Piston Slap: Of Power Curves and Turbo Boost…

Sajeev Mehta
by Sajeev Mehta
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piston slap of power curves and turbo boost
Chris writes:


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.

Sajeev Mehta
Sajeev Mehta

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  • Wmba Wmba on Dec 17, 2012

    This is a very interesting question. All the usual babble about torque peaks presumes full throttle, which rarely happens in road car use. These days, prodding the accelerator pedal to some position is interpreted by the ECU as a request for x amount of torque. The ECU reads road speed, engine temp, engine rpm, outside temp, position of the variable cam phasers, sun spot activity, cornering g, and issues an order to the throttle position motor to assume a certain initial position, where its lookup tables have said the required torque will be output by the engine. Turbo motors will have low torque capabilities when the turbine isn't spooling very much, so the throttle may be fully opened by the ECU, even though you have only given the accelerator pedal a small push. That is to disguise the low output capability of the engine at that moment at that apparently small accelerator pedal push. As the turbo spools up, and engine speed also increases, the ECU dials back the throttle opening because the engine can make the desired output torque at this lower opening. It's quite a dynamic situation, and nothing like pulling on a throttle cable and hoping for the best. Modern ECU tuning is quite the arcane art. Unburned fuel? Not likely at all, since all parameters are constantly being checked. Cobb tuning has more in-depth discussion on their site, if you don't go cross-eyed reading it and decide a Big Mac would be good right now instead. The bottom line? You as a human want the best economy while accelerating? Get in the highest gear you can as soon as possible. It's pretty obvious when you watch the instantaneous mpg display on your dash. Low engine speeds minimize friction losses. Or you just forget about it, and let the ECU do the worrying part while you sip your latte.

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      Me too. I wanted to engage Super Pursuit Mode and have all the hydraulic body panels shoot out.

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