Distributor "curve"

[Steve D.]

I have seen the terms "changing the curve" or "re-curve" used in discussions about distributors. What does this mean and how is it done ?

Steve

[Walt McGaw]

Re: Distributor "curve"

Steve,

It means to change the amount of distributor advance (timing) at different RPM ranges. For instance, you can install lighter springs / heavier weights to get the mechanical advance to come in sooner (lower engine RPM's) or go the other way to have it come in later. You can also install different vacuum advance cans that will supply different degrees of advance at varying manifold vacuum. I'm more familiar with the weights and springs route.

You might do this to improve the vehicles off the line performance, or midrange rpm performance etc. Need to evaluate the situation and then discuss with someone knowledgable about what sort of advance curve would be best for you.

Walt

[Steve D.]

Thanks alot. *NM*

[Wendel H.]

Re: Distributor "curve"

For what it is worth, the enemy of distributor modification is unleaded gas. The kits which include soft springs and weird weights are the cause of migraine headaches. I spent many years undoing distributor modifications. In both breaker point and HEI systems, the springs are designed to fully retard timing at idle. I saw so many cars with advance that changed from 600 to 1200 rpm due to soft springs. Unstable idle? just use a timing light to see the damage you did. Here is what works. To adjust for unleaded fuel, the computer controlled timing systems limit power advance to about 26 degrees, and economy advance to another 10 degrees. A stock distributor will allow about 32 to 35 degrees centrifugal advance and 25 degrees vacuum advance. The symtoms are detonation at full throttle and surging at cruising loads. The slot in the breaker cam and the reluctor in HEI systems should be shortened by about 1/3.This will allow design base timing but limit total mechanical advance.Make a timing tape of masking tape by measuring the distance between 0-10 degrees on your timing tab. Transfer the measurement to the tape for 40 degrees and attach the tape to the balancer.Now you can test your work. A lump of braze from a good welder will reduce the size of the slot. For the vacuum control, a good dump truck unit has 10 degrees max. Units are available from Delco for both breaker point and HEI distributors.

[Steve D.]

Thanks Wendel *NM*