Steve X16XE
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Registered: 31st Dec 06
Location: Barnsley, South Yorkshire
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You are probably familiar with the terms 'horsepower' and 'torque', and it is highly likely that you have at least estimated these values for your engine. Both are regularly used in the car industry and are considered the 'norm' in terms of rating the performance of a car's engine, and people are happy to compare using these figures.
The question is though, what exactly are these units, and how do they relate to how well an engine can actually perform?
Horsepower (Bhp)
This is the value being quoted more often than not, and is generally used as the main comparison between engines. Horsepower can be calculated to decimal place accuracy, but the size of the horsepower is somewhat of an estimate.
The value of the horsepower was invented by James Watt (the same James Watt who gave us the watt value seen on light bulbs etc), and it is said the measurement was created while Watt was working with horses in a coalmine. Watt observed that, on average, one horse could provide enough force to lift 550 pounds 1-foot in 1 second. Another way of quoting the same value, and as used more commonly, is 150Lbs 220ft in 1 minute. This value is really more of an estimation, however considering the same standard unit is used when comparing power, it doesn't matter. Horsepower can also be converted into other units. For example, 1 horsepower is the equivalent to 746 watts.
So, as previously mentioned, horsepower is the most commonly used rating of an engine's performance, but how do you find out that rating? In order to calculate an engines horsepower, you need to know two values - the cars torque level (lbs-ft), and the engine speed (rpm).
Calculating Horsepower
This is where it gets a bit heavy on the maths, but you don't really need to understand how it's calculated, just accept that it works!
As stated earlier, the unit of 1 horsepower is to move 150Lbs, 220ft in 1 minute.
This can also be stated as 1 horsepower is the ability to do 33,000Lbs-ft of work in 1 minute. (Work = Force Applied x Distance Moved).
Watts definition came from a horse applying torque to a lever, so for an engine we need to imagine a 1ft lever attached to the crankshaft.
If the lever were to rotate 360°, the end of the lever would travel 6.28ft (using the formula Circumference = Pi x Diameter)
The basic formula is this;
HORSEPOWER = (TORQUE x RPM x 6.28ft) / 33,000Lbs-ft
It can then be simplified to this;
HORSEPOWER = (TORQUE x RPM) / 5252
Now, we can try this formula out
Say we put an imaginary car on a dyno, and we took one of the readings such as 260Lbs-ft @ 3200RPM.
We can now calculate the horsepower of the car at that specific RPM
Horsepower = (260 x 3200) / 5252 = 158.4Bhp @ 3200RPM
Now, you may be saying to yourself 'why do you need a formula to find horsepower, why not just measure it?'. Well, this is where the confusion normally begins, so let me start by explaining just what torque is.
Torque (lbs-ft)
Torque is a measure of force, or more specifically a measure of rotational force - the twisting force that makes something go round and round. Torque is measured in pounds-foot (or foot-pounds in the US) due to the way it's calculated.
To illustrate the reason for its measurement, say you have a socket wrench with a 2-foot long handle, and you apply 50 pounds of force to that handle. What you're doing is applying a rotational force - torque, of 100 pounds-foot. That is, you're applying 50 pounds of force along 2 feet of handle, so that equates to 100 pounds-foot of torque. Similarly, you could apply the same 100 pounds-foot of torque by using a 100-foot handle with 1 pound of force on it. The handle, or lever, is acting as a force multiplier.
In terms of a car engine, it's very much the same as the above example - a wrench turning a nut. Think of the nut as the engines crankshaft. The piston moves up and down in the cylinder, and due to the con rod being offset, it applies torque to the crankshaft - the con rod is applying a rotary force, which causes the crankshaft to rotate.
Torque or Horsepower?
Now at this point, having given the basic explanations of torque and horsepower, I want to highlight a common misconception. Torque is a physical force, it can be felt and it can be directly measured. Torque is the force an engine exerts. In comparison, horsepower is not a force, it cannot be felt and it cannot be directly measured. Horsepower is purely a 'rating'. Again, you may say 'well horsepower makes a car move and accelerate doesn't it?'. This to is a common misconception, as it is torque, not horsepower, that both moves and accelerates a car. To make this point clearer, you need to understand the two values better.
Torque, as mentioned, is a force. In order to move an object, such as a car, you need to apply a force, in this case torque. Once the car is moving, to allow it to accelerate, you need to increase this force to overcome air resistance and friction, so you need to increase the torque, or force, being applied.
