Vacumn Gauge Useage

[11secAvanti]

On some of the topic discussions comments have been made about the vacumn readings at WOT. I use my vacumn gauge primarily at idle and cruise. I understand those readings.

Can I use it during the rpm pull and at the finish line and if so what information can I learn from it? Does it tell me anything about the carb size or port size or reversion, etc.? Where should the measurement port be located? At the carb, plenum, runner, head, etc.? How low should it drop for example and if it doesn't drop to a specific reading what does that mean? Application is quarter mile drags. Thanks and happy holidays to all.

[OldSStroker]

On some of the topic discussions comments have been made about the vacumn readings at WOT. I use my vacumn gauge primarily at idle and cruise. I understand those readings.

Can I use it during the rpm pull and at the finish line and if so what information can I learn from it? Does it tell me anything about the carb size or port size or reversion, etc.? Where should the measurement port be located? At the carb, plenum, runner, head, etc.? How low should it drop for example and if it doesn't drop to a specific reading what does that mean? Application is quarter mile drags. Thanks and happy holidays to all.

I don't think glancing at a standard vacuum gage in the lights will tell you much. I'd suggest you find a vacuum gage that reads in fractions of an inch Hg., probably a digital one. One that records would be even better. Mount the pickup in the plenum.

Depending on your engine, you may see less than 1.0 in Hg. vacuum. If intake vacuum gets much above 1.5 in Hg at power peak, you probably have something restricting inlet airflow, like a too-small carb or TB, and there is more power available to you. A 500 fwhp LT1 simulation showed about 16 hp loss (3.2%) at power peak when intake vacuum was increased from 0.8 to 1.6 in Hg. with a too-small throttle body.

My $.02

[maxracesoftware]

A 500 fwhp LT1 simulation showed about 16 hp loss (3.2%) at power peak when intake vacuum was increased from 0.8 to 1.6 in Hg.

just about same results i see on most Dyno tests.

Engines usually make Peak HP on Dyno
in a range of .6" Hg to 1.3" Hg of plenum vacuum
(even less "Hg w/2-4 barrell carbs on Tunnel Rams)

more than 1.20" to 1.30" , its starting to act like a "Restrictor-Plate"

1.50" Hg and upwards, is too much restriction most of Dyno tests

You definetly need to be recording plenum vacuum during a Run
or Dyno Test.....and not looking at the Vacuum Gauge as you're Racing the Car !!!

Dyno can record to Tenths of an Inch Hg. or better at every RPM
but that small diameter cheap Vac Gauge mounted in a Race Car
will be very hard to read going down DragStrip.

[maxracesoftware]

http://www.maxracesoftware.com/Getting_ ... r_Dyno.jpg

Pic of Dyno vacuum hookup

[SchmidtMotorWorks]

more than 1.20" to 1.30" , its starting to act like a "Restrictor-Plate"

Speaking of restricor plates, does anyone know how much vacuum a restrictor plate NASCAR engine has?

[SupStk]

http://www.maxracesoftware.com/Getting_ ... r_Dyno.jpg

Pic of Dyno vacuum hookup

Larry is that part of a Superflow sensor package?

I'd like to add a vac sensor to my dyno but the data recorder is custom made using Keithly Test Point software. Guess it is something to look into.

[maxracesoftware]

Larry is that part of a Superflow sensor package?

I'd like to add a vac sensor to my dyno but the data recorder is custom made using Keithly Test Point software. Guess it is something to look into.

by "Sensor Package" do you mean the Sensor Stand SuperFlow
sells alone ?

the Vacuum Gauge is part of the SF-901 Console
like in this Pic

http://www.maxracesoftware.com/SF901_Gauges.jpg

but even though i'm about a foot away from the Vac-Gauge
i never look at it during a Pull
its being recorded in Tenths of an Inch Hg.
automatically at every RPM , by a pressure transducer that
i'm guessing is calibrated 0 to 5.0 Volts
with each portion of a Volt equalling a certain amount of vacuum or
pressure.

look at the Vacuum Pic ...imagine how hard it would be
to determine .2" tenths or so of change Racing down a DragStrip ??
or even 1.0" Hg with RaceCar + Track induced vibration on a Run

[RJ]

more than 1.20" to 1.30" , its starting to act like a "Restrictor-Plate"

Speaking of restricor plates, does anyone know how much vacuum a restrictor plate NASCAR engine has?

About 11 inches.

