Primer Type Dissertation.....was: [Shotimes] Engine Vacuum

[George Fourchy]

On Sat, 19 Jun 2004 13:53:19 -0400 (Eastern Daylight Time), Carl Prochilo wrote:

>Why is this subject important?  Looking for a primer type dissertation. 

This will be simple and short.  It might be long compared to most other posts here,
but it still will be "short" compared to a vacuum discussion out of a basic
Automotive Technology Textbook.....!  I am drawing from memory...from the days when
my 289 4V Mustang was the main driver, and also when I put the Sig Erson RV cam in
the propane powered Boss 302...back in the late '60s and early '70s.  That's about
the time I realized I could get very good gas mileage out of the 289, and also the
429 Country Squire; also that the Mustang mileage I got was directly related to the
vacuum reading I was seeing on my personal "Rallye Pack"...a matching tach and
vacuum gauge (purchased from "Honest Charlie's Speed Shop", in Chattanooga,
Tennessee) on the steering column.  The CS had a set of 4 Stewart Warner Series III
gauges fastened to the bottom of the dash in the middle, over the tunnel.  Vacuum
was the first one in the group....most important.

An internal combustion engine is a vacuum pump.  When running, it is attempting to
create as high a vacuum in the intake manifold as it can.  The higher the vacuum,
the more healthy the engine.  Vacuum in the intake is generated by suction from the
pistons during their intake strokes.  High vacuum readings at higher RPMs indicate a
relatively efficient and healthy engine as compared to lower readings.  Suppositions
stated below are using a generic engine as an example, except as otherwise stated.

The average auto engine creates somewhere between 15 and 20 inches of vacuum at
idle.  Thirty inches of vacuum is about as high as one can go in our atmosphere, at
sea level.  Most engines can only reach a reading of 25 or more inches during
decelleration.  At higher elevations, the same engine will produce less apparent
vacuum, by about an inch per thousand feet.  The less time the exhaust and intake
valves are open at the same time, during the exhaust and intake strokes of the
piston, the higher the vacuum.  The more time they are open, the less vacuum will
appear on the gauge.  This time they both are open is called "valve overlap". 
Overlap is sometimes good, and sometimes not.  It is what causes a lumpy idle in a
performance modded engine.  It can cause popping if the cam(s) are not timed
correctly with the crankshaft.  It usually increases the power output at high speeds
in a modified engine.  The vacuum reading in a modified engine usually will increase
(but not always), depending on the engine, at high speeds vs what it is at low
speeds.  The Boss 302 engine I have, with its special Sig Erson cam, made just for
me by them (both "RV" grinds, but from the RV20 cam for the intake, and RV10 cam for
the exhaust), increases its vacuum as it speeds up, both unloaded and on the road. 
At idle, 1100 rpm, it has only 10 inches of vacuum.  Speed it up , and it will
increase to 15 or more inches, and rise to 20 when decellerating.  When it was new
and stock, it had 15 inches, and would go to almost 25 during decelleration.  When
on the road, at 60 mph, in 4th gear, about 2700 rpm, it now reads 12-14 inches.  At
75 mph it reads 16 inches.  At 80 mph, about 3700 rpm, it reads 18 inches.  (It is
the only engine I have seen that will do that....I have had gauges on driven cars
only with it, the '65, and the CS.  Other efficient engines, like ours, might
increase at speed, too.)  It becomes more efficient at high speeds, due to the
better management of the airflow through the engine by the existing overlapped valve
setup.  When it is idling, it sounds very lumpy (nice!), like it will die any
instant, but look at the engine, and it is as smooth as a SHO.

Vacuum, then, is also an indication of overall health of an engine.  If the vacuum
is high, above 25 inches and steady, the engine is healthy....good compression, and
tight, well sealed valves and seats.  A fluttering gauge needle indicates one or
more leaking valves, or valves out of time.  A steady low reading gauge indicates a
worn engine...rings are allowing blowby, which is the same as a leaking pump.  A
high reading that drops to low upon acceleration indicates a blocked exhaust system.
 Readings that are slightly low indicate problems with tune, such as bad plugs, or
more commonly, retarded timing.  I could tell when my '69 429 Country Squire needed
a tune up by its gauge....it read 20 inches when perfectly tuned....when it dropped
to 18 inches, it needed points and a timing check.  In the same manner, I could tell
that the 289 was hurt after I siezed it up due to running out of water....yes I was
dumb!!  After it was running agin, it idled at 15 inches, rather than 18.  After I
built it into a 271 hp factory HiPo engine, it had 15 inches again, this time
because of the cam.

I haven't had a vacuum gauge on the Lowrider, but with an easy 30 mpg obtainable,
I'd bet it's pretty high.  There's a good one hanging from the garage wall...I
should do that!!

Hope this answers some questions.

George