MagicMike
A good read: note that the car in question is a ttzed but I mainly post this for the explainations of torque and power and how they relate to each other 
There are several things that a dynochart will tell you about a vehicle's performance as well as where any problem may be. Horsepower and torque are two different ways of looking at the same thing, but they reveal characteristics of the powerplant that are unique to themselves. This is my interpretation of the information provided on a dyno chart based on my experience.
Horsepower: This is the measure of the engine's ability to perform work. This can be viewed as force per given time over distance.
Torque: This is the measure of instantaneous force the engine provides. This is what makes the wheels turn and it is direclt correlated to the push you feel on your back from the driver's seat. This is the 'fun factor' when it comes to driving a car.
Provided ample traction, a car with more torque is going to 'feel' faster than the same car with lesser torque, but it might not actually BE faster. "Faster" is a vague term that too loosely describes a car's performance character and we should use Fast and Quick. They have very different and almost opposite meaning.
A/F curve: This is a measure of what the mixture ratio of air to fuel is. There is a specific trend you should shoot for when it comes to A/F ratios, and it is directly dependent on the torque curve.
I'll start at torque.
Since torque is the instantaneous measure of force that the engine produces, you can imagine this unit as being the amount of energy released per given power stroke of the engine. The energy that is released in the chamber is directly correlated to the quantity of fuel that is oxidized by the air. The more air and fuel you can cram in there, the more power you will make. The torque curve effectively tells you how the engine is breathing.
Torque:
Since the engine is requiring more air as RPM increases and boost pressure only rises to a given point, we would expect that when the boost discontinues its rise, we should see a notable drop in torque. This point where the torque 'peaks' is actually where the turbos achieve their target boost pressure. The lower the RPM at which it achieves this peak, the less 'lag' your car is said to have.
Not all turbos have the same response, which is not the same as lag. Some turbos increase in boost pressure in a linear fashion and some do not. Some come in smooth and some come in hard. The response of the turbos are indicated by the slope of the torque curve. A very parabolically curved line shows a slow response, and a linear line to the top is a smooth response. You aren't going to get any better than a linear propagation of torque since the turbo's output is based on the changing rate of the engine RPM, which is linear-ly plotted on a dynochart. i.e. The closer to a linear rise in torque per RPM, the more drivable the car is going to be as power output is predictable. Slow response turbos are like driving a car with an on/off switch and are less desirable.
There is a basic law of fluids that dictates how much force is required to move a quantity of fluid through a vessel at a specific rate. As the quantity/velocity of fluid flow increases within a static volume, it requires more energy to move the fluid. Since the engine is moving more air out of the engine as RPM increases, but the exhaust components (specifically the manifolds) are not increasing their size, you are wasting more energy in trying to move the exhaust out of the engine. This energy is wasted in the form of heat, but its source is the rotation of the engine. i.e., it takes increasingly more power to move more air out of the engine through the exhaust tract as engine RPM increases. This effect is felt by the engine over its entire range of RPM, but it is most notable after the torque peak. The quicker the torque falls from its peak, the more you are seeing the effects of backpressure on the engine. You will note that Z's equipped with Greg D's inconel tubular manifolds have notably less torque drop after the peak than other similarly equipped Z's with stock manifolds. Although in every case with a turbo you are going to have a distinct torque peak and fall, the less your torque falls off after the peak, the more power you are going to make.
Seemingly less people want to know what the 'torque number' was for a car and desire to know first what the horsepower is. The peak torque achieved is actually more important IMHO than horsepower as it gives you a quantitative value of how much 'bang' you got going on in the chamber. The higher the peak torque, the better the engine is at being a pump.
Air/Fuel Ratio
The A/F plot on a dynochart has two key trends you should be looking for.
The A/F curve should be almost the opposite of the torque curve. i.e., as torque is increasing, the A/F should be decreasing (richer). At the point where you achieve your boost target, your A/F should at your target and it should stay at your target until redline - a little dip at the 6500RPM mark is good and safe. The temperature of the burn in the chamber is directly related to the amount of air and fuel in the cylinder as well as the air/fuel ratio. A leaner (less fuel) mixture is going to burn hotter, which will generate more power, but at the risk of thermal meltdown if the mixture is not rich enough.
Ideally, you want the A/F to drop as torque increases to its peak and at torque peak for a street car with pump fuel you are looking for anywhere between 10.5:1 to 11.5:1 dependent on your tuner or ECU programmer. For race fuel setups, 12:1 should be your limit.
Horsepower:
This is what everyone wants to know first, but its actually the leftovers when it comes down to it. If you have a torque curve and A/F trend of which I have explained above, your horsepower output is going to be just as desirable. During tuning sessions, I dont even care to see horsepower - it is simply an 'after the fact' figure to the process. Everybody wants to have high horsepower because they know that's the unit they'll be asked for, but, I can give you high horsepower that is completely useless, but it sounds impressive.
The horsepower can have independent traits from the torque curve. i.e., the torque can be falling, but the horsepower is still rising. This is a small fluke in representing power as it can be misleading to most. Horsepower is a function of torque and RPM. You can have less torque, but be actually producing more horsepower with a well-configured setup. Since you are not going to keep the torque from falling after your boost peak, your only hope is to simply slow its fall. If you can keep the torque from falling too quickly, your horsepower will not suffer and you will have a better 'powerband'.
You will note the characteristic of the horsepower curve between 0RPM and 5252RPM is very similar to that of the torque curve. Anytime the torque is increasing, so is the HP. Anytime the torque is decreasing on this interval, the HP will also decrease. Since we have already covered the torque curve and what you are looking for, the horsepower curve is really only usable after the 5252RPM point.
Things to look for on a horsepower curve
The "Powerband" is a somewhat vague term that is used to describe the range of engine RPM at which the engine is producing a similar amount of power. Typically when the torque peaks, your horsepower will tend to level off from its ascent and stay pretty flat and level until a later RPM. The higher up into the RPM range you can maintain that horsepower, the bigger the 'powerband' is said to be. The horsepower will not stay at that level forever as exhaust backpressure catches up and the torque will take another plunge and the horsepower goes down the tube with it. Obviously you aren't going to be producing an identical, to the 0.1HP variation amount of power over a range of RPM, but there are two points of distinction of the band.
Horsepower peak: I mention this at the very last for a very good reason - it is the least important defining value you could get when it comes down to making any determination on how an engine performs. It only defines one specific condition of the engine and, by itself, is missing a critical piece of information; engine RPM. Asking for the horsepower figure of a car is like being blind in an art gallery, but some people just dont know of the water they swim in. Consider it a conversation piece - that's about all it is worth.
Here's a pretty nice chart:
Chris Damato's 2530 powered TT with Greg D. Manifolds
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28th August 2006 - 01:10 AM
