In cylinder vacuum measurements ?
Moderator: Team
In cylinder vacuum measurements ?
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.
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.
Ignorance leads to confidence more often than knowledge does.
Nah, I'm not leaving myself out of the ignorant brigade....at times.
Nah, I'm not leaving myself out of the ignorant brigade....at times.
-
- Guru
- Posts: 4607
- Joined: Sun Sep 18, 2011 11:31 am
- Location: Heading for a bang up with Andromeda as we all are.
Re: In cylinder vacuum measurements ?
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”.
350 fps is equal to 2” of Hg.
2 inches of Hg is equal to a water depression of 27”.
You can cut a man's tongue from his mouth, but that does not mean he’s a liar, it just shows that you fear the truth he might speak about you!
- Stan Weiss
- Vendor
- Posts: 4821
- Joined: Tue Feb 20, 2007 1:31 pm
- Location: Philadelphia, PA
- Contact:
Re: In cylinder vacuum measurements ?
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
Stan
You do not have the required permissions to view the files attached to this post.
Stan Weiss/World Wide Enterprises
Offering Performance Software Since 1987
http://www.magneticlynx.com/carfor/carfor.htm
David Vizard & Stan Weiss' IOP / Flow / Induction Optimization Software
http://www.magneticlynx.com/DV
Offering Performance Software Since 1987
http://www.magneticlynx.com/carfor/carfor.htm
David Vizard & Stan Weiss' IOP / Flow / Induction Optimization Software
http://www.magneticlynx.com/DV
-
- HotPass
- Posts: 3466
- Joined: Sat Feb 23, 2013 5:42 pm
- Location:
Re: In cylinder vacuum measurements ?
Most of those pressure graphs are referenced to atmospheric pressure. That shows exactly what you’re asking.Tom68 wrote: ↑Sun Mar 19, 2023 12:45 am 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.
Vizard shared one where it was clearly referenced to the intake port which can be misleading (and it was).
-Bob
-
- Guru
- Posts: 1944
- Joined: Tue Sep 15, 2015 2:25 pm
- Location:
Re: In cylinder vacuum measurements ?
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.Tom68 wrote: ↑Sun Mar 19, 2023 12:45 am 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.
You do not have the required permissions to view the files attached to this post.
Re: In cylinder vacuum measurements ?
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
-
- Guru
- Posts: 4607
- Joined: Sun Sep 18, 2011 11:31 am
- Location: Heading for a bang up with Andromeda as we all are.
Re: In cylinder vacuum measurements ?
I should have included in that short sectence that a port air speed 350 fps usually is where peak torque is made.
You can cut a man's tongue from his mouth, but that does not mean he’s a liar, it just shows that you fear the truth he might speak about you!
Re: In cylinder vacuum measurements ?
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.Stan Weiss wrote: ↑Sun Mar 19, 2023 12:34 pm 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
- Paul
Re: In cylinder vacuum measurements ?
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.
-
- Guru
- Posts: 3285
- Joined: Wed Jun 25, 2008 1:46 pm
- Location: NC
Re: In cylinder vacuum measurements ?
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.
-
- Guru
- Posts: 9633
- Joined: Tue Nov 27, 2007 9:27 am
- Location: Chicago
- Contact:
Re: In cylinder vacuum measurements ?
Calculated data for informational purposes only.;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
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.
Re: In cylinder vacuum measurements ?
What would you do with this data if you had it?Tom68 wrote: ↑Sun Mar 19, 2023 12:45 am 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.
Re: In cylinder vacuum measurements ?
Get a feel for port flow v lift v crank degrees.
Ignorance leads to confidence more often than knowledge does.
Nah, I'm not leaving myself out of the ignorant brigade....at times.
Nah, I'm not leaving myself out of the ignorant brigade....at times.