I'm working on a restoration where I'm adding an electric motor to a classic Hemi V8. I’m curious about the best way to manage torque blending between the two power sources. Should the electric torque be delivered primarily at low rpm to fill the V8’s torque gap, or is a more constant assist preferable? What control algorithms or sensor inputs are commonly used to ensure smooth power delivery and avoid drivetrain shock? Would love to hear your experiences or recommendations.
Can classic Hemi torque be combined with electric motor assistance in a hybrid system?
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When I tackled a 5.7‑L Hemi rebuild a couple of years back, I decided to pair it with a 70 kW brushed‑DC motor mounted in the transmission housing. The first thing I learned was that letting the electric motor do the heavy lifting at low RPM makes the whole system feel far more natural. The Hemi’s torque curve is already flat, but there’s still a dip right around 1,500‑2,000 rpm when the intake starts to breathe. By programming the motor to ramp up its output from idle to about 3,000 rpm, the hybrid blend fills that gap, giving a seamless “low‑end boost” that feels like a supercharged Hemi without the lag. I kept the motor’s contribution relatively low (max ~0.2 Nm per Nm of engine torque) above 3,500 rpm, letting the V8 take over the bulk of the work.
For the control side, I used a simple torque‑request algorithm based on three sensor inputs: crankshaft position sensor (for exact engine RPM), a torque‑output sensor on the motor’s shaft, and a throttle position sensor that feeds both the ECU and the motor controller. The motor controller reads the RPM and throttle angle, then applies a proportional‑integral‑derivative (PID) loop to match the desired assist curve. To smooth out drivetrain shock, I added a “soft‑start” ramp in the controller that limits the rate of change of motor torque to about 8 Nm/s. That way, when you slam the gas from a standstill, the motor’s boost comes in gently, and the transmission sees a gradual increase rather than a sudden jolt.
I also placed a wheel‑speed sensor in the rear axle to double‑check that the combined torque isn’t exceeding traction limits. When the rear wheels start to slip, the controller trims the motor’s output in milliseconds, which has saved me from many a spin‑out on loose grip. In practice, this setup has given me a rock‑solid low‑end punch and a more linear power delivery across the rev range, without the harshness you sometimes get from a constant‑assist strategy. If you’re aiming for a smoother drive feel, I’d say stick with low‑rpm assist and let the V8 dominate the mid‑to‑high range—just make sure you have a robust PID loop and a quick‑acting torque limiter to keep the drivetrain happy.