240/300 distributor curve information


Well-known member
I have successfully made the 240/300 distributor swap! For those wondering how much difference it makes, I would say not as much as I anticipated.

In my case, I am using the Holley 1960 carburetor from 1956. This carburetor does not use a spark control valve, but has a check ball in the advance passages to perform a similar function. Due to the large Venturi the 1960 has compared to the 1904, the spark vacuum signal is much weaker and less responsive. To make the 240/300 swap, I made a small aluminum wire insert that fits inside the spark passage where the vacuum line screws into the carb. The wire fits into the venturi vacuum port and plugs it so the distributor now sees ported vacuum only. Before, the maximum vac advance at the port was 5.5” of mercury; now it will max out around 15”.

The stock setup consisted of an SCV-equipped 1904 carb and a step-stop loadomatic distributor with a single breaker plate spring. This setup was much more active and responsive than the 1960 carb with any loadomatic distributor. Converting to the dual-advance distributor has restored the advance responsiveness with the added bonus of much more total advance possible, allowing 40-50 degrees of part throttle cruise advance. This should help with fuel economy, but I haven’t logged enough miles to know yet.

To summarize, in my case, I feel like the swap has improved part throttle responsiveness and possibly fuel economy to some degree. If I was running the stock setup fully optimized, I’m not sure the difference would be as noticeable; however, this way I am able to keep the large-throat 1960 carb which helps with breathing from 3000-4000 RPM. The swap also makes it possible for me to use non-loadomatic carburetion if I want to at some point.

Now, with all that said, I’d like to dial the advance curve in with the new distributor. All of my testing indicates that this unmodified motor can’t handle more than about 30 degrees of WOT advance without pinging, but it likes a lot of advance early. So it looks like a super short curve is required, maybe around 20 initial climbing to 26 or so by 2000 and reaching maybe 32 by 4000 when the trans shifts. Does that sound close?

what about vacuum advance? Is 40-45 degrees advance at part throttle cruise (2500 RPM or so) reasonable, or too much?