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MG Midget and Sprite General - I learn-ed som 'em new about evac systems today

Here is a new take that hasnt been discussed


I hooked up my 10 gauge concel system today with the 2 vacume gauges


Im pulling 18 psi at 1200 rpm and 10 psi at 3000 rpm by way of the intake manifold.... Which im guessing is fine

Now here is where it gets interesting .... ZERO psi on both carbs taken from the (Y) connection .... Without air filters, so im starting to think there was a purpose to those old lucas air filters with the big horns off to the side, im thinking they provided restriction and thus vac that pulled out the positive air in the crank case,


My guess is the K&N air filters dont provide much restriction in the carbs,.... Because it a free flow air filter.... There for not much vacume for the crank case....

I cant wait to test my therory, ive still got the orginal air housing, a cheap pancake air filter and the K&N air filter system....hopefully next week I can put it to the test

Prop
Prop

The difference is simple you are measuring the vacuum between the engine and THROTTLE disk. Thus when the engine is not underload and revving naturally the vacuum is very large. as you open the throttle the vacuum decreases for that reason! the connection in the air filter case is at the opposite side of the throttle disk and will not see any vacuum whatsoever! as you have discovered.
Robert (Bob) Midget Turbo

Hmmm,

So under the factory set up, with the (Y) connection between the 2 carbs where does the vac come from to suck out the crank case,,,,
It just seems if it is sucking out the crank case then it should suck on the gauge

Not being obstinant .... I truly dont understand.

Thanks


Prop
Prop

Prop -
Sharpen thy pencil!
The vac at the carb breather ports is very small, usually measured in inches of water. You need a water manometer to measure this.

1 atmosphere = 15psi = 30 in mercury = 32 feet of water. So, 1psi = 25" H2O, very roughly, but close enuf.
The SU is a constant depression carb, and the constant depression region is from the piston to the butterfly, also conducted to the topside of the piston by two drilled holes, and is where the vac ports are placed. The amount of depression is controlled by the weight of the piston plus the piston spring pressure. Blue (standard Spridget) springs are 2 1/2 oz, and red (most modded HS2) ones are 4 1/2 oz. Guesstimating the weight of the piston as somewhat less than a half pound, we can take the total force including spring as somewhere near that half pound. The effective area of the H*2 piston is about 3 square in. So, .5lb/3sqin =1/6psi, or about 4" H2O. Even if my guesstimates are off by a factor of two, it would still only be 8" H2O, or 0.7" Hg, or 1/3psi. You can't measure it with a normal gauge.

It is pretty useless to try to measure it, since it IS a "constant depression" carb,and the reading will always be the same, except when something interferes. That would be the piston damper, which holds the piston down briefly on acceleration, causing the "constant depression" to be greater for a brief time, until the piston reaches equilibrium with the airflow. Dirty air filters would add to the constant figure, so you could put a precision gauge on to tell you when the filters are dirty by the increased reading - in fact, big trucks and other industrial devices sometimes have just those gauges. Got room?!

FRM
FR Millmore

Lol don't encourage him to have more gauges - poor midget must be dropping due to the weight already!!!!
rachmacb

Thank you FRM,,, I do understand that.

That leads to another question,,,,,

If I was only using perhaps 1 psi vac to suck positive pressure (factory set up) out of the crank case,

then is using 18 PSI at idle and approxmatly 8 - 10 psi during normal driving to much vac, or does it matter, could this much vac be sucking oil away from vital parts, not sure what those would be.

On the other hand, I seriously doulbt id ever have an oil leak issue ever agian....hahaha

Prop..... I do have a place for another gauge, whats another 8 oz.
Prop

Certainly need to sharpen your pencil Prop a full vacuum is approximately minus 14.7 PSI, it is against the laws of physics to create a bigger vacuum! Therefore you can not have 18 psi! (You cannie change the laws of physics captain!)

I understood you were comparing manifold vac with that experience at the airfilters but I see that was not the case.

I do not know how the systems are operated in the USA however on many engines in the UK we have a pcv directly onto the manifold to help cure oil leaks. That appears to work reasonably well.
Robert (Bob) Midget Turbo

Prop-
As Bob says, you can't have more than 14.7psi vac. The common unit for measuring vac is inches of mercury ("Hg) for general use, or inches of water for fine measurement. See the approximate equivalents I posted above. On the front of your vac gauges it should say, and it will be "Hg, and your readings are about right. 18" Hg is about -9psi. While you may have your engine attached to a -9psi source (the manifold) it is going through some kind of restriction in whatever hare-brained evac system you now have. Otherwise, you would suck all the oil out of your engine, but probably not because it wouldn't run at all.

FRM
FR Millmore

Guys....thank you for tje input

Im definatly needing glasses, I turned on some lights and had another look at the gauges this morning,,,, it appears im ACTUALLY running 11 to 13 Psi, not the 18 I thought,,,


Ah whats an extra 5 psi amoung friends,,,hahahaha


I think at this point with time being a concern, im going to go back to a basic factory set up, then reapproach this after midget 50,


It certianly wasnt the results I had anticipated, but wow a cool ne w issue to think About in the mean time....im certianly not dis-appointed...but rather inspired from what ive learned, kind a wishing I didnt have to go to midget 50 so I could play wih this over the summer

As to the usa 71 factory set up.... No pvc valve, its a hose from the timing cover to a (Y) connection that hooked into the 2 carbs close to where the air filter attaches ...it was the vac (depression) that sucked the positive air out of the crank case....somethimg ive always considard to be rather weak, and obviously I am correct, by the above calculations that this area is only pulling 1/3 to 1 psi of vacume out of the crank case
On another side note,,,, my modified evac system destroyed yetanother catch tank, sucked the air right out of it and turned it into a Plastic collapsed ball....more frisbe shaped then box shaped, so im guessing my thoughts are in the right direection
Prop

Am I the only person this side of the pond that would like to either see Props car in the flesh or see a full set of photos on Flikr or similar?

Prop - you must have time a plenty to do what you do - I'm lucky if SWMBO and the kids allow me 30 mins a day - accumulated if possible - to tinker on Blackie.
Dave
Dave Price

Dave,

I am vary lucky.... No wife, No kids, No crazy girl friends,my time is my own.
Prop

This thread was discussed between 16/06/2011 and 19/06/2011

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