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BHP or Torque? Which is more important?


Flatcoat888
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When assessing a tow cars potential ability. And you can’t say both! 

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It's the BHP that gets you over the hill.

BHP is basically torque times engine speed, so either you need lots of torque at low revs or lots of revs if you don't have so much torque.

Edited by Lutz
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33 minutes ago, Lutz said:

It's the BHP that gets you over the hill.

BHP is basically torque times engine speed, so either you need lots of torque at low revs or lots of revs if you don't have so much torque.

 

Which is why most (but not all)  people prefer a Diesel for towing a caravan, you can make reasonable progress without having to rev the (insert word of choice) out of the engine !

 

Modern petrol engines are much better at producing low down torque than in years gone by, but in my experience  a diesel is still better at it (and more economical) 

 

Tin hat at the ready to deflect the inevitable flak !! 

Experience is something you acquire after you have an urgent need for it.

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The "shape" of the torque vs rpm curve is an important aspect, a good level of torque maintained over an acceptable rev range would be my preferred option.

A narrow peaked, power point, presents the least attractive characteristic only tolerable with a multi ratio gear box. 

High power, is only of value if it can be used, ie it is available at a revs level, you and passengers can mentally live with.

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When I started my caravanning career, before the days of turbocharged engines, the needle would regularly hit 7000rpm when negotiating steep hills. Thankfully, that's a thing of the past, even with modern petrol engines. Some even achieve maximum torque at lower engine speeds than a diesel, but I agree with Mr Plodd that the diesel is more economical.

Edited by Lutz
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48 minutes ago, Lutz said:

It's the BHP that gets you over the hill.

 

No, power level only dictates how fast you get over the hill.

Whilst, to move, however slowly or fast, there has to be power, it's just the torque that power can be delivered at that does the "work", force x distance.

 

 

Edited by JTQ
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2 minutes ago, Lutz said:

When I started my caravanning career, before the days of turbocharged engines, the needle would regularly hit 7000rpm when negotiating steep hills. Thankfully, that's a thing of the past, even with modern petrol engines. Some even achieve maximum torque at lower engine speeds than a diesel.

 

Really???

 

There were precious few, if any, mass produced petrol engine that would stay in one piece at much over 5500 rpm, if they could even get that high without valve bounce!! At 7000 rpm most would very quickly find a con rod sticking through the side of the engine block.

Experience is something you acquire after you have an urgent need for it.

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1 minute ago, JTQ said:

 

No, power level only dictates how fast you get over the hill.

Whilst, to move there has to be power, it's just the torque that power can be delivered at that does "work", force x distance.

 

No, power is what dictates whether you get over the hill at all, whatever the speed.

Just now, Mr Plodd said:

 

Really???

 

There were precious few, if any, mass produced petrol engine that would stay in one piece at much over 5500 rpm, if they could even get that high without valve bounce!! At 7000 rpm most would very quickly find a con rod sticking through the side of the engine block.

 

Yes, my Opel (Vauxhall) Vectra 1.8 was redlined at 7000prm.

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Obviously both!

 

Modern waters are muddied with turbocharged petrol engines and hybrid things.

 

BHP - Power is a function of the speed to put you into a wall....torque is a function of how far you go through the wall.

 

For a towing vehicle, then more torque at the lower end of the rev range is the goal.

 

Most turbocharged engines, be it petrol or diesel start to produce most of their torque from around 1800rpm.

But in these days of economy, in top gear, this may mean a road speed of well in excess of a caravan speed limit, so its down to 5th, 4th (or 8th, 7th, 6th on some Auto's)

And dropping down the rev range to 1400 to 1500rpm can be detrimental to the longevity of your engine and particularly the dual mass flywheels of diesel engines.

 

So no hard and fast answer....and of course you may want to use your towing vehicle for other duties .

 

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2 minutes ago, Lutz said:

No, power is what dictates whether you get over the hill at all, whatever the speed.

 

Of course as I said there has to be power to move, because to move implies some speed element must be involved, but it is torque that give the ability to move or climb, as torque is force at the radius of the drive wheel. If the torque is too low it will not overcome rolling back down the hill let alone climb the hill. Power only dictates how fast you get up that hill, once there is adequate torque to climb.

"Power" is to do with the rate of doing work, ie the speed at which work is done, not the "work" aspect itself.

 

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Yes, but bags of torque at zero revs is not going to get you anywhere.

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Torque coming in at lower rpm is better. Some petrol engines don't  deliver higher torque until higher rpm like the awesome Honda VTEC.  

Turbos definitely better especially modern ones which have variable vanes.

The old turbo petrol were fun, not much power until the revs built up then wallop!

We had a Saab 900 turbo which did that, around 3000rpm it was like an afterburner!

You had to be careful with the right foot in the wet, no traction control.

Loved it, it had an economy and a boost gauge which went into the red when the boost went up!

I also had a Range Rover classic 3.5 efi, that had lots of torque and a healthy appetite for petrol.

