This has been done to death before - but for anyone interested... I was asked from another forum today and spent a few minutes putting the following together.....
Warning - it's a "rosco".....
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Re: Vacuum gauge?
Postby rosco01 on Sun Oct 25, 2009 3:51 pm
Hi again - s'mee - serial pest..... this subject is one of my "pets"....
I am going to try desperately to keep this to a reasonably short post...... occasionally, I succeed....we'll see.....
Again - and you'll learn to heed this warning - those not wishing to waste valuable time wading through waffle - go play elsewhere....
My reference to the following is based on a single carburettor application only - more specifically, a single throttle butterfly.
It is my understanding that a vacuum gauge is only appropriate for petrol carburettor use - but I am ever open to enlightenment....
The vacuum created for use in a carburettored four-stroked petrol engine is achieved by the downward stroke of the piston through the open intake valve.
Connection to all inlet valves is made through the intake manifold.
At the crux of this manifold is the carburettor.
Almost at the base of the carburettor, a throttle butterfly operates according to throttle pedal position between either fully opened or almost completely closed.
This butterfly controls the amount of atmospheric "air" to pass through the carburettor into the intake manifold and provides manual control of destroying the vacuum created as above.....
The carburettor incorporates a number of different "systems" (usually three - excepting "choke") which are brought into use singularly or in conjunction with each other according to demand of the engine.
The carburettor administers the correct mixture of air and fuel for combustion in the cylinders to meet these varying demands.
The carburettor requires a volume of air passing through it to enable fuel to be metered and administered to the cylinders.
This volume varies according to the opening of the throttle butterfly, the volume of air passing through it and the amount of vacuum in the intake manifold.
The amount of vacuum within the intake manifold also determines an amount of fuel to be delivered under heavy load or at very high speed.
(we're getting there - not much more to go).... "trust me - I'm with the Government".......
Under deceleration, the greatest amount vacuum is created by the rapid demand of the pistons through the intake valves and in the intake manifold under the almost closed throttle butterfly - (this is where the maximum boost is gained for use in your brake booster etc - a "check valve" in that line maintains this regardless of re-opening the throttle....).
Under very light load or at idle, the throttle butterfly is almost closed - resulting in considerable vacuum within the intake manifold.
The mixture of fuel/air is supplied by the partially opened butterfly (air) and a number of small passages under the throttle butterfly to deliver fuel.
Under moderate speeds or load, the opening of the butterfly is greater - and hence... more air can pass through the carburettor (vacuum is slightly reduced - destroyed).
In this position, a greater number of passages under the butterfly become open affording more fuel to add to the mixture.
As the speed of air passing through the carburettor increases because of the less restrictive butterfly - a main jet meters and admits fuel to this greater velocity of air - the greater the speed of air - the more fuel through the main jet, providing the necessary mixture.
At high speed or full load with the butterfly fully open - almost all vacuum is destroyed within the manifold.
The high volume of air passing through the carburettor is beyond that which the main jet can provide fuel to enrich the mixture.
To correct this, within the carburettor - a "power valve" which had previously been held closed by vacuum, opens - to provide sufficient fuel to compensate for the loss of vacuum and enrich the mixture to meet the requirement/load of the engine.....
In this position, maximum fuel/air is delivered to achieve maximum power...... a considerable amount of this mixture is "wasted" and it can be noted that excessive exhaust "smoke" (any colour except white or blue will do, thanks) indicates this raw fuel wastage.....
Now the bits you wanted to know about vacuum gauges..... (finally, you scream...!)
A vacuum gauge can only indicate to you the amount of vacuum resident in the intake manifold and on the underside of the throttle butterfly.... it can't do anything else..... well, if you have connected your instrument lights up to it, you can probably use it for some form of illumination......
By understanding the effects of vacuum within the intake manifold, and how they impact on the delivery of fuel from the carburettor - it is possible to operate the engine so that maximum power is attained without excessive wastage of fuel......
(Hooray, Rosco - why couldn't you have just typed that on one sentence and dumped all this waffle.....?)
Yes, I agree (before you even ask).... the same operating control can be achieved by "suck it and see" positioning of the throttle pedal..... here, a vacuum gauge can assist in this as a visual indication - to be used in conjunction with other indications (sound and feel).
I believe what I am about to type is critical in understanding the credibility of a vacuum gauge...
it must be fitted correctly ... that is - fitted to the intake manifold without sway of other "tappings".... ie brake booster, PCV valve etc.....
The vacuum line to the gauge must be intact - a leak will not indicate a correct reading.
