Underpsi

Title says it all, I just wanna know how it works. I know it retards the timing.

bgilbert

Retards the timing on non-intercooled pumps, advances it on intercooled pumps.

Underpsi

anyone know how?

Alec

I'm posting a write-up and diagram in my photo gallery. As soon as it is approved I'll add the link.

In short, either device raises the internal pump pressure at lower engine speeds, thus advancing the pump timing.

6DoubleDuece

How about what is it?

Thanks,
Brenden

G1625S

Like Alec said, it's a device that, under certain conditions (usually cold ones) will allow/encourage higher internal pump pressure. This higher pressure advances the injection timing. When the engine is very cold, advanced timing helps the engine run smoother.

Underpsi

isn't retarded timing smoother in cold weather? and thats why some people are worried about having trouble starting thier trucks in the cold if they advance the timing

Alec

Alec

This write-up describes a system using a wax motor similar to the old trucks. The newer trucks use a solenoid. Either system raises the internal pump pressure at lower engine speeds.

As for timing and starting: When the engine first starts, the timing is fully retarded, because there is no pressure built up inside the pump housing. This makes it easier to spin the engine over, and get the engine up to operating rpms. Once the engine is spinning at the minimum cranking speed for starting (which is normally around 500 rpm for a diesel), advancing the timing makes the engine run better, because more heat is captured in the compression stroke, and the combustion chamber temperature goes up, etc. With the VE pump this happens, because the internal pump pressure rises, and actuates the variable timing mechanism.

If you advance the static timing, it makes it harder for the starter to crank the engine over, but will actually improve things once the engine has started.

G1625S

Thanks Alec, well said.

6DoubleDuece

Thanks for the explanation.
Brenden

Underpsi

Thanks I appreciate it

Alec

I'm happy if it was helpful.

Timing is a big part of getting a lot of fuel to burn (and thus generate big HP) aparently. I am more familiar with it as an efficiency/longevity problem: advanced timing improves efficiency but tends to reduce longevity.