Hi Folks,
Have a 2012 MCS with the EcoRoute HD attached to my Garmin 3790.

I note that occasionally if I happen to get the Garmin over to the gauges screen quickly enough (and I think if it's colder out though I can't be sure on that one) I note that the manifold pressure is very low - somewhere around -0.6Bar (about -8psi I think?), and it will fluctuate with throttle and RPM as I would expect it to. Throttle closed and decel produces -0.8 to -0.9 bar, and opening the throttle increases it to near 0.0bar. Because the engine was cold, I didn't want to ask for power and spool the turbo.

After a couple minutes (no more than 2), the readings change, and the pressure goes up to -0.06 bar and only varies very slightly if I close the throttle and decelerate in gear (trying to produce high vacuum) to maybe -0.07 or -0.08bar. During acceleration, the turbo pumps the manifold up as expected, to close to +1.0bar.

Note that the outside air temperatures were in the 14C/60F range when I started the engine on those previouse incidents. Tonight when I went to do a coffee run, I had similar outside air temps, but the engine didn't perform it's tricks - it had been shut down for 5 hours or so, but went straight to -0.06 bar...

Am I seeing a function where the turbo is inhibited when cold, and then spins slightly during most operations, or have I got a leak that seals itself when cold? What are the normal manifold pressure values during a cold start, and when at operating temperature?

Regards,
Eric

 

No, you're seeing effects of the vacuum pump driven by the camshaft.

Remember, all engines work on a vacuum to draw in an air/fuel mixture.
The negative readings you see are when the throttle plate is closed and the engine is trying to "suck" the hardest.
But since the N14 / N18 don't create a enough vacuum under idle to operate the brakes at running temp, the vacuum pump does the work.

- Erik

 

So the vacuum pump operates only for a short period after start? I would expect to see the high vacuum at all times when the throttle is closed if the vacuum pump is causing the readings I'm seeing. If that pump shuts off after a bit, what provides the vacuum to the braking system, fuel emissions systems etc, given the turbo is presurising the manifold moste of the time.

Is there a clutch that engages the vacuum pump to boost the vacuum as required? (or an "unloading valve" for the pump?)

This has raised more questions!!

 

My understanding of NORMALLY ASPIRATED engines is that they produce vacuum in the manifold due pistons sucking air past a small opening - ie a closed throttle plate. This vacuum is used to assist the brakes, aid with emissions, etc...

Given we have a turbo on our engines that pumps up the manifold, it would be quite possible under spirited driving conditions (and possibly others) to not have sufficient vacuum in the system to drive the brake assist and possibly other systems, which could lead to disaster. So... BMW installed a vacuum pump to service these users.

This doesn't explain why I'm seeing low manifold pressure at start (probably) when the engine is "cold soaked," then, after a short while, I see near ambient pressures in the manifold.

I also gather that the 'Justas' use the full VANOS system for the valve train, and that the throttle closes during start to build vacuum and for testing purposes. Do the throttles in the S and JCW do the same? I thought the valve train on the S and JCW wasn't fully variable...

 

You are correct (applies to turbos as well at idle and light loads.) Just as in conventional engines, both MC and MCS have butterfly throttle plates that engage during cold start and cause the inlet manifold vacuum that you observed...

...however, after initial startup the butterfly valve is fully opened allowing the inlet manifold to see ambient pressure at idle and above ambient pressures during boost (as you have observed.) In a conventional engine this would send the revs to the redline (limited only by fuel cutoff) but this is where the Valvetronic system comes in!!!! Valvetronic (on both MC and MCS engines) allows for full control of inlet valve duration and lift. This is used to control the amount of air admitted to each cylinder instead of by opening or closing the butterfly valve. The beauty of this approach is that during the inlet stroke at partial loads the piston is pulling against ambient pressure rather than a vacuum. This requires much less work (minimizing so called pumping losses) and thus means the engine is much more efficient during part load operation. This is why you get great gas mileage!! It also explains why you need a separate source of vacuum. Hope this helps!!!

 

That explains it!

I did not know we had the Valvetronic system as well - I got the impression that only the Justas had the system fully implemented on the intake side - or at least, that was how the MA described it...

Thanks' for the explanation!
Eric

 

I just double checked and we were both a little confused It is only 2011+ turbo engines that get Valvetronic. More power and better fuel economy! Dave

 

I've searched around some more and it appears there is widespread confusion as to just which valve related features have been fitted to normally aspirated and turbo R56 engines

Many writers appear to be confused by VANOS vs. Valvetronic. For the record: VANOS controls valve TIMING whereas Valvetronic controls valve LIFT. There are two versions of VANOS: single and double. Single VANOS controls only inlet valve timing, whereas double VANOS controls both inlet and exhaust valve timing. VANOS alone cannot replace the butterfly valve function. This takes Valvetronic as well as VANOS (probably in order to achieve a wide enough range of control.)

There were so many inconsistencies in the various sources I found that I am still not completely sure which variations of VANOS are used in which engines. I THINK that for 2011+, normally aspirated engines get double VANOS, whilst turbos get single VANOS. Meanwhile, the MINIUSA website seems to make it pretty clear that only the 2011+ turbo engines have Valvetronic.

Phew! Didn't think it would be that hard Dave

 

Heh!

It is odd that MINI is so cagey about what is installed on their engines... Stands to reason then that my MA had no idea - by the same token, he tried to tell me that the twin scroll turbo had something to do with one scroll for high pressure air and the other for high flow... I swallowed it whole until I searched online and found out what it really meant! sigh...

Thank you for the help; I can relax now, when I see those readings on my Garmin!
Eric