The Displacement and Specific Output Thread

  • Thread starter Zenith
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As would I. It's nice to have a screamer that builds as power as the RPMs rise every so often but I like an assload of torque right off idle. Like, 350-450lb-ft at 2,000rpm. I don't really care about winding the piss out of an engine that much. The fact that it hits you immediatly instead of transfering into more of a pull as RPM's increase is what I like. Pulling hard is cool, and it feels awesome when you accelerate, but that multi-g-force "slam you into your seat" is what I love. Especially when you don't even have to work for it...it's all right there. I'd argue that it actually helps driveability because less shifting.
 
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Slash
I'd argue that it actually helps driveability because less shifting.
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It depends what you're doing. People love playing the "driveability" card with big engines but most people spend most of the time simply getting around the place at regular speeds. In that respect even some of the tiny-engined cars we have here in Europe are perfectly "driveable" because you only need 20-30 horsepower to accelerate up to a 30-40 mph speed limit in a reasonable amount of time and most of those cars produce 60-70 horsepower.

What they don't have of course is that flex-of-the-ankle highway ability, but then it's horses for courses. Someone who buys a tiny city car to mainly drive on highways is making as nonsensical decision as someone who buys a big V8 to roll around town in using a fraction of its performance.
White & Nerdy
And then there's me. If a fairly flat powerband is not possible, I'd rather have the power in the low- and mid-range areas than high up.
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I'm not sure whether it's worth me pointing out that the downsized turbocharged engines you regularly deride produce power exactly in this manner - either in a flat powerband or with a strong low-to-midrange.
 
Speaking of a flat powerband, there's one thing I have yet to understand. Many modern cars have their torque figures artificially limited, resulting in a flat torque curve with a steadily increasing power output. I can't see any sensible reason to doing that (other than having a weak gearbox) but I presume it may even be down to such a stupidity as people wanting the feel of power increasing with the revs. Because if the engine can do, say, 450 Nm between 2000 and 5000 rpm with a peak figure of 550 Nm at 3500 rpm, why limit it to that 450 Nm as is done so often? More is more, even if it means having a more noticable drop towards the top end - the thing is, is still has the same top end as the limited engine though. Just with a stronger and a more useful mid range.
 
Greycap
Speaking of a flat powerband, there's one thing I have yet to understand. Many modern cars have their torque figures artificially limited, resulting in a flat torque curve with a steadily increasing power output. I can't see any sensible reason to doing that (other than having a weak gearbox) but I presume it may even be down to such a stupidity as people wanting the feel of power increasing with the revs. Because if the engine can do, say, 450 Nm between 2000 and 5000 rpm with a peak figure of 550 Nm at 3500 rpm, why limit it to that 450 Nm as is done so often? More is more, even if it means having a more noticable drop towards the top end - the thing is, is still has the same top end as the limited engine though. Just with a stronger and a more useful mid range.
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Can you provide some examples?
 
I have never heard of anything called an artificially limited torque curve. The curve comes naturally from the design of the engine and the way the parts work. Everything from bore/stroke to blow by to cam profiles to head flow, valve sizes, combustion chamber style and cc size, compression ratios and cylinder psi's, and port velocity etc. Lots of variables. I don't even know how the hell that would work.
 
Zenith
Can you provide some examples?
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Everything with a torque curve shaped like this. Straight as a ruler.

vw_14_tsi_curve3.gif


I don't believe for a second that such a torque curve is "natural" as looking at the shape of the curve on both ends of the flat area indicates that there should be an arc above it, the peak being located somewhere between 2500 and 3000 rpm in that case. Gas flow in the cylinder and head simply doesn't change that abruptly at a specific rpm if boost, ignition and cam timing remain the same.
 
Could well be a factor in improving reliability.

That, and typically the sort of vehicles with a torque curve like that are diesels. The revs suggest that one is a petrol engine, but in the case of old-school turbocharged diesels the big peak-shaped torque curve was incredibly frustrating - you'd have about 1,000rpm of usable shove and it would die away abruptly.

Give me a nice plateau any day, in that situation. I don't care that the peak isn't as high as it could be because at least each gear is more usable for more than about three quarters of a second.
Slash
I don't even know how the hell that would work.
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To be fair, the answer to this is fairly simple: Fueling.

That power increases as the torque "curve" stays flat isn't a great mystery though. Power is the rate of doing work. As the engine spins faster its rate of doing work increases - i.e. it produces more power.
 
Space Dynamics
The L67 supercharged 3.8 in many GM cars only puts out 240 HP, but the vehicles with that engine will often outrun cars with engines that put out quite a bit more power (sometimes torque too) and similar weight. A good powerband is grossly underestimated, and is much more important than many may think. More displacement is a great aid to that.
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I loved the L67 in my Grand Prix. Gobs of torque available from idle on up, even when it was stock. Laying patches of rubber at only 2,000rpm was fun as well. Plus since I rarely had to rev past 3,000rpm, gas mileage was pretty good for the power it was making.

I wish the 3.0 in my Escape was like that. It has plenty of power but it doesn't come alive until 2,500-3,000rpm. It's a bit more high strung, probably the one thing I don't like about it. Have to keep the rubber band stretched kind of tight on it.
 
My motorcycle is a DOHC 14,000RPM screamer, and that's the way I like it. If I want power, I'll ask for it. Part of the point of enjoying manual transmission is the act of changing gears.
 
Adamgp
I loved the L67 in my Grand Prix. Gobs of torque available from idle on up, even when it was stock. Laying patches of rubber at only 2,000rpm was fun as well. Plus since I rarely had to rev past 3,000rpm, gas mileage was pretty good for the power it was making.

I wish the 3.0 in my Escape was like that. It has plenty of power but it doesn't come alive until 2,500-3,000rpm. It's a bit more high strung, probably the one thing I don't like about it. Have to keep the rubber band stretched kind of tight on it.
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That 3.0L has some guts for what it is though when you get on it.
 
Greycap
Speaking of a flat powerband, there's one thing I have yet to understand. Many modern cars have their torque figures artificially limited, resulting in a flat torque curve with a steadily increasing power output. I can't see any sensible reason to doing that (other than having a weak gearbox) but I presume it may even be down to such a stupidity as people wanting the feel of power increasing with the revs. Because if the engine can do, say, 450 Nm between 2000 and 5000 rpm with a peak figure of 550 Nm at 3500 rpm, why limit it to that 450 Nm as is done so often? More is more, even if it means having a more noticable drop towards the top end - the thing is, is still has the same top end as the limited engine though. Just with a stronger and a more useful mid range.
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Aside from drivetrain durability, there's a limit to how much torque the tires can cope with. A nice, flat powerband ensures even and predictable torque delivery, with no uneven peaks or troughs, will suffer less traction issues or variations in power delivery that affect day-to-day driveability or even driveability on the track.
 

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