How Much Intake vaccumn do you have?

[Lexxani]

To establish a test baseline, as I have been unable to find these specs any where else, what engine, year, approx mileage, and test conditions (preferably at idle, but also post under gear or what ever conditions you have readings for.)

Ill begin.

'78 M117.985
Approx 100K mi

15.5 InHG at warm idle in park

Tested with a MityVac.

[79Mercy]

'79 110.984
16.5 InHG
950RPM

[Graham]

1972 350SL 4.5L M117 engine, 148k miles.

Vacuum 19,5" HG at idle ~700rpm

Measured with good quality pressure/Vac gauge.

[250 Coupe]

13"Hg at 700RPM at idle,
1976 450SEL M117 with 135000.
.5% CO, 11.5MPG

Michael

[Gurunutkins]

1973 350sl M116 engine euro 17.1 inches with good quality vacuum gauge at 730 -40 rpm warm idle in P. pre megasquirt 15.3mpg, megasquirted 19.8mpg still tuning premium only gas HC engine

1974 450sl M117 engine US spec 17.2 inches with quality vacuum gauge at 710rpm warm idle in P. pre megasquirt 13.8 mpg, megasquirted 17.2 still tuning regular gas LC engine

[Mike D]

1974 M110 converted to 2 BBL Autolite carb 13.7, 900 rpms in gear, no auxillary vacuum pump, over 100,000 miles (odo not working, one of these days I'll get around to fixing the slipping gear)

1973 M110 Holley 4 BBL (Jam conversion) 14.0, 950 rpms (same car with the Solex 4 BBL 12.8, varying idle between 700 and 1000) in gear, w. aux. vacuum pump. 167,000 miles

1972 M115 dual Solex downdrafts 15.2, 700 rpms (manual transmission), gadzillion miles (this is the third body in which the engine has been used, 1971 220, 1968 220 and now a 1972 250C, so I guess it's a 220C)

[ctaylor738]

1972 350SL, 140K miles, known low compression (even), 17", Mityvac.

[DJet Tragic]

Originally Posted by Graham

1972 350SL 4.5L M117 engine, 148k miles.

Vacuum 19,5" HG at idle ~700rpm

Measured with good quality pressure/Vac gauge.

I am curious to know how & where you get a manifold vacuum reading on an early DJet V8.
Could you take a reading off the distributor advance/retard vacuum tubes - or do you have to add a "T" piece to a main vacuum hose & work from that?
Regards, Peter in Melbourne

[ctaylor738]

I like to T off the main hose that connects to the MAP sensor because that's where it's most critical to the car's running.

[Graham]

Originally Posted by DJet Tragic

I am curious to know how & where you get a manifold vacuum reading on an early DJet V8.
Could you take a reading off the distributor advance/retard vacuum tubes - or do you have to add a "T" piece to a main vacuum hose & work from that?
Regards, Peter in Melbourne

I teed into the tubing that leads to the vac switch & distributor. Put tee close to throttle housing. My '72 car has just one tube and it is a vacuum retard. You could tee in close to the distributor, but there is a switch and several connections along the way.

One thing about the vacuum readings - they ARE dependent on the engine rpm. My figures were just based on the engine tach. I have a dwell/tach meter, but it does not read correctly.

Teeing into the line that goes to the MPS does make sense, although vacuum should be same wherever in system you measure it.

[Tomguy]

What you need to measure is manifold vacuum. It will be different from throttle body vacuum, which is what teeing off the line to the throttle body will give you.

Best places to measure manifold vacuum since it's the same from these places would be to T off the MAP or just pull off the central lock vacuum and use that plugged directly into the vac gauge.

[Graham]

Originally Posted by Tomguy

What you need to measure is manifold vacuum. It will be different from throttle body vacuum, which is what teeing off the line to the throttle body will give you.

Best places to measure manifold vacuum since it's the same from these places would be to T off the MAP or just pull off the central lock vacuum and use that plugged directly into the vac gauge.

Tomguy, there is really no RIGHT place to measure the vacuum. Lexani just wanted a survey of what vacuum others see on their cars and did not specify where in the intake system it should be measured. The manifold vacuum would be slightly greater than the vacuum just below the throttle plate. One point controls the vacuum retard and the other is used by the MPS.

At idle speed air flow the difference would the exit loss of the air flowing from the throttle body into the manifold. I would not think this would be very much, but it would be useful if posters specify where they measured it.

I might re-measure mine at both points and see what the difference is at 700rpm.

[Lexxani]

I was measuring intake vacuum, not throttle body. I pulled off the line that connects to the engine to the car's vacuum lines on the back of my intake. That should be the easiest and most accurate place to measure your intake vacuum.

[79Mercy]

On my M110 I measured vacuum on the intake manifold towdards the fire wall, same vacuum port that goes to the tranny.