There must be a "restrictor" within the line - something which will "average" the "pulses" of each downward stoke of the pistons....
this is more relevant in a four cylinder engine - the needle will rapidly fluctuate at idle unless a restrictor is used....
The best location for a restrictor is at the "gauge" end of the line.....
Now the good bits....
A vacuum gauge can indicate the general "condition" of an engine and its best possible tuning. ..... the further to the "decelerate" end of the arc - the more "freely" an engine is operating.
- of course, this would need be used in conjunction with a timing light and more recently an emissions check..... damage could result in "advancing" an ignition setting simply by setting timing by use of a vacuum gauge.....
It can also give an indication of "wasted" fuel throttle positions.
Whilst driving, by careful observation - it can be determined where the "power valve" opens - remember, this is achieved by the destruction of vacuum - we have an instrument which indicates vacuum.... hence - we can determine when this "valve" will open.....
This power valve will open either off or on - no matter at what speed the engine is running at - it has no "middle ground".... and I believe most can appreciate that at a lower speed, fuel wastage probably more likely than at higher speed........?
There is also a point at which full power is achieved before full throttle - any further opening of the throttle is simply a waste of fuel..... albeit probably of a small amount at high speed.... but at slow speed - full throttle will do little more than a position considerably less with the same output..... the power valve will however open - and dump raw fuel into the mix......
We age getting pretty nit-picky here.... I firmly believe a laboratory would be needed and many tests conducted to establish just what throttle positions yield the most efficient outputs......
Finally, and it took me years to realise this, vacuum achieved is far greater at sea level than at the altitude of our alps..... it is also very minimally related to atmospheric pressure.... this would be of little consequence in operating a vehicle....
For all intents and purposes, a vacuum gauge is an aid - but useless if the operating indications are not understood.....
or the unit incorrectly fitted.
Forget those pretty colours - they are a "guide"..... what is shown as "good" for a V8 - would indicate pretty much useless for a 4 cylinder as far as power output is concerned.....
As in a previous post, with a van in tow - you'd probably find you would be running in the orange/red most of the time with a small 6 ... the same gauge indication with a V8 - would indicate you are running pretty poorly..... regardless of the amount of fuel that one engine is consuming against the other...... it is an indication of vacuum in the intake manifold only - that's all it can do - it won't give you L/100 km's or mpg's...... and works exponentially with engine speed...... the faster the engine is running at the same throttle setting- the better the indication.....
I noted back in the 4 cylinder Commodore days......glad they are gone! where we were "duped" into believing we were actually using less fuel because there were less cylinders! - but using substantially more throttle to achieve even a modest performance..... and flogging the poor little "never fire" 4 for all it could produce.....
The designers were reluctant to put colours onto the vacuum gauge..... and I noted the calibration was also a little "generous"..... the main part of the working arc was sensitive to mid to high power demand...... this, I recall - spilled over into the later series when common sense prevailed and the 4 cylinder become part of our nation's car's "history"....... The "black" 6 with fuel injection put paid to any inclusion of a vacuum gauge....... so, dear readers - I believe you can appreciate my philosophy that perhaps there really isn't a "need" for such an indication.......
But, to be honust injun, I have them fitted to both my carburettored vehicles - and make use of them when their indications can be utilised .....
As far as them being "needed"... sadly, no - many young blokes back when I was a pup - had just about every conceivable dial or "clock" they could somehow scrunch into the "cockpit".... to impress - most didn't know what they meant - or how to use them......
The faithful old "idiot" lights sufficed for many years to tell you something "was not as good as it could be, should be or probably needed to be"...... just before there was either a bang, many bangs or no bangs at all........
As most petrol fueled vehicles now run with injection - the dinosaur age of carburettors and the use of vacuum gauges is fast coming to an end.......
sorry for the long trek..... you're pretty lucky you didn't get the "long winded" version.....
My understanding of this subject is not in any way meant to be regarded as testimony..... but it is as I have come to understand it.....
frats,
Rosco
ps - I forgot and had to edit....... vacuum pump into fuel pump...... oh yes - I'm going to re-kindle some "affectionate" memories here...
This was not done for any reason to power the vehicle, supply fuel or help with ignition.......
Who can remember Mum and Dad's great trip out on a lovely day when coming home late at night, in the rain - we would travel along quite merrily in our old Holden (pre 1961) with the window wipers slapping time to the "chant" of the little grey motor ... until Dad decided we needed to go a bit faster or put the boot into it to get up a hill........? what was the result...?... the wipers went from a slap slap to a ....... s .....l ......a ......p.........shudder, shudder......s........l........a......p....
