254 | Understanding Torque and Power
The relationship between power and torque is one that many experienced tuners and engine builders don’t fully understand. In this webinar we’ll discuss how torque is measured and from here, how power is calculated. We’ll also see how the torque curve affects engine operation, useable power band and even when you should shift gears.
1:10 - What is torque?
3:30 - What is horsepower and how do we calculate it?
5:00 - How do F1 cars make sure much power?
6:35 - Torque is airflow
8:05 - Effect of changing torque and RPM on horsepower
8:45 - Fuel map should have same shape as torque curve
10:55 - Power and torque curve discussion
12:40 - Calculating tractive effort
17:45 - Optimal gear shift point is at peak horsepower
20:40 - Flat torque curves
23:20 - Questions
- Hey guys it's Andre from High Performance Academy, welcome to another one of our webinars and this time we're going to be talking about horsepower versus torque. What those two terms mean, what's important and how it's going to affect the operation of your car. This is a topic that I know causes a huge amount of confusion. There are arguments out there on the internet, there are some pretty weird false claims, false statements. One that I quite often hear is horsepower sells cars, torque wins races.
I think another one that I've heard is horsepower is how fast you hit the wall, torque is how far you move the wall. And while those are really nice statements, they really aren't that meaningful to us so we want to actually dive down and find out what horsepower and torque are and what actually is important when it comes to considering the torque and power curves of our engine. As usual, at the end of today's lesson, we are going to be having a question and answer session so if there's anything that I talk about during the lesson then please feel free to ask those questions and we'll put a shout out when we want you to start doing that and we'll get into those at the end. So for a start what we want to do is understand that torque is a rotating force essentially, so this is really the result of the combustion process occurring in our engine. What we end up with is a pressure being created inside of the cylinder.
That pressure acts down on the top of the piston, it's then transferred down through the connecting rod and that force is what makes the crankshaft rotate. So that's really important, that's the key and if you've followed any of our other webinars on tuning you'll have seen probably our demonstrations where we do a spark sweep test to find MBT timing. And the key with that, when we're looking for MBT or minimum timing for best torque, what we're doing is holding the engine under steady state conditions at a fixed RPM and throttle opening and then what we're doing is sweeping the timing from a very retarded starting point and we're advancing it through and we're plotting the relationship between power and torque. What we're going to find is as we do this, as we start advancing the timing, our torque will increase, it'll increase, then it'll plateau, it'll peak and then if we continue to advance the timing further we're actually going to see the torque drop away again. And the point where we see peak torque for that particular operating condition is known as MBT.
Now depending on our engine geometry, it's going to move around a little bit but generally that's going to happen somewhere around about 16 to 18° of crankshaft rotation after we go past top dead centre. That's because at that point we've got a little bit of angulation in the connecting rod and it allows the force being put down onto the top of the piston to actually rotate the crankshaft. If we produced peak torque when the piston was at top dead centre, everything's straight up and down, there's no real way that the pressure in the cylinder then can actually rotate the crankshaft. So around about 16 to 18 degrees after TDC, doesn't really matter specifically because unless you're lucky enough to have access to a combustion, in cylinder combustion monitor, you're not going to specifically know where abouts you are seeing peak cylinder pressure but if you've got a sensitive dyno, then you will be able to see what timing results in peak torque, so that's the first consideration there. So we've talked about torque.
Horsepower though is, in an application for our automotive sense, is the application of force at a certain speed. So this is where our engine RPM comes in. And we're going to be talking today here in horsepower and torque in pound foot, you can easily convert that if you wish to kilowatts and newton metres, it doesn't really matter, the concepts that we're going to be talking about are more important than the units of measurement. However, specifically when we want to calculate horsepower, we take our torque, we multiply that by the engine RPM at which the torque is occurring and then we divide it by a constant, that constant is 5252. Now I'm not going to go through how we derive that constant.
If you want, bit of a Google will find a fair few articles that will take you through the mathematics but we're going a little bit higher level than getting down in the weeds with the maths. Now there's a couple of takeaways from that. First of all, because 5252 is a constant that we're dividing by, should be no big surprise here if you've got a very basic handle on maths that the power and torque curves, when we are using horsepower and pound foot will always cross on the dyno graph at 5252 RPM. If they're not, then something is very wrong there. So the other aspect with this is we also need to understand that the multiplication factor there of RPM can be a very powerful one and this we can take advantage of.
So for example here, this is how Formula 1 engines, maybe not the current ones that are turbocharged but the older days, Formula 1 engines that were relatively small capacity, from memory I think they got down to 2.4 litre V8 before they finally went to the V6 turbo era. Prior to that they were three litre and earlier 3.5 litre. These engines made, depending on the specific generation, 750 through to probably 900 horsepower from a naturally aspirated engine, circa three litres which on face value seems impossible and there's no magic here. What they are doing in order to make such insane power levels from a very small capacity engine is they are focusing on taking advantage of that multiplication factor of RPM. And this is why these engines rev to 18,000, 20,000 RPM plus.
