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Old 07-10-2003, 11:36 AM
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Vacuum Advance/ timing confusion!

Hi All,
I read all the posts I could find re: Full and Port vacuum relating to Vacuum advance and I am a bit confussed! Here's why:

Full vacuum-(under the throttle plates) is highest at idle because the engine is trying to pull more air in and the plates are closed. As you open the throttle plates the vacuum drops, if you continue to open them it continues to drop, until you have reached WOT where the vacuum should be zero, right?

Port vacuum-(above the throttle plates) is very close to Zero at idle and increases with air velocity through the carb = (increased enginge RPM) and reaches max vacuum just before you blowup your engine, or reach maximum RPM, right?

I think I have this right!

Now relating this to timing, trial and error has shown me that with no vacuum advance, if you slightly advance your timing from lets say an initial of 10 deg to 14 deg, you get better throttle responce and more bottom end power. If you retard the timing, you get better top end power, but lose low end power.

Sooooooo! I'm running out of breath here, sorry.

Would the engine run better on full vacuum, so that you get lots of advance at idle=good bottem end then as you increase engine RPM you get less advance(more retard) and therefore more top end power?

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Old 07-10-2003, 11:57 AM
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Remember, the faster the engine turns, the earlier you need to ignite the charge so that cumbustion finishes at the time the exhaust valve opens to take full advantage of the power stroke. You can't adjust the speed of fuel burn rate (other than change fuel blends) so you can only start the process earlier.

If you run full timming at idle, there will be no advancement left to take full advantage of the power stroke at higher RPM's. Not to mention starting woes with high timming and detonation.

Some drag racers lockout the timming advance in order to have better controll and easier settings But they are limited to a very narrow range of power production as far as RPM's go. I assume you are researching timming for a street driven car where you need a very broad power range.
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Old 07-10-2003, 12:04 PM
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The only purpose for vacuum advance is to make the engine idle smoother and keep the plugs from fouling at idle.
That said;many people use vacuum advance to offset there mechanical advance to get more total timing.Instead of recurving the distributor to the correct specs.
Thats why almost all race cars don't use vacuum advance distributors,cause it is not needed and there's not much vacuum to make it work correctly anyway .It just becomes one more thing to tinker with.
Adjusting a vacuum advance is done with the needed initial timing set and the vacuum advance set up with enough advance to clean up the idle quality.Adjusting the spring in the diaphram is done to make the advance unit retard itself at a desired vacuum point.
thats all there is to it
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Old 07-11-2003, 02:18 AM
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Vacuum advance is helpful for part-throttle smoothness & fuel economy.
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Old 07-11-2003, 03:25 AM
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I'll buy that for a dollar...

For mechanical street performance applications I have found the dual canister vacuum advance/retard units to be the ideal solution...but.

The problem is they don't make a good adjustable retard unit, the advance side is usually fully adjustable but the retard side is usually fixed. Now you could drill and tap the retard side to limit diaphram movement but what you usually need is more retard not less...here's why.

You have two important vacuum ports usually available to detect and respond to engine load...ported and non-ported vacuum. The trick is to get the two to offset each other so you have a high advance at low loads that can instantly retard when the load increases...even slightly.

The gains in fuel efficiency are spectacular and this is the method mainly responsible for the newer vehicles superior mileage...of course a computer is doing all the sensing...and sensing much more than a comparitive vacuum source.

I have used old Ford dual diaphram units from the early 70's on Windsor engines and by drilling a tapping a single hole in the secondary diaphram you can limit the retard from 0-8 degrees I seem to remember...it might only be 5 so don't quote me. The advance side on these old units use shims and springs with a threaded cap that held the vacuum nipple, adjustment for up to 5-30 degrees of total advance timing was possible with anything in between. At one time I had aquarium needle valves controlling vacuum application speed etc. I managed to get my 69 Mustang with 290 duration cam and Holley DP with 3.0 gears to get 22 MPG...city.

Pretty good for a car that barely idled at 800 rpm I thought. The effort spent tuning is certainly worth it especially when it comes to throttle response, the car seemed to gain a lot of grunt off the bottom and eliminated the light throttle ping that you would get when you stepped on it quickly.

The good thing is you don't have to go through this nonsense anymore because the programable fuel/ignition systems available now do all that for you, having a custom chip made just for your car is so easy and affordable that adapting a stock fuel injection system and tuning it with new injectors and a custom MAP is an appointment away or a laptop hookup away.

I still miss the old days of springs and shims though, sure was cheap and easy and you had a million combinations to try. I always thought that was half the fun of hotrodding...but I don't mind playing with a laptop programmer either! It's so much easier and deadly accurate.

Anyone have one of those old Sun mechanical distributor machines still? Man, I used to spend a lot of time on that great old machine changing springs, bending pivots and setting points.

The good old days.:laugh:
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