Did you know? Triumph Motorcycles was also a German brand?

[docc]

May I frame the question this way:

How is "manifold pressure" different than "intake vacuum?"

[audiomick]

1 hour ago, docc said:

How is "manifold pressure" different than "intake vacuum?"

I started writing an answer to that, and then realised that it is probably too late at night for me to formulate my thoughts coherently. Nevertheless, in the course of thinking about it I found this article:

https://en.wikipedia.org/wiki/Manifold_vacuum

and realised that I had a "turned around" view of what the throttle does. One tends to think that opening the throttle is "urging on" the motor. As the article points out, in fact opening the throttle is more like "releasing the beast". Closing the throttle is not "turning down the fuel", but rather "cutting off the air supply".

Without the throttle, the motor (disregarding load) would run at maximun air intake and maxium revs all the time. Introducing the throttle into the equation provides the possibility to choke off the air supply, and keep the motor at idle, or lower than maximum revs.

So "intake vacuum" is kind of an indication of how much the motor is being held back by the throttle, and "manifold pressure" is the actual air (or gas, or air/fuel mixture) pressure in the manifold at any given state of throttle opening.

 

I think.

[Lucky Phil]

13 hours ago, audiomick said:

I started writing an answer to that, and then realised that it is probably too late at night for me to formulate my thoughts coherently. Nevertheless, in the course of thinking about it I found this article:

https://en.wikipedia.org/wiki/Manifold_vacuum

and realised that I had a "turned around" view of what the throttle does. One tends to think that opening the throttle is "urging on" the motor. As the article points out, in fact opening the throttle is more like "releasing the beast". Closing the throttle is not "turning down the fuel", but rather "cutting off the air supply".

Without the throttle, the motor (disregarding load) would run at maximun air intake and maxium revs all the time. Introducing the throttle into the equation provides the possibility to choke off the air supply, and keep the motor at idle, or lower than maximum revs.

So "intake vacuum" is kind of an indication of how much the motor is being held back by the throttle, and "manifold pressure" is the actual air (or gas, or air/fuel mixture) pressure in the manifold at any given state of throttle opening.

 

I think.

Well not strictly true. One advantage of Diesel engines is they have no throttle body and therefore no throttle blades. The engine speed and ability to produce torque is controlled by varying the volume of fuel only.

A piston aircraft engine with a variable pitch prop the power is controlled by adjusting the throttle to a specific manifold pressure and the engine rpm is controlled with the pitch lever. It's a bit like a helicopter driven by a gas turbine. In that case the engine turns 100% rpm all the time and when you pull up on the collective pitch it coarsens the blade angle of attack which provides the lift and this requires more power so it schedules more fuel to maintain the RPM at that load. RPM stays the same. Turbopro aircraft much the same.

An NA petrol engine will produce a manifold vacuum at idle and part throttle, more vacuum at high rpm on the overrun with a closes throttle and a little less than atmospheric at WOT. 

Before takeoff in a piston engine a/c the pilot will do a 30inch check at the threshold run up bay to ensure he is making full power before the actual takeoff. @Chuck needs to chime in he's the expert in this area.

[Pressureangle]

14 hours ago, docc said:

May I frame the question this way:

How is "manifold pressure" different than "intake vacuum?"

It's really a matter of semantics. There is no difference beyond the frame of reference. 'intake vacuum' is mostly an automotive prase, and measured relative to atmospheric pressure, as cars rarely go to altitude enough to change the frame of reference. 'manifold pressure' is used in aviation, because aircraft obviously gain enough altitude that the atmospheric pressure changes so much you can't use differential between atmospheric and intake pressure meaningfully. 'manifold pressure' is an absolute measured above zero, 'intake vacuum' is a differential between above and below throttle plates. Additionally, many aircraft (and modern autos) have turbo/superchargers which boost pressure above 'vacuum' so the phrase isn't applicable. 

Glad I could make that clear lol