I will make an addition to MPGFanatics car/engine. I have been driving the car to a few local cruises. I did change out the points distributer, for an HEI conversion. Just used a distributer I had (unkown curve). With the initial timing set where the engine has good idle, and low speed pull, in first gear (3 spd stick) , it will ping in high on level ground with moderate throttle. I have had to use 89 octane fuel. I do need to recurve the distributer.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

That is very typical of the long slot 1973-1975 distributor to ping in the mid-range throttle (and worse if you have a 1974 Holley 1945 with the #58 stock jetting). This is one reason I prefer to swap it out with a short slot later distributor, or recurve the long slot and put a short looped spring in to limit the advance. This also can be solved by moving the posts outward putting more tension on the spring and slowing the advance but then the engine loses it's "pep" so it's a band-aid not a cure.

I'm reading and digesting comments, keep'em coming.

I haven't seen any combos with 3.55 gears. Would the lower rear gear 3.55 lessen pinging, or exacerbate detonation all things being equal compared to some of the examples of heavy cars with 2.xx gearing listed?

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

"Performance" gearing lessens the amount of work the powertrain has to accomplish to motivate the car, decreasing the load on the engine when accelerate. It also moves the cruise rpm of the engine higher so that the cam must be chosen to provide a powerband that coincides with the gearing as well (thus the distributor curve and carb profile should match this as well for optimum performance).

My original supersix build for the blue duster would ping in the midrange with a stock late distributor and 2.94 gearing, switched to 3.55's and the ping went away (more fun to drive, car was easier to move).

-D.idiot