Technical Vacuum Advance Poll

Another advantage of ported is one gets to engineer some cute throttle solenoids ‘cause that big crippled V8 ain’t got enough power to run a/c compressor or barely run the alternator at idle especially w/torque converter dragging it down.

LOL

 

You need to develop some "left foot braking" skills, for your over-cammed, low compression, low stall combo
Then Bull***t everybody that you practiced this in "Indy cars"

 

Don’t put that ^^^^ on me !

 

GENERALLY, a street motor that has some of those high power gee gaws i.e. a hot cam, will pull very idle manifold vacuum or otherwise have difficulty a with smooth idle, running a direct manifold connection with lots of ignition timing advance will improve street manners considerably. This is why it is often preferred. But what is preferable for a hi-performance engine ain't necessarily a good idea for a grocery getter.

They do run a lot cooler at idle.. This is also a huge no-no for smog considerations. That's why everybody had to detune their iron in the bad old 70s. NOX emissions. This made for poor performing hot running stinky gas hogs.

Vacuum advance doesn't "improve" fuel economy, that isn't really the way to look at it if you stop and think about it. A street engine without vacuum advance is simply an engine with a retarded ignition timing curve, which wastes 10 to 15% fuel on the highway without any performance henefit. Drag racing engines don't include it because they don't do stop and go, cruise, or part throttle, so it's simply another component to potentially fail.

The reason for vacuum advance to begin with is simply because under flat ground steady cruise operation there was no way to achieve the correct ignition timing advance using centrifugal weights and springs alone. Any OHV engine easily runs 50°+ BTDC on the highway.

 

I going to have to disagree about the part about the vacuum advance doesn’t help fuel mileage. But agree with your last statement, that you need both to achieve both power and economy.
Those two statements seem to contradict each other, in your post.








Bones

 

Way too easy to understand !

Sometimes I cruise throwing dollar bills out the window every few 100’. This way everyone knows I gots da
timed port vacuum !

LOL

 

Read it again. A defective ignition timing curve results in a loss of fuel efficiency or power. So in a sense vacuum advance doesn't "improve" anything, it's simply that a defective ignition system will obviously reduce power and efficiency regardless of the mechanism itself. It's all done by crank sensors and computer now, but the principle has not changed.

 

I guess it’s just a difference in terminology.... maybe. But for optimum power and fuel mileage you need both centrifugal advance, based on rpm and an advance system based on load. Neither can accomplish the complete job by their selves.
Back in the previous century ,a ways , I had a 289/271 HiPo four speed, with only a centrifugal advance disturber. Didn't make any difference what speed you drove it, 11 miles to the gallon. We had another 289 two barrel automatic with vacuum advance, manifold iirc, 20 mpg! Both cars ran great, smooth as silk. While the two engines were somewhat different, they were both 289 cubic inches, but one got twice the fuel mileage. I always figured part of my poor fuel mileage on the HiPo was due to no vacuum advance.






Bones

 

I do agree on modern engines and the timing. That is one of the reasons modern engines run good, get good fuel mileage and run 300,000 miles. The modern engines control the timing so precise that the had to move the timing from the cam to the crank to better control it. Also instead of rpm and load influencing the timing there are probably six or more factors determining where the spark should occur and how long and how many! Our old systems can’t do this! And the timing can change in a milli second to be optimized for the conditions at that time.








Bones

 

At idle the air/fuel mixture is lean, has a slower rate of burn, the additional timing gives the combustion process more time to complete. As you open the throttle additional fuel is added and the mixture richens up, which burns faster and requires less time to complete. This is pretty basic information available in a lot of places on the net.

 

IME, if you dial in enough advance at idle without vacuum advance, you're going to have too much total advance. Vacuum advance at idle allows for more proper timing at idle and without causing too much total advance.

 

Proper fitting
for timed
vacuum port !

LOL

 

And also the probability that you stuck your foot into the throttle a whole lot more!

 

I still have it!








Bones

 

If HiPo car low geared,
Might be in secondaries
at hiway speeds.....only
so much VA can do !

