66RBS

Can someone explain to me basic pros and cons of ported vacuum and manifold vacuum as it relates to a non emission controlled engine like my ZZ4. The Holley street avenger carb has ported vacuum.

Randy

wmf62

simply put.... conventional wisdom (present day knowledge) says that an engine using a vacuum advance system functions better and more economically using full time vacuum advance rather than 'ported' advance that is only in play when the throttle plates are open.

ported advance is for emissions purposes

AND, supposedly GM doesn't recommend any vacuum advance for the ZZ4; why, I don't know... I use it and to the best of my knowledge haven't experienced any consequences...

Bill

Frankie the Fink

Ported vacuum is above the carburetor throttle plates and full-time vacuum below the plates.
To wit, ported vacuum isn't operational at idle (throttle plates closed) as the orifice isn't exposed to vacuum.
Hence you miss cooler, smoother running at idle due to the increased advance provided by the vacuum operated advance....once the car is moving (throttle plates open) the two types of vacuum begin acting similarly...

As mentioned ported vacuum was an emissions contrivance to keep timing retarded at idle; there is no advantage to it currently. However, you will still get some owners that insist their car runs better with ported vacuum, but have a hard time explaining why in detail.

DansYellow66

Ported vacuum advance is completely useless on a engine tuned for street performance. I'm sure someone can find some exception - Ford used some strange vacuum ported stuff that I'm still not sure I understand - but for the overwhelming majority of us it can only hurt performance and engine behavior.

In my somewhat backwoods view - there are two approaches to distributor timing for a performance car that must live on the street. A mechanical advance only distributor (no vacuum can or can blocked off) can be used and set up to run initial advance somewhere around 14 to 22 degrees of initial advance with centrifugal advance bringing it up to the 34 - 38 degree range depending on the engine, gas, car weight, gearing, etc. This should result in a reasonably good idling engine, with good response and should run reasonably cool if all the equipment is good.

Or with a mostly conventional vacuum advance distributor set up for full vacuum advance, you can run initial timing more in line with most factory specs of 6 - 12 degrees intial timing, and a vacuum can of specific vacuum range for the cam pulling in another say 10 to 20+/- degrees at idle and then a centrifugal advance bringing total initial and centrifugal advance at speed up to 34 - 36 depending on engine, gas, car weight, gearing, etc. This should provide a good idle, smooth response, good fuel mileage and a cool running engine within the limits and condition of the cooling system. Sometimes the trick is finding the right vacuum can that works with your engine's range of idle vacuum and also doesn't pull in so much total vacuum advance that at steady highway speed you avoid an over-timed situation that creates missing and trailer hitching.

66RBS

Bill,

Thanks. How much advance does your vacuum can provide over base timing which I assume is around 12*.

Randy

wmf62

Randy

I have an electronic distributor that works in conjunction with my EFI, so I don't have a vacuum advance....

presently my timing is (under what would be considered cruise or light load):

20* at 800 idle
20* at 1000
25* at 1200
52* at 1800 and above

at wide open throttle

20* at 1000
22* at 1200
22* at 1800
29* at 2200
32* at 2600 and above

the distributor and sensors will interpolate the timing values in the ranges between 'normal' operating load and rpms and WOT

right or wrong, this is what I have..

right now i'm STILL trying to figure out a perplexing lack of a smooth throttle response from idle to approx. 1800...., which may be a result of a combination of timing and AFR I have not yet figured out

Bill

Frankie the Fink

Dan said 'ported' vacuum was useless - not vacuum advance in general IMO (e.g. full-time)...

SWCDuke

Yes, indeed. I misread the first sentence.

Duke

GTOguy

Excellent explanations here. The best set-up for street driving is usually manifold vacuum as you will get a cooler, more efficient engine at low speeds and in town. Once underway, the centrifugal advance kicks in, and you get all the spark advance you need at higher RPM. Win-win. Better fuel economy and better power.

tbarb

I run manifold vacuum advance on my stock corvettes but with a modified engine that has little vacuum at idle and a high overlap camshaft a ported vacuum source can work very well. It's the vacuum below the blades that will gas the fuel load so a stock camshaft can run lots of timing at idle, heat is not as important because of all the vacuum.

Camshafts with high overlap and lots of exhaust dilution need richer mixtures and more heat from compression to gas the fuel. It's all about controlling the heat which is why a more conservative initial timing setting will produce that heat.

I realize that I will be attacked here for saying this but keep in mind the type of timing map I am suggesting is initial timing in the range of 14-18*, centrifugal advance in the 15-18* range plus ported advance in the 10-15* range. This in no way can be compared to what the intention of the 1970's GM timing maps were.

