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

[Carl Prochilo]

Yow!!  My head hurts, but thanks for a typical great posting George.

I have no idea how well my SHO works in this area.  I know I'm not getting
good fuel mileage.  I've actually considered for a while replacing the
coin tray/cupholder with a set of three gauges.  Maybe one of those should
be a vacuum gauge?
-- 
Cheers,
Carl Prochilo
92 Ultra Red Crimson

On Sat, June 19, 2004 6:16 pm, George Fourchy said:
> 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
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