Hi,

I came accross a good description of acceleration and top speed in terms of power and torque:

Acceleration: Engine torque to vehicle weight ratio. Increasing torque or reducing weight (or both), assuming reasonable level of grip, will improve acceleration.

Top Speed: Ratio of bhp to the coefficient of drag (CD). Improving bhp or reducing drag (or both) will raise top speed.

Source: Fast Road Car: Stapeleton,D

Any thoughts?

Best wishes

John

The amount of power you have determines the speed at which you go through the Hedge.

The amount of torque you have determines how far you push the Hedge........

Good analogy, torque is what accelerates a car, power is a product of torque x speed.

Martin Brundle during the F1 coverage gave a good boxing analogy, which he'd read in a motorbike magazine.

Torque is how hard the boxer can hit, the bigger the hit the harder the effect i.e.acceleration

Power is how fast the boxer can hit, the faster the punches the quicker the end result, i.e.top speed.

Ian

Difficult concept to understand though.

I think one of the biggest things I notice from my jabbads Octavia is the extra torque - I rarely change gear over 3000rpm

John

Difficult concept to understand though.

I think one of the biggest things I notice from my jabbads Octavia is the extra torque - I rarely change gear over 3000rpm

John

Nail into board analogy.

Torque: How hard you hit the nail each time.

BHP: The rate at which you hit the nail. Faster = finish sooner.

J.

HORSEPOWER is how fast you hit the wall, TORQUE is how far you take the wall with you.

UNDERSTEER is when you hit the wall with the front of the car,OVERSTEER is when you hit the wall with the rear of the car

Looking back, my favoured cars have always been heavy on the torque as opposed to the power but I guess a similar amount of both is favoured? My runaround is a remapped Fiesta TDCI and thats only 110bhp but 200lb/ft torque and it absolutely flies! (and still returns 50mpg around town!)

From low engine revs or a standstill Torque (lb/ft) gets the cars mass moving.............once it's moving and has momentum and the engine revs rise Power (bhp) then takes over!

This is why some of the new TSI engines have a super charger and a turbo charger...........super chargers are very good at the low end of the rev range (Torque) but tend to fade at the high end of the rev range.......where as turbo charges tend to be the opposite (nothing at the low end...and everything at the high (power) end of the rev range).......So the supercharger (producing mainly torque) acts up to around the 3500 rpm (there is a smooth overlap of a few hundred rpm) and then the magnetic clutch disengages and the turbo (producing mainly power) is now up to speed and takes over!!....

.....simples.....

Say you were to drive different cars down the same bit of twisty back road.....

Small (under 2lt) petrol engines with or without turbos would generally need to driven through the gears more to keep the engines "on the boil" with the revs in the upper end where the engine is most responsive.

Turbo diesels would not need as many gear changes as they produce enough torque low down (being a diesel) so you could let the revs really drop and they have a turbo charger to "boost" the power at the top end!!

super chargers produce torque and turbo chargers produce power..??

Hi, not quite - potentially turbosuperchargers produce more torque at few revs as they work off the crankshaft as opposed to the exhaust gasses. Switch the car on the turbo-supercharger starts producing boost - you have to rev the car to almost 2000rpm for exhaust gasses to drive a turbocharger to produce boost.

I am not quite using the right words.

John

super chargers produce torque and turbo chargers produce power..??

In addition it depends on the compressor map. For example, a small turbo (housing and turbine/compressor) will spool up quickly and add torque but will not be able to deliver enough air as the turbine can over-speed. A larger turbo would add torque higher up the rpm range but add power at the top end as it is within the parameters of the compressor map -i.e. not over-speed.

I was trying to be "general" to make it an easy visulisation on the difference between power and torque and used the turbocharger vs supercharger analogy in a basic old style form!........

And yes in these days with clever mapping etc you can get a turbocharger to product lots of torque low down compared to years ago when turbo chargers were all "top end" power and nothing else!

Hi,

I came accross a good description of acceleration and top speed in terms of power and torque:

Acceleration: Engine torque to vehicle weight ratio. Increasing torque or reducing weight (or both), assuming reasonable level of grip, will improve acceleration.