Horsepower, as its name suggests, is purely a measure of power, a rating. Look back to the formula used to calculate horsepower. What you're doing is taking into account two things - torque and rpm. You're saying this much force was exerted by the engine when it was spinning this fast. If you look, the formula takes the engines speed, its rpm, and converts it to a distance - how far a point at the end of a 1-foot arm on the crankshaft has travelled. Now power is defined as 'a measure of how much work has been done'. Again, looking back to the formula, we see work is calculated as force multiplied by distance moved. Well we have our force, the engines torque, and we have our distance by inputting the engines speed. So, by working out the horsepower, we've calculated how much work has been done.
What this shows, is that horsepower is simply a rating of how much work an engine can do. Now think of it like this. If you employed people to lift 5kg sacks, and in an hour one guy lifts 20 sacks while another only lifts 10, the person who lifted 20 has done twice as much work. By doing twice as much work, that guy can be said to have worked twice as fast as the other guy, even though he only had to exert the same amount of force to lift each sack. Now, back in horsepower land, an engine that produces 500Bhp is able to do twice the amount of work a 250Bhp engine can. It's that simple.
Now remember, torque is what accelerates a car. The more torque you have going to the wheels, the faster the car will accelerate. Horsepower is a rating of an engines ability to do work. The more horsepower you have, the more work your engine can do. Having this knowledge, it allows you to look at and understand an engines performance more accurately.
Example
Say we have two cars, and both produce a maximum torque figure of 300Lbs-ft. Now, if one car produced this figure at 3000rpm, then using the formula we can calculate that the cars horsepower when at its torque peak is 171Bhp. Now take the second car. Lets say it produces its figure of 300Lbs-ft of torque at 6000rpm as opposed to 3000. Again, using the formula, we can see when at its torque peak, the second cars horsepower is 343Bhp, twice that of the first car. This is because the engine is producing the same amount of torque but is spinning twice as fast, so that point on the imaginary 1-foot lever is travelling twice as far. Relating back to work done, the second engine has applied the same amount of force but the imaginary 1-foot arm has travelled twice as far. This means the second engine has done twice as much work, therefore it's rated as having twice as much horsepower.
Producing Horsepower
Basically, to change the amount of horsepower, you have to change these things - the amount of torque, the engine speed, or both.
Lets say we want an engine to have 200Bhp. We can go the Japanese route, and have a small amount of torque but a very high engine speed. To get 200Bhp, all we need is an engine that produces just 140
Lbs-ft, but at a high 7500rpm. Alternatively, we can go the American way, and have high torque and low rpm. Then to get our 200Bhp, we'd need an engine that produces 350Lbs-ft of torque but at a much lower 3000rpm. Finally, we can go the more European way, and have a mix of both. Then, to get the 200Bhp, we'd need an engine that produces 190Lbs-ft of torque at 5500rpm. This shows how different engines produce the same power but in different ways.
This also explains the characteristics of certain cars. For instance, a Honda Integra Type-R is a quick car, but it has to be revved hard all the time to perform. This is because it uses the low torque/high rev technique to produce its power, so when at low revs it only has reasonable performance. Then look at the AC Cobra, a car that accelerates hard from a very low rpm. This is because it uses the high torque/low rev technique to produce its power, but when at high revs the performance tails off. For a car that uses the more balanced technique for torque/revs, there's the Astra GTE. It's acceleration increases with revs until it passes the three-quarter mark of its rev range, at which point acceleration will start to decrease. This gives it good performance throughout the rev range, but means it doesn't need gearbox breaking torque or motorbike level rev limits.
Producing Torque
Just to elaborate further, and to explain how torque levels are altered, I'll answer a common question. People often wonder how large trucks could pull a house up a hill, yet they have a fraction of the power of say a Formula 1 car, and it's a feat a Formula 1 car couldn't repeat? Again, if you've understood the above, you realise torque is what provides the pulling force. In order to generate lots of torque, the simplest way is to build an engine with a large stroke.
Now, in this case stroke relates to the size of the con rods (the part between the piston and the crankshaft). As mentioned earlier, the con rod is offset, which means the base of the con rod actually moves in a circle around the crankshaft if you were looking at it end on. From the example with the socket wrench, you'll remember the longer the handle the more torque you can exert. Well, the longer the con rod is, the greater the force being exerted on the crankshaft. This shows how the con rod is in fact a torque multiplier, and the greater the length, the more torque the engine produces.
