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I am not familiar with the MSD distributor, but I think it is safe to assume that the centrifugal and vacuum advance systems are the same as a Chevy distributor.
I worked my way through college in the early Seventis with the GI Bill and a SUN distributor machine. I probably recurved a couple of hundred GM distributors. The only difference between then and now in that regard is the fuel availability and common compression ratio’s. That said, since the fuel AND compression ratio’s are lower today, the optimum curve for a street 350 of that day, is probably similar or identical to the one for today.
Assuming you are doing a hot street motor, the centrifugal should be about 22 degrees centrifugal advance in the distributor with about 10 degrees initial. In the kit is a bushing to put over the post that limits maximum advance. The centrifugal advance should be maxed at about 2500 engine RPM. This will limit the maximum range of the centrifugal advance, allowing you to dial in more initial advance. Vacuum advance would best be set by curving the centrifugal first, then using an adjustable vacuum advance like the one we used to get from Crane and dial in as much centrifugal as it will tolerate. It’s been 40 years so I might not remember the numbers exactly.
The centrifugal is easily curved on a GM distributor using one of the available kits. There are instructions on the package telling you which set of springs will give a particular curve using the included weights. It might very well be that the MSD distributor already has a similar advance curve. In fact, unless it is an all electronic curve distributor, I would expect that it does. I am not up on the latest MSD stuff, but I would not be at all surprised if they have an electronic advance system rather than centrifugal and vacuum.
All this said, if this is a street car that must pass emmissions testing, you will have to take a much different approach to distributor curving.
Hope this helps.Call the tech support number that came with the distributor they will have the best information for your application as to what weights and springs to use.
[quote=”MBDiagMan” post=164543]I am not familiar with the MSD distributor, but I think it is safe to assume that the centrifugal and vacuum advance systems are the same as a Chevy distributor.
I worked my way through college in the early Seventis with the GI Bill and a SUN distributor machine. I probably recurved a couple of hundred GM distributors. The only difference between then and now in that regard is the fuel availability and common compression ratio’s. That said, since the fuel AND compression ratio’s are lower today, the optimum curve for a street 350 of that day, is probably similar or identical to the one for today.
Assuming you are doing a hot street motor, the centrifugal should be about 22 degrees centrifugal advance in the distributor with about 10 degrees initial. In the kit is a bushing to put over the post that limits maximum advance. The centrifugal advance should be maxed at about 2500 engine RPM. This will limit the maximum range of the centrifugal advance, allowing you to dial in more initial advance. Vacuum advance would best be set by curving the centrifugal first, then using an adjustable vacuum advance like the one we used to get from Crane and dial in as much centrifugal as it will tolerate. It’s been 40 years so I might not remember the numbers exactly.
The centrifugal is easily curved on a GM distributor using one of the available kits. There are instructions on the package telling you which set of springs will give a particular curve using the included weights. It might very well be that the MSD distributor already has a similar advance curve. In fact, unless it is an all electronic curve distributor, I would expect that it does. I am not up on the latest MSD stuff, but I would not be at all surprised if they have an electronic advance system rather than centrifugal and vacuum.
All this said, if this is a street car that must pass emmissions testing, you will have to take a much different approach to distributor curving.
Hope this helps.[/quote]No, it’s not a smog car…. it’s a 69 el camino (mostly stock) 350 How would do you check ignition curve?
I expect that a little googling would provide information on distributor curve plotting far beyond what I can provide, but I will try to give you a synopsis:
Establishing the existing curve on a distributor machine makes it much more convenient, but it can be done on the car if you have a harmonic balancer with degree graduations up to about forty degrees, a tachometer and a timing light.
There are two curves to plot, the vacuum advance curve and the centrifugal advance curve. Let’s start with the centrifugal curve. Disconnect and plug the vacuum advance line. Set the initial distributor timing where it runs good enough to idle and allows you to raise the RPM smoothly and hold it at a steady speed. Have a pen and paper ready and write down the timing setting and RPM at idle. Raise the engine speed, say, 500 RPM and write down that engine speed and the timing setting in degrees. raise the engine RPM another 500 degrees and write down that engine speed and timing setting. Keep adding 500 degrees and writing down the information until the timing no longer advances further with increasing RPM.
With these numbers you can draw a curve on graph paper with timing advance against engine speed.
To graph the vacuum advance, use a vacuum source with a guage such as a Mity-Vac. Let the engine idle and write down zero in. Hg. and the timing setting in degrees. Raise the vacuum setting a few degrees, and again write down the vacuum value and timing in degrees. Continuing raising the vacuum a few in Hg. And writing down the timing in degrees until it no longer increases. You can then plot the curve for the vac advance with the plot showing in.Hg against degrees advance.
Hope this helps.
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