Top Speed: Ratio of bhp to the coefficient of drag (CD). Improving bhp or reducing drag (or both) will raise top speed.

Source: Fast Road Car: Stapeleton,D

Any thoughts?

Best wishes

John

Cannot understand the mystery of the torque vs bhp relationship. In simple terms, the more wheel torque you have at any given vehicle speed, the more the car can accelerate. However, wheel torque is a product of engine torque and engine speed.

Take a one ton car with an engine that makes peak torque of 200lb/ft at 3500rpm vs the same car with an engine that produces a peak 100lb/ft at 7000rpm. If you gear both cars such that they run at peak torque at the same speed, you will get the same wheel torque and acceleration from both at peak engine torque.

The big differences are in the driving experience, where the big torque motor will get the job done at lower revs, but will need taller gearing to reach the same top speed as the higher revving low torque motor.

More important is the real world driving experience. Ultimately, given a decent gear set and torque curve, the car that makes most power for its weight will be the fastest. Power is the amount of work done and ultimately indicates the performance.

The differences in the drive are typically (for road engines) that a high revving petrol engine will have big reach in each gear, but will need to be kept on the boil with lots of gear changes. Same goes for the turbodiesel in my experience, narrow powerband needing a lot of changes to keep it in the sweet spot. Best of both worlds in raw acceleration terms would be the turbocharged petrol engine where toque is spread wide and runs well up the rev range. The broad spread of torque means less gear changes for the level of progress being made.

Chris

Same goes for the turbodiesel in my experience, narrow powerband needing a lot of changes to keep it in the sweet spot. Best of both worlds in raw acceleration terms would be the turbocharged petrol engine where toque is spread wide and runs well up the rev range. The broad spread of torque means less gear changes for the level of progress being made.

Chris

Hi, Jeremy Clarkson makes the same comment about turbo diesels power delivery, you wai,t get nothing then it all comes at then it's over.

The reason this happens is various emission laws, fuel qualities etc affect the way ecu's are calibrated. If you can find a company to re-calibrate the ecu data it will widen the powerband .

Best wishes

John

Hi, Jeremy Clarkson makes the same comment about turbo diesels power delivery, you wai,t get nothing then it all comes at then it's over.

The reason this happens is various emission laws, fuel qualities etc affect the way ecu's are calibrated. If you can find a company to re-calibrate the ecu data it will widen the powerband .

Best wishes

John

I had my diesel Fabia VRS custom mapped and it did indeed grow the torque curve nicely out, but still very peaky compared to a turbo petrol car. I think from memory I had peak torque from about 1900 to 3600rpm, so in diesel terms a pretty wide 1700rpm. There was go beyond 3600rpm, but it was dropping off rapidly. The turbo MR2 (see link in signature) makes peak torque from 3000 to 6000rpm and is still within 10% of that between 2000 and 6800rpm. Where the Fabia would exit a 30mph corner in third and need another gear before you got to 70mph, the MR2 will be in the zone from 30mph and will sail happily past 90mph before 4th is needed.

I like the gear busy nature of turbodiesel cars, takes planning and ability to get it right, which is nice. For truly lazy driving, turbo petrol is the easier option.

Chris

I like the gear busy nature of turbodiesel cars, takes planning and ability to get it right, which is nice. For truly lazy driving, turbo petrol is the easier option.

Chris

You ought to have a word with my mate!!!.....

..We have both been in each others cars over the same twisty back roads........(he has a MKV Golf TDI)...and he accuses me of "doing to many gear changes" because I approach it like a rally stage with the engine always in it's sweet spot!!!... ........then I accuse him of not changing gear enough as he just lets the engine pull the car and more or less leaves it in 5th, or 6th....and wonders why the engine has problems as he says it "stutters" around the 1300rpm mark!!!......thats because he's in to high a bloody gear!!!!......

Hi, it would be interesting to know the rev range 200 lb ft is produced comparing the 2 cars?

