77 zz4

Does this cause higher emissions? Less driveability?

hensen1954

Lose gas milage, etc.

BarryK

car doesn't perform as well, runs hotter, less mileage, less responsive.

ir can be done and many people have done it but it's not a good idea for a street car.

I actually gave up taking my car to mechanics and started to learn how to work on my own Corvettes because everytime I took my '65 to a mechanic (to many different ones) for work, tune-ups, etc, they always ending up plugging the vacuum line to fix my timing issues (which was simply a matter of the distributor needing to be recurved). Anyone who runs a street car without the vacuum advance hooked up or a mechanic who plugs it as a "fix" simply does not understand what the vacuum system is for and how it works.

Before some people flame me for this notice I said STREET cars...... race cars such as drag cars don't need the vacuum advance system because they are always at WOT and the vacuum advance system isn't viable for such use. When the motor is under heavy load such as WOT or accelerating the vacuum drops and it's not supplying any advance to the ignition and race cars pretty much spend all their useful time in that condition. Street cars need the vacuum advance system though to run their best.

TopGunn

I have a ZZ4 like you and am in the middle of recurving my distributer for better performance. It's actually pretty interesting stuff to me, you can really make a science project out of it!

You need the vacuum advance for best performance at low throttle settings for the street as Barry said. If you were running on a circle track or a drag strip there wouldn't be a need for it because the mechanical advance would be doing the job.
GM does the best they can to set up a dist for normal use but you can set one up with the recurve kit to give you a better throttle response and still be economical to drive.
What the other guys said about gas economy is true, without it you'll probably start fouling plugs pretty quickly driving around town.

Bill

Bob Onit

I have one quick question.. Im not trying to hijack this thread
Please explain this for me (this seems so simple Im almost embarassed to ask)
Am I correct in assuming that, at an idle there is NO vacuum going to the advance diaphram?

Then at higher rpm the ported vacuum advances the diaphram which in turn advances the timing?

Thanks

BlueL36

There's plenty to read on vacuum advance, manifold vacuum vs. ported vacuum, vacuum cans, distributor recurve, etc. An advanced Forum search in current and archived threads in both the C3 and C1/C2 sections will give you a wealth of information. (The C1/C2 sections deal with a lot of pre-emission performance engines.) Or do separate searches for current and archived posts by Lars and SWCDuke. Both have made excellent posts on this subject matter.

norvalwilhelm

No at idle the vacuum advance has a strong reading. If you set the timing you disconnect the vacuum line, set the distributor advance and then when you reconnect the vacuum line the advance takes a significant advance. I too believe strongly in a vacuum adance system for the street.

Bob Onit

Thank you
This is totally contradictory as to what the instructions for my Holley have to say...

norvalwilhelm If you get a chance please visit the thread I started about this problem http://forums.corvetteforum.com/show....php?t=1414729

Sorry again 77 zz4.. I will not post on your thread again

BarryK

at idle the vacuum is the highest so it's giving the most advance.
As the motor becomes under load the vacuum decreases so the vacuum advance decreases.
it's not too hard to understand if you think about the reason BEHIND the use.
at idle or easy cruising the engine is working very easily without much of a load on it and the air/fuel mixture is lean. A lean mixture burns slowly during it's combustion so you need more advance to help burn it.
During acceleration the air/fuel mixture is rich. A rich mixture burns more quickly so you require less spark to burn it and under load the motor produced little vacuum so the vacuum advance is effectively off retarding the timing.

I have a couple great tech papers on timing and vacuum advance on my website tech articles page if you are interested in reading them.

http://69.253.166.197/page1/page65/page65.html

one is called: Timing101Article.pdf by John Hinckley
the other is: Vacuum_Explained.pdf by Lars

I'd than follow up these two papers by reading two more:

MappingAdvance.pdf by John Hinckley
Lars Timing.pdf by Lars

BarryK

Bob
I read that thread.
realize that most cars after approx 1970-71 or so switched the vacuum line on the carb to a "timed" or "ported" vacuum port rather than a full manifold port.
A full manifold vacuum port will give you full vacuum at idle but a timed port will not. This was done solely for emmisions reasons.
Your car will usually run better, perform better, and run cooler if you switch your vacuum line to a full manifold vacuum port.
If your carb doesn't have a full vacuum port you can pull vacuum directly off the intake manifold itself. if you don't have a free port to connect to on the intake you can drill one and install a connector. The connectors are commonly available at either local speed shops or places like Paragon, Dr. Rebuild, etc.

77 zz4

Ok, I should run with the advance but I have a stock HEI unit. I'm told that it (the vacuum pod) isn't enough. Should I then just intall a recurve kit and be done? Or should I ditch the whole thing and go with a new msd distributor?

BarryK

Nothing wrong with the stock HEI unit. when set up correctly and working properly they are fine distributors.

WHO told you your vacuum advance isn't enough and what was their reasoning?

With the vacuum line disconnected from the vacuum can and plugged, what is your initial timing and than what is your total timing?
Once your vacuum can is reconnected, what is your new timing numbers at initial and total with the vacuum can?