 

Yes, I know there were other factors involved in the fuel mileage of my car. Kinda like Blues said, my foot being a big factor! But occasionally when I was on a road trip, I would drive about 60, trying to squeeze some miles out of the tank..... still 11 mpg! So I gave up and drove it like I stole it!






Bones

 

I'll add some additional comments....

As has been noted, modern electronically-controlled ignitions don't have these issues with their multiple sensors and millisecond reaction times. My OT DD has 11.5 to 1 compression and runs great on premium pump gas and could be run on regular with a note in the owners manual that states 'total HP will be reduced from 420 to 402', a 18 HP loss. BFD! Our cruder purely-mechanical ignition advance systems have no chance of duplicating that feat. Build a HAMB-era motor with that ratio and you're buying race gas.

If you take the emission concerns out of the equation, the main complaint that's usually left about using manifold vacuum as your advance source is some motors will knock briefly during throttle 'tip in' until the advance reacts. Now the 'why' of this can vary; some motors are more sensitive to timing than others due to compression ratio, combustion chamber design, even 'hot spots' in the head casting. So using ported vacuum is an easy fix, if not the best. But there can be other ways to address this in some cases. Less initial timing/more mechanical advance could address it. Making sure that all parts move freely and your advance can is correct for the desired advance. But in many cases it's the speed of the ignition reaction that's the issue. If you hear one or two 'knocks' on tip-in, you can probably ignore those as they're very unlikely to cause damage. I'm a Ford guy and my experience is Ford motors are pretty tolerant of this. You guys with more 'fragile' motors, that may not be true.

But my personal experience has shown that where you source the manifold vacuum can make a difference. I've found that directly off the manifold nearly always works better than any 'manifold vacuum' at the carb. I can only surmise that the difference is in the vacuum path. Smaller/longer/more convoluted p***ages inside the carb can introduce small increases in response time at the vacuum can, just enough to give you brief knocking. The one downside I've sometimes found with this is if you're running power brakes, when coming to a stop and applying the brakes the sudden drop in vacuum can retard the timing too much and cause stumble/stalling. A auxiliary vacuum reservoir connected to the brake booster will usually correct this.

And as @Boneyard51 has pointed out, where the main economy difference between all-mechanical and vacuum advance shows up is highway driving. If your car is strictly a town-driven stop light bandit, you're unlikely to see any difference. The eye-opener for me was my '58 FE-powered wagon that went from 8 mpg to 13 freeway mileage after swapping out my dual-point, no vacuum distributor for a standard single-point with vacuum one. The difference around town was barely noticeable, but that probably had more to do with my driving habits than anything else... LOL

 

Computer controlled advance is pretty neat, yes I know OT here, but my kids OT Chevy ‘89 V8 the initial is set at 6BTDC, as soon as it fires up and idles, it’s way off the Tab, probably in the low 20’s, high teens somewhere.
Would be cool to do that mechanically and not be over advanced through the RPM range and cruise RPMs, on a non race engine

 

Which (surprise!) is exactly what I wrote. That's probably why I suggested reading it again.

Vacuum advance isn't required but then the timing curve is ******ed for street use. It absolutely "improves" mileage on the highway because of part throttle, lean fuel conditions during steady flat ground cruise. But it isn't an improvement by using it as such. Just a defect by omitting it. See the difference?

During many driving situations the two mechanisms kind of work of at cross purposes, because one is RPM based, and the other is load based, so one is adding timing when the other is not.

Steady cruise is one situation where the two are basically almost completely additive. Mechanical weights and springs can't be "loose" enough to achieve 50+° of timing in high gear without being knock city on acceleration.

The engineers had to find a way to do that, to get the timing where it needs to be, under that specific condition, using something other than centrifugal weights.

 

I guess we do agree on timing, because your definition is correct. Like I said in a previous post, we just must differ on terminology. Lol








Bones

 

?????.........Just fab up a 'limiter' so the total advance won't exceed 36* or whatever! Got my SBF limited to 36* total advance.
"Com-on Man"....your a west coast hotrodder where all this stuff started!!! J/K....
I made a bushing to stop the swing of the weights at 36* Took a little trial and error with the timing light.
Shivel-lay distrib would be a cake walk with its 'guts' on top!
6sally6