The limits were imposed by the FIA to try and draw back the RPM limit because obviously at that RPM range, things start getting expensive pretty quickly. So what these engines to is make relatively modest torque figures but they make that torque at very very high RPM. And then you've got the multiplication factor and this is how we can get those sort of power levels. So the other aspect we need to understand here is how does the torque value get affected, how do we make more torque essentially and this is really an engine mechanical design. Other than maybe influencing our boost level on a turbocharged engine and optimising the ignition timing, if we take those two aspects out of the equation, there's nothing we as tuners can do to influence the torque that the engine is producing, it is a mechanical design element of the engine.
And I like to think of torque as airflow so once we understand that relationship, it's much easier to understand what the torque curve means to us in terms of airflow through the engine or volumetric efficiency through the engine. So in this way, when designing very high output, high RPM engines like those F1 engines I've been talking about, the key is to really focus on improving the airflow through the engine at very high RPM. So very specific design of the cylinder head, the cam profile and every other aspect of the engine to get as much airflow through the engine at very very high RPM. Consequence of this though is you can't really have the best of both worlds and their low RPM performance would be absolutely terrible. In fact a lot of the F1 era engines don't idle below about 4000 or 5000 RPM anyway.
So if you're thinking of getting one for you road car it's probably not going to be that workable. Now another couple of aspects with the power and torque equation, so just to reiterate, horsepower equals torque multiplied by engine RPM and then divided by 5252. From this again some really simple math that we can sort of apply there. If we double the force, double the torque but we halve the engine RPM that the torque is occurring at, we're going to see exactly the same horsepower figure. Likewise if we half the force but we double the engine speed we're going to still be in exactly the same situation so just working through how that curve works.
So I've mentioned there that torque is essentially airflow so another really important aspect with understand that is this defines what you should be looking at with your fuel map. So essentially you should be looking at a fuel map that basically follows broadly the same shape and curve as your torque curve. If you're not, then this can actually indicate that you've got some problems. So a lot of novice tuners, when they're first getting started, would expect that the fuel curve's just going to constantly increase with RPM but that's not the case. What we should have is a fuel curve or a fuel map that slowly increases from low RPM, peaks, unsurprisingly, somewhere around about peak torque, middle of the RPM range and then at higher RPM as the engine efficiency and the airflow into the engine and out of it reduces, we also reduce the fuel map numbers as well.
So if we just jump across to my laptop screen for a moment, this is a really simple power and torque graph, we're going to come back and look at this in a bit more detail here but in this case the torque curve, that's this one here, so this is essentially what your fuel map should look like, the shape of your fuel map should look like. One of the key things, it gets a little bit away from our example today but I do want to just talk about it, if you have a torque curve on the dyno screen that looks like that but when you look at your fuel map, it's looking something like this, and this is quite common, if you've got a fuel system deficiency, this straight away is a bit of an alarm bell because the fuel map is not following the shape of the torque curve, it's not matched to the airflow into the engine. If I see something like this, generally that screams alarm bells that there's probably a likelihood that you've got a system deficiency in your fuel, fuel system, maybe your fuel pressure, your fuel pump can't keep up and your fuel pressure's dropping away so you're artificially increasing the injector pulse width in order to try and get to your target air/fuel ratio. Likewise you'll see similar if you're actually just out of injector but of course once you're out of injector, adding more to the fuel map doesn't actually give you any more fuel. Alright so now we've got a bit of an idea on some of the basics there, we're going to have a bit more of a look at this really simple sample power and torque curve now.
You can see here that this engine is a pretty low torque engine that is also fairly low revving, you can see we're only going through to 5000 here. So while you can see that the two curves are starting to come together, because we don't go to 5252 RPM for this particular example, they don't have the option to do so. I should also mention here, the rest of the lesson that I'm going through here, basically this is an article that Sasha from On Point Dyno over in Canada delivered for us a fair while back. It's on the articles section of our website. So if you do want some more details, I would urge you to go and have a read of that, I'm going to be giving you a bit of a synopsis of the article because I know that probably our articles section of our website is not as well used and supported as it should be.
On a side note, there is a really great, huge amount of information in the articles so please make sure you do check that out. So I'm going to be going through that but again if you want more detail, you can check that out at a later point. Alright so let's see what we can take from our power and torque curves here. On its own, just looking at a dyno graph of this nature, it doesn't really give us a lot of information because there's a lot of stuff here that can influence the output of the vehicle when we take into account the drivetrain. So what I'm talking about here is the gearbox, the gear ratios, the diff ratio or final drive ratio.
