Vacuum advance functionality questions

[coolingchips]

I'm sure this must have come up before, but I did a search and couldn't find it, so here it goes. My question is on distributor vacuum advance, old school, I know. I know because of lean mixtures at light loads it's desireable to advance spark timing for optimal pressure peak timing, with advance decreasing with increasing load. Also, most cars don't advance timing at idle. My question is how the correct vacuum signal is obtained. When I look at the vacuum port location in a throttle body or carb, it's just a hole upstream of the throttle plate in the closed position, on the atmospheric side. This location confuses me, the pressure signal looks like it's static and would have increasing vacuum with load. I don't see anything that's measuring stagnation pressure. I've seen advance curves for distributors and they increase advance with vacuum. So if I look at the port location and the distributor advance curves, It should be getting advance timing at higher loads instead of lower, which I know it doesn't. What am I missing?

 

[MikeHalloran]

Off-idle, when the throttle plate has moved off the idle stop, that port is on the downstream side of the throttle plate, so it sees manifold vacuum, which decreases with increasing load.





Mike Halloran
Pembroke Pines, FL, USA

 

[coolingchips]

It does expose the port to vacuum, but as the throttle opens further with no change in engine speed, the pressure decreases further because of the increase in flow, right?

 

[reidh]

Coolingchips,

I think you are confusing dynamic pressure with static pressure.

The dynamic pressure increases with increased flow as you stated, but the port is measuring static pressure which is independent of flow. As Mike stated, the vacuum decreases as the throttle blade opens, until wide open throttle where the static pressure is approximately ambient pressure.

Reidh

 

[FoMoCoMoFo]

some carbs have 2 or more manifold vac ports, and a venturi vac port, and a"ported"vac port that works as mike exsplained for the reason you mention.no vacuum at idle.a venturi vac port can act as you described,vacuum go up with load.its sometimes used to operate an egr valve but rarely involved in vacuum advance. vac advance is compensating more for lower pressure than for leaner mixture. Happy motoring.

 

[coolingchips]

Does anyone have a diagram or picture. I'm having trouble understanding that the throttle itself isn't acting as a venturi and lowering static pressure.

 

[coolingchips]

Here's the results of a CFD simulation that I ran. -2atm (-202 kPa) on the vacuum side. The throttle plate is at 15, 45, and 80 degrees from vertical. Notice that the pressure below the center of the throttle plate is decreasing with the more open plate.

http://i2.photobucket.com/albums/y37/bellylint/Throttle_flow/15_deg_2.jpg

http://i2.photobucket.com/albums/y37/bellylint/Throttle_flow/45_deg_2.jpg

http://i2.photobucket.com/albums/y37/bellylint/Throttle_flow/80_deg_2.jpg

 

[FoMoCoMoFo]

if you were to put a tee in that line and hook it to a vacuum guage on your dash and do the same with a manifold vac port (usually gets fed from a port about .25"lower in the carb) you would find that once the throttle is open a tiny bit they both read the same (manifold vac) at idle the "ported" one should read 0. manifold vac. decreases as load increases.

 

[coolingchips]

OK, I think I know where I was going wrong. I was looking at this as a constant pressure problem, but it's a constant mass flow problem. Then it all makes sense. The only thing though is that there is some dynamic effect at this location that wouldn't be seen a bit further downstream into the manifold, but I assume that's a negligable difference as FoMoCoMoFo says.

 

[MikeHalloran]

Carburetors made after ~1969 have all manner of extra ports, valves, solenoids, linkages, and just plain doodads that were added and modified as emission controls evolved.

Try to find a carburetor and a factory shop manual pre-1969, and study both.





Mike Halloran
Pembroke Pines, FL, USA

 

[JWaterstreet]

My Bosch Automotive Handbook has a really simple diagram of vacuum advance. Everyone that is into cars should have this book.

One caution if you look for older shop manuals, as Mike stated. There are some older cars (e.g. some VW cars/buses in the mid 60's) that had true vacuum advance, i.e. applied more vacuum to the vacuum solenoid on the distributor with increasing speed, instead of a mechanical advance mechanism. These worked by having a vacuum port in the center of the venturi, and a really sensitive vacuum solenoid on the disti.

Woe to the parts swapper that put one of these on a car that had a carb with a standard, off-idle vacuum port. The disti would go full advance at off idle (ping city) and have almost no advance at full throttle.

 

[coolingchips]

JWaterstreet, any chance we can get a scan of that diagram?

 

[coolingchips]

Can anyone explalin why they don't advance timing at idle? Wouldn't a lean idle mixture be beneficial?

 

[FoMoCoMoFo]

Do you mean vac adv? If so one reason is that in an auto transmission car stopped in drive vacuum can fall enough to start retarding timing, you then enter a feedback loop or death spiral if you prefer (I do) where idle drops so vac drops so timing drops and so on till it you die! sounds kinda scary huh? A similar thing can happen with man. trans but it's not as dramatic.. you seem again to want to tie ignition timing to fuel mixture. why? if you mean just plain initial advance at idle it gets complicated. if you want to study advance curves,carb design, fuel curves,cam timing or any other engine design parameters as they relate to perfomance do as mike suggests and look pre 69 after that its all about emissions and the changes are counter intuitive and counter performance and efficiency

 

[RossABQ]

Ford used the "Load-A-Matic" distributor from '48 through the mid-60's. It was a purely vacuum advanced distributor, using both a port just above the throttle plate and one in the venturi. According to their theory, the combination of the two ports would react to both light load conditions and increased RPM. (sounds like the VW system) Saved them the cost of a mechanical advance mechanism, and cost consumers millions of gallons of gas. I replaced the one on my '52 flathead V8 with an MSD unit that has "normal" vacuum advance and mechanical advance and the difference in performance and economy is astounding.

 

[patprimmer]

Your local Hot Rod Shop will have a book on how to tune , modify or hotrod Holley carburettors.

I have a copy stashed away. I can't remember the exact title, but it was by HP books or SA design or something like that. It has all circuits and ports explained, at least to pre pollution models.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dicer]

To make the advance work like you mentioned.
"Advance decressing with more load"
You just hook it to the manifold vaccum, that is any place below the throttle plate. If you can't figure it out at the carburetor. Just use a fitting that screws into a port in the intake manifold. At idle advance will be most, and when pulling a load it will be least.

 

[Tmoose]

" because of lean mixtures at light loads it's desireable to advance spark timing for optimal pressure peak timing"

I think part throttle could be either rich or leaner stoichomtric, and generally more advanced ignition timing would still be required for best torque due to the cooler, slower burning less dense mixture.