MakoShark72

Its the port just behind the carb. My real question is.... can I run my distributor vacuum advance off that port as well?? Or is there a better vacuum source? Maybe I should install a dedicated port elsewhere on the manifold?? If so, where? Its a ZZ4 engine...

Thanks!

BigBlockTank

I don't know if I have a clear understanding of what you're asking, but here goes. Vacuum advance should not come from the port at the rear of the carb, that should be the PCV valve hook up spot. Most carbs have a timed port on the passenger's side of the carb, or at least the ones I've dealt with have. The manifold vacuum that's in the intake manifold is usually for brakes, vacuum canister, etc.

DRIVESHAFT

You could run your advance off of that port and it would probably work fine, but usually there is a more appropriate port on the carb.
Unless you have a race type Holley carb on there, a vacuum port is probably available on the carb. Look at the front area of the carb.

BigBlockTank

of course I would, I already said it

DRIVESHAFT

I realized I was adding very little as I was typing, but by then I was already going. Oh well, hes has a second opinion now.

BigBlockTank

Unlike some on this forum, I was just yankin yer chain, not pushin ya off a cliff.

MakoShark72

Tank,
Sorry for the confusion. I was referring to the vacuum port ON THE MANIFOLD where the headlights/wiper system gets its vacuum. My question refers to the timing post by BarryK (beginning of his forum) where he recommends NOT using the vacuum port ON THE CARB, but switching to a "full manifold vacuum source" instead. He says the carb vacuum port was added in later years for emissions, and that its better to go with a port on the manifold instead...ALSO, Lars advised me to NOT use the CARB port, but rather use full manifold vacuum instead. So my question was should I used the mentioned port ON THE MANIFOLD? Does it qualify as "full manifold vacuum"?


Driveshaft,

I am running a Holley 670 Street Avanger and it does have a port designated for vacuum advance. See my response to Tank above for advice from Lars and BarryK. I originally had the vacuum advance hooked to that port, but Lars said better to go with a manifold vacuum source. So I'm looking for an appropriate "manifold" vacuum source.

Thanks guys!

DRIVESHAFT

If its the street avenger, you should have a manifold vac port on the front of the carb base plate, under the fuel bowl.

Look here http://www.holley.com/data/Products/...99R10219-3.pdf

BigBlockTank

I obviously don't have the knowledge that those two gentlemen do. I've had ported vacuum hooked to my vacuum advance for about 36 years now, that's all I know to do. My question is, why run a vacuum advance at all? As soon as you start the engine, with a full vacuum port, the advance goes straight to maximum, or is that wrong too? I'll keep doing what I'm doing, it has worked for soooo long for me, and I'm not downgrading BarryK or Lars, they have tons of knowledge as well. I just know what I've used and how it works for me. I've had engines that were alot happier with the vacuum advance plugged. The engine ran faster, cleaner, better response, etc.

Put your vacuum hose where they told ya and see how it works!! Doesn't cost alot, well, let's hope anyway

MakoShark72

Yea I see the port for the manifold vacuum, next to the PVC port. I currently have that manifold port running to the (emissions??) canister right below the vacuum tank in the drivers side fender. I'll do a little research about that.

'75

If there is another vacuum port on the carb, just check it to see if it has vacuum at idle, if it does then it is considered "manifold vacuum". If it does not have vacuum at idle but does just above idle then it is considered " ported vacuum".

BarryK

MakoShark,
yes, you CAN "T" off that manifold port to run your vacuum advance but you shouldn't have to. As mentioned already, most carbs have various vacuum ports on them including both ported vacuum and full manifold vacuum. If you don't know what ports give you want type of vacuum it's easy enough to find out. If you don't have a vacuum gauge use your finger - AT IDLE if you put your finger over a vacuum port on the carb and feel it sucking your finger than it's a full manifold vacuum port, if you don't feel the vacuum on your finger until after you raise the motor RPM above idle than it's a ported vacuum source.
You can use pretty much any full manifold vacuum connection you find to run the adv can on your distributor but I recommend against "T'ing" one that is used for the power brake booster or the PCV system.

