I've been advised it would be good to start a thread on this,so here goes. Thank you to everyone that gave me direction.
Anyway, I'm a newbie and just installed a vacuum gauge (via wiper vacuum hose) to see if it would give me more information on my 1950 V8 flathead that has great compression in six cylinders. Too bad it has 8 .
cylinders 5&6 are low...about 50psi. I splashed oil into those cylinders with no increase in compression. My manual oil compression gauge reads 45-50 cold, 35 or so once warmed up and 25 at idle.This engine has been running about a 1000miles per year over the last dozen or so.
So the vacuum readings.....pretty steady at 18... if I give it a good rev it spikes 4-25 ( if I remember correctly)before going back to 18. I have looked at various charts but so far I don't read the vacuum gauge doing anything that resembles burnt valves or a bad head gasket. Maybe it takes a little more experience to read these vacuum gauges correctly.

 

Reading a vacuum gauge and using it as a tuning tool certainly isn't rocket science. It's the way cars were tuned before we had rocket scientists. So it shouldn't take a whole lot to learn how to read it. Having a steady reading of 18hg is pretty good, and the fluxuation you experienced when blipping the throttle is perfectly normal. Here's a pretty good web page with good instructions on what you need to know.

Classic Inlines - Tuning with a Vacuum Gage

 

What's the history of the engine? Any sign of a rebuild in its past, sounds like not in 12 years at least?

It's entirely possible that the rings in those cylinders are in worse condition than the others. What were the readings in the others?

 

Found this in The Crankshaft Coalition the other day. Vacuum gauges used to be used a lot more than they are now. Hope you can read it. If you have to, go up to "View" and you can rotate the image so you can read it.

 

If your vacuum gauge is steady at 18, this indicates an engine in decent shape. Consider the elevation of your locale as a factor in this reading.
Oil pressure is not a factor here although yours looks ok.
What are your concerns, poor performance, rough idle, or just playing with the engine and found low compression? Normally an engine with low compression cylinder will be plagued with a rough low speed idle yet run fairly decent at speed. It may be down slightly on power. An uneven exhaust and popping at the exhaust is exhaust valve indications.
Are you sure the vacuum gauge is for automotive use and does not have a dampened movement? A dampened movement would not show up the small blips on the gauge that a low compression cylinder would normally show.
When the oil into the cylinders made no change to the low compression reading, that is telling you the rings are not the problem. Oil in the cylinder will sometimes have the effect on a leaking gasket to lessen the leak for a short time.
The easiest thing to try at this time is to torque the head to specification. Takes about ten minutes if you don't take too many breaks and costs nothing. It may or may not make a change but for what it cost it is worth the effort.
If the truck has been started a lot and allowed to idle quite a bit, never allowed to work hard, perhaps a good long run at highway speeds would help.
Good luck and keep us posted.

And, FWIW, pulling the head on a flathead is not that bad a job. If and when you decide to pull the head look closely for the dreaded valve seat or block crack that show up in these engines.

 

Thank you for all your suggestions and sites to gain info. I'll try retorqueing the head this weekend. Yes it has a rough idle,down on power up hills and popping at the exhaust. compression readings #1 110 #2 112 #3 120 #4 100 #5 60 #6 50 #7 90 #8 100
It may have been rebuilt 12years/12000 miles ago.
Also I called manufacturer of mechanical Vacuum gauge...its not dampened.

I think that answers all the questions.......