Other: Which is the better vacuum port?

[Stu Duglee]

My old Carter AFB has only 1 vacuum port & my new Edelbrock 500 has 2, partial & full.Which is the better of the two?I've been using the partial but I read a few weeks ago on a reply to a thread a member using the full vacuum..he stated that the partial makes the exhaust gases hotter to meet emission concerns & that running on full was better.....more power.I'm using a Mallory distributor in a balanced R1/2 engine & am curious which is the better vacuum source & why?

[Bud]

I use the ported vacuum port for the vacuum spark advance as that is the way Studebaker originally did it. I don't think the Studebaker engineers were concerned about exhaust emissions when our cars were built. GM in the 60's and 70's used full vacuum on the distributors in some of their engines but the distributors had their advance curves set to account for the full vacuum at idle on the spark advance. Bud

[Mike Van Veghten]

Using either port ISN'T going to make the exhaust gas hotter....!
Just how would that work ?

On the other hand, it does affect the way the timing works...which CAN effect the the chamber temperature if not adjusted correctly.

Rather than ask "which one is better" just try them both and make up your own mind which you like better.

1. Full vacuum or the "old" way (lower port)-
The vacuum advance has vacuum at idle. Therefore, the the timing is at full vacuum timing. As you push the throttle, the vacuum goes down the engine power lessens some. As the vacuum builds back up as you go into a cruise mode, some of that vacuum comes back up and the power comes back as you cruise down the road. When you hit the throttle to pass someone, the vacuum drops and the timing then drops, a slight loss of power, until you stabilze the throttle again, the timing comes back up.

2. Ported vacuum (upper port) -
This works, for the most part...just the opposite. Reread the above and swap the way the timing and hense the power comes into play.
When you hit the throttle, vacuum comes up, so does the ignition timing and...so does the power.

The conudrum here is you need to adjust the initial timing a little different so you don't end up with too much (or too little) ignition timing at the tip-in (initially hitting the throttle) of the throttle.
To get the most out of either will take a little experimentation. The full vacuum will take a little more initial timing to make the engine its happiest.
The ported vacuum will require a coupla degrees less initial timing to make the the engine happiest.

Note: overall, the timing has an effect on....power, driveability, gas milage.

This is part of the fun of these old cars...you can still experiment to find what works best for "you" and your driving style.
Play around a little, you might find it both interesting and give your Stude a little extra pep in the way you drive.

Mike

[Stu Duglee]

Thanks Mke,just got a new timing light with display & a vacuum gauge.Initially I set it up by ear & then I put the light on it & I was at 8 degees at 850rpm A friend set the carb with the vac gauge & on our first long trip(today) with a fresh engine we averaged 19+mpg over330 miles round trip in the mountains of southern B.C.I think I'll go with the ported for now Thanks

[alpayed]

Sorry to disagree with you Mike but retarded ignition definitely causes a hotter running temp at idle
Ignition timing determines the time at which ignition occurs. On the compression stroke retarded timing ignites the air/fuel mixture closer to TDC (maximum compression) The higher compression at the point of ignition means hotter temperatures form the burning of the air /fuel mixture and this translates into a hotter running engine.
In the early days 60’s manual transmission cars almost always ran full vacuum.
Automatic transmission ran ported. The reason for this is to avoid the timing change that happens on a vehicle with an automatic transmission when it goes into gear; the vacuum will go lower and cause the timing to change. In later years it was adopted to increase exhaust temps to reduce CO2 emissions on all cars.
Here is a good article on the subject.

http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/vacuum_explained.pdf

If you don’t have overheating use ported, but suffer the poor economy.
If you have overheating use full vacuum. The gear/idle rev change is not an issue.

Allan

[Mike Van Veghten]

Allan -

I know it does..!
That's not what I said.

I've had headers glowing for pictures in a friends blown Hemi boat with the timing so low it would just barely run. But the pictures turned out well. Along with hot exhaust, it also makes the engine "pound" a lot harder. The Combustion cycle doesn't happen very smoothly with lower timing either.


Reread, you have to adjust the initial to gain the correct overall (or as I say, make the engine happy!), when you go from ported to unported.
Mike

[showbizkid]

Let me ask a specific question. On Edelbrock carbs, or older non-PCV cars, you have your choice of using the front or rear carb ports for vacuum. But on later cars - say, '63-up - the rear port is used for PCV leaving only the front port available. So are your choices limited by default, or is there a way around this?

[Mike Van Veghten]

biz -

Not specific enough a question.

What ports ? Saying front or back doesn't tell us much.
Though MOST of the time, the rear port is a much larger threaded port (1/4 or 3/8NPT). And yes, this is for the PVC or the vacuum for power brakes.
Normally the smaller ports, the ones with a piece of tubing sticking out are for things like - trans. vacuum, for ignition vacuum, for a vacuum gage. etc. Some older carburetors have small 1/8NPT holes in them, same thing, as just noted.

Mike

[alpayed]

Mike,
I read your post again and I agree that the initial timing can be tweaked a bit. The port is just above the throtle plate and the instant you crack the throttle open just a bit the difference in vacuum the ported and non ported hole "sees" is virtually the same. the ported connection only effects the timimg at idle. If you connect a vacuum gauge to each port and watch them closely they move identically except at idle. The one connected to ported will read less at idle.The ported connection was initially put there by car makers to increase exhaust temp to lower hydrocarbon emissions. Regardless of how you adjust the initial timing (unless it is equal to the full amount of vacuum timing) the engine will run hotter at idle. On the subject of setting timing I have always started with the factory setting and then tweaked the timing acording to my ears. Adjust the timing to as much advance without "pinging". this is the most effecient setting. With regard to manifold ports you can of course run more than one device of one port.
Allan

[Stu Duglee]

Thanks for all the input gents now here's the poop,I started with ported then as Mike said . play around with it & I found with a vacuum gauge(old style economizer installed on dash)you can really tweak your carb & after a week of trials using full & ported....ported works better for my driving style & needs & I get 20 mpg on the average with lots of power.Thanks again for all the info.

[showbizkid]

Here's a thread I found that's pretty interesting:

http://www.corvetteactioncenter.com/forums/c3-technical-performance/60830-ported-vs-manifold-source-vacuum-advance.html

[Dan Timberlake]

Mike,
......The ported connection was initially put there by car makers to increase exhaust temp to lower hydrocarbon emissions. .....
Allan

Similar statements are all over the internet, and one similar is even attributed to a GM engineer.

Over the years, even in the pre-emission era, different manufacturers have used some different schemes at times, so any blanket statement is likely to be "wrong" sometimes.

The 1959 Factory Edsel manual says ported vacuum is used on their conventional vac advance mechanism distributors, as well as the curious all vacuum system on some 6 cylinders. They were not alone.

[Stu Duglee]

Thanks for the feed back.Very interesting sites to look at showbizkid & alpayed.....very interesting!