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OK:


So, your first post was that diesels do not have more "low down grunt" than petrols. Except you've proved it wrong yourself, unless you change the rules and say that 2600rpm = 2000rpm.

Low down grunt is not low mph, it's low rpm.
So, does my car, producing 258lbft of torque at 1750rpm produce more torque than your car at 1750rpm?
Yes
Is your car a turbo petrol?
Yes
Is my car a turbo diesel?
Yes.

Want to move the goal posts?
please re-read and understand my 1st post then edit your last post accordingly
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Phil, there's no point in me re-reading it. The words don't change. That's why I'm comparing turbo charged with turbo charged, so that I can't be accused of bias. Now, your second post is where you start defining grunt as speed, not revs.

Radosauf, without dropping the clutch, you're going to struggle to get a car moving without using revs around the 1750rpm mark, equally, since 1750rpm is around 100km/h in top gear in the 170D, if you're cruising, you'll be sitting at those revs too. Maybe not the same on track.

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Phil, there's no point in me re-reading it. The words don't change. That's why I'm comparing turbo charged with turbo charged, so that I can't be accused of bias. Now, your second post is where you start defining grunt as speed, not revs.

Radosauf, without dropping the clutch, you're going to struggle to get a car moving without using revs around the 1750rpm mark, equally, since 1750rpm is around 100km/h in top gear in the 170D, if you're cruising, you'll be sitting at those revs too. Maybe not the same on track.
You wrote "So, your first post was that diesels do not have more "low down grunt" than petrols. Except you've proved it wrong yourself, unless you change the rules and say that 2600rpm = 2000rpm."

My 1st post post was nothing to do with whether petrol or diesels have more low down torque, I was just saying that people often talk about the power delivery of diesels vs petrols when what they are really refering to is the difference between turbo charged and non turbocharged engines
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My 1st post post was nothing to do with whether petrol or diesels have more low down torque
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I don't get the whole 'diesels have more low down grunt than petrols' - diesels don't,
Why did you say "diesels don't" if you didn't mean it?

It's those two words that caused my confusion.

The bit about turbos is irrelevant, as I have tried to show that if you have a petrol turbo and a diesel turbo, well, one of them has more low down grunt than the other, and it's not the petrol one.
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Originally Posted by WTFH View Post
Why did you say "diesels don't" if you didn't mean it?

It's those two words that caused my confusion.

The bit about turbos is irrelevant, as I have tried to show that if you have a petrol turbo and a diesel turbo, well, one of them has more low down grunt than the other, and it's not the petrol one.
Depends what your talking about, peak torque or BHP per RPM? If it's peak Torque than petrols and diesels can be equivalent, if it's bhp per RPM than a diesel will reach it's maximum BHP before a petrol will..
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Why did you say "diesels don't" if you didn't mean it?

It's those two words that caused my confusion.

The bit about turbos is irrelevant, as I have tried to show that if you have a petrol turbo and a diesel turbo, well, one of them has more low down grunt than the other, and it's not the petrol one.
I probably should have wrote "diesels don't neccessarily"

The point I was trying to make is that the only thing that matters is how much power is getting to the road at a given road speed
- the actual torque figure that an engine produces is totally irrelevant - you could make an engine with 1000lb/ft of torque, but only gives you 50bhp so wouldn't get you anywhere fast
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you could make an engine with 1000lb/ft of torque, but only gives you 50bhp so wouldn't get you anywhere fast
I wish I fully understood the correlation between Torque and Power - but surely if the diesel and petrol both produce similar PEAK Power; at 2000 revs the diesel is only about 2500 revs off it's peak, whereas the petrol is still about 4500 revs off it's peak - doesn't that mean that the diesel is actually producing more BHP as well as torque at 2000 revs?
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I wish I fully understood the correlation between Torque and Power - but surely if the diesel and petrol both produce similar PEAK Power; at 2000 revs the diesel is only about 2500 revs off it's peak, whereas the petrol is still about 4500 revs off it's peak - doesn't that mean that the diesel is actually producing more BHP as well as torque at 2000 revs?
Nope , TBC car's can add "work" (rotating force) at lower RPM's equating to peak Torque low down by increasing rotational forces after a cycle. Amount of energy (HP) the engine produces is relative to torque, it explains how much "Work" can be done at any one time. So N/A would increase the amount of "Work" via RPM and a TBC would force induction to create more power converted to "Work" at a specified RPM via an ECU defined set of parameters.

