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Old 12-20-2010, 02:29 AM   #1
waypastcrazy
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Duraspark dizzy performance recurve.

This recurve is to show how to do it not as a fit all application deal.Because each application has its on base timing requirements and full advance timing requirements.Different head chamber designs,compression ratios,bore sizes,head material,cubic inches,nitrous etc etc have different timing requirements.
First i would like to say the vaccum advance is not the full advance mechanism.The mechanical advance inside the dizzy is.The vac advance is for low rpm loads and part throttle loads such as pulling an incline at low rpm in high gear etc.(fuel economy).Where the rpms are not high enough to activate the centrifugal (mechanical advance) completely.The vac is to low or none existent when the throttle is wide open for it to work.That is where the centrifugal (mechanical) is working compltely.I do not use it in performance applications.It is a pita to get it set correctly.Most have an adjusting screw inside vac hose connector if you want to use it.Just run it up till your engine pulls strong under a load at lower rpms than the centrifugal advance activates fully.BE prepared to chase your tail trying to get it and the mechanical to work together.lol
What I have found to work great in daily driver/hotrod is disconnect vac advance completely.Play with different spring tension on the centrifugal advance so that it gives the correct timing needed under different driving conditions.



I am not going to show every little screw.If you have mechanical abilities you can figure out how to remove a few little screws.lol

The key is to figure out where you want your base timing (static) at idle and full advance timing then do the recurve accordingly to acheive this.

FULL ADVANCE
The general rule of thumb is naturally aspirated 9.0 to 9.5 Static compression ratio engines need around 38 to 40 full timing 9.5 to 10.5 35 to 38 and 11.1 plus no higher than 35 degrees.

BASE TIMING
Static timing or what we refer to as base timing at idle with vac advance unhooked.depends on engine build.But the camshaft in general dictates the rpm and base timing.
Genaral rule is
270 dur 600 to 800 rpm @ 10 to 12 degrees timing
280 dur 900 to 1000 rpm @ 12 to 14
290 dur 1000 to 100 rpm @ 14 to 16
300 dur 1100 to 1200 rpm @ 16 to 18
310 dur 1100 to1400 rpm @ 18 to 20


EXAMPLE
I want 12 degrees base timing and 38 total.
38-12=26 then I would need my centrifugal to advance 26 degrees total.
This engine I am working on is 9.8 static compression and a 270 dur cam with factory iron heads.My experience tells me i will be close with this timing.But you will have to figure out what your engine likes then recurve to make it work.

Ok now down to business.
1.Once you have the cap and rotor button removed.Remove the vac advance arm and the 2 screws that hold the vac canister to dizzy housing.

2.Remove the 2 screws holding the gromet the electrical wires pass through.

3.Remove the dowel pin that holds the reluctor on the dizzy shaft.Just use a punch to tap it down underneath the reluctor.Then remove the c type clip from inside the center of the shaft.


4.Pry the reluctor off shaft.Be careful not to pry on the teeth put screw driver or pry bar under the main body of it.mark this because it does go on 2 different positions and if installed wrong your dizzy could be out of phase.
Attached Images
File Type: jpg dizzy 3.jpg (57.1 KB, 3 views)
File Type: jpg dizzy 4.jpg (54.3 KB, 4 views)
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Old 12-20-2010, 03:03 AM   #2
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5.Remove the 2 screws holding the breaker plate in place and remove the the plate.

6.Now you can look to see what your advance cam is set on.It will be labeled with the letter L and a number on both ends.The end that is mounted over the stop tang is the one that is the advance.The one on the opposite end is a different number and if it is what you need you can flip the arm over and position it on the stop tang.

EXAMPLE
8L = 16 degrees advance
10L = 20
12L =24
13L = 26
14L = 28
15L = 30
15L = 32
18L = 36

7.The centrifugal advance will be the number on the arm multiplied by 2.
EXAMPLE 8L would be 16 degrees of centrifugal advance.
Add that to what your base is going to be and see if it totals to be what you want total timing.if not change it accordingly. Local parts store should carry an advance curve kit.I have found that earlier dura sparks as in pre 76 usually have the size advance arms i need but you can weld the slot to make it not advance as far or bend the stop tang on the ones that have to much.widen the slots that do not have enough advance.I have measured them and aproximately 1 mm = 3 degrees of timing.
IF the opposite end has closer to the number you need just flip the arm over to that side.
Attached Images
File Type: jpg dizzy 2.jpg (87.1 KB, 6 views)
File Type: jpg dizzy 5.jpg (77.3 KB, 10 views)
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Last edited by waypastcrazy; 01-01-2011 at 05:06 PM.
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Old 12-20-2010, 03:23 AM   #3
waypastcrazy
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As you can see this dizzy has 18L which is 36 degrees of centrifugal advance which with my intended 12 degrees base I would have 48 total which would be way to much.The other end of the arm is a 13L which is 26 degrees. That plus my 12 base is 38 total.Which exactly what I am wanting.
One other thing I would like to emphasize is dont take short cut and take away or add a lot to the base timing to get the total advance.you can end up with run on or dieseling or hard starts. less than 10 or 20 bas will give problems on most.

Now as far as the springs go they control how fast the centrifugal advances.Basically how fast the engine gets from base timing to full timing.On a street/strip hotrod I change the springs so that I have full timing in around 2500 RPM and in most cases this will give me the correct timing when the engine is at cruise or part throttle without the vac advance.
I hope this explains the recurve.If not I will try to edit it best I can.And as I learn more if that be the case.thx and I hope it helps.
Clyde
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Last edited by waypastcrazy; 12-20-2010 at 03:34 AM.
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Old 12-20-2010, 08:00 AM   #4
QWKSNKE
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Nice write u p Clyde...

Moved to the FAQ/info area
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