advance to lower the hanging idle speed.  This will also result in a hanging idle when decelerating. Once you've got this sorted out and the engine can idle fine without the driver depressing the pedal to maintain idle, note the values you've arrived at and modify your Startup Airflow table accordingly.  For example, if your stock Airflow Final Minimum values were in the 6-7 gms/sec range and you adjusted them up by 4 to obtain a smooth idle, do the same for the Startup Airflow table and add perhaps a tad more if needed to ensure proper engine start on the first attempt.  The bigger the cam, the more airflow you may need in the Startup Airflow table to get the engine to start instantly on the first attempt. In otherwords, the spread between the Airflow Final Minimum table and Startup Airflow table may need to be greater the bigger the cam profile.


STEP 4.

At this point your engine should be able to start up instantly and idle fairly smoothly.  Now it's time to set the Throttle Follower values. The E38 and E67 ECUs use a torque value for Throttle Follower function rather than airflow values as used by the older GM ECU's.  If you performed steps 1-3 properly and your engine is able to idle smoothly, goose the throttle while at idle and note what your engine does. Generally one of four things will happen:

1. The engine revs up and then decelerates toward idle smoothly and steadily and returns to the target idle without overshooting or undershooting the target idle speed.

2. The engine revs up and then the engine speed hangs for a moment and then slowly returns to idle, undershooting the target idle speed and taking too long to return to the target idle speed.

3. The engine revs up and then decelerates rather quickly and overshoots the target idle speed and stalls.

4. The engine revs up and then decelerates rather quickly and overshoots the target idle speed but doesn't stall, and instead recovers and then momentarily hunts for idle, criss-crossing the target idle speed several times before finally settling in at the target idle speed.

ratio and get that part of the tuning process straightened out.  If the engine can run fine, but only if you pedal it to keep it running, you can breathe a sigh of relief 'cause you're in the home stretch. The stock tables in the OE calibration are mapped for a cam that will produce lots of vacuum at idle speeds of 550-700 RPM. Lots of intake manifold vacuum at idle allows for a greater degree of control over torque output as there's a lot you can do to control the pressure each cylinder produces during the power stroke.  You can easily alter cylinder air charge, timing advance and engine speed acceleration and deceleration to always hit the target idle speed without overshooting or undershooting the target idle speed.  Almost all aftermarket cams have enough duration and overlap to inadvertantly cause a very low intake manifold vacuum level at idle and that creates a few challenges when trying to tune for a dead-steady idle that doesn't hunt, undershoot or overshoot when the engine decelerates, or causes a stalling condition.

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