Vacuum advance: Timed or Manifold pressure?

[TrunkMatt]

So, my old Qjet had the vacuum advance running to a manifold/full pressure port. This new Edelbrock recommends the timed vacuum port...

 

I tested it on the Timed Vacuum port, and the Manifold port... aside from idle speed - I haven't noticed much difference.

 

Which is recommended for our engines? What gives the best power curve? Some internet people are saying run the vacuum advance to the timed, others are saying leave it on the manifold/full....

 

Thoughts?

[monte70car]

When I did the carb on my friends Monte Edelbrock 600 or 650 dont remember which one I used the port on the driver side, and the car is still running strong almost a year later.

[TrunkMatt]

That's the manifold pressure side...

 

I heard a slight knock at idle when using that side, which I think is an indicator it is drawing my advance too far forward at low speed.

 

I suppose I should get a vacuum gauge, and see what the readings are at each port at about 2500 rpm... if the timed advance works as advertised, they should be about the same.

[stangeba]

I use ported, like from the factory (less TCS) for my stock 71 350 4BBL. The thing about running manifold vacuum is you have it at idle vs just off idle with ported.

 

Some information from a retired Chevrolet engineer (My brother in-law and boss) about why you want it. He suggests trying the advance at both ported and manifold and see what your engine likes.

 

VACUUM ADVANCE AND WHY YOU WANT IT FOR YOUR CAR

An often-asked question relates to whether they really need a vacuum advance mechanism on their distributor. I think this question stems from their observation that many “high performance” distributors do not incorporate vacuum advance and the resulting implication is that it is not desirable or necessary for a “good” ignition. There are a handful of applications where vacuum advance is not of significant benefit:

1) Pure racing engines

2) Severe duty very large trucks

3) Constant speed and load applications (airplanes, generators, pumps)

Other than the above, for normal automotive applications the vacuum advance will benefit the engine as follows.

1) Improved idle cooling

2) Improved idle quality

3) Improved fuel economy

4) Improved throttle response

5) Improved drivability

6) Enables improved spark knock control under full throttle accelerations

7) Enables leaner fuel jetting at light load to further improve fuel economy.

The basic reason for all these improvements is that the vacuum advance mechanism allows the distributor to supply a more optimum spark timing proportional to the load and speed output. Without the vacuum advance the distributor can only vary spark timing in proportion to speed and ignores its need for approximately 20 additional degrees of spark timing (“advance”) at light loads: (idle and cruise conditions)

The basic reason for the change in optimum timing at light loads is that when operating at light loads, the mixture is leaner for fuel economy and less dense because of light load. These conditions cause the charge to burn slower, and thus, to reach peak pressure at optimum point in the cycle, the spark must be initiated earlier. Failure to do this will result in “retarded” spark timing and all the aforementioned losses.

All engines are different, and have different spark timing requirements, but they are all the same in that as load is decreased, additional spark timing is required for optimum combustion.

Do yourself a favor – 1) make sure your distributor has a vacuum spark system and 2) experiment to find out what your engine “likes” for timing at idle, light load, and heavy load. Then change the vacuum can to achieve a result closer to the optimum."

[TrunkMatt]

Indeed - I read a similar subtext here... http://www.stoveboltengineco.com/howto/vac.htm

 

I honestly really can't tell much of a difference. Some day I should just switch to an electronic ignition :-)

[stangeba]

Matt,

That's my brother in-law and boss, I work there. I stole it!

Bruce

[TrunkMatt]

Matt,

That's my brother in-law and boss, I work there. I stole it!

Bruce

 

Totally awesome... I've been reading my way through that site for a few days now!!! Tell him i am a fan

 

Also you guys are up in Utica!!! HAHAHA I go to my family vacation home every year for the last 30 years on Harsens Island (Exit 243 - New Baltimore to Algonac...)

 

I was just there in June.

[stangeba]

If you have a GM Chevy or even a flat head Ply or Dodge, Tom is very knowledgeable about those inline sixes. It's more than a job for him, it's a calling. Do you get his point about converting to an overdrive trans? He has one ready for me someday out of a Buick GN V6.

My wife and I have driven the route past the island that goes along the water all the way up to Port Huron many many times for almost 40 years now but have never been on the Island! I'm too cheap to pay the tool for the boat ride over I guess.

Bruce

[TrunkMatt]

You're not missing much that you can't see in Marine city with the ships etc. There is a few good restaurants on the island... but honestly --- mostly resident houses...

 

Overdrive tranny... yes indeed. It'd be a wonderful thing to have and I think something I too want to move toward.