Can I run 1 line from the manifold vac line out of my carb and tee it to both the vac advance and the trans? Or do I need to take 1 off a manifold port? 455 motor with edelbrock manifold and carb no emmision junk.

 

You dont use manifold vacuum for vacuum advance.

 

Some people do...

- Eric

 

True. I guess it depends on how radical the engine is. I was assuming it was a mild build.

 

To answer the original question, yes, you can simply use a tee, assuming you want to run the vac advance from manifold vacuum, not ported vacuum.

 

What is the purpose of using vac advance if you are using manifold instead of ported vacuum? I normally use ported vacuum or nothing at all and just rely on the mechanical advance in a higher perforance or high rpm engine.

 

Do we really want to go here again?



- Eric

 

I've tried it both ways after a long discussion with Joe. The above idle driving I found no difference. However, the idle quality was better with it connected to ported.

 

Just to recap, manifold and ported vacuum are identical once the throttle blades open enough to uncover the port. With the throttle closed, ported vacuum is zero. That's the only difference. As noted, try both and use what runs best on your car. You will likely need to change initial timing between the two to optimize.

As for why use one or the other, emissions considerations aside, the only thing that matters to the engine is advance degrees vs. load vs. RPM. The engine neither knows nor cares if this advance comes from initial, vacuum, mechanical, or electronic sources. With manifold vacuum you can run less initial timing, resulting in less total timing, but the manifold vacuum smooths the idle. Ported will require more initial timing to make up for the lack of vac advance but you may need to limit the mechanical advance so as not to run too much total. How much you need where will be governed by the engine's setup. This is definitely one place where an all-electronic system has an advantage - you can dial in the advance curve as needed.

 

So I take it that its a sore subject that has been hashed out a few times??? I missed all of them. I was asking only because I really wanted to know the benefit. I see NO benefit at all. I see Joes' post in his first paragraph and he is right. But, I guess before I express my opinion I'll read that "other" thread and keep my opinion to myself.

 

Good explanation there Joe. I either use ported vac or no vac advance at all. As you said, the timing doesnt care where it gets its adavance from, so if its lopey low vac motor, then I just recurve and reweight the mechanical advance and call it good enough as long as I am getting the full advance I want. And you're right, it all depends on the engine set up. Its purpose, if stock OEM, mild cam or biggo cam.

 

Perhaps because ignition timing is poorly understood, this subject tends to inspire spirited discussions, vile insinuations, and threats of violence, much like talk of putting LS engines in older cars does.

- Eric

 

Exactly. This is why hi perf motors typically used mechanical only back in the day. Vacuum readings were erratic and would have caused the advance to be irregular. If your engine can tolerate vacuum advance, it will usually improve mileage at part throttle. This is rarely a concern for a maximum-output motor.

 

You're right about part throttle cuz at full throttle you usually won't have any vacuum. It just seems to me using ported vac allows a benefit once you have your initial timing set at idle. The instant you take off from a stop at light or part throttle you have vac advance and its beneficial. And when you shut your engine down while idling, you are much less likely to have run on caused by advanced timing.
Where as with vac advance to manifold vacuum, when you take off from a stop while idling, the instant you give throttle to take off, initially you'll have a drop in vacuum, therefore retarding the timing from the initial idle timing. I don't know. Guess there are many opinions about it, I just thought it was a pretty straight forward process. Sure didn't mean to ruffle any feathers, but I'll continue to use ported vac for stock or mild engines, and no vac advance with modified recurved mechanical advance on hi perf engines.

 

That's actually a very good observation. Using ported vac to kill the advance at closed throttle would help prevent dieseling when shutting off. Of course, if you advance the initial timing to make up for the lack of vacuum at idle, then all bets are off.