GM: Silverado/Sierra better longitudinal tune

[morrislee]

This PR proposes reducing stopAccel from -2.0 to -0.39. The current value introduces overly aggressive braking, as shown in the comparison log where aEgo sharply spikes to -2.0 m/s², diverging from planner intent.

In contrast, a value of -0.39 closely matches stock ACC behavior and aligns well with planner targets, producing smoother and more natural stops.

📊 Three comparative logs are included:

Stock OpenPilot (-2.0) — aggressive decel with overshoot dc7716b32bf25574/00000029--3581c1f42c/2 (OP Long stopAccel = -2.0)

OEM ACC — smooth taper, no jerk dc7716b32bf25574/0000002a--a6b57eae9a/1 (Stock Long)

Tuned OpenPilot (-0.39) — matched decel to planner and actuator, no overshoot dc7716b32bf25574/0000002c--ee2c92ef33/1 (OP Long stopAccel = -0.39)

🧪 Delay Compensation Evaluation To validate the effective longitudinal actuator delay, we compare the planned acceleration across various delay indices (6–9) with the actual actuator command.

📈 Full-segment analysis (shown below) demonstrates: dc7716b32bf25574/0000002c--ee2c92ef33/1:6

This confirms that the system behaves as if it has ~0.625s delay. Combined with the stop behavior analysis, this supports reducing stopAccel to -0.39 to avoid overshoot due to delay-compensated controller output.

• Dongle ID: dc7716b32bf25574

I hope @sshane can sanction this slightly improvement :D

[sshane]

We should match the stock system, did you check what it does coming to a stop? Can you get a route? Also need to check this works on steep hills

[morrislee]

OEM ACC — smooth taper, no jerk dc7716b32bf25574/0000002a--a6b57eae9a/1 (Stock Long) stock acc is provided in the second graph

[sshane]

Again, we need to see what stock ACC commands, not the effect. That plot tells you nothing.

[morrislee]

Again, we need to see what stock ACC commands, not the effect. That plot tells you nothing. does this help?

[morrislee]

Stock ACC

OP Long with -0.39

[morrislee]

Stock

-0.39

[morrislee]

Stock

-0.39

[morrislee]

@sshane does the graphs help? I am not sure what I am looking for, could use a hand here

[morrislee]

@sshane

We should match the stock system, did you check what it does coming to a stop? Can you get a route? Also need to check this works on steep hills

Not many hills here, this is the steepest ones I can find to do stopping behind lead cars

Uphill dc7716b32bf25574/0000002e--781a6d659e/29:31

Downhill dc7716b32bf25574/0000002e--781a6d659e/45

Downhill dc7716b32bf25574/0000002e--781a6d659e/51

[sshane]

Can you get one other user with this car to try these changes and post a route?

[garrettpall]

Is this safe for the cars that fingerprint as Silverados that aren’t Silverado?

[morrislee]

Is this safe for the cars that fingerprint as Silverados that aren’t Silverado?

if it works for a large truck, I dont expect it would be worse for smaller cars. FYI, this never contributed to a shorter stopping distance that I can tell

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