The Triumph Dolomite Club - Discussion Forum

Don't know if there's anyone who can answer, but I was looking at the specifications for the Sprint recently and came across somthing that seems to me to be a puzzle:

The peak torque for the sprint is widely given as 122 ft lbs at 4500 and the peak power as 127 hp at 5200 rpm. However, if you use the power and rpm to cacluate torque you get 128.3 ft lbs at 5200 (there are a number of web sites that give this result). So how can a peak torque of 122 and a peak power of 127 bhp at 5200 rpm both be correct?

Graham

(122 lb-ft x 4500 rpm) / 5252 = 104.53 hp

(104.53 hp x 5252) / 122 lb-ft = 4499.93 rpm

(127 hp x 5252) / 5200 rpm = 128.27 lb-ft

(128.27 lb-ft x 5200 rpm) / 5252 = 127 hp

(128 hp x 5252) / 128 lb-ft = 5252 rpm

So at 5252 rpm the torque and HP will always be equal for any engine, the reason that torque is often quoted at a lower RPM is due to the peak volumetric efficiency of the engine, or how well it can get air in through the intake system to the cylinders. My apologies if your point went whizzing past my left ear and I've taught granny how to suck eggs...

Edit: Better mention that the magic 5252 comes from 1 hp for 60 seconds produces 33,000 lb-ft of work, convert rotational force of a crank into hp by 33,000 / (2 x Pi) = 5252

Interesting, but the question is still: how can peak torque be 122 (at whatever rpm) if the torque at peak power is higher than 122, i.e. 128? Surely then, the peak torque is at very least 128.

Graham

The workshop manual states Max hp is at 5700 rpm not 5200. Dont know if this makes it different?

Tony

The engine has surprisingly little torque and high up the range.

tony g wrote: ↑Mon Feb 19, 2018 12:43 pm The workshop manual states Max hp is at 5700 rpm not 5200. Dont know if this makes it different?

Tony

That is interesting. I take it it still gives 127 as the peak power, which gives a torque of 117.02 ft lbs. Then it all makes sense, and it's the 5200 that's wrong. I admit, I thought it odd that the Sprint's peak power was lower in the rev range than the TR7s, but didn't do more than check a few places on the internet. Either I didn't look far enough or the original upload got it wrong and everybody's quoted that. Just goes to show "much of what is on the Internet is true".

Graham

Torque is said to be 122lb/ft at 4500 rpm

Tony

Must admit I didn't think to check the HP/RPM figures in your post, my mistake!

Still, peak torque and maximum BHP will be at different figures for the volumetric efficiency of the engine reasons I gave above.

Galileo wrote: ↑Mon Feb 19, 2018 3:28 pm Still, peak torque and maximum BHP will be at different figures for the volumetric efficiency of the engine reasons I gave above.

Well that's true, but that power is torque times rpm (* const) explains simply why peak power can't occur at a lower rpm than peak torque.

Graham.

SprintMWU773V wrote: ↑Mon Feb 19, 2018 1:00 pm The engine has surprisingly little torque and high up the range.

I don't understand this comment. Does it not have more torque per litre than the Rover V8 3.5? The peak torque is 1000 rpm further up the rev range than the TR7 8-valve equivalent, if not much higher in value (122 vs 119). But isn't that exactly what you would expect where the only significant difference is more valves per cylinder (and a small increase in compression ratio), i.e. that it would have very nearly the same torque as 8-valve to a little below the rpm where the 8-valve has its peak torque, and keep increasing for a bit beyond that value because of the better breathing? Clearly, if torque is 122 at 4500 and 117 at 5700, it's fairly flat across quite a broard range. In which case, a little over 119 ft lbs at 3500 rpm does not seem unreasonable.

Graham

122lb-ft is something like 165NM which is not considered a great deal of torque. You don't tend to look at torque per litre, it doesn't really mean anything, either you have torque or do not. After all an electric motor in my wife's Lexus produces something like 207NM from go, it has no litres. A bigger engine, especially a longer stroke and/or with more cylinders will produce more torque. Rover 3.5 V8 is something like 260NM at 2600rpm, actually not hugely torquey for a V8 or particularly low down the range but then it's only a 3.5 litre with quite a short stroke.

They're better now but 16v engines always tended to push the torque much higher up the rev range vs an 8v engine. This is why you tend to have to drive such and engine harder to gain speed. Japanese engines in particular, especially Honda tend to be a bit peaky on the max torque too as opposed to a nice flat curve throughout the normal driven range. To me the ideal car has a nice flat torque curve from about 2000-4500rpm to give plenty of shove off the line and for overtaking without revving the nuts off the engine. Perhaps I've just got too used to driving a diesel with 320NM of torque!

One of my favourite engines, the ECU restricted to 150bhp VW ABF MK3 Golf GTi engine:

Galileo wrote: ↑Tue Feb 20, 2018 11:30 am One of my favourite engines, the ECU restricted to 150bhp VW ABF MK3 Golf GTi engine:

Which I read off the graph as 133 ft lbs - over 3 percent more than the "low torque" Sprint engine gives! And all it takes is twin cams and injection.

I realize that a 16-valve engine will give the same torque as an otherwise identicle 8-valve engine at lower rpm, but more torque, and thus more power at higher rpm. That's almost exactly what we have and see with the Sprint and TR7 engines.

Graham

SprintMWU773V wrote: ↑Tue Feb 20, 2018 9:02 am You don't tend to look at torque per litre, it doesn't really mean anything, either you have torque or do not.

Oh, what's "specific torque" in units of Nm/L or ft lbs/cu in, etc., then?

GrahamFountain wrote: ↑Tue Feb 20, 2018 9:34 pm

Galileo wrote: ↑Tue Feb 20, 2018 11:30 am One of my favourite engines, the ECU restricted to 150bhp VW ABF MK3 Golf GTi engine:

Which I read off the graph as 133 ft lbs - over 3 percent more than the "low torque" Sprint engine gives! And all it takes is twin cams and injection.

I realize that a 16-valve engine will give the same torque as an otherwise identicle 8-valve engine at lower rpm, but more torque, and thus more power at higher rpm. That's almost exactly what we have and see with the Sprint and TR7 engines.

Graham

Official figure is 132 lb.ft/4800rpm with its twin cams, injection and computer controlled ignition. There was always the myth that the 8v engine had more torque low down, which is utter bollocks and as you say the lighter internals and lower inertia makes the 16v keep on delivering more when the 8v runs out of puff. Posted the graph because I thought it's an interesting comparison, and I'm very familiar with the ABF.

Edit: 8V VW engine.