Distributor vacuum: Ported vacuum vs. full manifold vacuum: Which is better?
 

I'm running the following set up in my Cobra:

408” 351W based stroker motor
Late model 351W factory roller cam block.
Crank: Eagle 4.0” Stroke Rods: Eagle 6.200” Pistons: Probe Industries Forged Aluminum SRS 4.030” bore, Reverse Dome 22cc (Compression ratio should be about 10.09:1 with these pistons and the Edelbrock heads with 60cc chamber volumes)
Cylinder Heads: Edelbrock Performer RPM 2.02” Intake Valves, 60cc chambers
Intake manifold: Edelbrock RPM Air Gap 351W
Carb: Holley Model 4160, 750 CFM, manual choke, vacuum secondary’s #0-3310C
Fuel pump: Summit Mechanical 110 GPH

Camshaft: Mike Forte Custom Grind 510/534 Lift, 222/232 duration at .050, 114 LC Camshaft Hydraulic Roller Lifters: Ford M-6500 R302
Roller Rocker Arms: Competition Cams #1131-8
Hydraulic Roller Lifter Retainer and Spiders: Ford factory

Crankshaft Damper: Ford Racing Nodular Iron, external balance #M-6311-K351
Oil Pan: Milodon 8 quart
Timing Chain Set: Edelbrock #7811 Performer-Link True Roller chain set
Distributor: MSD Pro Billet Small Diameter, Magnetic Trigger #8478, with vacuum advance
Distributor shaft Steel Gear (#85834) and installation by MSD factory
Superformance stock Ford Dura Spark ignition module coil
Spark Plug Wires: Ford Racing black 9MM
Spark Plugs: Champion RC12YC (Recommended by Edelbrock for use with their alum heads). Spark plug gap: 036”
Ignition timing is about 15 degrees initial, 21 mechanical for a total of 36 degrees all in at about 3,100 RPM

The Holley Installation, Tuning and Adjustment Manual for my carb says to use the timed spark vacuum fiting in the chock side of the primary metering block as opposed to full manifold vacuum at the base of the carb. Yet many people swear by using full manifold vacuum. Why? Which is a better source for the distributor vacuum?

Direct intake manifold vacuum signal comes on faster and stronger than any other ported vacuum signal. Personally, I would never run a ported signal, only direct intake vacuum.

Now, PLUG or DISCONNECT the vacuum signal while you adjust the fuel/air ratio mixture screws at idle. It's just easier for MSD to tell you to use a ported signal so they don't have to tell you about adjusting the carb, both fuel/air and idle rpm. For many folks it's just to complicated. For those that have a clue, direct IS the best way. AFTER you have dialed in your air/fuel idle screws, unplug and allow vacuum signal back to the dist. Your idle rpm should now increase, due to more initial timing because of vacuum signal at idle. Now you can adjust the carb idle rpm screw.

IF the engine does not speed up when you connect or disconnect the vacuum advance at idle that means your engine cam profile at idle rpm does not provide enough vacuum signal to make the vacuum dist work. Typically MSD starts making vacuum advance at about 11 to 13 inches. Some hot rods only idle at 9 or 10 inches.

You know what? It's WAY easier to just tell you to hook it up to a ported carb vacuum signal, done, no more talk story all ready. :)

I'm no expert, but I have read a good deal on the subject.

Ported vacuum came out of emissions regulations. The goal was to use the throttle plates to block off the vacuum at idle. Retarding the timing at idle causes the combustion chamber to run hotter and reduce emissions it idle. It also uses more fuel. Long idle times can cause hot spots and pre-ignition.

Modern EFI computers also reduce timing at idle. Partly because timing is used to help control idle speed. Emissions and warm up are also in the mix. They do it intelligently taking into account many factors.

Based on the EFI strategies, it would seem that ported vacuum would make controlling idle speed a little easier, as with the timing pulled in to make maximum torque, the throttle may need to be practically closed at idle. Although I thought engines ran just fine on manifold vacuum back in the old days. I think I prefer manifold vacuum, but there could be a logical argument for ported.

The amount of ported vacuum at a given light load condition driving down the road may be quite different than manifold vacuum, so the vacuum canister may need set up differently to change from one to the other.

Like Excaliber says....and Direct will help throttle response noticeably.

