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Disty Vacuum advance - manifold or ported - lets hash this out
Posted 28 April 2004 - 09:42 AM
copied and pasted from article
Posted 28 April 2004 - 10:49 AM
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
Posted 28 April 2004 - 10:50 AM
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?
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.
Posted 29 April 2004 - 07:13 AM
Posted 29 April 2004 - 11:18 AM
Great fun debating though.
Your Sub friend
Posted 29 April 2004 - 12:46 PM
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.
Posted 17 May 2011 - 07:57 AM
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.
Posted 17 May 2011 - 04:23 PM
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?
Posted 17 May 2011 - 09:19 PM
Can anyone verify this data?
I have converted the vacuum from mbar to inches (writen to the left). Again can someone verify my numbers.
Posted 17 May 2011 - 09:34 PM
Posted 19 May 2011 - 05:58 PM
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?
Posted 19 April 2016 - 09:27 AM
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?
Edited by coronan, 19 April 2016 - 09:27 AM.
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