Read the paper on my site called MappingAdvance.pdf and it will take you step by step on learning what your distributor is actually giving you on the timing and the advance.
Only after you know these numbers can you really determine if your vacuum can is correct or not.

The simple method, although not as detailed and accurate is to see what your total timing is with the vacuum can disconnected, than see what your timing is for Total timing PLUS vacuum advance.
You should be at about 36* total timing, and around but not exceeding 52* timing with the vacuum can reconnected.

Ironcross

Most if not all high performance engines do not run a vacuum advance including the L88`s and early FI Vettes. No overheating, no plug fouling, and good gas milage with around 36+- total degrees total advance. So obviously some of the indicated problems with out the vacuum can hooked up are incorrect.

Watch what happens now!

bobs77vet

you should probalby switch vacuum cannisters to one that pulls full vacuum at a lower vacuum reading they cost about $14 from NAPA

BarryK

Walley, I can't answer about the early FI motors, but on the super HI-PO motors like the L88, they are basically nothing but race motors and never REALLY intended for street use. Since race motors spend most of their time accellerating or at WOT having a vacuum advance would be pretty worthless as the motor wouldn't be utilizing it anyway.
I am not even going to pretend to have the slightest experience with race motors and I know you have decades of vast experience with them but i'd guess that probably very, very few, if any, run a vacuum advance.
It's my understanding that the vacuum advance system is really only beneficial for a street driven car.

MotorHead

I happen to agree with Ironcross on this one.

I get over 20MPH on the highway, it doesn't overheat, it just runs good, real good

I never run a vacuum can and the main reason I don't is because I am at 11:1 CR. At that compression ratio I don't need a vacuum can adding any timing at part throttle. The vacuum can put your total timing over 55degrees and cause detonation

Just my opinion and we all know what opinions are like......

77 zz4

My initial is 12* and 36* total. I currently have the can capped. I'm leaning towards trying a new vacuum can as I'm interested in reducing the hotrod odor as much as possible. I'm for it even it slightly helps in that regard. Would it help with the smell?

bondoboy

I don't get where some of you say you can hook an advance to full vacuum. If you did this, you would have full advance at idle, and less advance when you speed up. When the advance is connected to a timed source, it advances when you reach a certain rpm. The only way a full vacuum advance would work is if the cannister were pulling from the other side of the dist. shaft. Chevy uses a clockwise distributor. When the vacuum advance starts working, it turns the point plate or pickup plate on hei counterclockwise. This makes the points open or the pickup to work earlier, which is spark advance. If you would have a full vacuum on a distributor like this you would be advancing fully at idle, and less as the rpms increase. This would actually hurt performance. I have installed rochesters and holleys both 2 and 4 barrel on various years of vehicles and they used a timed vacuum for advance. I asked people at work who have worked on old cars and they agreed. Unless the advance pulls clockwise or there is some other reason, full vacuum doesn't work.

77 zz4

We are still talking about hooking it up to the carb, right? The manifold vacuum nipple on my demon carb is currently capped. The ported nipple is hooked up to the trans (auto).

BarryK

full vacuum advance does work and it works better than a ported source.
The only reason the switched from full manifold vacuum to ported vacuum was for emisson purposes, period. The side affect was less throttle response, less fuel economy, and motors typically run hotter.

Vacuum advance is SUPPOSE to advance the timing at idle and decrease it under engine load, that the whole reason behind it and that's how full manifold vacuum advance works.

Ported vacuum advance basically works the same way except at idle there is no advance until the throttle blades are opened and than it works pretty much the same way - it is at it's highest levelto start and decreases as motor load increases.

it doesn't pull from a different side of the distributor at all, it activates and controls the advance on the distributor the same way mechanically, the only difference is when the vacuum advance comes in.

On full manifold vacuum you get full advance at idle and the advance decreases under increasing engine load such as accelleration and WOT which is when the fuel/air mixture is richer and you don't need as much advance. On Ported vacuum you get no vacuum advance at idle (which was desired at the time for emmisions reasons and went along with the use of the A.I R injector system) but worked the same at cruise speeds meaning under light cruising when the motor had higher vacuum levels it advanced the timing but under heavier engine loads it decreases or retarded the timing because of lower vacuum levels.

Read John Hinckley's and Lars papers on this. I have both on my website tech articles page here: http://69.253.166.197/page1/page65/page65.html

the two articles you should read to learn and understand about vacuum advancem how it works, the reason for it, and the differences between full manifold and ported vacuum advance are:
Timing101article.pdf
Vacuum_Explained.pdf

the cards you installed that used a ported vacuum port were later model carbs after the change was made for emission purposes. Older carbs always had a full manifold vacuum source.
Your "people at work" that worked on old cars that agreed with you don't know what they are talking about but that's not surprising since many, many people including most mechanics nowadays don't have a clue or proper understanding of the vacuum advance system. The fact that agreed with you that ported vacuum was better and that "full manifold vacuum can't work" shows they don't understand the system and how it operates