Disty Vacuum advance - manifold or ported - lets hash this out

[Guest taprackready]

Thought #1: Ported Vacuum or Manifold Vacuum? Manifold vs Ported, its a never ending debate. Depending on the age of who you ask, and their own experiences you still have a 50-50 chance of one or the other. Ported vacuum is the choice of the factory. It is also my recomendation to start with ported vacuum. Depending on what level of modifcations you have done to your engine, your driving style, and your vehicles characteristics its a point of debate. Each presents its own set of advantages and disadvantages. My suggestion is to just try both and observe how your engine behaves for your driving style.

 

copied and pasted from article

[Guest taprackready]

http://www.corvetteforum.com/techtips/viewsubtopic.php?SubTopicID=115&TopicID=3

[Guest taprackready]

http://www.netwelding.com/ignition_timing.htm

[Guest taprackready]

I've posted 4 or 5 articles that are in defense of manifold vacuum. I am by no means trying to convert or state that all vehicles should run this way. Do as many of the articles suggest and try both of the ports and see which works best for your engine combo and your driving style. Ported may work best for many people running most of the factory controls. My engine does not have the factory controls on it and it has many, many miles on it.

 

My engine was running fine and had good idle quality before I switched vacuum source to manifold. So to say I'm masking a problem and that's why I like manifold vacuum is untrue. I like it because it seems to have more low end throttle response. It did not reduce highway crusing power/speed. My mileage is good, my engine temps are low even on hot days and I had plenty of heat for the winter.

 

So in conclusion if you choose to read the articles I've posted in the 4/5 previous posts and still insist I'm all wrong...great, you win, but I'm still running my Brat that way I think it runs best.

 

Have a great day

 

 

Bill

[GeneralDisorder]

The links are all about GM cars.... none of them quote anything specific to Subaru's and in the last link the guys is quoteing something he read somewhere with absolutely no references that I could find to this source...

 

Stated right in the article on corvette forum:

 

The additional timing produced by the vacuum advance control unit must be tailored and matched to the engine and the distributor’s mechanical advance curve. The following considerations must be made when selecting a vacuum advance spec: How much engine vacuum is produced at cruise? If max vacuum at cruise, on a car with a radical cam, is only 15 inches Hg, a vacuum advance control unit that needs 18 inches to peg out would be a poor selection. How much centrifugal advance (“total timing”) is in effect at cruise rpm?

And:

 

If you choose to run straight manifold vacuum to your vacuum advance in order to gain the additional timing advance at idle, you must select a vacuum advance control unit that pulls in all of the advance at a vacuum level 2” below (numerically less than) the manifold vacuum present at idle. If the vacuum advance control unit is not fully pulled in at idle, it will be somewhere in its mid-range, and it will fluctuate and vary the timing while the engine is idling. This will cause erratic timing with associated unstable idle rpm. A second tuning note on this: Advancing the timing at idle can assist in lowering engine temperatures. If you have an overheating problem at idle, and you have verified proper operation of your cooling system components, you can try running manifold vacuum to an appropriately selected vacuum advance unit as noted above. This will lower engine temps, but it will also increase hydrocarbon emissions on emission-controlled vehicles.

This man knows what he's talking about - he's very careful to state that the whole system must be taken into account when making modifications like these. And as I have stated before, I'm talking about stock disty's, not special recurved ones, with different vac can's.

 

Another point I would like to make, is that nobody drives at idle. While it may help you not to overheat (not a problem with a well tuned cooling system) at idle, and it may run smoother (also not a problem if the engine is in good overall tune - no vac leaks etc), but it doesn't do much for driveability from there on.

 

Basically, it comes down to - do you want the additional advance when the throttle plates are open or closed? I want it when they are open (low manifold vac, high ported vac) for best acceleration performance. Having it come in when the plates are closed (high manifold vac, low ported vac) certainly makes for nice smooth, cool idleing, but that isn't a problem on a properly tuned EA series engine anyway even running on ported vac..... so I see little point in running this way unless you redesign the system to use a different advance can that fully deploys at a lower vacuum, and a mechanical advance that can make more advance without the vacuum assistance.

 

I think the mechanical advance comes in so quickly (1700) because it's not actually full advance - it's only a total of 23, instead of 33 when the vac advance fully kicks in from the rising ported vacuum.... that probably happens at the typical 3k - 4k RPM's. I need to get a vacuum gauge to take some detailed readings on that tho. I will test the advance without the vac unit using my light tho to see when it does in fact come in.

