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Help Me Understand Disty Curves


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5 replies to this topic

#1 garner

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Posted 08 December 2004 - 06:57 PM

In preparation for switching to Megasquirt and Spark I'm trying to figure out what the stock timing is on an 85 EA82T. I have an FSM and have an image the curve of the Turbo disty and I'm a bit confused. Check this out

Posted Image

If I set my timing to 25 degrees BTDC, how are these advances acting on it or, specifically:

Is the total advance the sum of the vacuum and disty speed advance? the average of the two? or ?

For example, If I am at 3000 rpm and -300 mmHg (boost) where is my timing?

-25 +10 +10 = 5 degrees btdc? Or?

or how about at 250 mmHg and 1500 rpm

-25 +5 -5 = 25 btdc?

Also, I assume when setting the timing, with the vacuum advance line disconected, if I set my timing to 25 btdc at 1000 rpm, about 2.5 degrees of that is from the disty speed advance?

As you can tell from my ramblings, I don't really understand this Disty advance stuff. Please give me a schooling.

garner

#2 naru

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Posted 08 December 2004 - 07:21 PM

Timing is the sum of intial(base) timing and the vacuum and mechanical advances.
At 3000 rpm and -300mmHg advance =25+10+10=45 BTDC
Note that -300mmHg is maximum vacuum not max.boost
At3000rpm,300mmhg(max boost)=25+10-5=30 BTDC
At 1000 rpm you will indeed have 2.5 degrees from the mechanical advance.
If you set base timing at 1000rpm subtract 2.5 from the totals above.
All this assuming the knock control unit isn`t hearing any detonation and retarding timing on its own.

#3 garner

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Posted 08 December 2004 - 08:17 PM

Timing is the sum of intial(base) timing and the vacuum and mechanical advances.
At 3000 rpm and -300mmHg advance =25+10+10=45 BTDC
Note that -300mmHg is maximum vacuum not max.boost
At3000rpm,300mmhg(max boost)=25+10-5=30 BTDC
At 1000 rpm you will indeed have 2.5 degrees from the mechanical advance.
If you set base timing at 1000rpm subtract 2.5 from the totals above.
All this assuming the knock control unit isn`t hearing any detonation and retarding timing on its own.


Thanks Naru,

Oh yes, that makes more sense that -mmHg is vacuum and mmHg is boost. I just got it backwards thinking -vacuum=boost. When I look at the curve it makes sense that under boost, I would want to retard....

Maybe, I'll plot out my ignition map and see if it makes sense before I try to run the motor on it....

garner

#4 PoorManzImpreza

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Posted 08 December 2004 - 11:08 PM

I might add that while all the stock disty curves might help you get your car tuned to run and run reasonably well..MS&EDIS opens up far FAR more tuning and power possibilities that disty based ignition curves ever could...

Strongly consider EDIS based ignition..even 88+ ECUs adandoned any kind of mechanical or vacuum advancement and simply used the distributor as a crank angle sensor and to place spark on the correct plug all advance/retard was done by ECU and knock control unit..

#5 garner

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Posted 09 December 2004 - 02:25 PM

I might add that while all the stock disty curves might help you get your car tuned to run and run reasonably well..MS&EDIS opens up far FAR more tuning and power possibilities that disty based ignition curves ever could...

Strongly consider EDIS based ignition..even 88+ ECUs adandoned any kind of mechanical or vacuum advancement and simply used the distributor as a crank angle sensor and to place spark on the correct plug all advance/retard was done by ECU and knock control unit..


Yep, EDIS is the way to go. The MS&S extra code will utilize a number of ignition triggering systems, Hall effect, optical, edis etc. I'm going edis with the trigger wheel mounted to the crank pulley.

I really look forward to tuning the ignition to perfection. For now, since I can't get real tuned ignition maps for an EA82T, I thought I'd get it running on the stock advance curves and tune from there. I started creating a map based on the stock curves and it seems to make sense. When I get it done, I'll post it and see what others think

garner

#6 oddcomp

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Posted 10 December 2004 - 01:50 AM

umm garner
do what i did and since it worked.. dam well i might add
retard all your high rpm and high boost values to say 20 deg btdc or so
i thinki started there

and unless your good at watching and listening .. to a screen and the motor
get a second person and just slowly increase timing values at each rpm/boost point
until you get detonation
and hen back it off 1-1.5 degree's
took a while
but i was more concerned about tuning the high rpm/boost/load range of the map versus the cruise and off boost points

what i plan on doing after i get the car on the road again
is wiring the wastegate open
so i get 0 to little boost
and then tuning that range
so in theory.. it should be as close to correct as i can get with out time and money for a dyno



and might i add having the correct ignition curve is almost useless unless you have your fuel sorta dialed in
as you might mistake ignition based deonation for lack of fuel based and then just end up chasing your tail :)




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