The Ford Barn - View Single Post - 8BA loadamatic dist. with 2GC carb question

40 Deluxe · 06-04-2018, 11:48 PM

Question: How much vacuum will there be at the ported vacuum port on a 2G carb at wide open throttle? Consider this: Imagine the 2G carb and Chevy distributor are still on the original Chevy, cruising at 60 MPH. At part throttle, ported vacuum is high so vacuum advance is at or near max. For sake of illustration, let's say mechanical advance is at 20 deg., and vacuum advance adds another 15 deg., plus 10 deg. initial. Now we go full throttle to pass someone. 45 deg. advance will cause severe detonation but at full throttle, ported vacuum drops to zero (or very nearly so) as does manifold vacuum, so vacuum advance goes away and timing advance drops to 30 deg.
Now, if we only have vacuum advance (LoadOMatic) and we just lost our vacuum signal (ported) what happens to the timing? It will retard! At a high enough RPM, air flow through the carb may increase enough to regain some ported vacuum but I doubt it would be the same as the venturi vacuum the Loadomatic was designed for.
Remember, venturi vacuum is what opens the secondaries on a typical Holley four barrel so it is a fairly strong signal.

06-04-2018, 11:48 PM	#11
40 Deluxe Senior Member Join Date: May 2010 Location: now Kuna, Idaho Posts: 3,790	Re: 8BA loadamatic dist. with 2GC carb question Question: How much vacuum will there be at the ported vacuum port on a 2G carb at wide open throttle? Consider this: Imagine the 2G carb and Chevy distributor are still on the original Chevy, cruising at 60 MPH. At part throttle, ported vacuum is high so vacuum advance is at or near max. For sake of illustration, let's say mechanical advance is at 20 deg., and vacuum advance adds another 15 deg., plus 10 deg. initial. Now we go full throttle to pass someone. 45 deg. advance will cause severe detonation but at full throttle, ported vacuum drops to zero (or very nearly so) as does manifold vacuum, so vacuum advance goes away and timing advance drops to 30 deg. Now, if we only have vacuum advance (LoadOMatic) and we just lost our vacuum signal (ported) what happens to the timing? It will retard! At a high enough RPM, air flow through the carb may increase enough to regain some ported vacuum but I doubt it would be the same as the venturi vacuum the Loadomatic was designed for. Remember, venturi vacuum is what opens the secondaries on a typical Holley four barrel so it is a fairly strong signal.