One thing I don't like about the MSD vacuum advance is it's non-adjustable, in any way. It has a set vacuum signal required and set amount of vacuum advance it will provide. Next tme I'm going with a dist that I can "play with". Or buy the parts to modify my existing MSD. :)

Try both, one will run better than the other.

Use that one!

MSD uses Delco advance units and there are tons of them that have different total advance and vacuum required to operate.

I know, I just haven't got around to messing with it yet. Just put in a heater, soft top, side windows coming, new TKO, had the dash out to do some re-wiring, interior is gutted.

...I'm gonna get around to that distributor any day now. :)

There's no advantage to ported vacuum. With full time vacuum the engine will idle better, run cooler and get somewhat better mileage. Having said that I can't believe I'm going to an all mechanical advance distributor on my engine. Is it true that the vacuum unit won't fit with the factory type expansion tank on an FE?

Dan

I'm running an MSD with vacuuma advance, FE and the typical expansion tank. The only problem I have is to make sure I install the dist in the right position so that the vacuum cannister doesn't hit the expansion tank when you make the final timing adjustment.

Nope, ain't true. The coolant expansion tank sits off to the passenger side of the engine and should clear the distributor. You might have to monkey around with the direction the vacuum modulator faces but it should fit. Considering the number of times this ported versus manifold vacuum question rears its ugly head on this forum, it's obvious that hardly anyone had been able to figure out the really correct answer to the question.

In the old hot rod days, no vacuum advance at all. Total advance was therefore initial only at idle and at WOT the sum of the initial advance plus the built-in centrifugal advance. Then came the vacuum modulator whose purpose was to provide vacuum advance at idle when the motor was under no load at all, therefore no ping. The vacuum advance was usedto result in a smoother cooler idling motor with reduced NOx. Vacuum at idle was initial plus vacuum (modulator connected to manifold vacuum), advance at WOT was initial plus centrifugal. Advance just off idle was initial only since manifold vacuum dropped cancelling out the vacuum advance and engine RPMs were insufficient for centrifugal advance to begin registering.

Just for kicks I tried connecting my vacuum modulator to ported vacuum and the engine idled like a tumbler full of rocks. Connecting it to manifold vacuum smoothed everything out and dropped coolant temp at idle. Of course, I had to readjust the idle speed screws but, for me, manifold vacuum was the way to go. With ported vacuum, the total advance at WOT was 10* initial plus 26* centrifugal plus 22* vacuum. WAY too much advance.

superheart you had too much mechanical which limited your initial. That killed your idle and caused higher temps.
I like ported. It gives me a good throaty idle, a nice deaccelaration when I let off the gas. I have 16 initial and 34 full, my idle temps are 180 just like my running temps. once the throttle plates open up it acts just like manifold vacuum. Except for idle they seem to be very close to the same. One port being above the throttle plate and one being below. Once the plate opens what is the difference? I believe you should try both and see which one you like.
Lou

There are two main reasons I like vacuum advance.

One:
I can get a higher idle rpm WITHOUT having to open to carb butterflies. This of course requires direct manifold vacuum, ported won't work for this. Why I like it: When I turn off the ignition with the carb butterflies closed more than they would normally be to achieve the same idle rpm, the motor has far less tendency to "deisel" or "run on". So this method helps control "deiseling" on engine shut down.

Two:
Vacuum advance helps improve mpg, in some cases, it improves it a LOT. Not worried about mpg? I only have a 17 gallon tank, the more mpg I get the more range I have per tank of gas. Under cruise conditions, light load, only partial open throttle, the engine can use a LOT of advance. Up to 50 degrees, 45 is safer, but it can use a LOT. Say your base timing is 15, add 20 for mechanical advance, add another 15 or so for vacuum advance and you get 50 degrees. I've been running this amount on my FE for the last 8,000 miles. Even with dual carbs I can get 13-14 mpg while cruising at 70-75 WITHOUT overdrive! There are some other "tricks" I've done to the motor to specifically address better efficiency/mpg, all the little tricks work together to provide a nice "package deal". Primary trick is "quench" in the combustion chamber, something the FE's never had back in the day. Custom pistons have to be used.

You do a lot of cruising,vacumn advance will net you better mpg,Bar hopping,not needed,who cares about mpg's when foot's in it.