Variable valve timing, throttles and intake vacuum

[JHZR2]

I installed a boost gauge on my 135i (twin turbo, DI, variable timing on intake and exhaust) yesterday. I was a but surprised by the readings, which were typically roughly -1.1 to -1.3 psi during idle and even deceleration.

This seemed odd to me, I had expected -15 to -25psi.

On boost I saw around 7 to 7.5psi, which is right for the turbo output. So it seems accurate under boost. I also verified the units it was displaying were psi.

I emailed the OEM, P3cars. He got back to me almost instantly, which was impressive and great service. He told me the following:

Quote:
The n55 engine uses valvetronic.... therefore the throttle stays open, and you get minimal engine vacuum. On normal engines, or even the n54, you will see -17 to -21 because the throttle is closed creating the restriction which causes vacuum in the manifold. The readings you saw are perfect for an n55 and exactly what you would see on any gauge, even a vacuum tap mechanical gauge.


Now my turbo vehicles have all been diesels, and they dont go negative. You have boost or zero.

But Im not understanding why valvetronic would hold a throttle open, what the reason is, and what the pros and cons of doing this are. Does this mean that the DI is more or less like a diesel with no throttle? I assume it is to reduce pumping losses, but how does it effect the fundamental operation of a gas engine?

Unlike my diesels, this car does not have a vacuum pump. So there must be sufficient to make the brake booster and similar parts work. Is -1.3psi enough?

Thanks for teaching me!

 

[JAG]

This article should help. http://en.m.wikipedia.org/wiki/Valvetronic
It says there is both a vacuum pump and a throttle plate.

 

[JHZR2]

Hey you're right, there is a vacuum pump like a diesel.

Any idea why there needs to be a throttle plate closed to initiate certain emissions control items?

If I'm thinking this through correctly, does all this mean that I'm only seeing -1.3 psi because the effective stroke length is much shorter due to the quickly closed intake valves, and thus there is less vacuum pulled?

 

[rationull]

Originally Posted By: JHZR2

If I'm thinking this through correctly, does all this mean that I'm only seeing -1.3 psi because the effective stroke length is much shorter due to the quickly closed intake valves, and thus there is less vacuum pulled?


I think what it means is that since the throttle isn't restricting and causing vacuum in the intake manifold, that the vacuum (or the majority of it) only exists in the cylinder, restricted by the valve.

That's how I reason it anyway.

 

[SteveSRT8]

A throttle plate is aptly named as it literally restricts the air flow thus throttling the engine down.

Valvetronic moves the valves open and closed across a large range of time and lift, thus the throttle is not needed. They have one for safety and to allow certain emissions functions.

Surprised me that it had much vacuum at all!!!