What type of camshaft is in the ZZ4 engine, I wonder why GM recommends a ported vacuum source. I also wonder why there was ported vacuum advance on the shp big blocks and the L88 has the V/A nipple removed. I realize these engines were not produced to use for cruise rpm's

Using no vacuum advance at idle (ported) will also make the engine less efficient at idle thus requiring a richer setting on the emulsion screws plus a slightly more open blade angle compared to the full manifold vacuum advance. My experience with this is that the engine will drive off easier when put under load like letting the clutch out.

I think what it comes down to is what works best for the engine combo.

wmf62

GMPP ZZ4 OE roller cam specs 208*/221* @ 0.050", .474"/.510" valve lift w/ 1.5:1 RAR, 112* LSA.

Bill

66RBS

[QUOTE=tbarb;1597180080

What type of camshaft is in the ZZ4 engine, I wonder why GM recommends a ported vacuum source.
.[/QUOTE]

Previous owner installed LT4 cam 24502586 218/228 duration. I have 12" of Hg at idle

Re vacuum source GM doesn't recommend ported vacuum

From ZZ4 instructions

"Set spark timing at 32º before top dead center (BTDC) at 4000 RPM with the vacuum advance line to the distributor
disconnected and plugged. This setting will produce 32º of total advance at wide open throttle (WOT). The HEI vacuum advance
canister should remain disconnected. This engine is designed to operate using only the internal centrifugal advance to achieve
the correct timing curve."

To add another opinion PO added MSD 8572 tach drive distributor with vacuum can. MSD instructions recommend ported vacuum.

So my ZZ4 with LT4 cam pulls 12" Hg at idle and B1 vacuum can ( may not be the original MSD can) is hooked up to ported vacuum on carb. Not good as B1 needs 16" Hg.

But I wanted to understand the benefits of manifold vs ported vacuum.

I'd like to try a vacuum can that maxes out at 8-10" Hg on full manifold vacuum and easy enough to eliminate vacuum advance totally as GM recommends.

Randy

tbarb

According to those instructions you are not to use any vacuum advance period. I would ask a GM engineer the reason for that instruction. If you want to try manifold vacuum advance do it and see how the engine runs but I would still pose that question to the right person.

It's also noteworthy the the WOT advance is 32*@4000rpm, and it may have something to do with the cylinder heads. You would never want to void any warranty.

Dar53

I was setting up a new carburetor on a Jeep with a wide band O2 sensor. I set the initial timing to specification and connected the distributor vacuum advance to the carburetor ported vacuum port. I order to get the to the desired idle RPM i had to open the carburetor throttle plates so far i exposed the carburetor main system. No matter how i adjusted the idle mixture screws and throttle plate i could not get the desired idle RPM and air fuel. I then connected the distributor to the carburetor manifold vacuum port. this enabled me to close the throttle plate to go back on the carburetor idle system and get the desired idle RPM and air fuel.

WhiteC1

I've posted this article in other forums over the years. I'm sorry, but I don't have the author's name to give him credit. He worked at GM during the development of the vacuum advance system, and his explanation is thorough and understandable. It's a bit of a long read, but I think you'll find it quite interesting.

Regards,
John

cardo0

I used to be a manifold vacuum only advocate but have read from authors like Cliff Ruggles that ported vacuum can be used successfully with large duration/overlap cans to help the leaner mixture. But I imagine it will take a fair amount of testing to optimize using ported vacuum and the amount of initial advance should play a major role as the more initial advance used the less vacuum advance is needed until RPM increases and overlap effects are reduced.

My suggestion is don't make it more complicated than it is and try both while documenting each results just one step at a time. Don't make 2 adjustments and expect to produce sensible results. Meaning don't change timing advance and RPM at the same time and try to record good information.

Good luck and give us some feedback on what worked for you.

JohnZ

That author be me.

Critter1

Thanks John. Good reading.

66RBS

Update. I was using the full manifold vacuum fitting at back of intake manifold. My mechanic is a fan of ported vacuum. He took some measurements off the "timed spark vacuum" fitting on the Holley Street Avenger carb and says it is not a true ported vacuum fitting. He moved the vacuum line to the "full manifold vacuum" fitting at the base of the carb rather than the fitting on the manifold. He says the carb fitting responds better. If I understand what he is saying he wants a manifold vacuum source that starts to drop vacuum as you get into the throttle thus reducing vacuum adance. Is there a difference between to the vacuum sources?

tbarb

No difference, if you want the vacuum advance control to retard quicker either select a different calibrated control or custom taylor the existing vacuum control to start moving at a higher vacuum level and extend it's travel on the other end so it provides the 10-15* advance.

The only way I can imagine the timed vacuum port not working correctly is if the throttle blades are opened to far at idle speed exposing vacuum at that port.