[SupStk]

Larry is that part of a Superflow sensor package?

I'd like to add a vac sensor to my dyno but the data recorder is custom made using Keithly Test Point software. Guess it is something to look into.

by "Sensor Package" do you mean the Sensor Stand SuperFlow
sells alone ?

the Vacuum Gauge is part of the SF-901 Console
like in this Pic

http://www.maxracesoftware.com/SF901_Gauges.jpg

but even though i'm about a foot away from the Vac-Gauge
i never look at it during a Pull
its being recorded in Tenths of an Inch Hg.
automatically at every RPM , by a pressure transducer that
i'm guessing is calibrated 0 to 5.0 Volts
with each portion of a Volt equalling a certain amount of vacuum or
pressure.

look at the Vacuum Pic ...imagine how hard it would be
to determine .2" tenths or so of change Racing down a DragStrip ??
or even 1.0" Hg with RaceCar + Track induced vibration on a Run

I was wondering if the vac sensors was part of the SF-901 or it was an option that you added. Guess what i was thinking was of some way to adapt it into my hardware. There is capability in the software to read the added input. Most of my other sensors and transducers are 0-5 volt signals with 12vdc inputs.

thanks

[maxracesoftware]

About 11 inches.

that's a bunch !!!

11 inches of Hg, (Mercury) at 1" Hg = 13.6" of Water

149.6" inches of water

i wonder what the Flow Numbers are at 149.6"


809.7 fps potential or .726 Mach

and i thought 7 or 8" Hg on some of the Local Dirt Track engines
was a lot ? need a ton of Comp Ratio to makeup for it

[maxracesoftware]

• The SF-1200 is typically used for carburetor testing. Prior to 1954, carburetors were tested at 3" of mercury, because that was considered to be a pretty good manifold pressure, running at full throttle for good atomization of the fuel with a two-barrel carburetor. At the time, there was a big demand for higher-power engines, and they needed more airflow capacity. Because they wanted less pressure drop across the carburetor to get maximum power, they developed the four-barrel carburetor, which ran as a two-barrel carburetor up to a certain engine speed. As the engine speed increased, a linkage opened the other barrels. When they did that, the 3" of mercury on their test equipment didn’t have enough capacity any more.
  
  So they de-rated the test to 1.5" of mercury pressure drop. That is the standard that is still used for all carburetor flow ratings. So, when you see a 700-cfm Holley, it was tested at 1.5" of mercury (20.3" or 51.6 cm of water). The reason you wouldn’t put a 700 cfm carburetor on an engine that needed 700 cfm is that, if it is a racing engine, 1.5" of mercury is too much pressure drop for the engine. The other guys will be running away from you. What happened was that they just rated the four-barrel at that point. It’s very confusing to racers who are the primary people that use cfm readings. The cfm reading of the carburetor and the cfm requirement of the engine don’t correspond.
  
  On the flowbench, we set it up so it will test at 1.5" of mercury, because that is the standard. But when you are testing boosters and things of that type, we recommend that you test at 10" of water. Mercury has a density of 13.6. Take the inches of water and divide by 13.6 to get inches of mercury. So you test at 20.3" of water to get the reading that Holley uses, but you would test at 10" of water if you were trying to develop your flow signals for racing at full throttle, because that is more representative of the pressure drop on an engine.
  
  If you want to know what the vacuum of your engine is when it is running, next time you test your engine on the dynamometer, look at the vacuum gauge when you are running at full throttle. You will see that the vacuum gauge goes up at higher speeds. You see greater vacuum as you go up in speed, because you’re drawing more air through the carburetor, just like you would a cylinder head. As you go up in speed, there will be greater vacuum. On a pro stock engine, for example, the maximum is 6" of water. On a race engine, it would be more like 10" of water. If you are at peak power you’re pulling 6" of water but when you’re down at idle and open the throttle fully, you will probably draw 0.10" of water and all you will get is nothing. The engine will just die, because it won’t suck any fuel through at that level.

from SuperFlow's website

6" of water = .441"Hg on vacuum gauge , about what i see on 2-4 Barrel Tunnel Ram tests on some Engines .4" to .6" range sometimes

10" water = .735" Hg vacuum

[MadBill]

FWIW, EFI engines often have only ~0.4 -0.5"Hg depression downstream of the throttle at high RPM WOT, as there is no need to atomize the fuel at that location, but the engine of course shows diminishing returns for ever-larger throttle bodies.