- dear readers - this is why that little vacuum pump was under the fuel pump....... it was all that was left to operate the windscreen wipers..........
rosco01
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Vacuum gauge
and my response to some further posts...
Re: Vacuum gauge?
Postby rosco01 on Mon Oct 26, 2009 9:04 am
Relax everyone - a couple of sentences ..... only - this time.....
Daryl - thanks for the enlightenment.....I may take up the challenge and delve into injected petrol engines.....I do not have any experience with fuel injected petrol engines... hence - I cannot comment on the creation of associated intake manifold vacuum... I believe my understanding of any "injection" engines to be related to the non-turbo diesels of the primitive nature..... perhaps it is time to become a little more "worldly"......?
Yes, agree wholeheartedly - trip computer.... but at what cost....? For the cost of fitting, with sensors, probes and calibration etc..... it would probably negate the cost of any wasted fuel for the period of time you have the vehicle..... just my playing the "devils advocate" here...... vacuum gauges used to be relatively cheap in comparison....
And - as we have only recently purchased a vehicle with a trip distance/fuel usage etc. etc computer and display..... it has come to my notice that they are not as accurate as I had hoped...... but, I have found it can equate the difference in consumption when either "pushing" a high gear as opposed to "nursing" a lower one at higher revs.....to effect the same demand - it, of course - cannot show the "load" on the engine - which a vacuum gauge can suggest.....
Like your words Macka - and I agree - let your "other" sensors determine or become educated to the use of fuel........
- with all the information generation "x", "y" and (I don't know where they will go after "Z.... it's going to become another Holden H? or &*#@ X?....... scenario.. me thinks)... seem to "need" whilst they have inadvertently forgotten the primary duty of actually driving the vehicle.....
albeit to the "criticality" vehicle manufacturers place on provision for such "adornments".... we've all seen the glossies - the more boxes checked or ticked... seem to attract media vehicle testers as to the most "appropriate value for money".....
Call me a dinosaur - but I'm firmly of the opinion that the more robust parts of a vehicle and their ability to perform without malfunction are the items I checked when making a choice in our latest vehicle..... the "creature comforts" came in pretty much later in selection "criteria"..... none-the-less... we enjoy some very aesthetic pleasing surroundings in which to enjoy our travels.....
Ooops - sorry.... did I go beyond two sentences.....?
frats,
Rosco
Re: Vacuum gauge?
Postby rosco01 on Mon Oct 26, 2009 9:04 am
Relax everyone - a couple of sentences ..... only - this time.....
Daryl - thanks for the enlightenment.....I may take up the challenge and delve into injected petrol engines.....I do not have any experience with fuel injected petrol engines... hence - I cannot comment on the creation of associated intake manifold vacuum... I believe my understanding of any "injection" engines to be related to the non-turbo diesels of the primitive nature..... perhaps it is time to become a little more "worldly"......?
Yes, agree wholeheartedly - trip computer.... but at what cost....? For the cost of fitting, with sensors, probes and calibration etc..... it would probably negate the cost of any wasted fuel for the period of time you have the vehicle..... just my playing the "devils advocate" here...... vacuum gauges used to be relatively cheap in comparison....
And - as we have only recently purchased a vehicle with a trip distance/fuel usage etc. etc computer and display..... it has come to my notice that they are not as accurate as I had hoped...... but, I have found it can equate the difference in consumption when either "pushing" a high gear as opposed to "nursing" a lower one at higher revs.....to effect the same demand - it, of course - cannot show the "load" on the engine - which a vacuum gauge can suggest.....
Like your words Macka - and I agree - let your "other" sensors determine or become educated to the use of fuel........
- with all the information generation "x", "y" and (I don't know where they will go after "Z.... it's going to become another Holden H? or &*#@ X?....... scenario.. me thinks)... seem to "need" whilst they have inadvertently forgotten the primary duty of actually driving the vehicle.....
albeit to the "criticality" vehicle manufacturers place on provision for such "adornments".... we've all seen the glossies - the more boxes checked or ticked... seem to attract media vehicle testers as to the most "appropriate value for money".....
Call me a dinosaur - but I'm firmly of the opinion that the more robust parts of a vehicle and their ability to perform without malfunction are the items I checked when making a choice in our latest vehicle..... the "creature comforts" came in pretty much later in selection "criteria"..... none-the-less... we enjoy some very aesthetic pleasing surroundings in which to enjoy our travels.....
Ooops - sorry.... did I go beyond two sentences.....?
frats,
Rosco