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This related question is concerned with why manufacturers de-rate engines.

But how can they get an engine to produce less torque/power?

As an example:

  • This is almost too broad. There will be dozens of plausible answers. – cory Dec 27 '16 at 18:58
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    @cory as long as they are valid means to reduce engine power output, they are valid. I don't want to know how VW did it here; this is a generic question (hence the engine-theory tag). The reason for the W12 example is to show a stark example of engine de-rating – Zaid Dec 27 '16 at 19:00
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    It seems like this would be most interesting if we can focus on what actually happens "in the wild." I would assume that these days the answer has a lot to do with the parameters that the ECU controls and some of the engine peripherals (e.g., the turbocharger). – dlu Dec 27 '16 at 19:07
  • Well then I should remove mine :-) – dlu Dec 28 '16 at 2:07
  • Why do you think this is the same engine? only because it is W12 5998 cc does not tell you much. The could use different internal parts such as bearings, pistons, valves, camshafts, cylinder head etc ... making this a completely different design. – Daniel Oct 25 '18 at 12:45
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There are several different ways a manufacturer can detune an engine:

  • Mechanically:
    • Cam profile can be changed. For instance, reducing the lift from one cam to another will not allow as much air to flow into the cylinders. The end result, not as much volumetric efficiency (VE) and not as much power output.
    • Create the engine with not as much static compression. As a rule of thumb, every point of compression is worth about 3% in power output.
    • Limiting the intake/exhaust ability. By adding parts which create more restriction, this also reduces VE.
    • Not allowing a turbo to produce as much boost with a smaller turbo, or not including a turbo at all (making the engine naturally aspirated).
  • Detune with a Tune With a tune you can:
    • Not allow a turbo to produce as much boost. Less boost equates to fewer HP.
    • Not allow the engine to rev as high. We all know HP is factored on torque and engine speed. By limiting the speed of any engine, you limit the amount of HP produced by the engine.
    • Be not as aggressive with fuel mapping. Many of today's fuel injected engines can see modest gains if the correct tune is applied. It only goes to reason, the other way can affect it inversely.
    • Retard the timing, causing the engine to effectively reduce the compression ratio and increase pumping losses.
  • Psychological Detuning:
    • Politics: In the earlier years of General Motors, they would not allow the Camaro to have as much horsepower as the Corvette. They would not allow their flagship sports car to have the same output as the mullet-headed freaks were using in their whoopties, so they said the Camaro put out less HP/TQ, while it was exactly the same engine. In actuality they did put out the same HP/TQ, GM just said it wasn't as strong.
    • Insurance: For insurance reasons, they did the same thing. Many manufacturers stated one figure when in fact the actual numbers were much higher. The Mopar Hemi engine of the 60/70's would fit into this category. While Chrysler claimed the engine only put out in the 400hp range, some estimates put it as high as 700+ (some even higher). These were nasty engines.

NOTE: While neither insurance nor politics are actually "detuning an engine", the fact the numbers were skewed to meet the needs of the climate could be construed as detuning.

As with anything, there can be a combination of any of the above to have the same effect of lowering HP numbers.

  • 3
    Insurance and politics are examples of "psychological detuning" – you persuade the users to believe the power output is lower. – dlu Dec 27 '16 at 19:14
  • @dlu - I like it and have edited it into my answer. Thank you! – Pᴀᴜʟsᴛᴇʀ2 Dec 27 '16 at 21:51
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    Another way currently in wide use is limiting throttle opening with computer controlled throttle body units. – Fred Wilson Dec 27 '16 at 22:07
  • One for your politics: The Japanese "Gentlemens agreement" Limited their cars to 280 hp. A lot of the 90´s Japanese sports cars where advertised with that exact rating but really did more. – Daniel Oct 25 '18 at 12:59
  • People claiming those hp figure for hemis are dreaming, They might have been a bit underrated, but in practice street hemis weren't that much faster than a 440 car with 375 HP – Dave Smith Oct 25 '18 at 13:09
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Don't know much about detuning gasoline engines, but in diesels one way that it is done is by using smaller injectors which deliver less fuel. Since a diesel engine is designed to run with "excess" air, restricting fuel delivery is a simple and flexible way to manage power output.

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    True, injector sizing is a primary actor in diesel engines since detonation is a non-issue – Zaid Dec 27 '16 at 18:53
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    Another way for this to happen on diesel engines is to limit the RPMs through the use of a governor. It is quite common for truckers here in the States (or used to be common) to extend the RPM range (by turning up the governor) of their diesel engines, which allows more RPMs, which in effect gives them more horsepower (since HP is a mathematical function of torque and engine speed). – Pᴀᴜʟsᴛᴇʀ2 Oct 25 '18 at 11:30
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One thing others have not mentioned here is that 'peak horsepower' is a point measurement, but performance comes from the integral under a power-curve, so equivalent engines with lower peak horsepower figures are not necessarily de-tuned, as in made to run with a lower thermodynamic efficiency. They are often tuned differently to distribute that integral over different parts of a powerband rather than piling all the performance into a narrow band, hence the lower peak-horsepower figure. That said, production vehicles do tend to tune on the conservative side of peak power output for longevity. 7-angle valve-seats may radically improve flow-bench numbers, but they will also need to be refaced much more often.

The VW Phaeton weighs 5400lb, while the Nardó weighs 2600lb. You can be sure that the Phaeton needs more torque at lower RPMs to get those big comfy couches and coffe-tables rolling. An equivalent motorcycle comparison might be the sporty Yamaha R1 vs the more touring/commuting-oriented FZ1. These use identical engines with similar levels of efficiency but very different performance characteristics. The FZ1 has a broader power-curve with much more power down low, around 3000rpm, while the R1 is producing 15 more horsepower at 8000rpm. enter image description here

4

In older carburatored engines power output could be minimized or maximized by way of fuel jet variations, cam shaft profiles, inlet and exhaust sizes, exhaust and inlet valve port dimensions, exhaust and inlet manifold formats and dimensions, lower or higher head compression ratios. Any of these variables can be applied to the same base engine. Mechanically injected diesal engines can be subject to some similar variations plus variables of injector volume and duration. The only alteration to the above for both petrol or diesal is the advent of EFI systems which by way of preset or reprogramable performance curve specifications can dramatically alter output. That is why the lucrative parallel market exists for those who want to either get their hands dirty or pay someone else to install any or all of the "fruit" available to transform the sedate sedan into a dangerous missile. :)

1

Since you said this is a generic question:

One example from the 1996-2000 Civics:

They just used a restricting gasket before the throttle-body to limit airflow for the 75 hp model. For ~$7 you could buy the 90hp gasket for a swap and have a cheap tune.

Nowadays this is usually done electronically via ECU. But don´t be fooled: The higher power models often have a lot of secondary modifications which are not immediately obvious.

This goes from the motors internals (higher spec) to the cooling system, transmission, power train, breaks, suspension etc...

Honda gasket

Image: https://www.all4honda.com

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