Also the rolling diameter of our tyre or wheel combination as well. What we really want to know is what's actually happening at the wheels. That's really where it matters, what's being delivered to the wheels and that's what we're going to calculate there, we're going to calculate the torque being delivered at the wheels. Now we call this tractive effort and basically this defines how much traction is necessary for us to be able to put the power to the ground without wheel spin so this is really the key, this is what we want to understand. Now in order to be able to do this, we are going to need a few pieces of information, we're going to need our gear ratios, should be pretty easy for you to find that information regardless whether you're running a factory gearbox or a motorsport specific gearbox.
And you're also going to need the final drive, same goes there. And your tyre rollout as well. So for this particular calculation Sasha went through, he generated some numbers that were basically designed to keep the math very simple. Probably explaining to you right now those numbers in itself is not going to really be that useful and it's also not going to be necessary to be able to follow through with the rest of this demonstration so I'll talk through what is necessary and we'll explain that in a bit more detail. So what we're going to do is start by calculating the torque being delivered at the wheels.
And we're going to just take for a start here, first gear, so in this hypothetical vehicle we've got a first gear ratio of 3:1, we've also got a final drive ratio of 4:1 and we've got a tyre rollout of 2000 mm so again I won't go through this for every gear but in first gear, what this means is that at the 5000 RPM limiter, we're going to be doing 50 km/h so again the math makes things nice and simple there. So what we're going to do now is calculate and plot the torque versus road speed for the gear. For first gear, again I'm not going to go through this for every gear, what we want to do is take the engine torque from our dyno graph, we want to multiply that, first of all by our gear ratio, which remember is 3:1 and then we also need to multiply that by our final drive ratio which in this case is 4:1 so this gives us a multiplication factor of 12 times our engine torque so this is why a gearbox is a very powerful aspect of the total installation in our car. For first gear, just given that ratio, the tyre rollout etc, we are going to be moving from 30 km/h through to 50 km/h, sorry 10 km/h through to 50 km/h for first gear. From our original torque curve, the peak torque being produced on the dyno was 340 pound foot at 3000 RPM.
If we multiply that by 12 that gives us 4080 pound foot being delivered to the tyre at 30 km/h. So let's have a look at the tractive effort graph, and we've got our axle torque in pound foot here on the vertical axis and we've got our road speed in km/h. So we have now got this little plot here in orange of the torque curve in first gear. So straight away, 4080 pound foot sounds like a pretty impressive amount of torque but also note, it's first gear, we are maxing out there at 50 km/h so we're not going very fast. We've also got this little orange dot here which shows us the point in that curve where we were actually seeing peak power.
So that's our first gear complete there, I'll just get us across to second gear. So just need to zoom these in. Hopefully these are easy enough to see, the colours may be a little bit tricky for you to see but maybe I'll actually just draw over these, I think the orange probably comes across pretty easily but this is essentially the same graph and now we've added in our plot for second gear. And we've got that sort of shape here. So straight away, as soon as we've got first and second gear plotted together here, it makes it instantly obvious where the shift point, the optimal shift point should be and that's at this point here.
Now obviously at the moment, the horizontal axis is road speed but of course we can easily take that back and derive our engine RPM at which the shift point should be. Basically if we shift earlier than that, or we shift later than that, we're going to have less tractive effort being delivered to the wheels which in turn means that we're going to have less acceleration. Alright so what we'll do is we'll just go to our next plot and now we've got essentially all of the gears being plotted here so we can sort of see the shape of the torque curve, the tractive effort curve I should say, once we've got all of our gears, we've still got the peak power values being, points being plotted there by the little balls on that graph. And the key here, the key takeaway once we've got this is other than the fact we now can very easily see what our optimal shift points are, is that we'll have the highest torque to the wheels if we stay as close as possible to peak horsepower as we can. So this is the part that's overlooked.
It's actually really important for us to choose our gearing and basically choose our shift points so that we are staying as close as possible to the peak horsepower value. Now if we had the perfect situation here, what we would want to do is have a drivetrain that would keep out engine at peak horsepower right through our acceleration curve. Those actually exist, no one likes them but they're called CVT gearboxes and if I can get this to go to our next curve, essentially that's what we would see, the red curve that's drawn in there is what we would see with a CVT style gearbox that was optimally set up to maintain the engine at peak horsepower during acceleration. So what we can see as well, again it might be a little bit tricky but there are a few areas here, if we actually follow the curves, the tractive effort curves for the individual gears, we can see the areas where they actually drop below that potentially or theoretically optimal line here as we shift gears. And that's where we're going to be basically losing acceleration compared to that optimal CVT style gearbox.
