Performance tuning 101: Stop the guesswork and start measuring

Did you just click to my blog because you have been wasting hours and hours online, in order to find some crucial metrics for your build ?

My two cents: Stop wasting your time, and start measuring. This is a pretty basic knowledge, if you happen to have an engineering degree. Let’s have an example. You have been thinking if your fuel pump and lines has a sufficient flow for your build. Then you shall do the following:

1. Attach 14 volts supply to your project cart,
2. Detach a fuel line from a fuel rail,
3. Find a bucket with measurement units,
4. Bypass the fuel pump relay, and feed constant current to the pump,
5. Measure how much your fuel pump and line will flow gasoline per minute (with 14 volts),
6. Multiply this by 60,
7. Done. Now you have the measured information about your fuel pump flow, in liters per hour.

Note: It is important to feed 14 volts to your fuel pump while measuring, thus this will increase the fuel pump flow compared to 12 volts, when your car is turned off.

Now you have the measured data of your fuel pump capacity. How about measuring a fuel consumption per hour in a full throttle then? This is a bit trickier to measure, because you need an access to live or logged data. The parameter we are looking is Engine Load (mg). This number will tell you how much your ECU is putting gasoline trough injectors per stroke.

Example variables for calculating fuel consumption:

1. Engine load = 826 mg
2. Revolutions per minute = 5900 rpm
3. Two revolutions is required for a full power stroke

Calculation:

0,826g *5900rpm / 2 = 2437 grams of gasoline in a minute,

Whereas 1 litre of gasoline weighs 0,75 kilograms,

Thus fuel consumption is 2,437kg / 0,75kg = 3,29 l / minute

Fuel consumption per hour is 3,29 *60min = 197,5 litres per hour.

Now you have the exact, measured data. And we can tell, that your fuel pump shall have at least 200 litre per hour flow capacity + some overhead + reserve for future updates. In this example, 250 l/h pump would be a good choice.

Remember: stop guesswork and start measuring. It can take hours and hours of surfing on the internet, and yet – you have only assumptions – meanwhile you could be measuring e.g. a real capacity of your fuel pump.

My friend gave me a reminder after reading this article: fuel flow is in relation to fuel pressure. There is good charts out there about the correlation, like the one from tanksinc.com. More pressure will decrease a pump flow, so remember to put enough overhead for your calculations.