BigBlockTank,
the Ported vacuum connection on your car to go to the vacuum advance can was the way it came from the factory. They all used to be connected instead to the full manifold vacuum and the ONLY reason they moved it to ported vacuum was for emissions reasons, period.
If your car runs finne there is no reason that you HAVE to move it to full manifold vacuum but there are advantages to it if you chose to do so:
1. better idle characteristics
2. better throttle response
3. lower operating temps
4. better fuel economy
You asked WHY run a vacuum advance at all....and than I assume you meant why run a full manifold vacuum advance since you stated correctly that at idle the vacuum advace goes to full. The reason is that typically, up to a point, the more timing you can run on your motor the happier it is. Also, the key to understanding the reasons behind the entire vacuum advance system is simple, motor load. The timing needs of a motor constantly changes based on two things, motor RPM and motor load. The mechanical advance in the distributor controls timing advance soley based on motor RPM but does nothing based on load. The vacuum advance system adjusts spark timing based on motor load. Both the mechanical advance ad the vacuum advance are two completely seperate systems but they work together to achieve correct spark timing for the motor.
Can a motor operate without the vacuum advance system? sure it can and all race cars do that since they typically only run at either idle or WOT but for a street driven car with it varied and ever changing driving conditions the vacuum advance system has many benefits as i've already listed.
Full manifold vacuum gives the motor full vacuum advance at idle and the higher advanced timing helps keep the motor running cooler, better throttle response, etc. As you drive the car and the load varies so does the vacuum and therefore the amount of advance the system adds in. Under heavy loads such as WOT, or the motor is lugging at slow speed in a high gear and you push on the gas the motor loses virtually all of it's vacuum and will give no vacuum advance to the timing.
The ported vacuum set-up works the same way except that at idle there is no vacuum available so the spark timing is actually retarded (well, not really retarded from the initial timing but there is no additional advance over the initial mechanical timing amount). As soon as you increase rpm speed and open the throttle plates it send the vacuum thru the port and the sytem works the same now as full manifold vacuum. The only real difference is at idle. This is the reason they brought it in this way for emissions reasons, they actually needed a much retarded timing at idle to help increase operating temps to lower emissions levels. It worked but it left performance on the table.

BigBlockTank

BarryK, that was a good vacuum advance 101, I'm sure someone will read it and learn something from it. I already understand why it was moved, what it was moved from, and when. I just made a statement that you and Lars have some different info from me. I use whatever the motor is happy with, not a "text book" answer. I run a MSD distributor, 6AL box and have it tuned for my motor. It has very good response, idle characteristics, and pretty good fuel economy. It runs at 195* in Tucson, AZ at 108* outside temperature. My around town mileage is 12.7, with about 60% cruising on a tank of gas. I don't think that's too bad for a 71 454/365.

Don't think that I'm trying to knock any of your info, I've seen your posts, and you have skills. If you put 5 tuners together, you'll get 6 good answers for the same question. You hopefully get my drift. I've been doing this for awhile too. But only as a hobby and crew chief for an 8 sec. alcohol dragster. And, with any luck, I learn something everyday!!

BarryK

like I said, if you are happy the way your car runs than great

And if you are crew chief for a drag car than I see where your "no vacuum advance" comment came from!

Even Lars says that while full manifold vacuum is typically better and the preferred method for some reason some cars do tend to run a little better off ported manifold vacuum for one reason or another (probably a combination of various things that makes that happen) so try what works best.

MakoShark72

Thanks guys..I'll get 'er figured out!

68/70Vette

I have a ZZ4 also and currently the distributor vacuum advance is connected to the carb ported vacuum outlet. I have a timing problem currently. It's not so bad in that the engine runs fine during normal street driving. The engine stumbles and appears to be backfiring occassionally through the carb during a full throttle acceleration. In fact the car will accelerate better at perhaps 1/2 throttle rather than WOT.

I'm planning to switch to connect the vacuum advance to full manifold vacuum. The ZZ4's are "pre-emmissions" engines as far as I know. (Even though they've been on the market for just a few years.) Yes, it's time to have a session with the timing light. I think I remember a Lars recommendation to use full manifold vacuum.

Mike Ward

The vacuum can has to match the characteristics of the engine, compression, cam, vacuum at idle etc. etc. Unless everything is coordinated from a systematic approach, it's possible to make it run worse rather than better.

BarryK

Mike, I agree with you 100%.

MakoShark72

The ZZ4 comes with an HEI NON tach drive distributor. I bought an HEI tach drive distributor, and swapped the "ZZ4" vacuum advance can to the tach drive unit. They looked identical and there were NO markings on either vacuum advance can. I have only started my engine, havent taken it out on the road to do the "break in" yet. It idles fine, and I set it at 12 degrees base timing. Centrifugal advance only gave me about 18 more degrees, and vacuum another 18 degrees. I switched to black (from silver) distributor springs to get more centrifugal advance and got about 34 degrees at about 3000 rpm. I also (per Lars) swapped the secondary jets (68) to the primary, and installed 76's in the secondary...Holley SA 670..Now hopefully just fine tuning left..

BarryK

all vacuum advance units look the same, that doesn't make them equal although if that's the can that the builder choose for the motor it's probably the correct one (but you never know).
Easy way to check is to measure the vacuum the motor produces at idle than with something like a MityVac and vacuum gauge you can check to see when the vacuum adv can comes in.

18º centrifugal advance for a HEI is typical. If you set total timing (vac adv disconnected and plugged) for 36º all in between 2500-3000rpm that will bring your initial timing in at 18º. That's a bit high for a stock motor but probably OK for the ZZ4.
BTw, changing the springs doesn't give you more centrifugal advance, the springs only determine how quickly the advance comes in. To change the actual amount of centrifugal advance you would need to change to a different OD sized stop pin bushing or modify the advance slot itself.
If you get the timing between 34-36º all in by 3000rpm though you should be good than reconnect the vac adv can