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I was watching "The Perfect Road Trip 2" by Top Gear, where at some point Richard Hammond is driving a Mercedes A45 AMG. He says that the car develops a whooping 355 BHP, and all that comes from a turbocharged 2.0L engine.

I am now genuinely wondering what are the advantages of having a relatively small engine (2000cc) with such a high power output (~350 HP), rather than going for a bigger engine (say 3000cc or more, which is more common to see with around 350 HP).

What good is it? I don't think it has something to do with taxes or costs to the end user, since we're talking about a +40K car. Is it because a small engine means easier development? More common parts (doubt that as well)? Is it because the original A Class was a small car, hence it had to stay that way?

In addition, are there downsides in having this kind of engine? I expect it not to last as long as other engines, since it's so overpowered.

As you can imagine, I am a total rookie when it comes to this subject, so please bear with me for any error/oversight.

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There are advantages, but also disadvantages to using a smaller turbocharged engine with a higher power output.

The pro's of smaller, turbocharged engines:

  1. When not on boost (below e.g. 3000RPM) the engine uses much less fuel.
  2. turbocharged engines have a flatter torque curve, meaning they're working closer to optimal level throughout a larger portion of the rev range.
  3. The engines weigh less, helping to increase power/weight ratios.
  4. They're much easier to customize to get the characteristics you want. You can either have a powerful, high-boosting engine or an economical, low-boosting engine simply by modifying the electronic control unit parameters, e.g. max boost pressure and timing advance.

The con's of smaller, turbocharged engines:

  1. Turbochargers produce lots of extra heat and stress, significantly shortening the life of an engine. a 3000cc NA engine can easily last for 200 000 miles, but you'll struggle to achieve the same figures in a 2000cc turbocharged engine unless you drive very conservatively.
  2. Because of the additional strain that turbochargers put on an engine, the internals need to be of higher quality, ergo they're more expensive. Cars like the Astra VXR, A45 AMG, RS3 and WRX STi usually require forged pistons and connecting rods be installed to deal with the higher pressures and heat.
  3. Turbo engines require premium fuel, which is more expensive. Because of the nature of the engine and the increased temperatures produced by forced-induction, premium fuel is needed to prevent premature detonation (knocking). A higher grade fuel actually requires more effort to ignite, which sounds counter-intuitive, but it makes sense when one researches how engines work. This may be a problem when you live somewhere where premium fuel is not a available. E.g. my Subaru requires a minimum of 95RON fuel, which is also the maximum RON fuel available in our country. I actually have to use an additive to artificially boost the RON value of the fuel I put in just to keep it from destroying itself.
  4. Turbos break. And they're expensive to repair or replace. Expect a turbo to last around 100 000 miles, which means that if you're buying a secondhand turbo car, you'll probably need to replace the turbo at some point. And that usually costs at least $1500, bu may be significantly more.
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I would like to add two valid points to this discussion as well:

1) When producing engines like this which produce more power with lower CC's, they won't be as smooth as their bigger naturally aspirated predecessors. They will have to rev higher just to produce the same power. So lets say you are driving at 70 MPH with a 5.0 V8, you could comfortably cruise at 1400 RPM (maybe even a bit less). A smaller turbo charged version of that engine, will probably rev at 1600 RPM (if not more). So the smaller CC engine will make more noise and won't be as smooth to drive.

2) The engine won't last as long because it has to deal with more stress just to produce the same power as the bigger naturally aspirated engine. In order to deal with this stress, the engine block is made to withstand much higher pressures, which could potentially increase the cost of the car.


A good example

I think a great example of this, is the Mercedes-Benz AMG 4.0 V8 BITURBO engine. It is being used to replace the lovely old 6.2 V8 on cars such as the C63 AMG. Anyway, I think in the long run, we will see customer reviews state that there C63's engine doesn't last as long as their previous C63's because the engine has been reduced in CC. Its also worth noting that the new C63 costs more than the previous C63, this could be to account for the extra cost of the new engine.

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There are several advantages to a lower displacement engine over a larger one.

Firstly is weight, an engine of 2,000cc will weigh significantly less than a 3,000cc engine because it will have a smaller block, this in turn decreases inertia of moving parts (such as the cylinder head and cam shaft).

Secondly we can factor in fuel economy, if an engine has the capacity for 3,000 cubic centimetres of fuel/air mixture it will naturally burn more fuel on average. If an engine can produce the same power with better fuel economy, it is infinitely better.

Thirdly would be size, a smaller engine can be placed in more convenient locations in the engine bay/car, cars can be generally smaller if the engine is smaller which would also contribute to my first two points. This point also significantly affects the weight distribution and handling, which would be easier to manipulate the smaller the engine.

Other points to think about is the cost of the engine (which would be significantly lower the smaller the CC), Carbon Dioxide and other pollutant emissions (worse and more expensive to pay tax for in some contries in a larger car) and finally a small (but noteworthy factor) is public stigma, a lot of customers would be dissuaded to buying a car with a 3 litre engine because of the consensus that they are over-powered and too expensive to run.

  • But does it actually burn less fuel? Regardless of the considered speed? – Sebastiano Jan 11 '15 at 20:27
  • If by speed you mean RPM then yes, a larger engine will burn more – user90843843434 Jan 11 '15 at 20:31
  • Provided it has respective compression ratios, the same metal parts, and same layout that is, I assume we are speaking of the same engines but of different size – user90843843434 Jan 11 '15 at 20:58
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In addition to size, weight and fuel economy, a small, highly tuned engine is going to be much more ecologically friendly. This is because a large engine producing 350 bhp can do it without being highly tuned - fuel is unlikely to be fully burned, emissions will be higher than with a smaller engine which can burn fuel more fully.

  • Isn't there also a tax in say GB for the larger engines? I was thinking the larger engine costs more on a yearly basis than does a smaller one. – Pᴀᴜʟsᴛᴇʀ2 Jan 11 '15 at 23:32
  • @Paulster2 - it used to be based on size (< or >1600cc), now it's done on CO2 emissions... Other countries may vary... – Nick C Jan 12 '15 at 10:07
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    Gotcha. They'll get their money one way or another, eh? – Pᴀᴜʟsᴛᴇʀ2 Jan 12 '15 at 10:35
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    Yup. It's not about the environment, it's about taking your money in creative ways. – Captain Kenpachi Jan 12 '15 at 11:31
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Another point that hasn't been mentioned - a smaller, turbocharged engines uses less fuel when not on boost (as Juann Strauss says) - this means that it produces less emissions (which has to be a good thing, especially in urban areas where air quality is becoming a major issue).

Less emissions also means they reduce the average emissions over the manufacturer's range, which helps them comply with recent EU laws (see here) - especially useful for 'premium' manufacturers like Mercedes who mostly produce high end, high emission cars

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