Horsepower versus torque

[Moloch]

Ok. So, do we all agree, that:

- Total hp is the best checkmark figure for overall power.
- The torque curve determines the handling of the car.
- The gearbox ratio and weight of the car are very important in determining acceleration.
- High torque at low rpm makes a car feel much more responsive.
- Turbocharged engines have better performance than naturally aspirated ones.
- A comparable diesel engine saves you up to 50% in fuel costs.
- For all-out performance, you want a turbocharged gasoline engine that revs to 10,000+ rpm.
- For responsive driving you want a V8 gasoline or turbo-diesel engine.
- For cheap and responsive driving you want a turbo-diesel engine.

And:

- Great engineering and handling (suspension, steering, gearbox, brakes, tyres and all) make a car perform much better than one that has a better engine but lacks in those.

haha

 

[KimB]

- High torque at low rpm makes a car feel much more responsive.

Well, I would claim that this is only true for two situations:
1. Low speeds.
2. Gearing is set high for the torque that the engine provides.

The first is only really going to be a problem if your car is pretty gutless. The second may be an issue if the car is produced to maximize fuel economy over drivability.

In my mind, the main thing that a high-torque engine gets you is improved fuel economy at freeway speeds without sacrificing drivability.

 

[Frank]

Well, I would claim that this is only true for two situations:
1. Low speeds.
2. Gearing is set high for the torque that the engine provides.

The first is only really going to be a problem if your car is pretty gutless. The second may be an issue if the car is produced to maximize fuel economy over drivability.

In my mind, the main thing that a high-torque engine gets you is improved fuel economy at freeway speeds without sacrificing drivability.

It gets you basically improved fuel economy all the way, as you don't have to rev your engine to get the same response. And it saves on shifting, if you drive a manual.

 

[Dooby]

Umm which car is that?

The new Honda Civic 2.2 CDTi. Official figures are 8.5-8.7, but man in garage assured me that its closer to 8, they don't want people buying more diesels than petrols, which is closer to 9

 

[dizietsma]

In a V8 diesel from Mercedes or Audi (both 4.2l displacement), you will.

No you don't. Even the Audi diesels at Le Mans this year sounded terrible ..

 

[dizietsma]

Ok. So, do we all agree, that:

- Total hp is the best checkmark figure for overall power.
- The torque curve determines the handling of the car.
- The gearbox ratio and weight of the car are very important in determining acceleration.
- High torque at low rpm makes a car feel much more responsive.
- Turbocharged engines have better performance than naturally aspirated ones.
- A comparable diesel engine saves you up to 50% in fuel costs.
- For all-out performance, you want a turbocharged gasoline engine that revs to 10,000+ rpm.
- For responsive driving you want a V8 gasoline or turbo-diesel engine.
- For cheap and responsive driving you want a turbo-diesel engine.

Torque curve does not determine the handling of a car. It could determine traction but traction is not handling

For acceleration you also have to figure in where the engine is, how many wheels are driven, if an lsd is fitted and the tyres when accelerating from zero or low speed

Turbochargers give more performance than naturally aspirated per given engine size but whether it is better or not is a matter of opinion.

A 10,000 rpm turbocharged engine is silly for a road car at least as they are contradictions in what is being achieved.

For cheap and responsive driving you want a light car with a small light petrol engine such as a Caterham 1600cc. Fun in a Renault 1.4 diesel = 0, fun in the Caterham = 100

Don't forget also that disel engines are heavy and upset the handling of a car compared to a lighter engine. This extra weight also means you have to give it bigger brakes etc etc and this also adds to weight.

 

[MPI]

Ok. So, do we all agree, that:

- Total hp is the best checkmark figure for overall power.

Yep.

- The torque curve determines the handling of the car.

Well... "handling" with regards to cars usually means roadholding abilities, but the torque curve hints at the characteristics of the engine, yes.

- The gearbox ratio and weight of the car are very important in determining acceleration.

Weight, definitely. F = ma. Gearing ratios... well, strictly yes, of course. But since you usually have 5 or 6 to choose from, it isn't that interesting, really. If you've got the proper gear you maximize the F at max power. Power-to-weight ratios are where it's all at.

- High torque at low rpm makes a car feel much more responsive.

Depends.

- Turbocharged engines have better performance than naturally aspirated ones.

Well, duh.

- A comparable diesel engine saves you up to 50% in fuel costs.

Since diesel is about the same cost/litre here(Sweden), 50% seems very optimistic, but 30% savings probably isn't impossible.

- For all-out performance, you want a turbocharged gasoline engine that revs to 10,000+ rpm.

That would give the absolute most power-to-weight ratio in a piston engine, yes.

- For responsive driving you want a V8 gasoline or turbo-diesel engine.

Now we have to define responsiveness, as it is quite subjective. I gather responsive to you means pull or "grunt".

To me responsive means that the engine reacts quickly and predictably to driver inputs. Turbochargers add lag to throttle inputs and big V8s usually have considerable rotational inertia(->slow to change speed).

IMO, nothing beats a normally aspirated petrol engine for responsiveness.

- For cheap and responsive driving you want a turbo-diesel engine.

Cheap; definitely. Responsive; again, depends.

And:

- Great engineering and handling (suspension, steering, gearbox, brakes, tyres and all) make a car perform much better than one that has a better engine but lacks in those.

Well, I don't know if I would choose an F1 with a Trabant engine or an Trabant with an F1 engine... It's tough.

 

[davefb]

before you recommend turbo-diesels... best to be behind one when the driver puts his foot down..

i'd call the effect similar to when a ww2 ship captain said "SMOKE SCREEN!"