I had my diesel Fabia VRS custom mapped and it did indeed grow the torque curve nicely out, but still very peaky compared to a turbo petrol car. I think from memory I had peak torque from about 1900 to 3600rpm, so in diesel terms a pretty wide 1700rpm. There was go beyond 3600rpm, but it was dropping off rapidly. The turbo MR2 (see link in signature) makes peak torque from 3000 to 6000rpm and is still within 10% of that between 2000 and 6800rpm. Where the Fabia would exit a 30mph corner in third and need another gear before you got to 70mph, the MR2 will be in the zone from 30mph and will sail happily past 90mph before 4th is needed.

I like the gear busy nature of turbodiesel cars, takes planning and ability to get it right, which is nice. For truly lazy driving, turbo petrol is the easier option.

Chris

Regarding a peaky diesel engine. This is down to the mapping. Because theirs room at low rpm, tuners make use of the air which creates loads of low down torque which then drops quickly. So you can have 300lbft at 2krpm which drops to 220lbft at 4krpm. Or you can map it so it keeps the stock 240lbft across the rev range with a slight drop down to 220lbft at 4krpm.

Well that's how I'm mapping mine anyway

Hi, it would be interesting to know the rev range 200 lb ft is produced comparing the 2 cars?

Have no dyno graph scanned for the MR2, but the Fabia seems to be above 200lb/ft from around 1800rpm to around 4300rpm, the MR2 reaches 200lb/ft at 2700rpm and drops back down to it at around 6500rpm.

The Fabia peaked at around 283lb/ft, the MR2 at around 230lb/ft. Dyno for the Fabia:

Thing is the torque numbers are irrelevant really because without taking into account the gearing the figures mean little or nothing. This is why power is the figure that gives the best indication of outright performance. Just looking at torque curves, the Fabia should be the quicker car even with it's 350Kg weight disadvantage. The reality is very different though. Where the Fabia could manage 50-70 in around 2.5 seconds, the MR2 does it in 2 seconds in third and just over 1 second in 2nd. The Fabia was managing 60 -80 in around 4 seconds, the MR2 does it in a little over over 2 seconds.

When talking torque, the most useful information about what to expect of the drive is how much and the rpm at which the peak occurs.

The remap was a Jabba custom job, designed to produce a nice flat curve limited to around 280lb/ft for preservation of the DMF, clutch and gearbox and making traction easy to manage.

Chris

Hi, I agree regarding the gearing but lets say the gearing is the same.

I think the vrs has more to give - is the mr2 near the limit?

John

Hi, I agree regarding the gearing but lets say the gearing is the same.

I think the vrs has more to give - is the mr2 near the limit?

John

The MR2 is pretty much at its limit of durability on the standard engine internals. Gearboxes are marginal at 240lb/ft and I think the engine could be pushed to around 275bhp as a maximum with a slightly bigger turbo. The PD can be pushed further in the mid range, but at the top end on a standard turbo, I was pushing as far as I felt comfortable at the top end.

Obviously, running the same gearbox for both engines would make a different story although the gear ratios don't seem to be too different. Drop the mapped PD130 engine into the MR2 and run it from 30mph in third, the diesel gets away at 2000rpm with more torque, staying ahead in the torque stakes until 4000rpm, around 53mph, then from there onward the turbo 1zz motor is pulling ever stronger and will continue to pull through to 94mph. The PD130 will need a change to 4th at 67mph, where the higher gear will drop the mechanical advantage and the wheel torque is 16% in the 1zz turbo's favour even when the mapped PD130 is pulling its hardest.

Thing is that, if you where "making progress", you would never be driving the petrol car at the low revs where the diesel works best, similarly, you would not de limit your diesel and run it at 7000rpm. The crux of it is that through the gears, the diesel can make around 22% more torque, but at a nominal 50% less revs, so put both engines on the same gearbox and give it maximum through the gears, the turbo petrel would still be around 28% quicker, which, not by coincidence, is the approximate difference in power.

Chris

I did see a racing 1.9 tdi pd150 engine on a rolling road and accidentally they revved it to over 6000rpm and it survived ok - I think 5000 rpm is ok.

I was trying to get at the torque range of 2 similarly tuned petrol/ diesel cars, as in phase 1 (standard remap) to phase 3 (larger turbo, injectors etc) with similar gear ratios (hypothetical)

John

The MR2 is pretty much at its limit of durability on the standard engine internals. Gearboxes are marginal at 240lb/ft and I think the engine could be pushed to around 275bhp as a maximum with a slightly bigger turbo. The PD can be pushed further in the mid range, but at the top end on a standard turbo, I was pushing as far as I felt comfortable at the top end.

