Intermittent Spark

[ramon alban]

Ramon, here's the article that made me feel manifold vacuum was the be all and end all of vacuum controlled ignition advance, long, but interesting: (Blah blah blah) - followed by:

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum.

Paul, I dont use the term blah as being derisory, but just to represent the whole essay. So, trying hard not to be pretentious, having read it thro' three times its almost as tho' the final sentence (conclusion) has a massive erroneous conclusive/typo error and should have read:

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to ported vacuum.

and if, perchance, that was the case, might it explain why your experiences and preachings were so embarassing and why, to my very limited experience in these matters, I followed almost everything in the essay except the enigmatic last sentence.

The essay also says.

Most stock vacuum advance cans aren't fully-deployed until they see about 15" Hg. Manifold vacuum, so those cans don't work very well on a modified engine; with less than 15" Hg. at a rough idle, the stock can will "dither" in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle.

This seems to chime with my supposition attached to Question #2 - ie

Question #2 - Have you any idea why that phenomenom was occuring?

I cant figure it out, but might it be because the manifold vacuum changes with engine load, this induces the dizzy vacuum advance system to oscillate on and off because the vacuum itself was being affected by feedback from the change in spark timing?

Either way, Paul, thanks for the responses. Its probably only talking shop stuff, with little relevance to the OP's original problem or my local SD1 Efi universe but it all helps with understanding ignition systems which is my current mission.

Many thanks,

Ramon

[Paul B]

Ramon, here's the article that made me feel manifold vacuum was the be all and end all of vacuum controlled ignition advance, long, but interesting: (Blah blah blah) - followed by:

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum.

Paul, I dont use the term blah as being derisory, but just to represent the whole essay. So, trying hard not to be pretentious, having read it thro' three times its almost as tho' the final sentence (conclusion) has a massive erroneous conclusive/typo error and should have read:

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to ported vacuum.
and if, perchance, that was the case, might it explain why your experiences and preachings were so embarassing and why, to my very limited experience in these matters, I followed almost everything in the essay except the enigmatic last sentence.

Hmm, I'm not too sure how you arrive at that conclusion. I just re-read the whole thing, especially the last section that deals specifically with ported/manifold vacuum and it seems to make sense to me that they DO indeed mean you to use manifold vacuum in that last statement.

The essay also says.

Most stock vacuum advance cans aren't fully-deployed until they see about 15" Hg. Manifold vacuum, so those cans don't work very well on a modified engine; with less than 15" Hg. at a rough idle, the stock can will "dither" in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle.

This seems to chime with my supposition attached to Question #2 - ie

Question #2 - Have you any idea why that phenomenom was occuring?

I cant figure it out, but might it be because the manifold vacuum changes with engine load, this induces the dizzy vacuum advance system to oscillate on and off because the vacuum itself was being affected by feedback from the change in spark timing?

Either way, Paul, thanks for the responses. Its probably only talking shop stuff, with little relevance to the OP's original problem or my local SD1 Efi universe but it all helps with understanding ignition systems which is my current mission.

Many thanks,

Ramon

My motor was bog standard, had good vacuum at idle, 20" or so, so now, looking back, I'm thinking that maybe the vacuum advance can was simply not designed to run on the manifold vacuum, which would make a lot of sense, thus it was giving me far too much advance at idle. That might explain things.

I sold the car last year though, so no more experimenting can be done.

[Darkspeed]

If you have a read of the Des Hammil book about building distributor ignition systems he mentions the issue with Rovers using Vacuum advance and the change from SU's to Holley (or the manifold vacuum) - the Holley provides a greater Vacuum signal than the SU and causes misfire's at idle and at steady state cruising because the Vacuum is advanced so far that the rotor arm is no longer pointing at the correct lead connection and the spark is jumping to the next plug.

The article about manifold advance appears to hold true with what I understand and have read up on recently and the key is that the Vacuum canister must be rated and calibrated for the vacumm signal available

I have been pulling apart a number of dizzy's and playing with the advance curves and looking at vacuum cans off of EFI and Carb engines and the varations are enormous.

On two OPUS dizzys - one has 26 crank degree mech and another only 18 degrees with completely different springs and weights. The movement and the pressure required for the Vac cans was also very different - one with 15 degrees advance and one with 30.

With so much mixing and matching of components going on as the engines get swapped about its easy to see how it can all go wrong.

Andrew

[ramon alban]

Hmm, I'm not too sure how you arrive at that conclusion. I just re-read the whole thing, especially the last section that deals specifically with ported/manifold vacuum and it seems to make sense to me that they DO indeed mean you to use manifold vacuum in that last statement.

OK Paul. Not so much a conclusion, more a speculative suggestion.

Therefore, your following observation ......

I'm thinking that maybe the vacuum advance can was simply not designed to run on the manifold vacuum, which would make a lot of sense, thus it was giving me far too much advance at idle. That might explain things.

.... does explain things and suggests the conclusion in question should not be assumed to apply to bog standard road going engines and vacuum advance systems, as you subsequently found in reality.

Indeed, I misread that the author made his conclusion in relation to, non-stock, race, modified engines with a lot of cam that were also being used on the street, as opposed to it being applicable to normal street cars.

Make a diary note - must stop dabbling in things outside the box!

Ramon

[Paul B]

If you have a read of the Des Hammil book about building distributor ignition systems he mentions the issue with Rovers using Vacuum advance and the change from SU's to Holley (or the manifold vacuum) - the Holley provides a greater Vacuum signal than the SU and causes misfire's at idle and at steady state cruising because the Vacuum is advanced so far that the rotor arm is no longer pointing at the correct lead connection and the spark is jumping to the next plug.

And I think we have an answer to the problem.

Thanks Andrew, all becomes remarkably clear all of a sudden. Quite why I never tried putting the vacuum hose back on ported outlet at any time during my experiments will forever be a mystery.

Ramon, I too knew that the article was for tuned motors, yet I still applied the vacuum theory to my stock motor. Why?

I think I'll go buy a bog stock Peugot 106 now, and forget messing with V8 motors forevermore.

[topcatcustom]

This is going to be a nightmare when I get to starting up and timing/tuning my blown lump!!!!! I dont think I will even bother with the vac advance unit on my dissy as the timing will simply be miles out!!!