I am redoing the vaccum hoses on my carbureated (Carter AFB in place of Holley 4180) 460 CID.

According to the vacuum hose routing diagram under the hood - a vacuum line starts at an intake manifold port - then goes to a "Y" connector. One side of the Y has a hose connecting it to the lower port of a 3 port PVS sensor mounted on the thermostat housing. The other side of the wye connector has a hose that has a plastic vacuum restrictor orfice in it and connects to the top port on the PVS. The hose on the middle port of the PVS goes to the distributor vacuum advance.

I'm confused as to why the vacuum advance is setup this way.

I have set the static ignition timing (no vacuum applied to distributor) to 10 degrees BTC at idle. When I reconnect the vacuum line to the distributor - it advances my timing to more than 20 degrees BTC at idle.

From what I've read - the reason behind this distributor vacuum advance setup has to do with engine overheating. As long as the engine temp is below 235 degrees F - the vacuum supplied to the distributor advance is from the intake manifold thru the restrictor orfice to the distributor. The restrictor acts as sort of a time delay to buffer the actual vacuum signal at the distributor advance diaphram.

If the engine begins to overheat (temp goes above 235F) - the PVS switches and bypasses the restrictor orfice and supplies full intake manifold vacuum (no restrictor orfice) to the distributor. The reason given is that by supplying full manifold vacuum it will further advance the ignition timing and cool down the engine???

My biggest concern is that at idle with the vacuum line connected to the distributor I am running above 20 degrees BTC ignition timing.

Can anyone shed more light on this factory vacuum setup ? I'm pretty sure I'm reading the vacuum hose routing diagram correctly.

 

Are you in an area that requires you to run the vacum lines to factory specs? If not just route the vacum advance to a ported connection on the carb. Makes like so much easier. I honestly couldn't answer your question for the reason why the factory is routed the way it is.

 

Mark:
I don't need to keep vacuum lines setup per the routing diagram under the hood - and I doubt any of the stations where I have my vehicle inspected would bother checking. I will take your suggestion and connect the vacuum advance line to the ported connection on the carb. I already am using this ported connection for my EGR valve - so I'll put a tee connector in it and run a tap over to the distributor.

I am still curious as to why Ford setup the vacuum advance to be run from intake manifold vacuum rather than a ported connection on carb??

Thanks for your comments

Walt

 

l just went through the same drill.Ford set up the timing(cam and ignition)and the vacuum lines to minimize emissions,plain and simple.l set my timing at idle to 18 degrees btdc(l`m at 7000 feet) and rerouted the dist.to a vacuum source on the carb that had no vacuum at idle and upon throttle showed vacuum to allow 38 degrees total advance.The truck nows runs great.Keep in mind at altitude we run much higher advance.Look for a ported vacuum source that is inactive at idle/shows vacuum on throttle and port the dist to that-set timing at idle to 12 btdc,measure max advance with a timing light to show ,say, 34 degrees btdc at 2000+ rpm and you`re there,it should run great like that check the timing specs for your engine of course but they`re usually retarded for emissions and can be advanced a bit safely for better performance

 

I've been doing some studying on this issue myself. The reason they hooked it up that way is that when the temperature is high, when you open the throttle wide open, manifold vacuum goes down, which causes vacuum to the distributer vacuum advance to give no extra advance. This happens when you have a heavy load on the engine. When the temperature is below 235, through the restrictor, the restrictor causes the vacuum in the distributor vacuum advance to bleed down slowely, so it gives partial vacuum advance even if you floor it, unless you're doing so for a long time.

To make a long story short, it's to lessen the chance of detonation due to advanced timing under load and high temperature.

 

More advance increases the idle speed.
The water pump turns faster and the fan pulls more air through the radiator if you are idling in traffic or lugging the engine.

 

This is a way old thread, so just to kick up the old arguement I'm going to say that all street engines run better if tuned to run the vacuum advance off manifold vacuum instead of ported.

 

So your theory is that 20° to 30° timing at idle is a good thing. Which planet were you born on and what kind of physics to you use? Here on Earth we use minimal initial advance because the engine will idle better and smoother. The when throttle is applied the mechanical advance advances timing just like it did before the advent of vacuum advance. Then when the engine is done accelerating and the vacuum increases you get an appropriate amount of vacuum from the carbs ported vacuum port. Vacuum advance was created as a mileage increaser and not a way for poor engine designs to run better by over advancing the timing at idle.

Using manifold vacuum for controlling timing came about when the engineers were looking for a way to meet emission requirements, otherwise you would never see vacuum advance connected to the manifold.

Where the hell do you guys get this crap?

