In cylinder vacuum measurements ?

[Tom68]

Has anybody ever accurately measured real Vac numbers in a cylinder at speed.

There's plenty of graphs etc out there with pressures v degrees of rotation but I haven't seen real numbers for the vac side.

Not surprising when the vacuum could be less than negative 1 PSI I guess considering the pressures gettting measured on the positive side.

[mag2555]

Well you can do it a round about way if you know your intake port air speed.

350 fps is equal to 2” of Hg.

2 inches of Hg is equal to a water depression of 27”.

[Stan Weiss]

If you look at rate of volume change as the piston moves down the bore you can get a rough idea of what is happening.

Stan

ab-cvc-15-20.gif

[hoffman900]

Has anybody ever accurately measured real Vac numbers in a cylinder at speed.

There's plenty of graphs etc out there with pressures v degrees of rotation but I haven't seen real numbers for the vac side.

Not surprising when the vacuum could be less than negative 1 PSI I guess considering the pressures gettting measured on the positive side.

Most of those pressure graphs are referenced to atmospheric pressure. That shows exactly what you’re asking.

Vizard shared one where it was clearly referenced to the intake port which can be misleading (and it was).

[rebelrouser]

Has anybody ever accurately measured real Vac numbers in a cylinder at speed.

There's plenty of graphs etc out there with pressures v degrees of rotation but I haven't seen real numbers for the vac side.

Not surprising when the vacuum could be less than negative 1 PSI I guess considering the pressures gettting measured on the positive side.

When I was teaching, we had pressure transducers to install in the intake and in the spark plug hole. We could get running compression, and clear views of the pressure waves in the intake. On a lot of modern engines, we used pressure transducers in the exhaust pipe as well to detect weak cylinders and other problems. Somebody good with a scope and a pressure transducer in the cylinder can spot a loose timing chain. On many or the VVT engines it was the only way to really diagnose them.

[digger]

Well you can do it a round about way if you know your intake port air speed.

350 fps is equal to 2” of Hg.

2 inches of Hg is equal to a water depression of 27”.

port air speed is waaaaay more than 350fps near peak hp rpm.

i would say the in cylinder vacuum of typical engine is in the order of around ~0.4b at peak hp

[mag2555]

I should have included in that short sectence that a port air speed 350 fps usually is where peak torque is made.

[pcnsd]

If you look at rate of volume change as the piston moves down the bore you can get a rough idea of what is happening.

Stan

ab-cvc-15-20.gif

I think knowing the above, a person could use Boyle's law to calculate the pressure drop from 1 atm through 180 degrees of rotation as a representation of the intake pumping cycle.

[RAS]

Scientifically there is no such thing as a vacuum. Only less than atmospheric pressure. If you ever pull a vacuum, you would have nuclear fusion.

[Warp Speed]

Scientifically there is no such thing as a vacuum. Only less than atmospheric pressure. If you ever pull a vacuum, you would have nuclear fusion.

That is a true statement. A Vacuum is absence of gasses and all matter aka a black hole.
That is why we refer to it as depression.

[David Redszus]

port air speed is waaaaay more than 350fps near peak hp rpm.
i would say the in cylinder vacuum of typical engine is in the order of around ~0.4b at peak hp

Calculated data for informational purposes only.;

Vacuum = 0.406bar =5.894psi=12.0 in Hg=163.14inH2O
8% fuel vapor
30%RH
80F
PR=8.80/14.7= 0.5986
Critical PR=0.5351
Sonic velocity=1011 ft/s
Air Particle velocity=368.8 ft/s
Peak velocity occurs at peak torque.

If motored with 0% fuel and dry air:
Sonic velocity=1138 ft/s
Air Particle velocity=402.2 ft/s

Air particle velocity will depend on properties of air being ingested.

[Nut124]

Has anybody ever accurately measured real Vac numbers in a cylinder at speed.

There's plenty of graphs etc out there with pressures v degrees of rotation but I haven't seen real numbers for the vac side.

Not surprising when the vacuum could be less than negative 1 PSI I guess considering the pressures gettting measured on the positive side.

What would you do with this data if you had it?

[Tom68]

What would you do with this data if you had it?

Get a feel for port flow v lift v crank degrees.