I have heard a lot of people talking about re-curving the distributor. What is that and how is it done?

This process refers to changing the amount of centrifugal advance and the rate at which it occurs; the advance mechanism is composed of two pivoting weights retained by little coil springs - you'll see it when you remove your distributor cap and rotor. Changing the type of weights and springs alters the amount and rate of centrifugal advance at given engine rpm's. Your spark advance at idle is determined by the position of the distributor and the specs of the vacuum advance canister on the distributor; as engine (and distributor) rpm's increase, the combination of weights and springs determines how much spark advance is added and at what rate it increases (as engine rpm increases, the spark has to start sooner on the compression stroke so the fuel burn takes place at the optimum point of piston travel for best power and economy - this mechanism manages that process). It can get much more complex than that, but this is a basic explanation of "how it works".

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JohnZ
'69 Z28 Fathom Green

Thanks a lot John that makes sense. Where can I get a set of these weights and spirings and how much would I expect to pay for them. Also where would I feel the change for the better? acceleration? passing? Let me know.

OK, I'll give it a shot and the Iman can fix the error of my ways.

Imagine the engine at idle and the timing set to 8* Before Top Dead Center (BTDC). What that means is the spark is igniting the air-fuel mixture in the cylinder at 8 degrees of crankshaft rotation before the piston reaches the very top of the cylinder, or Top Dead Center (TDC). This allows the combustion event (flame) to begin and be in full force by the time the piston starts back down the bore for the power stroke.

As engine speeds increase the spark must happen sooner or the combustion event will happen too late. Increasing the spark timing is called advance.

Advance can be changed in non-computer controlled cars in two ways, mechanical and vacuum.

Mechanical advance occurs when a set of weights and springs, connected to the distributor driveshaft, are spun during engine operation. As engine speeds increase the weights fling themselves outward due to centrifugal force but are limited by the springs. The heavier the weight, the sooner the timing will advance. Likewise, the lighter the spring the sooner the timing will advance. Combining these two factors will determine your overall mechanical timing curve. Too much, or too little, mechanical advance can cause detonation, backfire, loss of power, etc. You will find the weights and springs under the rotor.

Vacuum advance is used on some street cars along with mechanical advance. As engine loads increase timing must be reduced in order to keep the engine from pinging. As engine load increases, vacuum falls, and supplemental vacuum advance goes away. For example, at cruise speed you may have 8* initial, 12* mechanical, and 10* vacuum advance for a total of 30* BTDC. If you step hard on the gas, engine vacuum falls, and the timing reduces to 20* BTDC. This helps to reduce the chances of ping.

You also must limit the total amount of timing. Typical street cars have a total initial + mechanical advance of between 36-38*. However, this, along with the mechanical advance curve and vacuum supplement, is dependent on your engine combination and driving style. If in doubt a good distributor shop should be able to help you out.

If you perform a site search on IgnitonMan's posts you will find some very informative information.


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mr gasket makes them i think (what dont they make?), around $10. any of your local auto parts stores should have them or get them for you.

If you're not experienced at this, I'd recommend you leave this job to a pro, preferably one who has a distributor machine so the desired curve can be established accurately. Doing it by trial and error with "kit" weights and springs is pretty chancy unless you have the right tools and a dial-back timing light so you can chart the curve with each change. Few of the "kits" give you what you want out of the box, requiring lots of trial and error, and there are niceties to be dealt with such as advance-limiting bushings, etc. to really do the job right. Pros with distributor machines have all this stuff handy and know what to do with it.

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JohnZ
'69 Z28 Fathom Green