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Non-Vacuum Distributor

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marcm

marcm

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After a great 3 days of motivation at the Lake of the Ozarks, I came home and started putting my 948 together so it would be ready when the body comes home from the paint shop. Found out there is no vacuum line to the (Lucas) distributor. Google revealed information at https://www.aptfast.com/APT_Parts/Aseries_Parts/a_Ignition_Components.htm , says "If your compression ratio is above 9.6:1 you may be best to go with the non-vacuum road performance type distributor." My cylinders are running 150 PSI. Anybody have any experience with these? How does it change the spark advance?

- Marc "the other Masquelier"
 
All distributors (excluding fully electronic ones like the 123 product) use mechanical (a.k.a. centrifugal) advance. There are "bob" weights and springs under the advance plate that advance the spark in response to changes in engine RPM. Vacuum advance is mainly (not exclusively... mainly) added to distributors to improve part-throttle fuel economy.

It looks like APT is selling the mechanical advance version of the 45 series Lucas distributor. If you were looking for an older one from the 20 series you'd be seeking a 23D4 which was used in the Cooper-S A-series engines.

A more succinct answer to your question is that you don't have to have vacuum advance and that performance distributors may only use centrifugal advance. However, if you are running your car on the street, I don't think you need to stay away from vacuum advance distributors just because of the compression ratio. You can adjust your timing to avoid pinging/knocking and run almost any distributor. With today's gas prices I would use a dizzy with vacuum advance to maximize your fuel economy.
 
Marc,

My spare 1275 has a mechanical advance dizzy on it if you need to look at a working one. I believe they can pretty much interchange between vacumn advance and centrifigal advance as long as you plug up the vacumn port on the carb. In fact I'm thinking of sticking on the 1098 as I think I've got an issue with the timing and vacumn advance to see if it makes a difference in how the engine runs > 4,000 RPM. Bugsy seems to run out of gas in trying to rev the 1098 over 4-4,500 rpm. If indeed my vacumn advance is not working properly that would explain that.
 
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