scrappie

Just need to be educated a little on the principles of how a points dist. works. I understand how it spins with the cam and distributes spark in a specific order to cylinders. What I want to know about is curve or re-curving and changing counter weights, springs and how that affects performance and how one goes about doing it? I see kits sold to re curve? Is it better to just go Pertronix or HEI? is that the same in regards to curving? can you re curve an electronic distributer? I know a guy who has a 72 olds BB with a 66 dist. He said the 66 dist. is a "good one" what does that mean? Anyone feel like trying to explain to a knuckle head like me?

joe_padavano

First a little background. The flame front inside a cylinder is not instantaneous. There is a finite amount of time required for the flame to propagate from the spark plug all the way across the combustion chamber. Since this propagation speed is relatively fixed, as engine RPM increases you need to fire the spark plug sooner to ensure that combustion is complete in time to provide pressure to move the piston down on the combustion stroke. Unfortunately, you can't just arbitrarily fire the spark plug too early, as this leads to preignition. Firing it too late leads to lost power, since the combustion process doesn't complete until the piston is well on its way down the cylinder. This amount of time (measured in crankshaft degrees) that the spark plug is fired in advance of top dead center is called the advance (imagine that!) and the relationship of advance to engine RPM is the mechanical advance curve. We'll ignore vacuum advance for the moment.

Non-computer distributors use a set of weights and springs to control this advance curve. Centrifugal force flings the weights outward and the springs hold them back. A set of ramps or cams on the weights change the relationship of the rotor to the distributor shaft as the weights fling outward, thus changing the advance. Changing the spring force, the size of the weights, and the shape of these ramps on the weights will change the advance curve. Note that computerized distributors do this electronically, making it much easier to try different advance curves.

Changing to either an aftermarket electronic ignition conversion or a stock HEI does nothing about the advance curve, since these options still have the weights and springs. Only changing to a newer computerized distributor replaces the weights and springs with electrons.

Back before the Earth cooled, we used a Sun distributor machine to check the advance curve with the distributor off the car. The machine allowed you to spin the distributor at specific speeds and check the advance while doing that. You can also do this on the car with a degreed balancer.

I mentioned vacuum advance. This is a separate system that changes advance in response to engine load. Lightly loaded engines (as indicated by high manifold vacuum) can use more advance for improved economy. As engine load increases (and manifold vacuum drops) you want less advance to avoid detonation. Again, the computerized distributors do this with a MAP sensor and software. MSD is one source of these computerized aftermarket distributors. They use the brand name "E-curve" and the distributor uses a self-contained MAP sensor and microprocessor to control the advance curve electronically. You can change the curve with small switches on the side of the distributor body. I don't think they make one for an Olds yet.

Oldsguy

Very good explanation Joe thanks. I was waiting for someone to explain this because, as we all know, know something and being able to explain that knowledge are two different things. Doesn't MSD sell the 6 and 7 series contollers to allow adjustment of the advance curve on certain distributors (certainly their MSD distributors) where there is no computer in the car as you mentioned?

scrappie

Holy S@%t, thanks Joe, you the man! Who needs books if we have you?

Run to Rund

I have the MSD non-vacuum advance distributor and 7AL-3 box on one of my Oldsmobiles. The distributor has a number of "adjustments" meaning bushings to limit the amount of advance, and springs to vary the rate vs. rpm the advance comes in. For decades Mr. Gasket has offered about the same thing for points type distributors, meaning a stop bushing (only one thickness, however, whereas MSD has three different ones) and different tension springs. If you don't have vacuum advance, you have to compromise and give the engine as much mechanical advance as is reasonable without pinging, to keep the engine cool and gain some economy (while at low throttle openings). Lets say the "ideal" mechanical curve would be 12 deg. initial and 36 deg. total @ 4200 rpm. Well, you might go for 15-18 deg. initial and all in by 3000-3500 rpm as a compromise if the gasoline is good enough. In the old days racers might lock out the advance (and do it today too) with a constant 36 deg. The engine is very hard to start with that much initial advance.

The "ideal" curve is something that in the old days Olds engineers arrived at by driving the car and carrying with them a box full of springs etc. The factory curves in the 60s up to maybe 72 were pretty close. Of course the vacuum advance was an important part too, until the late 60s when advance curves were reduced for emissions reasons. On a performance basis, with a larger hydraulic cam, usually something like 10-12 deg. initial and all in by 3000-4200 rpm works well. Traditionally, quicker advance curves were used with automatics and a bit slower with 4 speeds. Then you work with the vacuum advance (perhaps with one that has an adjustable advance stop) to get as much vacuum as you can for economy without part throttle pinging under load.

For a very strong cam in street-strip usage, you might be using a non-vacuum distributor for the most stable ignition curve, and compromise with something like 18 deg. initial, 36 deg. total at 2500 rpm. The MSD distributor offers a bushing and springs to do just that. For street use, the overall curve is not quite as good as having a vacuum advance too, but will work. Some racers with low compression engines use 46 deg. total in the lower gears and the MSD box retards the total to 36 deg. in high gear.

joe_padavano

But does this system do a vacuum advance function?

joe_padavano

Just to add to this thread, there are a few people on the DIY-EFI list who have reprogrammed the ECU from the TBI cars to perform this electronic advance function, just as the CCC system does on the 307. You can use a MAP sensor to get the vac advance function and the 307 distributor and build your own "E-curve" system for a fraction of what MSD charges.

Run to Rund

The MSD 7AL-3 box does not have a vacuum advance function per se. You can use the "start retard" function that retards the timing 25 degrees (useful for "locked" advance distributors and crank trigger ignition systems), and can use the high gear retard function for situations like I described where the racer retards from 46 deg. to 36 deg. The MSD distributor can be locked or can be set for centrifugal advance. MSD has vacuum advance distributors as do other mfg. of course.

joe_padavano

That's what I thought. The difference is that only the E-curve distributor uses a built in MAP sensor (yes, right in the distributor) to perform the vac advance function electronically - those others you noted use a normal vacuum canister and a moving "point plate". The advantage to something like the E-curve is that you can easily tailor both the "vacuum" and "mechanical" curves through software. No more changing of springs, weights, bushings, or vac canisters. No bouncing of the point plate either. The DIY system I mentioned above does the same thing without the price tag.

tru-blue 442

This is such a good thread I thought
I would bring it back, it's older.

It seems we have attracted some beginners
over the last few yrs, and a refresher is aways
a good thing. Thanks Joe and the guys who
contributed as well, good question Scrappy.