Holley Carb Vacuum Hookups

[iwantacobra427]

There are three vacuum hookups on my carb: timed spark, manifold, and power brake. My MSD distributor has vac advance so that hooks up to the timing vac. Where do I connect the manifold hookup on the Air Gap intake manifold? Also, are you guys using power brake vac or should I just cap it off on the carb? I don't think my master brake cylinder is designed to work with vacuum boost. I don't see any way of hooking up a vac line to the brake master cylinder..

[J Persons]

A vacuum line doesn't actually connect to the master cylinder, it connects to the power brake booster. If you have a vacuum manifold, you can T it in the opening that is below the carb.

[CHANMADD]

from what you are saying ,I would'nt connect anything unless you have someone who knows ,check it.Post some pics of your carb and manifold and ignition and I am sure you will get the correct advise on Club Cobra!

[iwantacobra427]

The red arrows indicate where I'm proposing to install the plugs (2 x left side and 1 x rear) based on reviewing a few engine pictures I found. The black arrows show the distributor timing hookup (right side of carb), the pcv and manifold hookups (front of carb), and the power brake vacuum hookup (rear of carb). The grey masking tape covers what I think would be the water temperature sensor location. Is this right?

[CHANMADD]

The grey tape is for the Temp sensor.The two red arrows ...correct plugs. I would hook the vacuum advance..the distributor..timing. to the small pipe at the front of the carb.That way when you hit the gas the vacuum will drop off and retard the timing. The vacuum advance is only for part throttle driving. The pipe on the side should be for port vacuum which is for the vacuum advance also,but a lot weaker ,some prefer manifold and some prefer port?? Try both and see which works better for you. The pipe at the back of the carb is very often blocked off..IE not drilled through so if you do have a brake booster you need to connect it to the big pipe at the front. You could install a brass hose barb fitting in the manifold at the back for the PCV, or put a "T" in the hose that goes to the brake booster. Make sure that the "one way" valve on the booster is on the booster or if in the hose is on the booster side of the "T" .
John.

[SuperHart]

This is a subject that has been reviewed many times. First, the vacuum advance module on the distributor should be attached to the small hose nipple in the base plate of the carburetor so as to pull manifold vacuum thus giving full vacuum distributor advance at idle. The reason it's done this way is to make for a cooler running smoother idling street use motor. Connecting the distributor vacuum advance module to "timed" vacuum (the small hose nipple usually located on the side of the Holley primary metering block) would just add vacuum advance to centrifugal advance as engine speed increases and end up with too much total advance (confusing, ain't it).


Your power brake vacuum line also is plumbed into manifold vacuum. You may connect the power brake hose either to the large hose nipple on the rear of the carburetor base plate. If you use a TEE fitting you can also use the large port for your PCV valve. Or you can use a hose nipple screwed into one of the threaded female ports in the intake manifold to supply vacuum to the power brake unit.

The gray taped threaded port on the intake manifold is indeed used for the coolant temp sensor OR the 5/8" return hose from the heater core. Usually there are two threaded ports into the coolant passage, one for the temp sensor and the other for the heater hose.

[Excaliber]

This is such a complex subject I could write a book on it. There are a couple of threads that deal with it in great depth, no doubt difficult to find with a search. Vacuum advance is not generally recommended for a performance engine at all. The justified fear being to much total advance under certain conditions, no matter what port you use.

There are basically only TWO types of vacuum on an engine. ABOVE the throttle plates (ported vacuum) and BELOW the throttle plates (intake manifold, direct, unported full vacuum).

Which one to use? Well now, that IS the question, isn't it? There is no right answer. It all depends on what you are trying to accomplish, what you believe (right or wrong) about how the vacuum signal works. Which brings us back to the 'book'...

My personal belief is that there is no 'steadily increasing' vacuum signal ABOVE the throttle plates as the rpm increases. That is, no appreciable increase in vacuum signal ONCE the throttle plates begin to open. There is vacuum, or there is not. Therefore, the second you 'step on the gas' it makes little or no difference WHICH port you use as BOTH ports WILL provide a strong (or weak, as the case may be) vacuum signal.

Which port to use, therefore, becomes a matter of how you want to tune your engine for the best 'idle' condition. Using vacuum below the throttle plates WILL require an adjustment of the idle speed screws (and possibly the air/fuel ratio screws as well). Using PORTED vacuum (above the throttle plates) will not change the idle speed or require mixture screw compensation.

I prefer BELOW the throttle plates myself, the vacuum signal is a 'hair quicker' to respond and slightly more vacuum is present overall. But the REAL reason is so my idle speed can be set SO LOW the engine WILL shut off immediatly when I turn the key off, rather than be subject to a possible 'run on' or 'deiseling' condition. My cam profile requires a rather high idle rpm, 1000 rpm or so. That is 'just fast enough' to cause engine 'run on' in my case. Maybe it's not a problem with your motor...

Tricky stuff this vacuum advance. Ask the BIG dog engine builders and I'll bet every one of them will say 'dont' go there' at all. To set it up correctly it's best left to the pro's. So, some more pictures and data, 'cause, WE are the 'pro's'!

[SuperHart]

The reason most factory setups use full manifold vacuum at idle is to smooth out the idle, lower emissions and have the engine run cooler. I tried both settings, manifold and ported, and decided that manifold vacuum worked better for me. Believe it or not, using manifold vacuum tamed the idle from full beast to tamely aggressive, reduced noxious exhaust fumes, and reduced coolant temp at idle by nearly 10*. Go figure. My understanding is that the factories added vacuum advance at idle primarily to reduce emissions at idle. The added timing didn't harm the motor because there is no engine load at idle. As soon as you step on the throttle manifold vacuum drops which removes the vacuum advance (usually 22-24* built in from the factory). In summary, assuming 10* initial advance using the timing light, adding the vacuum advance at idle gives a total advance of 34* which drops off rapidly to 10* when the throttle is applied, building back up to 36* at high RPMs when the centrifugal advance kicks in.

Still confused? Try reading this article: http://www.fordmuscle.com/archives/2...ng/index.shtml

[iwantacobra427]

Thanks guys! You've given me some things to ponder.. The engine is a 331, with a 750 cfm four barrel Holley 4150, Comp Cam 280 HR 10 (hydraulic roller), 6AL MSD ignition, 185 cc AFR heads, RPM Air Gap. The car won't be raced but I should have some decent torque when needed. I think I'll first try the distributor vac line to the ported vac hookup since I won't have to mess with the carb settings. I'll keep the manifold and brake vac hookups capped.

[cobred]

The port in the back of the manifold is for power brakes, ERA does not use power brakes. The large port at the back of the carb. is for the p.c.v. valve.
You can leave the others plugged for now and have no vac. advance until you break in the motor, tune it and get it running good, then play with it later.