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PostPosted: 02 Jan 2004, 12:37 
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When the timing needs to be retarded under boost conditions, how does that happen? Does vacuum pressure build up inside the hose that connects to underneath the distributor cap?

There's a valve with probably a filter inside after the hose that connects to underneath the distributor cap. Would engine performance suffer if that valve were clogged or not and if so how?


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PostPosted: 02 Jan 2004, 18:21 
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Yep, pretty much got it in one.

I can't tell you what the engine does without it as it's never died on mine, but afaik the engine will run reasonably ok off boost without it but will detonate and run poorly on boost... that's about the best I can do :P

Have a look at steve's second post in this thread for how to do a quick vac test on it.

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PostPosted: 02 Jan 2004, 18:30 
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Cordias do not have boost retard, they have vacuum advance. it has a similar effect as a boost retard, but not quite. at vaccuum the timing is advanced to improve bottom end response. at 0psi and above it's standard timing. if you disconnect that line the difference is almost unnoticeable, you lose a tinny bit of bottom end power, top end remains the same.

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PostPosted: 02 Jan 2004, 19:59 
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PostPosted: 02 Jan 2004, 20:05 
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I think that the standard Cordia distributor does retard under boost.

Vacuum advance is used at cruise conditions, never under power.

If you disconnect the vacuum advance unit, power may improve
but you will also more likely start to ping under load.

Matt


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PostPosted: 02 Jan 2004, 20:20 
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Bill at Adelaide Turbo Service once wrote an article on cordias. Aparantly they have both an advance and retard curve. Or something like that. Might try and dig up the article, had something to do with regraphing the standard dizzy.


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PostPosted: 02 Jan 2004, 21:02 
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so which way around does vac adv have to face. mine is black end throttle body and pink end to dizzy. would that be right?


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PostPosted: 02 Jan 2004, 21:08 
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"mine is black end throttle body and pink end to dizzy. would that be right?"

other way round. that valve should stop the advance unit seeing boost. with the pink side facing the dizzy it will only see boost. that causes lurching and jumping at boost.

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PostPosted: 03 Jan 2004, 12:32 
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Grumbly

If you could find that article, I would be interested in reading it.


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PostPosted: 03 Jan 2004, 18:34 
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Don't quote my last post. remembering that my valve was in the "wrong" way when i bought my car too. switching it solved the lurching problems. (after i'd gone and bought another valve thinking the stock one must be owned.) :(

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PostPosted: 03 Jan 2004, 18:41 
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I disagree witht he point made of advance only occuring at cruise conditions...to make power you need to get to the safe limits fo the car, ie as much advance as possible until it starts to ping, then back of a bit....this is how fuels such as optimax work, they push the point of pinging further up the curve so the motor keeps on advancing timing, makeing more and more power until pinging occurs, which on ultimate fuels is higher up, therefore more power is felt.....such vac is giving to the dizzy at cruise etc, but what if under vac your at say 0 advance, and as vac drops the advance weights actually start to move, in essence in an advanced direction.......(hope that's not too confusing..)

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PostPosted: 03 Jan 2004, 19:50 
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I didn't understand a word of that :P

afaik the cordia has no intelligent advance control, just vacuum advance. so if you put low octane fuel in it it will ping, if you put high octane fuel in it won't, it doesn't know how to retard itself to stop pinging, it just goes fuel cut.

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PostPosted: 03 Jan 2004, 19:59 
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im confused now. so which way should the vac adv be and how can you tell if its not working properly?


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PostPosted: 03 Jan 2004, 21:05 
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Timing on the Cordia is controlled by 2 things: rpm & manifold
pressure.

Timing advances with rpm controlled by weights/springs in
distributor.

This is adjusted by either vacuum in diaphragm advancing the
timing or boost in diaphragm retarding the timing.

The only exception is when detonation is detected by the knock
sensor which will retard timing.

The ecu controls fuel only(and timing retard when knocking)

The proof about the timing retard under boost can be seen by
looking at the distributor and seeing which way the distributor
linkage moves when vacuum or positive pressure is applied
compared to the relaxed position.


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PostPosted: 04 Jan 2004, 17:58 
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Good wording coop1er i agree.

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PostPosted: 05 Jan 2004, 17:18 
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Does anyone else have anything to add to this thread before I move it to FAQ's?

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PostPosted: 05 Jan 2004, 20:12 
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PostPosted: 31 Mar 2004, 19:51 
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Late reply from OzStick added to archive:

--------

I'm doing as suggested and scouring the entire forum to find answers to all my modding questions, when I stumbled over this thread which appeared to cause a bit of debate.

Ox!gen suggested there is only vacuum advance capability on the Cordia dizzy, however Grumbly recalled a write up done by someone in Adelaide on regraphing the Cordia dizzy which explained it has both advance and retard on it.

I remembered reading a magazine article showing exactly that, which I managed to find. It is from the first edition of a magazine called Hardcore Performance and the article is called Graphing A Spark.

It follows the process involved in regraphing the standard Cordia dizzy as done by "Local Adelaide tuner Peter Hall" to cater for an engine outputting at least 20% more power than standard.

The article quotes the following:
The distributor responds to both vacuum and boost courtesy of a simple external diaphragm canister (vacuum advance/boost retard unit) that bolts onto the outside of the distributor body. The advance/retard unit has a manifold pressure pickup on the outside of it that hooks directly to the plenum chamber to sense vacuum or boost.
A movable rod that is directly attached to the distributor baseplate can either push it in to retard or pull it into an advance condition.

It's a really interesting article and food for thought for those tossing up the idea of aftermarket management ie going MPI & how to handle ignition.

If you're interested in a copy of the article let me know here and if enough people are keen I will scan it into a file and drop it onto the web (if it's not too large!) for anyone to download.

----

Thanks ;)

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