 

[_xxx_]

before you recommend turbo-diesels... best to be behind one when the driver puts his foot down..

i'd call the effect similar to when a ww2 ship captain said "SMOKE SCREEN!"

Those times were over some ten years ago, at least with european diesels.

 

[_xxx_]

I don't buy that at all. I've never seen any data that suggested anywhere close to a doubling of mileage in going diesel. The difference is closer to 20%.

No. It's almost double. I drive a C220 (2.2l) diesel which needs 6.5l/100km, compared to the C230 (2.3l) gasoline engine requiring 12l/100km.

They may be cleaner today (though I'm not convinced of this). But they only give out less CO2 because of the lowered mileage. They give out more CO2 per gallon of fuel used.

Yes, but is that relevant? The bottom line is, the emissions per mile are less.

I don't buy that diesel engines last longer. Gasoline engines basically never go out unless the owner doesn't perform basic maintenance (like changing the oil).

Nope. Here's an example (bad Google translation, but whatever):
Link
There are LOADS of diesels here with over half a million kilometres. Mostly Merc or BMW.

EDIT:

- Turbocharged engines have better performance than naturally aspirated ones.

That depends on what kind of performance you're looking at. A turbo engine looks rather bad when compared to the similarly powered high-rpm non-charged engine.

 

[MPI]

No. It's almost double. I drive a C220 (2.2l) diesel which needs 6.5l/100km, compared to the C230 (2.3l) gasoline engine requiring 12l/100km.

Erm, 12l/100km seems WAY high. Besides. the C 230 has a 204hp V6, as opposed to the I4 150hp diesel.

The Merc webpage cites:

C 230 l/100km
City: 13,5–13,8
Highway: 6,8–7,1
Mixed: 9,3–9,6

C 220 CDI l/100km
City: 8,6–8,8
Highway: 5,1–5,2
Mixed: 6,4–6,5

That depends on what kind of performance you're looking at. A turbo engine looks rather bad when compared to the similarly powered high-rpm non-charged engine.

Just about every engine can be turbocharged, so saying that turbocharged engines generally have better performance isn't unfair IMO.

But I still don't like turbos very much anyway...

 

[_xxx_]

Erm, 12l/100km seems WAY high. Besides. the C 230 has a 204hp V6, as opposed to the I4 150hp diesel.

Over 160 hp in the meantime, but whatever. I took the motors with the same displacement regardless of power. Torque is about the same though.

And those 12l are rather optimistic in reality. My dad drives one really without pushing it and he needs about 13-14l mean. And I drive them often as well with more, errm, dynamics and I need about 15l (we have 5 or so of them in the company fleet).

The 220 diesel also needs 7.5-8l if you really push it, of course. Still worlds of difference there.

 

[Frank]

Don't forget, that accelerating at high rpm is the worst thing to do for your mpg. So, if you have high torque at low rpm, there is much less reason to drive uneconomically.

But yes, accelerating will burn more fuel in either case.



Anyway, if you're only interested in that great sound, very high power, sporty driving and all, and you don't care about costs, then a (turbocharged) gasoline engine is the way to go. And I think many Americans are slightly slanted, as they know what they like, but don't really know the alternatives. Or at least, I don't get the impression that the US people who say gasoline is best in all cases have much, if any experience with those alternatives.

 

[KimB]

Yup. So I talked to my grandparents about it, for instance, and they said, "But aren't diesels gutless?"

I wouldn't be surprised if this is one major barrier.

 

[RussSchultz]

Don't forget, that accelerating at high rpm is the worst thing to do for your mpg. So, if you have high torque at low rpm, there is much less reason to drive uneconomically.

!?!?!?!?

C'mon. Use a little bit of science here.

No matter high or low RPM, it takes power to accelerate. Gas and deisel are simply carriers of that power; the engine being a way to extract that power. No matter whether you extract it at high RPM, or low RPM, it takes power, hence fuel. That's why your mileage goes down when you accelerate, not because you're at higher RPM.

If your statement were true, the V8's of the 70's that ran at 2-4K RPM would be the absolute king of efficiency, and we all know that's not right.

 

[KimB]

But Russ, at higher RPM's, you're incurring higher friction and lower efficiency. The fuel energy to kinetic energy ratio is never one-to-one in a car, and neither is it constant.

Yes, acceleration always requires more fuel than constant motion (at the same speed). But acceleration at high RPM's also adds the additional spectre of added inefficiency to the mix.

 

[RussSchultz]

But acceleration at high RPM's also adds the additional spectre of added inefficiency to the mix.

Its an engine. Things are interdependant.

Your 5.25" harddrive spinning at 5200 RPM isn't more efficient than a 1.8" at 7200RPM.

Just the same, an V8 running at 2000 RPM isn't more efficient than the 1500cc crotch rocket running at 8000rpm.

 

[KimB]

Of course they are. But that doesn't mean that low torque doesn't help with fuel economy. Yes, there are obviously other factors, but this is certainly one of them.

 

[Blazkowicz]

you know, I have that mental image : Pentium 4 and FX 5800/5900 Ultra are weak high-reving gasoline engines, Athlon 64 and 9700 pro are good turbo-diesel

(comparison stops here as I think we should all run a FX5200 on quake 3 engine, that is 800Kg cars with a 1L gas engine . >1 ton car only if needed, and turbodiesel)

 

[dizietsma]

Final nail in the coffin for disels.

Go into a bar next to two fantastic young ladies and say within earshot

"Golly the Renault 1.4 diesel is running ecconomically tonight "

or

" Golly, the Ferrari V8 is running smooth tonight "

and see which has the Lynx effect ....

You'd even be better off saying your hybrid was sweet than anything about a diesel because they at least think you are a caring man. With diesel they just think your trousers smell funny because you split it all down them at the garage.