I don't understand the power valve...

On an automatic car you can "feel" the Power Circuit happening. Just put it in gear (observing all cautions, don't run over your washer nd dryer, don't overheat your transmission, and slowly depress the accelerator. When the Power Circuit opens the engine will noticeably respond, if you have a tach you will see it.

That's the PV/PC opening. Of course this only works if the valve is working and the mixture is reasonably close.

If you look up at the power chart think of it this way. I am at 1/2 throttle and making 75HP and then the Power circuit opens and voila 90!

 

Thanks, Hoop,
So it's kind of like the "squirt" that comes when the secondaries open on an AFB. Way forward thinking for the time.

Now, I have a question. If you look at the picture, there is an opening on the manifold between the carbs that is drilled and tapped. I'm assuming that is for a vacuum fitting.
The backs of the carbs also have openings, again I assume, for vacuum.
If I run vacuum to my dizzy, where do I get it? I'm assuming again that the fittings on the backs of the carbs are throttle body vacuum, and the fitting in the manifold is, wait for it, manifold vacuum.
At this point in time, my dizzy is getting no vacuum, the tube is just hanging out in mid air. And the engine runs like a sewing machine! Can I expect it to run better with vacuum to the dizzy!?!
I actually lost a bet whether my distributor had vacuum or not, I couldn't believe that the engine could run as well as it did, if it was supposed to have vacuum and wasn't getting it.
As usual, half of getting an old hot rod running smoothly is undoing the mistakes made before you got there. Thanks, Mike

 

Actually, most of the time with dual carbs you won't want to run vacuum. The original system was the fore runner to Ford's Spark-o-Matic from the fifties and sixties. Basically it's mostly vacuum comtroled through a carb mounted valve called a spark control valve. GMC Bubba from here on the H.A.M.B. has set the last couple of crab cap distributors I have worked with up to run without vacuum and they have been awsome additions to the cars they have been on.

 

Your '47 crab distributor has full mechanical advance with vacuum retard, while the late 8BA distributors have vacuum-only advance, and is what your 8BA 94's are designed for. Simply plug the rear carb vacuum ports. Manifold vacuum should be connected to your crab vacuum advance piston brake.

 

I'm in agreement on an ignition system upgrade, (Ford made some marginal ign systems) as well as getting a UniSyn and dialing in the airflow. It's inexpensive, easy to use, and worth the time (not much) to do it.

Happy Roddin' 4TTRUK

 

This is an early Ford here, no vac controlled Loadamatic!
With no vac hooked to your ('32-48) distributor the centrifugal advance is running with some restraint, and should have a bit more response and better part throttle power with vac hooked up and the brake adjusted out.
It uses manifold vac; plug the ports on carb and hook manifold vac* to the port on front cover that connects the distrib on a 59. Plug the ports on carbs...very messy situation there and you don't need any of it!
Now loosen locknut atop brake cylinder and screw adjuster bolt out BY HAND till you feel it hit the top. If you wrench it out you may not feel the stop and then will pop the spring seat off of the widget and have a bad afternoon figuring out a handful of little widget fragments...
From top, screw it back in maybe 1--1 1/2 turn to slightly load the brake disc. If unloaded completely you will get some float in your timing, slight drift as the brake plate moves radially. For modern gas this generally does it...you would screw adjuster down to get rid of pinging, but unless you are running NOS prewar gasoline or kero you should not have that problem.
Adjusted out you have the quickest curve from the centrifugal...the restraint on full throttle advance is only needed with poor combustion/pinging. The brake was meant to slow the curve with bad gas when cranked down.
This is a Mallory patent distributor design...function is not comparable to '60's vac cans or the goshawful '49-56 Holley distributors, whose patent was assigned to Satan.

*hookup...first buy the repro of original vac line, as you need the funny fitting to attach it at the front. Cut off the manifold end and get a fitting to affix that end to the hole shown, which is almost certainly 1/8 pipe threads.

 

Thanks, Bob. That explains why the engine feels so good now.
The stock manifold has a huge bolt screwed into the port where I'm assuming vacuum used to come from. Hooking that up to the point on the Edmunds manifold will be easy.
Just another thing to cross off the "fixed that" list.

Thanks Bruce.
The end of the vacuum line has been cut smooth at the manifold end, again, I'm assuming, to install a flexible line. I'll save myself some grief and put in a 3" or so flex line to the manifold. I was given another crab dizzy, I'll check it and the diagrams in the "greenbook" out to see how the brake works.
My brain is going to explode before long......

 

I would agree that you don't want to run the Vacuum Spark Control valves with dual carbs, (or at any time for that matter) however you are wasting fuel if you don't have a load sensitive ignition advance.

A couple of points, let's say it takes 35 HP to go 55 MPH.

Let's say it takes 20 Inches of Manifold Pressure to make 35 HP.

