Advanced Engineering Tech - MAP higher at idle than at part throttle

What is the science behind this?

My car idles at an average of 57 kpa at idle at 800 rpm. If I rev it up to 1500 or so, I notice the KPA goes down to 40 kpa. I can understand why MAP dips below idle during decel, not I cant see why it would happen when freely holding revs. I would think that when you crack teh throttle open with yoru right foot, more air enters the intake causing greater MAP readings. The greater manifold pressure is what puts more air into the intake runners and into each cylinder, spinning up the motor. Why do I see the oppodite happen to a point? More air in the intake manifold does indeed spin the motor up, but the lower MAP reading is the confusing issue.

Im thinking that this accel dip in MAP readings may be the cause or atleast contribute to low load cam bucking.

Any knowledge on this?

it happens to any car with a decent size cam in it.

at low RPMs, the engine isnt very efficient anymore... so you open it up with more air (higher MAP reading) and it holds that RPM....


but at a cruise of say, 2500 RPM, its now MUCH more efficient then it was at 800... and you hold a lower MAP reading when maintaining RPM..

remember, its still more air at 2500...the RPM is higher..its just at a different pressure.


make sense?

Basically, at low rpms the engine is not producing good vacuum which drives a MAP sensor. As you reach higher RPMs (to a point) the engine is now producing better vacuum due to the air flow being at optimum point of the cam (idle wise).

The overlap lets it draw back from the exhaust as well as
intake and this decreases the vacuum draw from the intake
side on the intake stroke of the cycle. Because the piston
is slow the cylinder can be filled with a large proportion of
spent gas rather than intake air, even though the overlap
is largely at low lifts. More RPM cuts the overlap actual time
down, less drawn from the exhaust in the early going and
more from the intake as the valve opens.

IVC is very important too. It's pushing charge back out of the cylinder at lower RPMs until IVC, which goes directly into manifold air pressure, thus raising MAP reading.

ive been wondering this same thing. glad to have an answer on it!!

I think some of you are misunderstanding the question based on this line:

More air in the intake manifold does indeed spin the motor up, but the lower MAP reading is the confusing issue.

The MAP sensor changes from idle to light throttle, to loaded throttle has very little to do with the cam, but more related to engine load and RPM.

My car idles at an average of 57 kpa at idle at 800 rpm. If I rev it up to 1500 or so, I notice the KPA goes down to 40 kpa.

This is what should happen with no load with any camshaft profile.

The pressure in the manifold is relative to ambient atmospheric pressure.
With the engine off, the MAP should read atmospheric pressure because the
pistons are not altering the plenum pressure.

When you start the motor, the pistons create a lower pressure in the intake
manifold which causes air to move from outside of the motor into the cylinders.

At this point the MAP will read lower pressure within the intake manifold than
it had when the engine was off.

The air is moving from high pressure (ambient outside pressure) to low pressure (engine intake manifold pressure).

If your intake had a higher pressure, the air would be pushed out of the intake
and into the atmosphere.

I can understand why MAP dips below idle during decel, not I cant see why it would happen when freely holding revs.

Engine load will alter the piston speed and change the pressure in the manifold.
The intake manifold will have a different pressure while idle at 2000 RPM than it
would at 2000 RPM in gear, going up hill.

More air in the intake manifold does indeed spin the motor up, but the lower MAP reading is the confusing issue.

It's actually the amount of charge the enters the cylinders that would have
the impact of accelerating the motor. The manifold pressure is not the gauge
of VE%