Cranking vacuum check out.

[Dave WM]

in the interest of science, I did a "cranking vacuum" test. I have never heard of such a thing but read about it somewhere. Thought it would be something to measure and make note of

Completely stock NA L-28 manual setup for a 800 rpm idle.

Warm up engine to about 150f (indicated on gauge)

pull brake boost line and hook up HF vacuum gauge.

using remote starter (ign off, plugs in) crank engine for about 10 seconds

I get about 7" of hg with no noticeable movement of gauge.

maybe 120ft above sea level.

throttle plate closed.

My normal idling vacuum is about 17-18 a little better if I run the idle mix a bit richer (about 19). I only found out about the richer is better when swapping AFM's one had the idle bypass nearly closed and produced the better idle vacuum. I assume for this test it would not matter since the throttle plate is the restriction not the AFM.

 

 

 

Edited April 4, 2018 by Dave WM

[siteunseen]

Now I think of Thomas Dolby, you've blinded me with science! 

"I can hear machinery
Blinding me with science, science
Science"

[Captain Obvious]

Good heavens, Miss Sakamoto! You're beautiful!

[zKars]

Just cuz you have nothing better to do, have you checked that the throttle butterfly is closed all the way with foot off the peddle? Just in case 'someone' may have tweaked the little adjuster screw on the side to get some idle bypass air via butterfly offset, rather than adjusting the official idle/air bypass? 

Just pointing out one of the assumptions one makes when doing tests like this that may escape consideration.

[Dave WM]

nope did not check that good point, anything is possible since disco died.

[SoCalJim]

Disco died? 

[Captain Obvious]

Yup. A couple years after Chicago.

[Captain Obvious]

1 hour ago, zKars said:

in case 'someone' may have tweaked the little adjuster screw on the side to get some idle bypass air via butterfly offset, rather than adjusting the official idle/air bypass?

As long as the idle is at the proper level, I don't think any of that matters. The idle speed will be a factor of the addition of all the leakage paths around the butterfly, and long as the idle speed is where it belongs, it doesn't really matter which of all of the sources is passing the air.

For example... If someone mucked with the butterfly stop and opened it up a little bit, you would just compensate for that by closing down the idle screw a little bit. As long as they didn't open it so wide that it was beyond the compensation range of the idle screw (completely closed), then I think it should all come out in the wash.

The idle air flow will be the sum of what goes past the butterfly valve, what goes past the idle speed screw, and what leaks through the BCDD.

 

[Dave WM]

That is what I was thinking, and the reason I specified the RPM. Not really sure why a cranking vacuum is something you would want to know unless for some reason you had a no start and did not have access to a compression tester. I just did it for a data point. I suspect my backup AFM (the one that gave the better idle vacuum) would give even better vacuum readings, since I had to decrease the bypass air on the throttle body to reduce the idle back to 800 rpm with it installed. The warm engine is key as well, I did not realize how much it effected the vacuum readings. I was getting about 155-160 compression readings with a cold engine. Now I wonder what the effect of a warmed engine would be.

Edited April 5, 2018 by Dave WM