Spotting the LDA
The LDA (boost compensator) is found on turbo Bosch VE pumps. You can easily tell a naturally aspirated pump from a turbo pump by whether or not it has one on top.
Naturally aspirated pump on the left, turbo pump on the right.
What does the LDA do?
On a naturally aspirated diesel: full-load fueling (pedal to the floor) is almost constant across the RPM range. It only drops slightly as RPM rises, because the mass of air drawn into each cylinder per cycle decreases as RPM goes up.
On a turbo diesel: this is where it gets trickier. On top of that, full-load fueling also has to change with boost pressure. Fueling needs to increase as the turbo builds boost.
So the pump's fueling is set as if the turbo were always on boost, and the LDA's job is to restrict fueling until the turbo has spooled up.
How does the LDA restrict fueling?
Without going into the full details of how a Bosch VE works: just as there's a lever on the outside to control the throttle, there's also a lever on the inside. That's the famous control collar lever, and its position sets how much fuel is delivered.
When the driver floors the throttle, this lever is pulled all the way to accelerate as fast as possible.
That's where the LDA acts as a stop for this lever, preventing it from moving all the way until the turbo has built boost.
Simple and effective!
How does the LDA work?
Now let's see how the LDA moves its stop. At the top of the LDA there's a diaphragm. The upper side is connected to the boost circuit, and the lower side is vented to atmosphere, with a spring.
It works like a turbo actuator: the more pressure you send in, the further the diaphragm is pushed down.
An LDA pin is attached to this diaphragm, or rather a cone if we're talking about a stock LDA. You can start to see where this is going.
As you can see on my little diagram, the LDA bell crank lever pivots in the middle, with the stop at the bottom and a small pin at the top, called the guide pin.
The guide pin rides against the LDA pin, so:
- Boost pressure comes in
- The diaphragm moves down (and the pin with it)
- The guide pin can now move further to the right
- So the stop can move further to the left
- And more fuel is allowed.
Once again, simple and effective!
How do you adjust the LDA?
Not every engine runs the same boost pressure, or the same off-boost and on-boost fueling. Here are the adjustments:
- Off-boost fueling: yes, you can tweak off-boost fueling by adjusting the starting position of the pin.
- Max fueling: you can adjust the end of the pin's travel.
- Taper rate / sensitivity: a stiffer or softer spring, with more or less preload, lets the diaphragm move down more or less easily.
- For stock pins: the cone is off-center, so rotating the pin increases fueling overall.
What about performance builds?
For builds, the big issue is travel. The LDA's travel, that is.
Stock pins are designed for stock engines, which basically means engines that don't even run 1 bar of boost.
And that's where it falls short. They don't have enough travel to properly manage the fueling difference between off boost and full boost, so you get heavy smoke at low RPM, which is usually not what you want.
"Performance" pins, like the ones available here, offer more travel and therefore better fueling control.
They can also work as a "stage 1" on a stock vehicle, so to speak. Because stock LDAs never fully unlock fueling, a good, well-adjusted pin can give results similar to a remap on an electronically managed engine.
A complete LDA tuning guide is available on the store.