GEBackbone

I have a stock 283 with a 600 cfm holly carb. I set the timing to 5 degrees BTDC, with vacuum line plugged with golf tee. When I connect the vacuum line to the base of carb, the timing goes off scale approx 20 degrees BTDC. When vacuum advance is connected to the vacuum port on the metering body, towards the top, it doesn't really change timing.....Which port do I connect the vacuum advance to??.. the base or the metering body of carb.
Thanks in advnce Gary

DSR

Gary: Ported vacuum is off the metering block. This means no vacuum advance on idle. No vacuum until the throttle is opened a little bit. So you set the timing at 5 degree's btdc. with the vacuum line plugged. Connect vacuum line to the metering block you should still read 5 btdc on idle. Connect vacuum line to manifold vacuum ( base of carb ) you will get full vacuum on idle, which you see as 20 degree's btdc. Full vacuum on idle raises the rpm on the idle, so the idle would have to be adjusted lower. People run them both ways, I'm not sure why. Fuel economy, emissions, drive ability? Some cars do not have a ported vacuum and have to run manifold vacuum. Lots of people say run manifold vacuum, personally I use the ported vacuum off the metering block.
Dave

1snake

Connect it to the constant vacuum source and adjust your idle accordingly after setting your timing with it plugged. I can't think of a single reason to use ported vacuum. It was a design to reduce emissions(and performance).

Jim

Frankie the Fink

EDIT - Jim beat my post but I agree with him totally.

Correct....I run full vacuum (NOT ported) to my 283ci WCFB carb base to my aftermarket vaccum can and it works perfectly. Ported vacuum IIRC was for emissions engines...

With a little extra effort you can even hide the vacuum can under the original ignition shielding!

JohnZ

The angled nipple on the right front of the baseplate is the full manifold vacuum source.

Dicecal

I beleive this is what you want, recently switched mine:

DSR

How does the ported vacuum reduce emisssions ?

Or How does less timing on idle reduce emissions ?

And how does less timing on idle reduce performance? (The motor is only idling and at the very bottom of the performance scale)

Dave

BarryK

As Jim stated, Ported vacuum was only used for an emissions reason - to help lower overall emission output to met federal requirements.
it was used in conjunction with other emissions equipment such as the A.I.R system, moving the t-stat level up from 180º to 195º etc and overall lower timing setting.

Full manifold vacuum gives you full vacuum advance at idle and ported vacuum advance gives you zero vacuum advance at idle.
Since running a lower timing setting (A retarded timing setting) typically causes higher operating temps (among other effects of lower timing) the motor engineers at the time took advantage of this effect - by lowering the initial timing setting AND running ported vacuum advance - hence no vacuum advance at idle - this caused operating temps to rise. They used these higher temps and the A.I.R. system to recirculate and burn off HC levels thereby lowering the amount of HC emissions

On your last question, full manifold vacuum gives you a more advanced timing at idle, one of the benefits is typically better throttle response compared to a lower initial timing setting and ported vacuum

for a lot more details on this topic, may i suggest reading the great timing article by John Hinckley (our forum member JohnZ).
It explains everything very well.
http://www.lbfun.com/warehouse/tech_...101Article.pdf

MikeM

This is a false statement.

DSR

Thanks for the input, I will read that link.
Dave

Frankie the Fink

???

From the John Hinkley article:

"“Ported Vacuum” was strictly an early pre-converter crude emissions strategy, and nothing more."

BarryK

OK Mike, than tell that to John also since that is who I learned it from... (among other sources)

and if it wasn't for emissions reasons than what was it for?

1snake

Oh my gawd, Mike. What are you smoking? I know you'll never admit it but YOU'RE WRONG.

Jim

MikeM

Don't be so sure of yourself. As late as 1963, Corvette had one engine that used ported vacuum advance.

Back when there we a number of different brands of cars experimenting with vacuum advance and sorting out the tune, I believe a fair number of them used ported vacuum. I don't trust my memory to specifically tell me which ones used it though.

Why Chevrolet used ported advance in that one engine, I have no idea as it had nothing to do with emmisions and everyone knows they run better with manifold vacuum going to the advance. It may have had something to do with eliminating spark knock but that's a guess.

Frankie the Fink

Here we go..

1snake

Just to give you the benefit of the doubt, I checked all my manuals regarding dist. and tune up specs. for the 60's Corvettes and NONE used a vacuum advance connected to ported spark. I did however get a PM from someone that says you used to be a Ford mechanic. That really explains alot.
As I stated before in posts involving you, I'm not going to get in a pizzin' match because that is all you seem to try to start when you're wrong and won't admit it. As far as I'm concerned, you can put the vacuum advance hose in your ___

Jim

kimo1965

MikeM

So far in this thread, you've made at least three inaccurate or untrue statements. It's probably good that you left before you make anymore statements of this nature.

I must have missed where anyone said anything about this thread being limited to mid-year Corvettes except you.

In any case, the '63 327/360 Corvette engine ran on ported spark advance and that had NOTHING to do with emmisions.

You know what you can do with your hose.

.

buns

Excuse me, I just need to




ENGINE GUIDE

For good street driveability, your distributor's vacuum advance should be connected to "ported vacuum," which is drawn from a source in the carburetor above the throttle plates. This results in increasing vacuum as the throttle is applied. The function of vacuum advance is to provide timing advance in low-
rpm acceleration conditions for smoothness and improved driveabiiity. The alternative, manifold vacuum, is taken from a source below the throttle plates, resulting in decreasing vacuum as the throttle is applied. If your distributor is connected to manifold vacuum, full vacuum advance may be applied at idle. Then, as the throttle is applied, vacuum decreases and timing advance is decreased, causing poor engine performance and acceleration at low speeds. Think about ignition timing: If your engine starts, your initial timing is fine. Ported vacuum gives the best performance around town. Mechanical advance brings you up to desired total timing for midrange and top-end performance.



Source: GM Performance Parts: Frequently Asked Questions

MikeM

When vacuum advance was in it's early stages of automotive use, the engineers used various combinations of ported/manifold vacuum and different combinations of vacuum can calibrations and mechanical curves looking for the best combination for driveability and fuel milage. By the mid 50's, most everyone decided that manifold vacuum driving the vacuum advance gave the best results for economy and driveability. One exception was the Corvette engine I mentioned. These examples had nothing to do with emmisions. That came later on.

Maybe the motor mouth will read this and be a little better informed the next time he gets diarrhea of the mouth.