Unfortunately CVT is not particularly well liked, I've driven a few of them and while they may work well on paper, they just give a very weird horrible driving experience so I don't think, I think that's probably one of the reasons why we really haven't seen a huge adoption of those in general. Alright so what we want to understand from this, now that we've gone through and we've calculated our tractive effort graph, there's a couple of things here, so first of all, the peak torque of the engine is of no interest to the ideal tractive effort curve, that's because the maximum acceleration will be realised with shorter gearing and higher engine speed despite the lower engine torque. So in other words, all we really care about is staying as close to peak horsepower as possible and then we let the gearing make up for the lost torque at the engine. So the horsepower and our horsepower curve, where peak horsepower is occurring actually becomes more important than our torque curve. So if you've got questions, this is probably a good time to start asking those, we've got one more little aspect that we're going to go through here before we finish up.
I know this is a confusing topic, I know there's a lot of misunderstanding about it so let's see if you've got any questions and we will jump into those in a second. One of the misconceptions or misunderstandings that I often hear, Sasha basically found exactly the same when he was developing this article is a lot of people think that the ultimate would be an engine with an absolutely flat torque curve. We hear this bandied around as the ultimate and this is a bit misunderstood and misconstrued. So the reality is that that would end up being a pretty horrible situation. Hopefully by now you've got the idea that if we have a flat torque curve, what we're going to end up with in turn is a power curve that just continually increases as our RPM increases.
So what this of course means is that we're going to reach peak horsepower at the shift point, at our rev limiter I should say, not our shift point, obviously the shift point then by virtue becomes the RPM limiter. So this is what we'd sort of be looking at, if we jump back across to my laptop screen, if we had a completely flat torque curve, we've basically gone through and plotted that tractive effort graph again. And what you can see here is at that shift point, let's just take, what's that, first, second, third to fourth shift. So we shift at 75 km/h here from third gear and then we shift to fourth gear and we can see the big area that we are losing there. So the car would significantly suffer compared to having a conventional torque curve.
Pretty much the only real advantage there in having a flat torque curve like this is it's going to mean that from the driver's perspective, regardless of what RPM they are at in the RPM range, they're always going to have essentially identical amount of torque so this is going to make it a little bit easier for the driver to manage the torque delivery from the engine, particularly if you are in a vehicle that's traction limited or you're on a wet racetrack, it means that the delivery of the torque is going to be very predictable but ultimately it's going to be slower than if we had a conventional torque curve, we could run the engine past peak horsepower and shift into the next gear. Alright we'll head across and have a look and see if we do have any questions. And it looks like we haven't so either I've done an exceptional job of describing all of that and everyone's just understood it or I've jumped the gun and no one has had a chance to ask questions. Oh no, we've got questions down here, just didn't scroll dar enough, there we go, that'll get you. Craig has asked, has anyone put together a calculator to make building these graphs easy, doesn't look too much more complex than an Excel spreadsheet to build.
Craig 100% that's exactly what you'd do. There probably are calculators online but it's really not that complex and you can basically build a simple Excel spreadsheet to do exactly that which is exactly what Sasha did there. Barry's asked, as you said the gearbox is a torque multiplier, if you dyno a car in a gear that's not 1:1, would that skew the dyno torque readings and if so, how do you go about working around getting an accurate reading on a dyno with a car that doesn't have a true 1:1? Yeah Barry there's actually a really good article that Todd from Mainline wrote about exactly this situation. The reality is yes, the gearbox is a torque multiplier but what we need to also understand that the torque that is being delivered to the dyno roller, let's say we're talking about rolling road, is going to be dependent on the torque multiplication factor but then we also have the fact that if you use a lower gear, say you dyno the car in second or third gear, yes you're going to have a higher torque multiplication factor but the road speed, the tyre speed, wheel speed, roller speed is all going to be lower. So in essence it all cancels itself out however in reality yes there will be a small difference in power and torque output depending which gear you dyno the car in.
This is more to do with parasitic losses etc through the drivetrain than anything else. Personally I don't see it as being a big factor, if you were chasing internet fame and you wanted every last horsepower, sure, run the car in third, fourth, fifth and sixth gear and see which one gives you the biggest number. Most people who are really tuning for the process of tuning the car and optimising it, they don't really care about the peak number and what they're trying to do is get repeatability and then you're just going to be dynoing the car in the same gear. Generally I will be trying to dyno close to fourth gear, sorry close to 1:1, that'll be typically fourth or fifth gear. Just a Pretzel with Internet Access has asked, great name by the way, I want to put a blower on my '06 Merc but it's an ethanol, it's the ethanol capable motor, is there anything different I need to do? Look probably a topic that's a little bit outside the scope of today's lesson, we try and keep these questions really on track so it doesn't just sort of deviate onto a bit of a sort of anything goes.
So looks like the rest of our questions also probably a little bit off topic so I think we'll just leave it at that. And for our members as well, if you've got questions that crop up after this webinar has aired, please feel free to ask those in the forum and I'll be happy to answer them there. Thanks to everyone who has joined us and hopefully we'll see you online again next week, cheers.