 

GD

[4wheeln2]

taprack I have to admit, I only skimmed through the artticles you provided. The relevance that they have is questionable, however. The car craft articl3e seemed to remotely support the case for the try and see approach. the corvette c6 article seemed to be an overview of available vac advance units, and an explanation of its function, which you seemed to gloss over. another article was an example of one person's personal preference. as i skimmed through, though it did seem like there was some insight in the operationoof the ignition timing system, particularly vac advance, which is what the discussion is about. not vac advance on drag mustangs, 400 hp corvette c6's, zz502 big block crate motors( which if you read the article required a lot of tuning and an electric vacuum pump to run right) or the like. a more helpful article might have had more to do with street-petrformance or off-roading, where the goal is to have a broad torque curve, and good throttle response. you obviously are a smart fella to find all those articles. All I have beeen trying to get across is that while it might be cool to have a 15 hp spike at some point in your powerband, that it will pay off more to have you motor run its best throughout the rpm range, and have predictable throttle response. that's pretty important if you take your brat offroad- nothing like stalling out at the top of a hill climb, etc. in the final analysis, you are absolutely right- because you are working on your car, not mine and you are the one who has to be satisfied with its performance. I just think if you really understood what you were doing, you'd seee the point in leaving this aspect of the sube stock. its almost like someone trying to reverse the battery terminals for the sake of experimentation. I hope you don't think I'm being a jerk, you can take what I say with the biggest grain of salt, i'll tune my brat my way, you tune yours your way. we can agree to disagree. Patand GD, the deal with the vaccum advance is that it is there to supplement the mechanical timing during part throttle conditions. if you have 24 degrees in the disty, and 12 deg in the vac advance, and 12 deg. initial timing, you dont have 48 deg total. you havbe 36 total, because the vac advance only comes in to the picture at part throttle conditions- or sitting still in your driveway while you work the throttle, maybe. butr in the real world, you don't really "add" the timing from the vac advance, it is just there when the engine requires it.

[Guest taprackready]

4wheeln2, and Gen Disorder, I appeciate all the feedback you've given me. I can understand your frustration with me when all the available information seems to favor leaving that part of the ignition stock. I'm just experimenting and trying what the big boys do. Only I'm doing it on a junkyard budget. I'm still going to get out my vacuum gauge and fiddle around with the car more.

 

Great fun debating though.

 

Your Sub friend

 

Bill

[GeneralDisorder]

Patand GD, the deal with the vaccum advance is that it is there to supplement the mechanical timing during part throttle conditions. if you have 24 degrees in the disty, and 12 deg in the vac advance, and 12 deg. initial timing, you dont have 48 deg total. you havbe 36 total, because the vac advance only comes in to the picture at part throttle conditions- or sitting still in your driveway while you work the throttle, maybe. butr in the real world, you don't really "add" the timing from the vac advance, it is just there when the engine requires it

Absolutely - I was refering to total advance as the sum of all possible advance in the system. Ported vacuum also rises in pressure even with a fully opened throttle plate. I want to say that it's a linear relationship, but I'm not totally sure on that. The rising vacuum comes from the low pressure that develops in the venturi of the carb - which is there regardless of throttle position. It's actually a function of the speed of the incomming air, not the throttle plate position per se. But the plate position does of course change the maximum flow rate, so of course has an indirect effect on this value. At wide open throttle, and high RPM's, the intake air speed should be at it's maximum and thus the vacuum advance unit would be fully deployed. In the case of manifold vacuum, at wide open throttle, there is no appreciable vacuum present in the manifold, so the advance unit is not deployed at all.

 

I hope I explained that ok.... hehe

 

And I do concur that this is an excelent debate thus far.

 

GD

[Dirk]

And she rises like a monster from the deep.

 

Looks like it has been a few years for this one.

 

I would like to throw a spanner in the works by suggesting that the ported vacuum on the EA engines is not formed by the venturi but is true manifold vacuum that can be 'turned off' by the throttle plate.

 

I think that the vacuum secondary runs from venturi vacuum.

 

Does anyone now know the factory specs for the mechanical advance of the EA engines? I suspect my mine is coming on too soon.

[Dirk]

It would also be handy to know at what vacuum the advance starts and at what vacuum it peaks. (factory spec)

 

I understand that raising the compression of an engine increases manifold vacuum. If this is correct, would the advance systems need to be modified accordingly and if so how?

[Dirk]

I think this is the data I am looking for. Interesting that it suggests the mechanical advance is active all the way to 5000rpm.

 

Can anyone verify this data?

 

I have converted the vacuum from mbar to inches (writen to the left). Again can someone verify my numbers.

 

Cheers.

 

Dirk.

 

[Dirk]

And here again. I guess the last set of data includes vac advance.

 

[Dirk]

Hey Rick!

 

Now that I have insulted you by calling you grumpy:(, I still need your input on these advance numbers.

 

Don't suppose you could have another peek at the FSM?

 

Cheers.

 

Dirk.

[coronan]

Awesome info in this thread!!!

 

Can anyone provide the Advance specs for Turbo EA81's???

I know its 15* @ idle / no vacuum

 

What about the rest of the RPM range?

on boost?

 

Thanks!

Edited April 19, 2016 by coronan