Inliners International Forums General Discussion Engines Best vacuum port

Got my "punched" 194 equipped Nova running much better since heating the Clifford intake and the 390 Holley is much smoother under a load since replacing the power valve to a 3.5 (was a 6.5). Gas mileage has increased to 24 mpg hwy. I am currently using the vacuum port located on the inboard/rh side upper tube near the rear of the float bowl for vacuum advance to the distributor. This Holley has many ports that are capped. Is this the BEST place to draw vacuum from?

The vacum for the dist does come from the metering Block.Which is Just above the Idle mixture screw. Which is Metered Vacum. SO you are in the right place.

The original 64' dist. uses straight manifold vacum from a fitting that was routed underneath the throttle plate of the stock rochester. Do not use a timed spark port,or you will not have full vacum advance at idle. Use a port that shows full manifold vacum (the highest reading) at idle. The metering block ports are not the same on all Holley carbs.

i also agree with diesel d. tom

When i did that with my Holleys The Full vacum at idle over time would Put a hole in the diaphrams In short time.hum?Once i changed it to a meterd vacum I no longger had issues. Secondly the advance should increase as rpms go up and at a max set rpm. (Ex. 32 degress at 3600-3800 performance cam) stock cam is lower.

just my 2cent issue & opion.

I have a 450 holley with a vacuum port that comes straight out on the on the right side of the primary metering block and is manifold vacuum. The ports I have encountered that come bent at a 90 (out of metering block) have almost no vacuum at idle.I have heard it called metered vacum, ported vacuum, and spark timed vacuum but I cant think of any pre-74 distributor that uses it. Not positive about that for sure though. I like to use full manifold advance on my 250 because it gives my cam the timing it needs to idle well, and gives me better mpg at cruise, but it also works as kind of a mechanical spark retard when starting engine. But I have a flat 4 (bug) that runs way better with the vacuum advance disconected altogether, go figure

Gentlemen;

Am I correct in asuming that my 1966 292 (Holly 4 bbl planned), will be the same??

For performance reasons.
Running full vacuum @ idle is not really ideal.I always use ported vacuum.
For instance, lets say you have 40 degrees @ idle, as soon as you start giving it some throttle the vacuum will drop causing the timeing to retard, then when you let off full advance, then on the throttle, timeing retards again, it goes all over the place.
Sure, there is mechanical advance also, but initially, when you give it throttle it retards, then the mechanical advance will kick in more advance later.
You can set your distributor w/more initial advance & limit you mechanical advance ( so you do not get too much total advance)
You will have greater throttle response, as long as you can get away w/more initial advance.
Hope that makes some sort of sense?
My two cents. :-)

Dear Hank;

It makes perfect sense to do that, I think.

I'll just have to see what works best if I ever get all the parts to build this truck.

The "old car" (& 261 engine) sales have stopped here due to the gas etc.

Perhaps next spring.

Thanks.

Hank is right-on. I’d never hook-up a distributor to a standard vacuum port (like what the brake booster needs). It annihilates the whole advantage of vacuum based timing advance. The ‘timed’ vacuum port provides almost no vacuum at low idle, good vacuum at initial part-throttle, medium vacuum at mid part-throttle and almost no vacuum at full-throttle again (this scheme may change slightly according to carb brand and model). It enhances both part-throttle response and fuel economy. If you need more timing advance at low idle, just spin the distributor body. That will shift the whole timing curve. If that gives you too much timing as RPM increases, you can work on the centrifugal (mechanical) advance, using stiffer springs in the distributor (or reducing the weights…).

PS1.: Remember, carb is not good to your body!

PS2: Gas dropped back to $2.05 here!!

Twisted,
Your thoughts and recommendations are "right on the money" my neighbor owns a California Emmisions Test facility and he put my car on the Dino and the best performance at idle and at high speed demand (low vacuum) proved best with the set up I am using. My best investment to increase low end performance and top end economy is to work on transmission and rear gears...ie..a BW-T5 and go from there with rear gears to match what "I Want" the machine to do. Thanks again to all that have responded and I look forward to meeting with more members at the next club event here in NorCal/Lodi.

