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Old 07-04-2008, 01:29 AM
 
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timing issues

Im putting a pro billet distributor in a chevy 350 with basic edelbrock intake and a good sized cam with a msd 6al box. My question is: what stop bushing do I need there are 4. The biggest one will stop the timing advance at 18 degrees and the smallest will let it advance to about 30 degrees. Dont know what I would need. The other thing is: the springs on top of the distributor for the mechanical advance. As you know the lighter spring the faster the arc of the timing curve. Im just not sure what to use, when is my timing curve supposed to start, as in rpms and how steep. If someone could shed some light that would be sweet appreciate anything. THX

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Old 07-04-2008, 07:59 AM
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posting that you have "a good sized cam" tells me nothing...
(I did chuckle, does "good sized" mean it fits really good on the bearings?)

read this excellent timing basics article based partly on a cam's duration.....

http://www.gnetworks.com/v4files/bar...withimages.pdf

then, with all the specifics about your car and motor combo written down call or email MSD tech....

ps: (can't find it on my computer "but") MSD has their own excellent applications questions bulletin board just for their products
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Old 07-04-2008, 08:04 AM
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Quote:
Originally Posted by fireman147
Im putting a pro billet distributor in a chevy 350 with basic edelbrock intake and a good sized cam with a msd 6al box. My question is: what stop bushing do I need there are 4. The biggest one will stop the timing advance at 18 degrees and the smallest will let it advance to about 30 degrees. Dont know what I would need. The other thing is: the springs on top of the distributor for the mechanical advance. As you know the lighter spring the faster the arc of the timing curve. Im just not sure what to use, when is my timing curve supposed to start, as in rpms and how steep. If someone could shed some light that would be sweet appreciate anything. THX
Centrifugal timing will usually begin as low as 800 RPM (this depends on the spring combination you select).

The selection of the return springs is determined by the RPM you want the Total Mechanical Timing to be "all in". Usually this would be in the 2500-3000 RPM range. A combination of the two springs will yield the results you want. Many times using one medium and one light spring will get the results desired.

The selection of the "stop" bushings will be determined by what initial timing you select and then installing the bushings needed to limit the Total Mechanical advance to whatever you want it to be.

i.e. Let's say you want the Total Mechanical advance to be 36 degrees and you want the initial to be 18 degrees. Then you would want to limit the amount of mechanical advance to 18 degrees (36 minus 18). In this example you would use the 18 degree stop bushing.
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Old 07-06-2008, 10:17 AM
 
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hmmmm

well the reason I used the term good sized cam is that I have know idea what size I have it just lops pretty good, I bought the car with the cam in it. to give some a duration and lift size would be impossible. As for the stop bushing I have a question about initial timing whats that???? Off the harmonic balancer??? Im just having a hard time understanding your procedure to find what stop bushing I need. Thanks for the help....
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Old 07-06-2008, 11:09 AM
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I am going to do the same installation as you pretty soon, I have the distributor and coil just need time.
I will have 16º initial, that´s the timing at idle with the vac cannister disconnected.
Your motor would like 34-36º total so using my 16º we subtract 16 from 36 and get 20º, the nearest bushing to that is the blue one, the one which came installed in your distributor.
Now go on to the advance springs charts in the instruction sheet and decide how quick you want the mechanical advance to come in.
I will be going for example E as my cruising rpm´s are under 3000.
If you are going to use 12º initial, go for the silver bushing but the springs you´ll have to experiement with.

As you are so unsure I´d suggest starting off with the blue bush and the springs that came installed.

Here´s the MSD manual for other to see. It´s all in there.

The spin doctor Good info here too.
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Old 07-07-2008, 09:46 AM
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fireman,
FIRST, use your timing light to read and write down what are the idle base degrees (at what idle rpms) and what is the centrifugal all in rpms (and curve) on the old dist BEFORE you pull it off the motor....

those numbers tell you which bushings and springs to use for a baseline set up on the new dist....

a crude but simple/quick way to find out "very roughly" what is your cam duration is to read the max idle manifold Hg possible with a vacuum guage versus needed idle rpms to make that much Hg...

click on each of the cam graphs on this link to read the idle Hg and needed idle rpms....
as cams cams get bigger, they make less manifold vacuum Hg at idle and need more idle rpms (and that's what makes a lopey idle...low idle Hg=crummy A/F mix="rumpity rumpity")

http://www.compcams.com/Technical/DynoSheets/
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