Edited by 664DaveS
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7 minutes ago, Lutz said:

Yes, but bags of torque at zero revs is not going to get you anywhere.

 Precisely what I said, and is blindingly obvious, zero revs in not moving at all, there has to be a modicum of power to move, as moving implies speed is a factor. But a ludicrously low power can develop high torque, that is why we have gearing to offer the power at torque levels to do the required "work".

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46 minutes ago, Lutz said:

When I started my caravanning career, before the days of turbocharged engines, the needle would regularly hit 7000rpm when negotiating steep hills. Thankfully, that's a thing of the past, even with modern petrol engines. Some even achieve maximum torque at lower engine speeds than a diesel, but I agree with Mr Plodd that the diesel is more economical.

 

And we were always watching the water temperature.

My choice of engine would be an electric motor.

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Torque at the wheels is what gets you over the hill (or moving in the first place) which is a function of gearing, engine torque and speed. A wide flat torque band which starts low in the rev range is what gets you over a hill without feeling stressful.

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44 minutes ago, AlwynMike said:

Obviously both!

 

Modern waters are muddied with turbocharged petrol engines and hybrid things.

 

BHP - Power is a function of the speed to put you into a wall....torque is a function of how far you go through the wall.

 

For a towing vehicle, then more torque at the lower end of the rev range is the goal.

 

Most turbocharged engines, be it petrol or diesel start to produce most of their torque from around 1800rpm.

But in these days of economy, in top gear, this may mean a road speed of well in excess of a caravan speed limit, so its down to 5th, 4th (or 8th, 7th, 6th on some Auto's)

And dropping down the rev range to 1400 to 1500rpm can be detrimental to the longevity of your engine and particularly the dual mass flywheels of diesel engines.

 

So no hard and fast answer....and of course you may want to use your towing vehicle for other duties .

 

 

Some modern petrol engines now give max torque low down well below 1500rpm.

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48 minutes ago, Lutz said:

Yes, but bags of torque at zero revs is not going to get you anywhere.

Good for hill starts though.

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Interesting views. I tend to err to diesel but now having a 4wd diesel hybrid I have probably the best tow vehicle I have ever driven. However I need to change for various reasons so looking at petrol hybrids (both plug-in and self charging) as well as a non- hybrid diesel. Putting aside the dubious environmental credentials of  batteries there is no doubt the silent power of EV can be intoxicating. My current car accelerates to 60 in under 6 secs and hills hardly impact on progress. I am test driving two different petrol hybrids this weekend with combined power of 230 and 260 horses respectively. 

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With 627 lb/ft of torque and 377bhp I don't worry about either :D but I've always regarded sufficient torque at reasonably low revs (i.e. flattish torque curve) to be desirable for towing and usually the best compromise of torque/power/economy is delivered by a turbo diesel but technology is moving on and no doubt eventually diesels will gradually disappear - I can no longer get a replacement new Cayenne diesel.

Life is not a rehearsal . . .:)

Porsche Cayenne S Diesel & Knaus StarClass 695. Previously Audi S4 Avant & Elddis Super Sirocco

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The UK has become particularly averse to diesel but that is driven by fleet business and the 100% business tax relief for new EV’s distorting the market. A lot of now petrol only cars in the UK are still available as diesel sur le continent. The new Hyundai Tucson can be had as a diesel hybrid elsewhere. 

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That would make a lot of sense for us caravan haulers!

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But of course any such vehicle sourced would not be “U.K. spec” left hand drive, headlights dipping the wrong way amongst many other issues, so firstly it would attract a higher insurance premium and possibly not be covered by the new car warranty.

 

In addition this new “No ICE passenger vehicles to be sold” regulation may also ban personal imports of such vehicles :angry:

Edited by Mr Plodd

Experience is something you acquire after you have an urgent need for it.

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6 hours ago, Mr Plodd said:

But of course any such vehicle sourced would not be “U.K. spec” left hand drive, headlights dipping the wrong way amongst many other issues, so firstly it would attract a higher insurance premium and possibly not be covered by the new car warranty.

 

Unless you bought it in the Republic of Ireland, of course. Then you'd only have to change the speedometer.

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21 hours ago, Flatcoat888 said:

Interesting views. I tend to err to diesel but now having a 4wd diesel hybrid I have probably the best tow vehicle I have ever driven. However I need to change for various reasons so looking at petrol hybrids (both plug-in and self charging) as well as a non- hybrid diesel. Putting aside the dubious environmental credentials of  batteries there is no doubt the silent power of EV can be intoxicating. My current car accelerates to 60 in under 6 secs and hills hardly impact on progress. I am test driving two different petrol hybrids this weekend with combined power of 230 and 260 horses respectively. 

After 20miles towing that 230 horses will no longer be available, so I see hybrids as just another thing to go wrong.

Best just to buy a petrol or diesel imo, or ideally an electric car.

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You have it wrong with how a self recharging hybrid works. If I use the appropriate settings My current hybrid always has battery back up available. As for EV,  no good to me for my car needs and I don’t have EV money. 

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