In the "how to" section on Tom langdons site they cover this topic fairly well. They recomend aproximatly 20 degrees vacuum advance at idle/cruise. It would be impossible to run that timing under a load, the whole point is to retard timing when you nail it. That is also why big trucks, full race cars, and generators dont need it(they are always under a load). Some timed ports will give you zero vacuum at idle and all timed ports give more the faster you go. It seems easier to just work it out with the initial timing and mech.advance, than to use timed vacuum, because timed vacuum responds only to speed, (not load) and we already have our mech.advance to do that. I have noticed that on the C cab Fords from the late 70's and early 80's (all use Holleys with ported vacuum advance) you can pull of the vacuun advance line while running and the idle does not change. Sorry to beat a dead horse so hard I am just having trouble understanding the need for ported vacuum advance.

I believe timed vacuum is load sensitive.
Putting a load on you engine will decrease your engine vacuum?
Vacuum advance is for better fuel milage & other things.
The 20 degrees is just as a general rule of thumb.
You can lock in 32 degrees or more if you want to, as long as it will start & does not detonate @ lower RPM's.
You can do away with the vacuum can altogther, if you are not concerned about milage.

Basically if you run full engine vacuum @ idle you get full advance. As soon as you crack the throttle your vacuum drop which also drops off your timeing advance.
If you use ported vacuum, you have little to no timeing advance at idle,,,,but as soon as you crack open the throttle you start getting vacuum to the vacuum can,, then it will start to advance your timeing.

I think timed vacuum works more on venturi velocity than actual vacuum (kind of like vac.secondaries)and Roc is right in that it only works at part and mid throttle. I guess between ported vacuum, manifold vacuum, mechanical avance,(springs, weights,stop bushings)intial timing, and even adjustable vacuum advance, there is probably more than one road to the same optimal timing curve.

Diesel D

Before "timed" vacumm, high performance engines didn't have vacumm advance. In the mid '60s when we were dumping crankcase gases out road draft tubes, the first timed ports started showing up on Holleys. All of a sudden, nearly every high performance car had a Holley, and vacumm advance. As those engines didn't have a vacumm advance before, every racer ran with them unhooked.

The "timed" port is located just above the trottle blade when closed, and is often partially uncovered when the choke is in operation and the engine on fast idle. It operates with manifold vacumm when the trottle is open far enough, as everything under the throttle will have the same vacumm reading. The port in the venturi on a Holley is the port for the vacumm secondaries.

Problems that occur with maniold vacumm. Idle problems. Most engines won't pass emmissions with manifold vacumm hooked to the dist. Maybe that's why everyone thinks it's an emmissions thing. The problems with meeting emmissions is unburned hydrocarbons. UNBURNED FUEL. Some engines are worse than others and have a noticeable miss. Others just drop a hole every few revolutions and you don't really notice it, but the sniffer does.

Another problem assiciated with using manifold vacumm is what happens when putting the car in gear with auto trans. There is, in most cases, a larger drop in RPM when shifting into gear. The common fix is to raise the idle RPM in neutral, and that creates more "bang" going into gear when using stock convertors. This happens because you are firing it WAY early, and when you put it in gear, the flame front/pressure rise is occuring in the same amount of time, but the engine in gear is idling at a 30% slower pace, so we get more pressure and burn before the piston reaches TDC, the wrong side to do work, and the engine slows more as more energy is used overcoming the negative pressure.

Have you ever noticed the MSD, and owners of, claim better idle quality? MSD made a fortune with the multi spark as it continues to spark until the car fires, overcomming the misfire from too much advance at idle.

Engines that came from the factory in the 30s-60s with vacumm advance had very low intial advance. Ford FH was 0 I believe. My Nailhead Riv was 2. Now people want to combine 12-18 initial with manifold vacumm for 30 or so at idle. Although this will work on some engines very well, most suffer the problems I mentioned above.

The question I have, is why in a situation where the engine is doing "no work" as in idle, do we insist on creating a situation where emmisions and drivability are both compromissed? Is this a Conspiresy to sell MSD units to cure idle problems?