A truck engine has massive torque for this very reason - it has a long stroke. However, the problem is, the longer you make the stroke, the lower the engines rev limit. This is because of the increased inertia the con rod experiences, meaning the faster it moves the greater the strain it exerts on the rest of the internals. If the engine revs to high, it will destroy itself. So, a truck engine has a long stroke meaning lots of torque, but has a low rev limit because of this. In comparison, a Formula 1 car produces its power through the low torque/high rev technique. So, in order to achieve its 800Bhp +, it has a very short stroke meaning less torque, but it can rev to 18000rpm because of this.
Hopefully this has helped you to understand the key differences between horsepower and torque, but there may still be a few doubts. The main problem people have with understanding this, is that they'll see someone modify an engine, they'll be told it's producing an extra 50Bhp, and they'll see it now accelerates faster. Well, assuming you understood any of the above, then you'll realise that in order to increase the engines horsepower, they had to increase either the torque peak, or the engine speed at which the torque is produced.
In that respect, horsepower and torque are the flip sides of the same coin. If you want to increase horsepower, then you have to increase torque, and if you increase torque then you have to have increased horsepower.
The Need For Both?
OK, so what do we now know? Well, if you're happy with the above, then you should have an idea of how torque is produced, what it does, and why it's important. You should also know that horsepower is a rating of what an engine can do, and you understand there are different ways of getting the same power. Well, what you may still be wanting to know, is that if torque is all so important, why do we need horsepower? A simple answer would be because horsepower is an easy way of understanding an engines performance. It takes into account all the confusing bits and provides a straightforward value. However, horsepower is more important than that, and it should be taken into account when looking at torque.
For instance, say someone tells you their car has 200Lbs-ft of torque. Using your new understanding, you'd realise torque provides acceleration, so you'd think it was a fast car. Well, what if this 200Lbs-ft was produced at just 3000rpm? If you use the formula, then you'll realise this engine has 114Bhp when at peak torque. It doesn't sound so impressive anymore, and it's no surprise they would quote torque without horsepower. This is why you need to have both to get an idea of performance.
Acceleration And Top Speed
Now some sources define torque as being responsible for acceleration, and horsepower being responsible for top speed. This is technically wrong, as torque is responsible for all motion, but the theory is right.
Looking back at the different ways of producing the same power, you can see that high torque/low revs gives lots of acceleration, but low top end performance. Also, you can see that low torque/high revs gives the opposite. For this reason, you could say horsepower relates to a cars performance at high speeds. This is because if you have a big torque value but a small horsepower value, you know it will have low down acceleration but weak top end performance. If it has a low torque value but a high horsepower value, then you know it's performance comes in at the top end. If both are large, then you have the complete package.
Now this brings us to an important point. If you have to choose between high horsepower or high torque, always go for the horsepower. OK, so you're probably thinking that contradicts everything that's been said, but there's a reason. Remember how the con rod acts as a force multiplier? Well, gears are also force multipliers.
The very reason you have a gearbox is so the torque your engine produces can be multiplied in order to provide enough force to move. When you see a gear ratio, it relates to the difference in rotation between what goes into the gearbox and what comes out.
For instance, first gear will always have the largest ratio, because first gear needs the most torque as it's required to move a car from standstill. So, say that ratio was 3.5, then for every 3.5 revolutions going into the gearbox, one revolution goes out, but the torque has been multiplied by 3.5. However, due to the drive reduction this ratio causes, the car will reach its rev limit whilst travelling at a very low speed. In order to increase the speed, the gear must be replaced with one which has a smaller ratio (why you change gear), but the torque is multiplied by less meaning less acceleration. So, the larger the gear ratio, the greater the torque is increased by, meaning the greater the acceleration provided.
The reason for choosing horsepower over torque is simply because torque can be multiplied through gear ratios, whereas horsepower cannot. The downside to this is that if you need to use gearing to compensate for a lack of torque, you'll have a low top speed. This is because in order to maintain the torque levels the ratios will have to be larger than normal in every gear.
The ultimate solution is to have lots of gears like a Formula 1 car. As mentioned, Formula 1 cars use the low torque/high revs technique, and is why the latest Ferrari's and McLaren's run 7-speed gearboxes. This should also explain to you why close ratio gearboxes sacrifice top speed for acceleration.
So, hopefully, you understand horsepower and torque now. With this knowledge, you can get a better understanding of your car's performance, and apply this to how you drive it.
Maximum Acceleration
The most important place where you'd need maximum acceleration would be during a quarter-mile run. The goal is to get from A to B, from a standing start, in the fastest time possible, and in order to do this you need the greatest acceleration your engine can provide.
