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Motors Engine size and hp per litre

Discussion in 'General' started by Jamie, 3 May 2007.

  1. Jamie

    Jamie ex-Bit-Tech code junkie

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    If you can buy a car with a 3.6 litre engine giving 600bhp why can't you buy a car with a 1.2 litre engine giving 200bhp?
     
  2. crazybob

    crazybob Voice of Reason

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    Which car are you referring to with a 3.6 liter, 600 horsepower engine? You might be able to buy a car with that kind of specific output, but it's not common. At any rate, there are two ways to get mad horsepower out of an engine - You can either use forced induction or use a high-revving engine.

    If you used forced induction on a tiny engine like the 1.2, you'll run into problems with lag much more rapidly than you will with the bigger engine. If you use a turbocharger, the small engine will take a while to spin the turbine up. If you use a supercharger, the small engine will struggle to turn the extra load at low RPM. The bigger engine won't have these problems and can probably be boosted higher before you run into issues. The other thing that counts is longevity - if all cars had tiny, high-strung engines that needed full boost every time you pulled away from a stop, the engines wouldn't last very long. A bigger engine can be treated much more gently most of the time, when you don't actually want 600 horsepower.

    The other way to get lots of horsepower out of an engine is by using lightweight parts and a short stroke to get an extremely high redline. Using a shorter stroke costs you torque and using lighter parts costs you durability - this is the approach used by racing engines. I don't know what 3.6 liter engine you're referring to, but you actually can get 1.2 liter engines with this kind of output. Look at 4-cylinder sportbikes. But those engines are basically racing engines. They actually have quite a bit of torque, and they work very nicely in certain cars (Westfield Megabusa, anyone?) but they are much too high-strung for daily use.

    Basically, there's no reason we can't get the same specific output from a 1.2 as from a 3.6. The difference is, a 600 horsepower 3.6 has more than enough power for normal driving without having to rev the nuts off it or in any way abuse it. In the rare case where you actually want all 600 horsepower, it's just sitting there waiting to be called upon. When you get the smaller engine, though, you'll have to be much harder on it to do your daily driving. It'll be louder and it won't last very long.
     
  3. Jamie

    Jamie ex-Bit-Tech code junkie

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    Very detailed reply, thanks.

    The engine I was looking at was the 6 cylinder boxer from the 911 turbo - slightly modified.
     
  4. NiHiLiST

    NiHiLiST New-born car whore

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    crazybob's pretty much covered everything really. Modern turbo engines have really come on leaps and bounds in the past few years. In the past a 600hp 3.6-litre engine would be quite peaky and you'd have to keep it on the boil to get anywhere near the full performance out of it, but nowadays I expect that it's very nice to drive.

    There's a guy in Australia who has the same car as me (Suzuki Cappuccino - 660cc turbo engine) who tuned his engine up to over 170hp, but said it wasn't very driveable. Unless you were ragging it everywhere the was very little power. He's now fully rebuilt an engine from scratch (the spec list is like pornography) and it should now have even more power but deliver a lot more across the rev range.
     
  5. ufk

    ufk Licenced Fool

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    Those nutters in australia like the little 1 litre turbo charades as well, they've got those out to nigh on 200 bhp
     
  6. NiHiLiST

    NiHiLiST New-born car whore

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    Yeah, the Charade GTTIs are fantastic little cars from what I've heard/read. The engine's as tough as old boots :)
     
  7. Fatboy

    Fatboy Bored

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    Well you can get 1000+ BHP from a cosworth BDA block and thats only 2.0l, and its easy enough to get 800bhp from a YB. A normal ford focus engine can be taken way above 500 bhp as well and thats only a 2.0.
     
  8. Krikkit

    Krikkit All glory to the hypnotoad! Super Moderator

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    But they don't really last reliably at such high power outputs Fatboy - the most important thing to manu's these days is finding the optimum point of power, refinement and reliability.
     
  9. jaguarking11

    jaguarking11 Peterbilt-strong

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    Then you take those same engines back to the shop and rebuild them after 500miles or plainly throw them out and build a new one.
     
  10. Fatboy

    Fatboy Bored

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    But if they were built like that from the factory they would last.

    They weren't because escorts and sierras dint cost 60,000+ , as no one would have bought one.
     
  11. crazybob

    crazybob Voice of Reason

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    No, it doesn't matter who builds them, an engine with that kind of specific output won't last very long. Racing engines don't last more than a few dozen hours, no matter whether a tuner or the manufacturer is responsible for the power output.

    Even with perfect tolerances, you're creating somewhat ridiculous stresses in the engine block and crankshaft. No matter what material they're made of, every stress cycle (every combustion stroke in each cylinder is a stress cycle) weakens the material. Take a look at this graph to see how the strength changes. In addition to material fatigue, you're operating that engine at very high temperatures, which can also weaken the materials used.

    That's completely ignoring the fact that nobody in their right mind would want a 1000hp 2 liter engine for road use. Refer back to my original post - the power output would be very peaky. You'd feel like you were driving a 500cc until you got it up to 7000 RPM, at which point it would scare the living daylights out of you.

    If manufacturers could actually build reliable engines with 500 HP/liter that were usable in normal driving, we'd all be driving around with engines smaller than 500cc because the manufacturing and maintenance costs would be tiny and we'd all have enough power to keep us happy.
     
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