 

Hey Teddy Bear this brings back memories, seems we've done this before.

Actually YES "20° to 30° timing at idle is a good thing", try a little experiment and tune just initial timing for highest idle speed and vacuum with the same carb settings. You will find that you get the highest vacuum and rpm at the same point and that it's somewhere between 20° to 30°. This alone proves my point, the engine will run smoother at idle, use less fuel when you dial back the carb again for idle speed and have better throttle response.

Minimal initial advance is exactly the wrong thing to do, don't suppose you've ever noticed how few factory carbs have ported vacuum. You really only see it on Edelbrock and Holley carbs. And even when the factory has a carb with ported vacuum how rarely it is used.

The best initial timing is a function of what is appropriate for a bogged engine, low rpm full throttle, 8-16 deg. Then a rpm mechanical advance for high rpm full throttle, generally 34-38 deg. And vacuum advance settings to advance during hi vacuum situations of idle and cruise, idle in mid 20's and cruise in high 40's of advance.

"Vacuum advance was created as a mileage increaser" not really sure if that was why they were first created, but YES again. More mileage also means a more efficient engine, and a more efficient engine is also a more powerful engine.

"Where the hell do you guys get this crap?" I get it from years of tuning engines for a living.

 

I have more years experience doing this in everything from street engines to race motors and if your moronic theory is right why don't any of the factory's do it. Because it doesn't work and just covers up bad cam, heads and carb choices.

Then if vacuum advance wasn't created as a mileage enhancer then why did it exist back in the 50's, long before emissions or the real horsepower race started. Also vacuum advance does not add any power what so ever since it only works when the engine is loafing along not make any real power. Where did you go to basic engine school? Because they didn't know crap about how engines work or what vacuum advance is and does.

You are still trying to sell "crap" and it won't fly in the face of facts.

 

Quote:

Originally Posted by Bear 45/70

I have more years experience doing this in everything from street engines to race motors and if your moronic theory is right why don't any of the factory's do it. ....

I learned a looong time ago that there are a lot of morons in the auto world that have been doing the wrong thing for a long time, your age doesn't make you right just wrong for longer.

Like I mentioned in my previous post and the post that started this thread, many Fords and every Chevy with a quadrajet were set up this way from the factory.

Quote:

Originally Posted by Bear 45/70

Then if vacuum advance wasn't created as a mileage enhancer then why did it exist back in the 50's, long before emissions or the real horsepower race started. Also vacuum advance does not add any power what so ever since it only works when the engine is loafing along not make any real power.

Your the one that said it was about mileage, did you forget already or are you just arguing with yourself again?

It does add power, it allows you to make more power with less fuel during all but full throttle, that's why the idle speed goes up, making it more efficient.

Quote:

Originally Posted by Bear 45/70

Where did you go to basic engine school? Because they didn't know crap about how engines work or what vacuum advance is and does.

Well I went to basic engine school at the UTI, and your right they didn't know crap. But my real knowledge came largely from a man named Jim Green and many others, and a lot of experience, reading, and engineers.

OK Mr. grumpy know-it-all, just what is it you think "vacuum advance is and does"?

Why is it you think mileage and power contradict each other? They complement each other. It's emissions and power that contradict each other, most of all limiting NOx.

 

Morons don't win races or championships or set speed records. How many of those things have you done personally? I will wager good money I have forgotten more about engines and hi-preformance than you have ever known, mainly because I not only did it for myself but was paid to do it for other.

 

Ahhh I'm get'n kinda bored with this, we've done this before and you refuse to learn or think so I really don't care about convincing you, just kinda having some fun with the argument. If someone else pops in I'll help them learn.

But FWIW no I haven't won any official race championships, I don't do any official racing just some street racing at times. I have spent time tuning for others at the track and they won but I have no interest in it. I'm interested in creating the best, most useful vehicle for myself and my customers.

And in one thing you are kinda right, for pure racing vacuum advance has no effect on total power, if all you need is WOT power then it doesn't matter at all. But most of us want it to drive well and get more then 8MPG, this is a TRUCK forum after all.

 

I need to learn? I was factory certified mechanic for both 2 and 4 strokes plus I've won my races and National Championships and set my speed records, but damn according to up you I don't know *****. Yeah that must be it I'm the one that needs to learn, yep that must be it.

It is true you can't teach those who refuse to learn and idiots.

It appears that it is too true.

"Never argue with an idiot. He will just drag you down to his level and then beat you with experience!"

 

If I remember correctly Bear, you had never heard of aligning thrust bearings either... something I learned on my FIRST bearing job.

And I''l take that wager about you forgetting more about engines...etc.