Now something that needs highlighting is that when an engines torque and horsepower values are given, they are peak values. They're given at an engine speed because it is only at that specific rpm that the maximum is produced. Above or below that speed and the engine will produce less, meaning less acceleration. This can be seen most clearly from the graph you get when you run your car on a rolling road. The highest point on the graph will be the peak value, and it's important to see how the engine builds up to that value. To put it simply, the more area there is under the graph the better, as power is being produced evenly throughout the rev range. If it's a very sharp rise and fall, this isn't so good, as the engine is producing a lot less whenever it's away from peak rpm. So, even though your car may have 200Bhp, when it's not at it's power peak, it will be producing a lower value.
Another point is that (unless stated otherwise) both values are given at the flywheel, not at the wheels. This means the affects of gearing and transmission losses (through friction/internal resistance etc.) are not taken into account.
One of the most important parts of the run is the start, as this is where people can often win or lose a race. The crucial factor is the engine speed (rpm) when you release the clutch. Now if you're all excited and feel having the needle bouncing off the redline is productive, then go ahead, but you'll lose the race. In order to get the fastest getaway, you need full acceleration. Acceleration is provided through torque, so for the ultimate launch you need to release the clutch at the engines torque peak. Now on all cars, the torque peak normally comes before the horsepower peak, and the horsepower peak has to come before the redline. So, for those who bounce off the limiter, you're a good 1500rpm past your torque peak.
Now being at the torque peak provides the ultimate launch, but only if traction permits. It's no use letting the clutch up at the torque peak only to disappear in a cloud of tyre smoke. If this is the case, then you need to launch before the torque peak. By doing this, you'll sacrifice initial acceleration for traction, but then once moving the engine will come into it's torque peak.
Here's another mistake. Upon understanding the above, people think they also need to shift at the torque peak to get maximum acceleration. To do this would be wrong, and could also lose you the race. So, how do you work out the shift points in order to achieve maximum acceleration? Well, to do this, you need a performance graph for the engine (showing torque against rpm, which you get from a rolling road session) and you need to know the gear ratios of the car.
At this point it's important to note that the table below does not take into account the final drive reduction, or the transmission losses that would be experienced. This is because the table is purely showing the performance difference between gears, and the above factors would apply similarly to each gear.
This is based upon a standard Mk2 Vauxhall Astra GTE 16V, and the data is from the official brochure. I'd like to point out that due to the quality of the graph, these values are approximate, and may differ slightly from actual figures.
Torque 1st 2nd 3rd 4th 5th
RPM (ft-lb) 3.55 2.16 1.48 1.13 .89
1000 97...... 345 210 144 110 87
1400 108..... 382 233 159 122 96
1800 115..... 408 248 170 130 102
2200 120..... 427 260 178 136 107
2600 125..... 442 269 184 141 111
3000 130..... 461 280 192 147 116
3400 134..... 476 290 199 152 119
3800 141..... 500 304 209 159 125
4200 147..... 521 317 217 166 131
4600 148..... 526 320 219 167 132
5000 149..... 529 322 221 169 132
5400 148..... 524 319 218 167 131
5800 141..... 500 304 209 159 125
6200 131..... 463 282 193 148 116
6600 117..... 416 253 174 133 104
The first column in the table is the engine speed in rpm, followed by the torque output at the respective engine speed. The following columns have been calculated by multiplying the given gear ratio by the torque output. This calculates the actual amount of torque that is being sent to the final drive.
So, what does this table tell us? To make any use of it, you need to know the formula for calculating the change in engine speed going from one gear to another.
The formula is;
NEW RPM = CURRENT RPM X (NEW RATIO / CURRENT RATIO)
So, for the above car, if you were doing 3000rpm in first and you changed up to second, your new engine speed should be 1825rpm.
This formula is very important as it allows us to see the difference in torque based on gear changes. Now, lets say we were to take the advice that you should change at the torque peak. For this car it's at 4800rpm. So, we're in first gear at the torque peak, and the gearbox is sending 530Lbs-ft of torque to the final drive. We change into second, we're now at 2920rpm, and it's sending 279Lbs-ft to the final drive. Immediately we can see this was bad advice. If you look at the table, even if we left it in first for an extra 1000rpm, at 5800rpm 500Lbs-ft would be sent to the final drive. Yes we are past the torque peak, and yes the torque is dropping, but it's still sending more torque to the final drive than at any point in second gear.
This car happens to have good torque throughout the rev range, and it is apparent from the table that, with this car, in order to get the greatest acceleration you need to change as late as possible, about 6600rpm being the optimum.