At sea level when we have 20 inches of manifold pressure and are reading a Vaccum Gauge (Depression from Atmospheric) it will indicate 10 inches of Vacuum.

So we are at about 67 per cent of maximum load.

If we disallow the hopefully better fuel distribution of dual carbs the Manifold Pressure (Manifold Absolute Pressure) therefore the Vacuum would be the same using 1, 2, or 3, carbs.

As a matter of fact with more equal fuel distribution we would expect to require a little less MAP to make the same 35 BHP and resultingly more vacuum, maybe 0.5.

As a matter of fact if you have a test road, anytime you can run the same speed (avg 2 ways to compensate wind/slope) and have more vacuum, the engine is using the air and fuel more efficiently.

So let's look at WOT spark timing.

In most cases we are running up against spark knock, we call this KLMEP, Knock Limited Mean Effective Pressure. Don't worry about the fancy word, Mean Effextive Pressure x Displacement = Torque.

So we have a KLT or knock limited torque limit.

We set full throttle mechanical advance to stay just on the safe side of this limit.

We could have a situation, and often do with new efficient chambers where we are not knock limited, more advance produces less power.

Think of starting a race on a bicycle but you only get one push. Where would you put the pedal. Not BTDC you would go no where, not TDC you would waste time, but just slightly ATDC, too far ATDC and you would waste energy.

The engine is the same, we want to have ~90pct of the charge burned as we start down.

We control this point by how early or advanced we lead the ignition.

Now when we go from 60 pct load at 12.5 to 60 pct load at 15.4 the flame shows down dramatically. Leaner mixtures compressed less burn much slower.

So to end up at the same end point we need to start sooner.

The Vacuum or Load based advanced takes into account the lower density and leaner mixture in the cylinder.

So we need it.

Sometimes it is simpler just to make it run without it, but is definitely not better.

at least, I think so

 

Woah, Hoop!
Are you a teacher? You should be! I've spent my whole life playing with the infernal combustion engine, and I've learned more theory off this thread than I have in 64 years!
I see where making the vacuum work in my distributor may not be noticeable to me, but my engine will be happier.

 

"...to see how the brake works..."
Mallory...like hotrodders...HATED movable point plates. The Flathead '32-48 and the true Mallory hotrod distributors (modern Chinese mallorys are just generic dual points, like accel or something) had a long centrifugal curve that was undisturbed by the brake at high vacuum/part throttle cruise. Under full throttle, vac drop allowed the brake spring to put pressure agains the disc and it slowed down the curve in proportion to the adjustment on the doodad. This was an adjustment to cope with variable and poor quality gasoline. On modern 87 generally the flathead likes the full rapid curve fine. It could probably actually use even more part throttle advance, but that's all she wrote!

https://docs.google.com/viewer?url=www.google.com/patents/US2092269.pdf is a Ford explanation of an older one...note the various curves shown, essentially samples from infinite variability.

Note here some sort of Asian interest in this in 2012:

http://www.google.com/patents/US209...a=X&ei=zwOEUZ6zEKrM0gGq-4H4DA&ved=0CDkQ6AEwAA

 

I see.
The "brake" slows the action of the breaker plate, either in advance, or retard, so as to make changes in timing less "jittery". A smoother transition from one tangent of the plate to the next. The dizzy will function fine without, but smoother with. Worth the effort to make work properly.

 

It actually works on a brake disc that is part of the system controlling advance...advance is accomplished by rotating cam around the shaft, breaker plate is entirely immobile. Effect of brake application is to slow down the advance by pulling the disc back against the forward impetus provided by the flyweights!

 

Thanks Brauce and Hoop! I screwed the pooch on that one completely and got the two different eras messed up. Been doing too much Falcon stuff lately! How ever, I do know that with the two crab cap type early Ford units I had last year to deal with, a quick trip to GMC Bubba for a recurve and cleaning/reworking, and a quick bolt it on and check it kinda thing and we were down the road, happy as a clam.

 

It's a highly miniaturized system too complex for most minds. Here it is blown up 50 times, should be much easier to understand.

 

Any good college level text on parapsychology will sort this right out for you!
MUCH simpler than a Model T; Ford dealership mechanics of the '20's were required to complete at least a Master's degree in parapsychology before they were allowed near the timer or the four coils!

 

Ok For real



This widely misunderstood device is the vacuum brake. Found on all front mounted distributors, it's sort of a vacuum advance in reverse as it retards timing to prevent detonation. With the brake applied, ignition advance at 1,200 rpm is normally limited from 9-12 degrees; the brake normally applies when manifold vacuum drops below 6 inches and releases when it reaches 7 inches or more.

 

Mike, remember the old show you asked for it?

But first, the Vacuum retard function occurs when vacuum drops below 6", the enrichment valve opens when vacuum drops below 6 inches, richening the mixture speeds up burn, coincidences? Maybe they knew a little more than we credit them.