Obviously, running the same gearbox for both engines would make a different story although the gear ratios don't seem to be too different. Drop the mapped PD130 engine into the MR2 and run it from 30mph in third, the diesel gets away at 2000rpm with more torque, staying ahead in the torque stakes until 4000rpm, around 53mph, then from there onward the turbo 1zz motor is pulling ever stronger and will continue to pull through to 94mph. The PD130 will need a change to 4th at 67mph, where the higher gear will drop the mechanical advantage and the wheel torque is 16% in the 1zz turbo's favour even when the mapped PD130 is pulling its hardest.

Thing is that, if you where "making progress", you would never be driving the petrol car at the low revs where the diesel works best, similarly, you would not de limit your diesel and run it at 7000rpm. The crux of it is that through the gears, the diesel can make around 22% more torque, but at a nominal 50% less revs, so put both engines on the same gearbox and give it maximum through the gears, the turbo petrel would still be around 28% quicker, which, not by coincidence, is the approximate difference in power.

Chris

I did see a racing 1.9 tdi pd150 engine on a rolling road and accidentally they revved it to over 6000rpm and it survived ok - I think 5000 rpm is ok.

I was trying to get at the torque range of 2 similarly tuned petrol/ diesel cars, as in phase 1 (standard remap) to phase 3 (larger turbo, injectors etc) with similar gear ratios (hypothetical)

John

Revving to 6000rpm may not be too bad for the engine, but making any significant power at that engine speed is not something that I have seen in a diesel engine. Even LMP1 cars only run to a red line near 5000rpm.

Torque range for the diesel is going to be around 1500 to 3500 for a standard ish road engine, race diesel engines as seen in LMP1 cars will have a rev limit of around 5000rpm and produce useful power that far up the rev range.

Torque range for a turbo petrol engine on a road car is typically 2500 - 5500, a competition boosted petrol engine may rev to anything up to 20,000rpm.

Big issue with diesel in competition cars is weight, all that torque and very high peak cylinder pressures mean the engine has to be heavy. You can make a high revving diesel, I had a two stroke one that would rev to 18,000rpm, but it was a tiny marine unit. Technically, the diesel makes its torque by having a relatively long stroke, and you can increase bhp at the expense of torque by reducing stroke length.

What are you planning on building?

Chris

LMP1 diesel cars have air restrictors which is critical in terms of a large turbo supplying a high volume of air at a high boost pressure.

I am dreaming of an old audi a8 4.0 tdi with uprated twin turbos larger injectors producing 450 bhp and 700 lb ft.

John

BTW I think the 2014 f1 1.6 turbo engines have an rpm ceiling of 12,000

From low engine revs or a standstill Torque (lb/ft) gets the cars mass moving.............once it's moving and has momentum and the engine revs rise Power (bhp) then takes over!

This is why some of the new TSI engines have a super charger and a turbo charger...........super chargers are very good at the low end of the rev range (Torque) but tend to fade at the high end of the rev range.......where as turbo charges tend to be the opposite (nothing at the low end...and everything at the high (power) end of the rev range).......So the supercharger (producing mainly torque) acts up to around the 3500 rpm (there is a smooth overlap of a few hundred rpm) and then the magnetic clutch disengages and the turbo (producing mainly power) is now up to speed and takes over!!....

.....simples.....

Say you were to drive different cars down the same bit of twisty back road.....

Small (under 2lt) petrol engines with or without turbos would generally need to driven through the gears more to keep the engines "on the boil" with the revs in the upper end where the engine is most responsive.

Turbo diesels would not need as many gear changes as they produce enough torque low down (being a diesel) so you could let the revs really drop and they have a turbo charger to "boost" the power at the top end!!

Add VVT into the mix and "Miller cycle" is possible to lessen pumping losses and apparently reduce inlet charge temperatures. I'd never thought of that last bit.

http://en.wikipedia.org/wiki/Miller_cycle

J.