A low capacity N/A with a gearbox designed for torque is also a way to use rotational travel, but you loose maximum speed per gear. A large N/A will produce more torque, so the gearbox can be matched to add the same amount of torque and more speed.

..

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Nope , TBC car's can add "work" (rotating force) at lower RPM's equating to peak Torque low down by increasing rotational forces after a cycle. Amount of energy (HP) the engine produces is relative to torque, it explains how much "Work" can be done at any one time. So N/A would increase the amount of "Work" via RPM and a TBC would force induction to create more power converted to "Work" at a specified RPM via an ECU defined set of parameters.

A low capacity N/A with a gearbox designed for torque is also a way to use rotational travel, but you loose maximum speed per gear. A large N/A will produce more torque, so the gearbox can be matched to add the same amount of torque and more speed.

..
Yep....that's exactly how I thought it all hung together..... I think I'll stick with Jeremy Clarksons explanation that 'witchcraft occurs'......
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Yep....that's exactly how I thought it all hung together..... I think I'll stick with Jeremy Clarksons explanation that 'witchcraft occurs'......
, I need a new less expensive hobby!.
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I quite agree that gearing is a big factor; but surely it's because of the extra low down torque in a diesel that it's able to pull much higher gearing than the petrol?
More that as the diesel can't rev out so far it is forced to use higher gearing.

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I wish I fully understood the correlation between Torque and Power - but surely if the diesel and petrol both produce similar PEAK Power; at 2000 revs the diesel is only about 2500 revs off it's peak, whereas the petrol is still about 4500 revs off it's peak - doesn't that mean that the diesel is actually producing more BHP as well as torque at 2000 revs?
Power is torque x rpm. The important bit is thrust, which is essentially torque x gearing. This is why as you accelerate in a single gear you feel the 'power' trailing off long before peak power. What you are feeling is the torque curve. The higher the rpm you can sustain torque the lower the gearing you can use, hence you get more thrust at the same speed.

Power as a figure is mainly useful as it includes rpm hence gives a fair approximation of how quick a car is with gearing taken into account (and if you optimise the gearing for top speed then that speed will be reached at the rpm for peak power). Torque on its own without rpm is a fairly meaningless figure.

Personally I don't like an engine with peak torque at just over idle speed, with the torque then dropping away as the revs go up. Feels like the car is punishing you for actually driving it.

If anyone has a couple of dyno charts for equivalent petrol and diesels, along with gear box / final drive ratios and wheel / tyre sizes I can knock up a thrust graph to compare them.

All the best

Keith
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More that as the diesel can't rev out so far it is forced to use higher gearing.



Power is torque x rpm. The important bit is thrust, which is essentially torque x gearing. This is why as you accelerate in a single gear you feel the 'power' trailing off long before peak power. What you are feeling is the torque curve. The higher the rpm you can sustain torque the lower the gearing you can use, hence you get more thrust at the same speed.

Power as a figure is mainly useful as it includes rpm hence gives a fair approximation of how quick a car is with gearing taken into account (and if you optimise the gearing for top speed then that speed will be reached at the rpm for peak power). Torque on its own without rpm is a fairly meaningless figure.

Personally I don't like an engine with peak torque at just over idle speed, with the torque then dropping away as the revs go up. Feels like the car is punishing you for actually driving it.

If anyone has a couple of dyno charts for equivalent petrol and diesels, along with gear box / final drive ratios and wheel / tyre sizes I can knock up a thrust graph to compare them.

All the best

Keith
Hmm, Motor Torque x gear ratio = torque at the wheel. So wouldn't you have longer gearing for engines that produce more torque? RPM X Torque really applies to non forced induction assisted engines as is the way I've always done things, so a car which needs to travel at 70 plus at some point will have to be geared with the output smaller then the input, which equals more speed and less torque. Just asking really, might learn something new ..

These are my figures for you to use it's a 2.0 TBC petrol, (There off the Mountune website anyway).
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Hi

Cheers for the graphs, although the gearing bit is pretty essential (not sure exactly which model they are from.

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Hmm, Motor Torque x gear ratio = torque at the wheel. So wouldn't you have longer gearing for engines that produce more torque? RPM X Torque really applies to non forced induction assisted engines as is the way I've always done things, so a car which needs to travel at 70 plus at some point will have to be geared with the output smaller then the input, which equals more speed and less torque. Just asking really, might learn something new ..
Few if any cars are geared to minimise revs at (say) 70 by gearing to have the torque available just enough to maintain speed. If they did then as torque was dropping away while wind resistance is going up that would probably be the top speed without changing down. Gearing is more down to getting the top speed to match up with peak power, but maybe biased on the low side to have the car more responsive in top, or biased on the high side sacrificing top speed and engine response at the possible benefit of fuel consumption.