Plus closed loop timing with a knock sensor, albeit the sensor was human and the loop was long...

So we started out with you not knowing the purpose of the Power Valve and I had no idea whatsover of the operation of the Ford Vacuum Advance Brake, we both got learned!

 

I need to figure out how to use this as a screen saver!

 

I'm going to bump this thread one time just to see if there is anything that hasn't been said.
If it sinks, Thanks to all that contributed to it, I have learned a great deal off this one thread, and will keep it to refer to later. Thanks again, Mike

 

Some bullet points.

For good street performance it is desirable to have a load or demand sensitive fuel mixture control device and spark advance.

Engine power is primarily controlled by Manifold Pressure which is your local pressure subtracted by Manifold Vacuum.

ATM - VAC = MAP
30.0 -20 = 10
30.0 -18 = 12 (Idle or no load)
30.0 -13 = 17
30.0 -9 = 21 (50 Pct load, early start of enrichment)
30.0 -6 = 24 (66 Pct load, normally we are enriched here)
30.0-3 = 27 Typical WOT for under carbureted
30.0 -1.5 = 28.5 WOT Mutliple Carb/4 bbl
30.0 -.5 = 29.5 WOT Max Carburetion

At around 66 Pct load we are beyond cruise and are sensing the driver wants power now, we richen the mixture and with a richer mixture and more load we are pulling timing out.

In Theory Multiple carbs should only effect engine load from 66 pct up.
In reality linkage and foot control may give us a non-linear response.
Because we are opening throttle area 2 x as fast the same throttle setting will have more manifold pressure/less vacuum.

To cruise our manifold pressure and vacuum should be the same with either setup.

Having to go to a 3" PV indicates we are going rich on part throttle opening. Consider smaller primary jets, less accell pump and a return to 6 inch valves as a possible better compromise.

Flatheads on lean cruise will want 10 degrees or so more timing than when under higher loads.

Set mechanical just short of WOT spark knock.
Then tune Vacuum Brake to just short of cruise spark knock.
An occasional light tip in spark knock is the sound of economy, sustained knock is the opposite.

 

I'm going to keep this in my subscriptions as a text book.
I love the HAMB!

 

Effect of timing on Vacuum:

So under cruise load we would advance timing to max vacuum unless knock limited/



Effect of leaning carburetor on vacuum and fuel economy.

We can see on this engine peak economy was about 1.5" of vacuum below peak.

So if you put in a smaller jet and lose a bit of vacuum good, a bunch of vacuum probably too lean.

 

Some very info,and a fun read too.

 

Wow, this is a meaty thread, gonna take me a week of reading to understand any of it, not counting that diagram. In the mean time, can I time my stock crab/vacuum brake distributor with a timing light? Spent a couple hours today finding tdc and marking my pulley. If I take off the brake and set it for 24 total it's going to be too retarded with the brake, no? Or will the centrifugal overcome the brake at high rpms?

 

Just found this thread - great info. Does anyone know about Rochester 2gc Power valves? I have a fairly big cam which causes the vacuum to be a little low and my engine is flooding all the time or very rich. Are there different power valves available depending on how much vacuum I'm pulling through my carb? I'm trying to dial this thing in on one center carb and have the two secondary carbs blocked off with no fuel lines run to them and aluminum plates under them until I solve this problem. My carbs are real tripower Rochesters off a 348 but I have replaced the center carb with a new (rebuilt) one from Summit. My engine smakes black all the time I'm driving it and spits raw gas out the tailpipes when I rev it up. It wants to stall all the time because of too much fuel. Frustrated. I have the top off the carb right now so I can't start it but will get it back together this am and check my manifold vacuum. My idle adjustment screws don't seem to make much difference in vacuum or in the sound or smoothness of the engine and I noticed when I took off the top of the carb that the fuel reservoir has only about 1/4" of fuel in after sitting for about a week. My float level is good and my fuel pressure is set down to 2 1/2lb. right now.. Frustrated!!!!!!!!!!!!

 

Some visual aids for figgerinout how Ford carbs and also Strombergs work, system by system:
Go here: http://www.norgv8club.org/norg/inde...rs-and-tech-tips/category/9-service-bulletins
And open up the second clump of bulletins that mention "operation"...this is a set of Ford bulletins explaining and nicely illustrating circuit operations in both families of carbs, published in 1938.
The Ford rebuild instructions are on same page. Good stuff, print it out before Ford discovers the violation if you don't own the '38 Service Bulletin set!

 

I have never heard about this. Good info to have!

 

You mentioned Houston International...what year (s) g

 

The only experience I've had with a power valve was on a 69 ElcaMEANo with a 435 hp.427 corvett motor in it.every time I would nail it the powervalve would blow...then it would idle like someone was pouring a quart jar of gas down the carb. So being the dumbass I was I took the powervalve out and soldered it shut.
It didn't run so good after that...but it sure didn't load up. I guess we live and learn.