Piston Slap: Of Power Curves and Turbo Boost…

Sajeev,

In a couple recent Piston Slap articles you’ve mentioned that when driving car with a manual transmission its most efficient to accelerate with the engine near its torque peak, then cruise in the highest gear possible. This raised two questions in my mind:

1. Does the engine’s torque peak vary based on throttle position? From what I understand, power and torque curves are generated at wide open throttle. But would the torque curve look different at, say, 50% throttle? I’ve heard that exhaust backpressure can affect the torque curve (maybe this is a myth). Could throttle position have the same effect via intake vacuum? Speaking of intake pressure, that leads me to my real question:

2. How does your strategy of accelerating with the engine near its torque peak apply to a turbocharged vehicle? My car has a turbo and according to the manufacturer the torque peak is 2000 rpm. But clearly it’s not always capable of generating max torque at 2000. If I’m loafing along at 1800 rpm and floor the throttle it takes a very laggy second or so for the boost to build and its definitely past 2000 rpm by the time it starts really generating power. I’m thinking there must be a different torque curve for part-throttle acceleration, when the engine is either off-boost or not making full boost. I think this would also apply to an engine like Audi’s supercharged V6, where the supercharger can de-clutch from the engine under low load. Any thoughts on the most efficient way of accelerating in a turbo? Better to accelerate “on boost” at relatively low rpm and relatively wide throttle? Or accelerate with less throttle, keeping it out of the boost (but probably winding the tach up more to avoid moving at a snail’s pace)? Or just forget the whole thing, floor it and enjoy the wild turbo-torque surge?

If these are stupid questions, please disregard. These are just things I ponder while sitting in traffic… Keep up the great work!

Sajeev answers:

This is a fantastic question that I am totally not qualified to answer…but that hasn’t stopped me before, and it hasn’t stopped you lovely people from reading, so let’s do this thang!

Point Yes, throttle position will affect the torque peak. Because an engine is basically just an air pump, if you have less throttle you have less air, less fuel and therefore less power. Thankfully, with the advent of electronic fuel injection there are multiple mappings: older systems have a full and a part throttle program, and newer systems probably have several. So I betcha you can maximize an engine’s efficiency at just about any throttle opening. Every application is a little different, and many are tuned to maximize performance with a computer reflash from an aftermarket programmer.

As a rule of thumb, and I’m ready to get slammed by engineers for saying this, backpressure (or a lack thereof) does indeed affect the torque output of an engine. More importantly: backpressure isn’t a good thing, finding the ideal exhaust velocity to minimize backpressure while keeping the speed “slow” enough to not hurt torque output is crucial. That’s why, in the past 10-15 years, we see far higher quality exhaust systems in all OEM applications: no crush bends in the tubes, cast iron manifolds that are shaped more like aftermarket tubular headers, and mufflers/catalytic converters that aren’t a significant restriction.**

Point turbocharged motors are just like point #1 when it comes to power in part throttle applications. And every boosted application out there is different. Once again, and even more so, tuning makes ALL the difference in the world. Because the turbo is a muffler/restrictor in the exhaust system, you want as little restriction behind it to ensure maximum efficiency: hence why the Dodge SRT-4 is muffler-less from the factory.

My gut feeling is that with any modern car, turbo or not, you need to give it more gas to cut through the slop of electronic throttle control/torque management to get into your torque peak quicker. Spend less time accelerating and more time cruising, with traffic conditions in mind of course. That doesn’t mean you run wide-open throttle, either. There’s a happy medium out there, somewhere.

Off to you, Best and Brightest: I’m ready, I’m wearing my flame suit.

**Grab a catalytic converter from the 1970s-early 1990s. They neck down, restrict air flow, etc far more than the goodies I see today in cut-away diagrams at the auto shows. We have come a long way, baby.