Power = torque x rpm is a fixed factor, irrespective of whether there is a turbo or not. Trouble is that people mix up torque as a figure with an engine being described as torquey (as being flexible), when the 2 are pretty much unrelated.

As an example, here is a thrust graph for a 1.7IE 33, peak torque at about 4500rpm.

All the best

Keith
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Hi

Cheers for the graphs, although the gearing bit is pretty essential (not sure exactly which model they are from.



Few if any cars are geared to minimise revs at (say) 70 by gearing to have the torque available just enough to maintain speed. If they did then as torque was dropping away while wind resistance is going up that would probably be the top speed without changing down. Gearing is more down to getting the top speed to match up with peak power, but maybe biased on the low side to have the car more responsive in top, or biased on the high side sacrificing top speed and engine response at the possible benefit of fuel consumption.

Power = torque x rpm is a fixed factor, irrespective of whether there is a turbo or not. Trouble is that people mix up torque as a figure with an engine being described as torquey (as being flexible), when the 2 are pretty much unrelated.

As an example, here is a thrust graph for a 1.7IE 33, peak torque at about 4500rpm.

All the best

Keith
Don't think that's quite the case, let's take a diesel or TBC petrol which produces peak torque from 1750. Then the gearbox will be tuned to deliver in 5th or sixth slightly higher revs to meet factors such as wind resistance, but the RPM is still relatively low I think mine's around 2300 RPM @ 70 in sixth. They use the minimum RPM possible to reserve fuel consumption, on a bike I've not a clue I don't work on them.

Motor torque is a massive part of the whole design, so is the gearbox. The less torque the engine produces, the more the gearbox has to be tuned to deliver mechanical rotation to the wheels, but you forfeit speed per gear.. The more torque an engine produces, the more speed per gear you have access to.

E.g. Focus ST 2013 250HP / 270 lb-ft @ 1750 2.0 Turbo charged, revs @ 70 MPH 2380:

I = 3.23, II = 1.95, III = 1.32, IV = 1.03, V = 1.13, VI = 0.94

BMW 130i 260HP / 232 lb-ft @ 2500RPM naturally aspirated, revs @ 70 MPH 3000 , larger capacity unit producing more torque to drive train, but still not as much as the ST.

4.350 2.496 1.665 1.230 1.000 0.851

Higher gear ratio's made up in first and second for lack or torque, then balanced out in upper gears for more speed.
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Don't think that's quite the case, let's take a diesel or TBC petrol which produces peak torque from 1750. Then the gearbox will be tuned to deliver in 5th or sixth slightly higher revs to meet factors such as wind resistance, but the RPM is still relatively low I think mine's around 2300 RPM @ 70 in sixth. They use the minimum RPM possible to reserve fuel consumption, on a bike I've not a clue I don't work on them.
Min rpm is not the major factor. In basic theory an engine is at its most efficient at peak torque (although with fueling changing with rpm, throttle position, etc, that can be miles out).

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Motor torque is a massive part of the whole design, so is the gearbox. The less torque the engine produces, the more the gearbox has to be tuned to deliver mechanical rotation to the wheels, but you forfeit speed per gear.. The more torque an engine produces, the more speed per gear you have access to.
Just gearing. True that the more torque you have the higher gearing you can run, but that just means having the same torque at the wheels, just like if you can produce torque at higher rpm you can use lower gearing, hence more torque at the wheels for the same engine speed.

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E.g. Focus ST 2013 250HP / 270 lb-ft @ 1750 2.0 Turbo charged, revs @ 70 MPH 2380:

I = 3.23, II = 1.95, III = 1.32, IV = 1.03, V = 1.13, VI = 0.94

BMW 130i 260HP / 232 lb-ft @ 2500RPM naturally aspirated, revs @ 70 MPH 3000 , larger capacity unit producing more torque to drive train, but still not as much as the ST.

4.350 2.496 1.665 1.230 1.000 0.851

Higher gear ratio's made up in first and second for lack or torque, then balanced out in upper gears for more speed.
Unfortunately those stats miss the final drive gearing which is the easy bit to change and which makes a big difference. From what I can find the BMW has a final drive ratio of 3.23 while the focus has 4.06. That is a massive difference.

Also with those the BMW has a fairly flat torque delivery and its peak is at fairly low rpm.

