I'm having a hell of a time getting the timing right in my Cutlass. The engine is a 1970(?) 455 bored .060 over, 10.5:1 compression, stock W34 heads, Torker 455 intake, Edelbrock 800 performer carb, Mallory Unilite (YT) with vacuum advance. I'm trying to get the total advance set to 38 degrees. If I set the initial advance to 8 degrees, the total advance is about 52 degrees. I've tried bending the advance limiting tab inside the distributer, but there is little or no change. At 8 degrees initial advance, the engine starts very easily, and at 38 degrees total advance, the engine runs beautifully at 3000 rpm. I just can't seem to get it set to get the timing to do both. Can anyone help, or at least point me in the right direction?

 

I have a 1970 455 Bored .030 over . Stock c heads Edelbrock performer intake and a performer 750 carb. I am also runing my timing at 8 but I'm getting a total timing of 50 . I went and got a new distributor and all . but i can get it right . If you find out any thing let me know

 

There are 2 separate timing functions - vacuum and centrifugal. Basically, you want 34 to 36 total centrifugal at about 2800 rpm total (centrifugal). The best way to measure is with a timing tape or dial back timing light. If you post your distributer numbers, I can look up the amount of mechanical (centrifugal) advance built in. The factory slows the curve, so you can use 1 lighter spring on 1 of the weights. This brings the curve in quicker. Set this with vacuum advance disconnected and plugged. Then hook up vacuum advance to manifold vacuum. You will probably need to adjust your idle. Then you can dial in vacuum; you probably need about 10* vacuum. This is done with a specific canister or a timing kit (like Crane). Most V8's run well with this curve. Vacuum advances timing based on engine load while centrifugal is purely controlled by rpm. This is how I do it...

 

Make sure the vacuum advance is disconnected while you are setting the centrifugal.

Norm

 

Your cam lift and duration will determine your intitial timing - most modified motors run the best in the 36 to 38 total timing range - what you need to do is take out some of the mechanical advance - take your total timing (36) less your initial timing (8) to determine your mechanical advance (28) - there is a pin in the distributor that the mechanical weight plate ramps up against - you keep putting bigger bushings on the pin until you reach the desired advance - this is with total disregard to the vacuum advance (unhooked) - once you get the initial timing and the total timing where you want it, you work on the vacuum advance - you will need an adjustable advance cannister and go for a total advance of around 48 or as much as the engine will take under load without knocking - 8 degrees initial timing sounds low - if you could post your cam specs I could get you a more accurate initial timing setting!!

 

Not always the best advice.

The factory used the ported (above the throttle valve) source to help compensate for the delay while the fuel, from the (accelerator) pump shot, traveled to the combustion chamber.

Like anything else, if there is a choice try it both ways, to find out which works best for you.

Norm

 

My distributor works well with no dead spots ...

I installed a petronix pointless conversion with matching coil, which lets me run a little more advance without pinging and one step lighter springs ...

 

Ported vs Manifold vacuum advance - there are many opinions on this. I use full manifold for several reasons. More advance at idle, which gives better off idle transition. More idle timing results in cooler operating temps. Ported timing came in with the somggers which wanted to raise operating temps. Just my 2 cents, others can chime in...

 

Easily done with a curve that is properly matched to the application. If additional (vacuum) advance is needed, the curve is not right.

Not true. See my previous post:

Beginning with their first use of vacuum advance systems, in 1936.

Manifold vacuum was only used, as one leg of the TVS (thermal vacuum switch) circuit, to control overheating in emission controlled engines.

Norm

 

Since vacuum goes away as the engine is loaded, how could one expect the engine to "knock" at full advance?

Norm

 

....Since vacuum goes away as the engine is loaded, how could one expect the engine to "knock" at full advance?....

Because it diminishes with the load, it does not just "go away" - if there is too much vacuum advance, it will knock as load is increased - tedster - if you are not street driving the car very much, I would just get an aftermarket distributor without vacuum advance - I don't have vacuum advance on any of my hot rods, and I have five of them!!