setting mechanical advance

[m_m70]

OK I have read a ton on this but still confused. I have been playing with my timing curve and found that it runs best with the light spring on one side and the medium spring on the other. also have 20* limiter bars on it (pertronix flame thrower dizzy).
So where I am getting confused is we seem to shoot for 36* total timing which I get to 34* with the vacuum advance disconnected at 3000 rpms. When I connect the vacuum advance it shoots to 48* at 3000 rpms. I have read that it doesn't matter what the timing is with the vacuum advance hooked up and some say disconnect all together and some say it's going to torch the motor the way I have it set now. I'm not hearing pinging (tough to hear much with the top off and loud mufflers).
So what do you all think?

Thanks for any input........

 

[broncnaz]

Vacuum advance is more based upon engine load than actual advance. When the engine is loaded the vacuum goes down and so the vacuum advance retards.. Some people factor in vacuum advance but I dont. It never seems to run as good with vacuum figured in. A street driven engine can use lots more than 36 degrees at certain times. Most of the 36 degree and thats it crowd pertains more to racing where you pretty much go from idle to close to redline and after 3000 RPM the engine cant really handle anymore timing. plus a lot of those engines may not pull enough vacuum to even work a vacuum advance properly.
As long as you get good driveability and no pinging your fine. Vacuum advance helps fill the gaps that the mechanical misses out on. basically you want as much timing as you can get wth no pinging. Once you start pinging you just back it off till it quits and you should have the best all around setup. Pinging actually causes power to drop so while you may not hear it you should feel it. Ive been in some loud vehicles and you can usually hear it when it happens.
Overall you just find whats best for your setup. but the no more than 34-38 by 3000 rule is good to follow for the initial and mechanical.

 

[bax]

I run a mechanical advance. no vacuum I tend to set up to be all in at 2500. Also different engines can take more advance than others. As stated above the pinging is pretty much your limiting factor. Better gas will help but pre ignition will kill a piston. 34 degrees of total advance, you should be very safe.

 

[m_m70]

So this mornings ride in was interesting. Where I live I have to go up and over a pretty steep hill about three straight miles on an incline. Usually do this in second gear (three speed manual) but the motor was totally cutting out. Shifted to third and it was a little better at the lower RPS but still had issues. Felt like rear end gears jumping teeth (rear end is fine, just felt that way) so I pulled over and disconnected and plugged the vacuum advance. It only "cut out" once after this and drove the rest of the flat freeway drive fine. Thinking I should retard the timing a bit and pull the limiter bars out to see what happens. I really don't want to destroy this motor. Kind of hot-rodded around the complex when I got in and it drives great without the vacuum advance hooked up. No hesitation from take off and plenty of power. Got me stumped...........

 

[xcntrk]

Once you know your desired total advance, the rest is easy. Dyno testing has shown that most small block Fords with 9:1 to 9.5:1 compression make peak HP with 38-42 degrees total advance. How much mechanical advance is possible out of your dizzy is based on which reluctor arm you have in place. If you have a 10L then your dizzy is capable of 20 degrees centrifugal advance, if you have a 15L arm - 30 degrees centrifugal, and so on. So if your desired total advance is 38 degrees, and your dizzy can spin out 20 degrees, then your initial timing should be 18. The springs simply change the rate at which the centrifugal advance is applied (i.e. less spring tension means a quicker onset of advance - at lower RPM).

I use lightweight springs with 10L reluctor arm, 20 degrees total centrifugal advance, which comes on by 2,500 rpm with the soft springs. Works great and only costs a whopping $5 plus a good timing light.

http://www.summitracing.com/parts/MRG-925D/

 

[67RT]

Set initial by vacuum and ear.with vacuum plugged. Adjust idle, then idle screws to max vacuum, back off 1/4 turn, adjust idle again, the see where your all in timing is. Limit it to the number for small blocks. Keep vacuum advance connected. Good for fuel economy. Also, yor motor may not want all in so early. Depends on cam, carb, cubes, etc..

 

[broncnaz]

You also have to make sure your jetting is correct as well as if your lean you wont be able to use as much timing.

 

[m_m70]

thanks for the replies. I'm thinking the issues I had this morning may be a short somewhere cutting the juice to the motor...........Wiring is not great. picked up new AAW harness 10 months ago and just haven't had time to install.
It's a really weird feeling though when it cuts out. Like a split second and it has the feeling of a chain on a bike/motorcycle jumping the sprocket. really intermittent though. Cant re-create if I try.
any thoughts on that?? may start a new thread.

Thanks!

 

[WheelHorse]

When does your vacuum advance sign on/off?

You'll need a vacuum hand pump to watch it as you apply vac to see when it starts to advance and how quickly.

 

[pcf_mark]

What he ^ said

Vacuum advance only adds timing when you have vacuum. You need to measure the vacuum amount versus timing added if you want to know what it is doing. You should know the max amount of advance if you put manifold vacuum to the distributor at idle.

Also vacuum advance is linear. If you know two points you know the curve. Cruise around town with a vacuum gage and record your readings to learn what vacuum you are pulling. Then you know how much advance is being added at each point.