Putting the gearing and fairly crude figures in it looks like that Ford is geared fairly low, to the extent the gearing is probably limiting top speed, while the BMW is geared with peak power in top gear well beyond top speed. 5th gear on the BMW is pretty close to top gear on the Ford (in 5th the BMW has more thrust than the Ford in 6th, even though peak power in 5th equates to a higher top speed than 6th in the Ford).

All the best

Keith
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Min rpm is not the major factor. In basic theory an engine is at its most efficient at peak torque (although with fueling changing with rpm, throttle position, etc, that can be miles out).
Sorry mate, just making sure where on the same page here, if the engine is at it's most efficient at peak torque and let's say a TBC petrol engine's peak torque is at 2300 RPM. Doesn't that throw the RPM X Torque thing out the window? If it was a N/A with liftcam like a Type R that would make sense.

Quote:

Just gearing. True that the more torque you have the higher gearing you can run, but that just means having the same torque at the wheels, just like if you can produce torque at higher rpm you can use lower gearing, hence more torque at the wheels for the same engine speed.
But that would be ideal would it not? Especially in a performance orientated hot hatch style of car. If a competitor has a top speed of 130 and you create more torque balanced across the gearbox to achieve 150MPH, then said manufacturer wins top trumps. There's a specific reason they turbocharge cars to produce more torque and HP than there naturally aspirated brethren or it wouldn't be worth doing.

Quote:
Unfortunately those stats miss the final drive gearing which is the easy bit to change and which makes a big difference. From what I can find the BMW has a final drive ratio of 3.23 while the focus has 4.06. That is a massive difference.

Also with those the BMW has a fairly flat torque delivery and its peak is at fairly low rpm.

Putting the gearing and fairly crude figures in it looks like that Ford is geared fairly low, to the extent the gearing is probably limiting top speed, while the BMW is geared with peak power in top gear well beyond top speed. 5th gear on the BMW is pretty close to top gear on the Ford (in 5th the BMW has more thrust than the Ford in 6th, even though peak power in 5th equates to a higher top speed than 6th in the Ford).

All the best

Keith
In sixth there's one MPH between them, although I can see what BMW have done..

Enjoying this Keith, I know text can come across as a little stabby so don't take anything I put down as undermining or aggressive ..
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Figures & graphs mean nothing if you hate the rattle, smoke & smell
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Figures & graphs mean nothing if you hate the rattle, smoke & smell
100%, I love how petrols are so quiet until they roar.. The noise is excellent, that's if you don't walk off and forget you left it on after owning a derv ..
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Sorry mate, just making sure where on the same page here, if the engine is at it's most efficient at peak torque and let's say a TBC petrol engine's peak torque is at 2300 RPM. Doesn't that throw the RPM X Torque thing out the window? If it was a N/A with liftcam like a Type R that would make sense.
No. Power = torque x rpm thing is fixed. They are directly related, and the relationship is mathematical one. Doesn't matter whether it is a turbo, diesel, petrol or whatever. In imperial terms Power(in hp) = (Torque(in lbf) * rpm) / 5252 (this is why if you look at any legitimate imperial unit dyno graph the power and torque lines will cross at 5252rpm).

From that for your figures for the Focus ST, 270lbf torque at 1750rpm means the car manages 90hp at 1750rpm, while the 250hp at 5500rpm means it is managing to still produce 236lbf of torque at those revs. The BMW has peak torque at 2500, managing 232lbf. But from the 265hp at 6600rpm it has managed to still be producing 211lbf at 6600rpm.

Remember that as long as torque is not falling away too quickly power can still be going up. Hence for your example turbo petrol engine after 2300rpm acceleration is reducing even though peak power might not be until 5500rpm.

For fun, compare the figures above with this graph for a somewhat played with 2 stroke 125cc bike. This is producing peak torque at 10000rpm, same revs as it produces peak power. This is producing a hell of a lot of torque for its engine capacity, with 13.13lbf (1/17th of the torque of the BMW 130 for an engine 1/24 of the capacity), but with such a peaky power delivery there is no way it would be described as torquey (great fun though).

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But that would be ideal would it not? Especially in a performance orientated hot hatch style of car. If a competitor has a top speed of 130 and you create more torque balanced across the gearbox to achieve 150MPH, then said manufacturer wins top trumps.
To an extent. Produce X torque a Y rpm and you will have to use higher gearing than another car that produces the same X torque but at 2Y rpm. Hence at any particular speed despite having the same amount of torque the 2nd car has twice the thrust. Hence why you want torque over a wide rev range, with the torque hanging on for as many revs as possible. Same torque produced at higher rpm will result in a higher top speed. If you have an engine biased to torque at low rpm then you will lose out on top speed.

