Basic Engine Calculations
Tuning is the single most important variable in getting power from your performance upgrades. An engine creates power by burning fuel and oxygen. Adding one component without the other will not create more power and can be fatal to a motor. A motor with too much fuel displaces air and reduces power. A motor with too little fuel runs hotter and has reduced power. If the A/F mixture is too lean (not enough fuel) the temperature will become hot enough that detonation will occur. Detonation will destroy a motor.
Tuning is usually done in 1 of 3 methods and can be done on a track or on a dyno.
1. Acceleration tuning is usually done by measuring the amount of time it takes to travel between two RPM (or MPH) points in a given gear or the amount of time to coverage a distance. This method is very good at determining gains from parts. It is also a great method to micro tune a base set up for better performance.
2. Tuning for horsepower and torque is the most common type of tuning. Most commonly peak HP is the goal in this tuning. Tuning is usually done on a dyno or on a ¼ mile track using MPH to determine peak HP.
Unfortunately peak HP doesn't always mean your engine will accelerate faster. It simply means it makes the most peak power in this setup. Tuning for peak hp will often make your car slower, by sacrificing midrange power. When tuning for horsepower, to acheive maximum acceleration you want to create a torque or hp curve that has the largest area under its curve.
3. Tuning for Air Fuel ratio is often used to create fuel maps. It is usually used in combination of with methods 1 and 2 above. NA Honda motors generally make the most power slightly lean, between 13.0:1 and 13.5:1 A/F ratio. Turbo motors are generally tuned around 12.5:1 to prevent detonation. AF ratios greater than those numbers above are not suggested even if they make more power because of detonation and heat concerns.
There are multiple devices that can be used to tune in the methods mentioned above. They are.
1. Accelerometer such as the Gtech Pro. These devices measure the acceleration of the car and then use the weight of the car to determine HP (F=MA; Force = Mass x Acceleration). The devices are accurate if set up properly. This device is used on a track. The new competition model allows for data download to a computer, data logging. Combining this product with data logging wide band oxygen sensor would be great for tuning.
2. A Data logging device can be used such as the Diggimoto or Scantool on OBDII ecu. These devices read data such as velocity over time and can calculate acceleration and HP similar to accelerometers. They don't need to be calibrated, but are more prone to errors caused by wheel spin. This device is used on a track.
A data-logging device in combination with a wide band oxygen sensor (that is logged) is the ultimate tuning tool. Unfortunately data loggers with a WB sensor are extremely expensive.
3. An engine dyno is a device that measures HP at the crank of a motor. This means that the engine must be out of the car to be hooked up to the dyno. Manufactures and domestic engine builders usually use engine dyno's.
The engine dyno is a fairly simple device that is very accurate. It is basically a generator hooked up to the crank of the motor. The generator creates electricity and the amount is proportional to the hp created at the crank. Simple, effective, and accurate.
Unfortunately these dyno's are not particularly useful to an enthusiast building up their motor, as the engine must be out of the car.
4. A chassis dyno is a device that measures HP from a stationary vehicle while the engine is still in the car. There are several makers of chassis dynos; the most common are DynoJet, Mustang Dyno, and Dynopack.
All are excellent for determining gains from performance parts and are good for tuning.
There is a great misconception about inertia-based dynos. That misconception is that they are inaccurate. This certainly isn't true. Inertia based dyno are very accurate, but they do have an achilles heel that some people exaggerate to discredit them.
The issue with inertia-based dynos is that during acceleration through the rev range erratic errors can be caused due to the inertia of the drum. Power is absorbed from the engine during acceleration and released on deceleration. This occurs as the rotating drum is resisting changes to it's motion. The effect is similar to a gyroscope and is based on the same principal. These errors are called sweep rate variations and cause torque spikes in a dyno plot; which is why raw dyno sheets have spikes. The errors magnitude depends on the sweep rate; the faster the engine is revved the larger the error.
It should be noted that even if the dyno is not inertia based these errors are still present to some degree. The transmission of a car, the wheels, the brake discs, the flywheel, even the rotating assembly in the engine all add inertia errors to any test done on the engine.
The errors caused by inertia are small if sweep time is reasonable. If the errors were large as some people suggest you'd see much larger fluctuations between dyno runs. This isn't the case.
Still inertia effects should be considered, and certain things should be done to minimize their size. These are:
1. smooth the results
2. have a long sweep rate (ie: longer acceleration times)
3. reduce the inertia (low inertia is better than large inertia)
4. multiple tests
Each dyno has its advantages and disadvantages and all are excellent tools for tuning an engine. Dyno results fluctuate between dyno to dyno due to calibration.
As you can see there are many tuning options. There is not necessarily a best method to tune, but this is what I suggest.
Use a device with a wide band oxygen sensor to adjust fuel maps. Running lean is dangerous so using a WB sensor is necessary to get your base fuel settings correct. To do this you need a data logging WB sensor mapped to RPM. A safe starting point to aim for is 11:1 AF.
Once your base fuel maps are setup you tune. I suggest starting with cam gear settings (note this will affect ignition timing), ignition timing, vtec cross over, and finally fuel fine tuning. Using a dyno to make the initial setting is great because all your information is easily visible and available on the fly. I don't suggest tuning for peak hp, but area under the HP vs RPM curve. If tuning for drag racing or autocross you may want to ignore low RPM results and focus in on the rev range where your car is between shifts while racing. Tuning for maximum area under the curve in this area. Once the initial tune is done I suggest fine-tuning for acceleration at the track.