Not all cars have such a good torque spread, and shifting may take place before the redline (American cars especially). However, the rule is that if you could be sending more torque to the final drive in a higher gear than the one you're currently in (by using the rpm formula), you've missed your shift point. As long as you shift at the optimum rpm, then your car will be accelerating at its maximum rate.
I wrote this in order to try and explain a very difficult and confusing subject. I know the account was very long winded, but I felt it was necessary in order to get the points across. Just by searching the internet you'll find explanations are very few and far between, for the very reason that not many people really understand it. On researching the subject I came across many different ideas and opinions, and for that reason I'm happy to try and answer any questions or justify any comments.
I hope this has helped.
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Steve X16XE
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Registered: 31st Dec 06
Location: Barnsley, South Yorkshire
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I have also typed this up (ok found).
Would this be a good thing for the Tutiorals (sp) area?
How good do you think it is?
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Colin
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Registered: 4th Apr 02
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Post the torque explanation in the Honda thread 
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Steve
Premium Member
Registered: 30th Mar 02
Location: Worcestershire Drives: Defender
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where did you get it from?
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Ben G
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Registered: 12th Jan 07
Location: Essex
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gtf if someone think's i'm going to read that to understand bhp and torque lol
summary
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Steve X16XE
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Registered: 31st Dec 06
Location: Barnsley, South Yorkshire
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quote:
Originally posted by Steve
where did you get it from?
Have you seen my Are you a racer thread?
I had them on a floppy disc that i found when i clean up my bedroom. I think i've had them since i was 17 (6 years ago) when i was at college.
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Steve X16XE
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Registered: 31st Dec 06
Location: Barnsley, South Yorkshire
User status: Offline
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quote:
Originally posted by Ben G
gtf if someone think's i'm going to read that to understand bhp and torque lol
summary
Summary..... Put your right foot down to make you go faster 
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Ben G
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Registered: 12th Jan 07
Location: Essex
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what if your in a 1.2 8v though in 5th gear at 20mph though
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Steve X16XE
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Registered: 31st Dec 06
Location: Barnsley, South Yorkshire
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quote:
Originally posted by Ben G
what if your in a 1.2 8v though in 5th gear at 20mph though
Get out and push
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mattk
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Registered: 27th Feb 06
Location: St. Helens
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quote:
Originally posted by Ben G
what if your in a 1.2 8v though in any gear at 20mph though
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Scotty C
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Registered: 6th Nov 05
Location: Kidderminster Drives: 1.6 16v Sport
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Good read. Didn't read it though
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P1CK4D
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Registered: 19th Jun 06
Location: Around Essex Somewhere
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Ive just read it all.. and quite confused. What do we mainly want? horsepower or torque? 
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Warren G
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Registered: 14th May 06
Location: Kent
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quote:
Originally posted by P1CK4D
Ive just read it all.. and quite confused. What do we mainly want? horsepower or torque?
abit of both is always nice 
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Brett
Premium Member
Registered: 16th Dec 02
Location: Manchester
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quote:
Originally posted by Steve X16XE
quote:
Originally posted by Steve
where did you get it from?
Have you seen my Are you a racer thread?
I had them on a floppy disc that i found when i clean up my bedroom. I think i've had them since i was 17 (6 years ago) when i was at college.
Steve wants to put it on his site and is concerned about the copyright
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Steve X16XE
Member
Registered: 31st Dec 06
Location: Barnsley, South Yorkshire
User status: Offline
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quote:
Originally posted by loafofbrett
quote:
Originally posted by Steve X16XE
quote:
Originally posted by Steve
where did you get it from?
Have you seen my Are you a racer thread?
I had them on a floppy disc that i found when i clean up my bedroom. I think i've had them since i was 17 (6 years ago) when i was at college.
Steve wants to put it on his site and is concerned about the copyright
LMAO I see.
Sorry i don't know where i got it from.
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Nic Barnes
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Registered: 5th Apr 04
Location: nowhere near ginger people
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quote:
Originally posted by P1CK4D
Ive just read it all.. and quite confused. What do we mainly want? horsepower or torque?
bhp is for pub talk, torque wins races
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davcohen
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Registered: 1st Nov 03
Location: North London
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going to have to read this a little later thanks for the heads up though
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TomSBD
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Registered: 22nd Jun 06
Location: Spalding, Lincs
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good read
also on a dyno graph power and torque should always cross at 5252 rpm if in bhp and lb/ft
easy way to tell if you get a dodgy graph
[Edited on 24-07-2007 by TomSBD]
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