It isn't the gearbox giving you that torque. The higher the gearing the more thrust (which is the torque you feel) that you lose at the wheels.

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There's a specific reason they turbocharge cars to produce more torque and HP than there naturally aspirated brethren or it wouldn't be worth doing.
Turbos give you a bit more power on the cheap, as far as fuel consumption, weight, etc, as they are taking advantage of wasted energy in the exhaust, hence why they are popular. But they are gaining more torque than they are losing rpm for that torque.

That said, personally I would prefer a normally aspirated engine which revs out, with peak torque at fairly high rpm (hence when you start to push it, it is like the engine is encouraging you saying 'thrash me, thrash me', while the one with torque at low rpm and dropping off is saying 'I am bored, change up and go to sleep'), and rather more instantaneous throttle response.

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In sixth there's one MPH between them, although I can see what BMW have done..
The BMW is geared for about 175mph at peak power in top gear (not a hope it would manage that) while the Ford is geared for just under 150mph at peak power in top gear (which it should manage). Seems the BMW is geared for cruising (can always change down for decent umph) while the Ford is has a closer ratio box

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Enjoying this Keith, I know text can come across as a little stabby so don't take anything I put down as undermining or aggressive ..
No problem.

I have spent a fair bit of time playing around with power / torque figures and gearing, when trying to work out gearing that works for doing top speed runs on bikes. With some highly tuned bikes it is entirely possible to have enough power to hold a speed, while the rev range that useful torque is produced over is so small that you can't reach that speed. One example was a bike that had enough power to hit 265mph, but gear it to do that speed and between ~235mph and ~260mph in top it didn't have enough thrust to even maintain speed, let alone accelerate.

All the best

Keith
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Hi Keith, from that were on the same lines and really enjoyed it.. But I wish it was that simple:

Power Explained
Torque and work measurements tell us how much has been accomplished, but provides no clue how fast a given amount of work (or torque) is done. That's the job of power, an expression of the rate or speed at which work is performed. The more power that is generated, the more work is done in a given time-period.

Second, assuming engine displacement is fixed, raising the engine's operating rpm range is the most effective way to make big naturally aspirated horsepower numbers. Suppose an engine makes 400 lb-ft at 3,000 rpm. The equation tells us that at 3,000 rpm it would produce 228 hp. But if the engine made 400 lb-ft at 6,000 rpm, it would produce 457 hp. It's true that doubling the torque output to 800 lb-ft at 3,000 rpm would also yield 457 hp, but that large an increase on a given displacement engine is unlikely in the real world without forced induction.

So Turbo's etc. are a big part of it, there's a very good read about all the maths and background here from hot rod magazine:

It is simply unrealistic to expect a normally aspirated engine to produce both big torque and big power numbers under 6,000 rpm-unless the engine is really huge.

Torque vs. Horsepower - Engine Power Delivery Explained - Hot Rod Magazine

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Does any of the above esoteric debate actually help the OP with his query?
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Does any of the above esoteric debate actually help the OP with his query?
Not at all, but it's fun debating this stuff with a fellow technical car nut .. Didn't it come down to someone saying petrol's have less grunt down low then petrols? I'm not sure I lost track
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Hi Keith, from that were on the same lines and really enjoyed it.. But I wish it was that simple:
It is that simple.

The bit that is difficult to getting an engine to produce a lot more torque at the same rpm, or the same torque at higher rpm (or even better a lot more torque at higher rpm).

Most turbo engines now seem to be biased towards increasing torque at low rpm rather than at high rpm. Hence a greater increase in torque than power.

If you have 1 engine that produces 100lbf at 6000rpm and another that produces 200lbf at 3000rpm then they are both producing the same power at their respective peak torques (about 115hp). But if they both geared for the same top speed in each gear then the one with 200lbf needs twice as high gearing, and both land up producing the same thrust (torque at the wheels). Neither engine has an advantage over the other

This is why the rpm that torque is produced at is important. You are using the lower gearing to produce the amount of work (ie, torque) more quickly.

All the best

Keith
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Quote:
Originally Posted by Shadowamd View Post
Didn't it come down to someone saying petrol's have less grunt down low then petrols? I'm not sure I lost track
Think so (assuming you meant petrols have less grunt low down than diesels), but for me unless I am towing something heavy then that is a down side

Diesel engine does have the advantage that it is using a fuel with a higher energy content, and that (along with reduced pumping losses from not have a throttle) is the main reason that they are better on fuel.

All the best

Keith
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