recurving a stock distributor

My motors manual only goes back to '75, but it shows 4000-4600 depending on which distributor was used. FWIW this is pretty much typical of what I remember older cars using as well. The old point distributors sometimes didn't achieve full advance until 4800 rpm.

Once you've got your slots welded up so you have the correct amount of mechanical advance just install the one light spring and leave the heavy one out. Don't worry about the empty spring slot. With just the one light spring in place you should have full advance by 2000-2400rpm. You may want to increase the total timing to 38 degrees or more on a low compression motor. I've seen as much as 42 degrees mechanical advance work real nice on bb with little or no compression. You can also run up to 52 degrees advance with the vacuum advance hooked up. Dave

Hey six-barrell, who is FBO? Thanks

Will the stock specs leave power on the table?
It really depends on what the stock distributor has in it, and how much that cam changes things.
A lumpy cam likes more initial timing than a stock cam. Also lower compression will want a bit more advance high compression. You might end up setting as high as 18 degrees at idle but 15-16 seems like a good starting point. If there is enough vacuum at idle to get the vacuum advance to contribute at idle (in gear) then it may be fine with 12-15 inital.

For total advance (w/o vacuum) Mopar Performance recommended 38 deg BTDC for production iron head B/RB engines. With vacuum they recommended 56 degree BTDC for these engines. (Instruction Sheet DCF-194 for Mopar Performance Electronic Ignition Kit p.6)

Be conservative. Start with these numbers or less. Because under load (like top gear, WOT) too much advance can be destructive.

You could wait until the engine is done. Find what initial it likes. Then knowing how much advance is in the slots, weld up as needed to get the initial and total you want.

Which Springs?
Yes. the early performance guidance (eg. Master Tech bulletin 1969) advised removing the heavy spring and stretching the light spring. However, if you shorten the outside of the slots, the heavy spring will not come into play very much and could actually be useful - especially on a heavy car.

There are competing theories on the best approach. This early advice is not the only way, and even DC/DC used other approaches. For example later on DC/MP liked to use a lot of mechanical advance, a very light spring and a long looped heavy spring. The engines would start easily and get roughly the advance needed at idle. Full advance might be 800-1200 rpm, with heavy spring allowing only a degree or two from there to the top. The p-partr tach drive dizzy was set up that way. My opinion is that this makes most sense for a very high compression race car, but not optimum for lower compression mixed use cars.

My suggestion for your application is to first get the slot length. Then, if the heavy spring with long loop is only controlling only a degree or two at the top of the curve (2500-4000) leave it for now. Then try lighter springs on the other side to see if bringing the curve to full advance earlier is beneficial.

With a heavy car and stock convertor or manual trans, the shorter slots might be enough. In fact you may find that instead of 1 light spring and one delayed heavy spring, it prefers two light springs.

for that cam in a low comp motor, I'd probably look at 18-20 initial, and 38-40 total timing

also, your dizzy advance plate will have a number stamped on it. that's the advance in distributor degrees (1/2 crank degrees). a 17 plate will show 34 degrees advance with a timing light. measure the length of the slot with some calipers, and divide by double the stamped number to get the amount you need to shorten the slot per degree (i.e. if you have a 15 plate, and the slot is .3" long, you need to shorten the slot .01" per degree, or .1" if you want to reduce the advance from 30 degrees to 20.)

when you weld up the slot, weld up the inside. this adds preload to the springs, which will cause the advance to start in at a higher RPM range. most stock distributors I've played with, the light spring is so light, it starts advancing at ~700 RPM or less. if your neutral idle is above where the mechanical advance starts advancing, you'll get excessive RPM drop in gear and an unstable idle. I like setting up my mechanical to start coming in at ~1000 RPM

FBO website:

www.4secondsflat.com