Rail Pressure or Injector Duration

How you should be tuning your common rail diesel

Words by Andrew Leimroth

Photography by Caravan & Motorhome

There are a lot of statements being made about how to safely tune new common rail diesels. So, how do you know how to tune your diesel efficiently?

Diesel engines are a ‘constant volume’ engine, meaning the air volume going into the engine is unrestricted, and combustion is only controlled by the amount of diesel being injected. Valve timing doesn’t change in current diesels, and the amount of time the injector is open (injector duration) varies only a small amount from idle to redline. In fact, as RPM and load get higher, combustion has effectively less time to happen, so opening the injector longer (altering injector duration) will be inefficient in increasing power.

Think of extending injector duration like punching a boxing bag. If you hit the bag close to you (piston at start of combustion stroke), the bag gets a full hit. But if you hit the bag as it’s moving away from you (piston moving down on the combustion stroke), the bag receives a lesser hit (piston receives a lesser push).

The same goes for extending injector duration. To add more power, the injector needs to be opened much longer to allow more fuel in to get the added bang onto the already moving away piston. But it’s not the most efficient way of tuning a diesel for more power, considering you will need to add a lot of fuel to hit that descending piston.

So how does the manufacturer get more fuel into the combustion chamber quicker as RPM increases? Simple... it digitally increases the fuel pressure so you get more in during the same time. Rail pressure can be as low as 3000PSI at idle and as high as 22,000PSI at the highest load/rpm combination point.

Just think of a garden hose with low spray. When you turn the tap on more, you get a significant increase in water volume and an even finer spray from only a small increase in pressure. Same goes for diesel common rail pressure – low load receives low pressure, while high load gets more pressure and a finer spray pattern.

Good quality diesel chips altering rail pressure correctly with digitally controlled accuracy are safely tuning the same way the manufacturer intended it to be. You won’t find any cheap chips tuning digitally, so be careful. Remember, diesels are durable, but not invincible. Make sure your next power up is guaranteed to be safe, and offers an extensive engine and driveline warranty backed by the manufacturer of the chip.

Safe Driving

Andrew