The Avanti R1 was a 289 cubic inch carbureted version with a 4 barrel and the R2 was a 289 cubic inch version equipped with a supercharger. The R3 was a special performance supercharged variation displacing 304 cubic inches and very few of these were built. The R3 version came with a bored out 304.5 cubic inch V8 stuffed into the engine bay, modified suspension and 4-wheel disc brakes. Responsibility for the aggressive engine development plans fell into the hands of Andy Granatelli of Indianapolis 500 fame. Like the earlier 289 cid R2 version, the R3 was also supercharged, fully blueprinted to racing specs and hand built by Paxton. There were also R4 and R5 performance variations, but none of these were sold to the public. The experimental "Due Cento" with the only R5 dual supercharged engine built by Paxton reached 196.62 mph.

R4's were a non-supercharged version of the R3's. Approximately 140 R3/R4 engines were made. Only a fraction of these still exist. These engines display many serial number irregularities: some prototypes were not numbered; very early ones have RS stamped on the block; two experimental numbers are EX2123 and EX2132; one prototype number is RS1021; and others start with B1 and go as high as B153.