iBooster and stop-and-go

Pre-AP Model S uses a conventional vacuum-assisted brake booster — the same type of brake system found in most non-electric cars. There is no electronic brake actuation. NAP cannot command the brakes. Full stop, no asterisk.

This is the core limitation that separates pre-AP from AP-era cars for stop-and-go use: AP Teslas got an electro-hydraulic iBooster that can apply the brakes on command. Pre-AP didn't. The iBooster retrofit adds it back.

Why vacuum brakes are the wall

When openpilot needs to slow down a car with electronic brakes, it sends a brake pressure command and the car handles the rest. On pre-AP, that path doesn't exist. NAP can only decelerate by reducing throttle and letting the motor's regenerative braking do the work. Regen braking works reasonably well on the highway, where a gradual reduction in speed is usually enough. It is not enough for abrupt slowdowns, and it disappears almost entirely below ~5 mph (8 km/h) — exactly where you need it most in stop-and-go traffic.

Without the iBooster, you are the brakes. NAP in pedal mode brings adaptive cruise down to ~1 mph and handles smooth following, but when traffic stops fast, your foot needs to be there.

The iBooster retrofit

The iBooster is an electro-hydraulic brake booster from the AP-era Model S — it replaced the vacuum booster on those cars. When wired up on a pre-AP car, it gives the control system the ability to apply friction braking electronically.

The iBooster unit itself is a salvage part from an AP-era Model S. It replaces the vacuum brake booster on the pre-AP car. The brake master cylinder mounts to it directly, so the installation involves the braking system: draining brake fluid, unbolting and removing the old vacuum booster, installing the iBooster in its place, and bleeding the brakes.

The SGH Innovations controller ECU

The iBooster requires its own controller ECU that the software can communicate with over CAN. In the Tinkla era this was the SGH Innovations iBooster Controller ECU, a purpose-built unit that sat between the software's brake commands and the iBooster actuator. Without a controller ECU, the iBooster is just a power brake booster — it assists the driver's foot, but it doesn't respond to software commands.

With the SGH ECU installed, the software could command a specific brake pressure over CAN. The ECU also included a vacuum sensor circuit to prevent commanding throttle and brake at the same time. The Tinkla Unity branch (approximately 2022) proved out the combination, and several community members ran real stop-and-go on pre-AP cars with it.

The Comma Pedal is required alongside the iBooster — the pedal handles throttle, the iBooster handles braking.

SGH Innovations is out of business. The controller ECU can no longer be purchased, which is the main reason this retrofit is currently a dead end for new installs. Tinkla-era cost, for reference, was roughly $800–$1,000 in parts (salvage iBooster + SGH ECU) plus brake work.

Current status in NAP

In the works, not promised. NAP wants to bring iBooster braking back — the plan is to reverse-engineer the SGH controller so the hardware path doesn't depend on a company that no longer exists — but there is no timeline, and it may not happen at all.

What exists in NAP today: the NAPiBoosterEnabled parameter key and a settings-panel toggle, both placeholders. There is no iBooster control logic in the current pre-AP port. If you have an iBooster already installed from the Tinkla era, NAP will not drive it.

If reverse-engineering ECUs is your thing, this is one of the most valuable contributions you could make — say hello in Discord.