Al, thanks, yes that is clearer, but:
Quoting from this web site:
http://web.ncf.ca/ag384/vacuum.txt
I see the following statements regarding gasoline car engines and their vacuum at the intake manifold.
"Haynes emissions control manual
a. testing at various speeds
- engine starting vacuum should be 1 to 4 in Hg. To test
disable ignition (ground wire from coil), hold throttle
wide open, crank engine slowly with starting motor.
- healthy engine at idle should read steady 15 to 20 in Hg.
- healthy engine at 2000 rpm should read steady 19 to 21 in
Hg.
- healthy engine at open throttle should read close to 0 in
Hg.
- healthy decelerating engine reading should jump to 21 to
27 in Hg as open throttle released."
So even a gasoline operated engine reaches 0" of vacuum at WOT. The gasoline operated engines use a vacuum reservoir or tank along with a one way valve that allows the engine to pull a vacuum on the vacuum bottle and vacuum booster, but does not allow the vacuum to leak back out of the bottle and vacuum lines during low engine vacuum operating conditions.
You say that insufficient vacuum is developed during idle. What is the engine vacuum (at the intake manifold, not the venturi port) at idle on the SD22?
Correct me if I am wrong on this, but doesn't the pneumatic governor operate on two vacuum signals, one from the intake manifold, the other from the connection just above the throttle plate, the one you called the venturi port. And thus the governor operates on the vacuum DIFFERENTIAL pressure between the two vacuum locations? My thinking is that Nissan did not want an intermittent air supply, for example multiple vacuum lines occasionally feeding non throttle body air to the engine from the brake booster and other vacuum operated lines, because it would interfere with the engines pneumatic governor.
What I am saying is that perhaps Nissan had other reasons, like the pneumatic governor vacuum control across the throttle plate for using a separate vacuum pump.
Do all these small diesel engines use a separate vacuum pump, or just the ones with the pneumatic governors?
How much vacuum does the SD22 develop in the intake at idle?
How much vacuum is really needed to operate a brake booster? How much vacuum is the SD22 vacuum pump generating?
asavage wrote:Mike:
The SD's throttle plate does not close nearly as much as in the typical gasser. For that reason, insufficient vacuum is developed in the intake to provide adequate brake booster operation, and running down the road under load, there is effectively almost no intake manifold vacuum (which is the same situation as many gasser FI vehicles, at least my old '73 Volvo 145 B20F had nearly zero vacuum at highway load). It's not the volume, it's the amplitude of the pressure. Is that clearer?
Philip:
Though only 6" will move the pneumatic governor full sweep, that 6" is developed by the venturi port, not the intake manifold. I know you know this, I am merely amplifying your statement for the benefit of those who don't.
Bacho:
No. If you had a venturi small enough (necked down enough) to create sufficient vacuum to operate the brake booster, you may as well have not put the turbo on at all!
