Manifold Pressure/RPM

[Oktas8]

Here's another version of the same explanation

The engine sucks air through the induction system when pistons move down on the induction stroke.

To simplify the process, think of the engine as a vacuum cleaner motor. The vacuum cleaner hose represents the induction system.

1) When the vacuum cleaner is running, is the pressure in the hose more, less or the same as outside air pressure?
Generally get the answer - less.

So remember that a normally aspirated engine that is running, has a manifold pressure less than atmospheric pressure.

2) If I stick two fingers over the end of the hose (or close the throttle of the engine), will the pressure in the hose change? Remember that the motor is sucking just as hard, but there's less space for the air to get through.
Generally get the answer - the pressure gets less.

So remember that manifold pressure drops when the throttle is moved towards closed.

3) If I turn down the speed of the motor somehow, what will happen to the pressure in the hose? (Think carefully before answering.)
Hopefully, get the answer - it sucks less, so the pressure will increase.

If I don't get that answer, then I ask
3b) If I slow the motor all the way down until it is stopped, what will happen to the pressure in the hose? Refer to q 1.
Generally, get the answer - slower motor, less suction, higher pressure in the hose.

So remember that the faster the normally aspirated engine turns, the more it sucks, and the lower is the pressure in the induction manifold.

Hope this helps.

BTW the pressure drop is due to a lot more than the carburettor venturi. Possibly Barit1 you are confused between the pressure measured by the manifold pressure gauge, and the low pressure region generated in the carburettor to help meter the fuel. A well designed carburettor venturi will have exactly the same static pressure upstream as downstream (apart from minor changes caused by evaporation of fuel).

So what does cause the drop in pressure? Think what might cause a drop in pressure in the vacuum cleaner.

- a really long hose
- a narrow hose
- a kinked hose
- a dirty air filter

Which correspond to:
- a really long induction system (ideally, have one carburettor per cylinder and stick it right on top of the induction valve. Length = very short, efficiency = very high)
- a really wide induction system - bigger is better.
- no corners at all (ideally, mount the induction system on top of the engine like racing cars, so the air pipe goes straight into the induction valves)
- no filter at all (ok that's impossible, so use a new, clean, efficient filter)

Operation of valves as per Old Smokey's post? Yes, definitely vital for an efficient engine. But not really relevant for discussion of the manifold pressure, as the valves are at the downstream end of the manifold system and so don't affect the manifold pressure as recorded by the gauge.

As usual, I've started out writing a short note, and ended up with an essay. Sorry about that, and thanks for reading to the end!

O8