fuel injection nozzles & jetting challenge

The determination of nozzles & jetting in a new fuel injection setup can be a challenge. Our last racer combination was a auto drag race bracket blown methanol engine. It was 438 cubic inches with 12 to one compression. Our jetting calculator determined nozzles & jetting for an optimum air to fuel ratio of 3.4 pounds of air to one pound of fuel. This remained steady in different air densities, blower overdrives, and engine RPM ranges for over 400 drag races.

Our new engine combination is mechanically identical to this previous combination except for a new rotating assembly. Different rods, pistons, & crankshaft now measure 478 cubic inches with 8.3 to one compression. With only these changes, the old nozzle & jetting setup was drunk rich. New nozzles & jetting were quickly revealed in testing that makes a lot higher air to fuel ratio. It is now 4.2 pounds of air to one pound of fuel. All else in our racer combination including the fuel injection & fuel pump remain the same.

We were all surprised how much difference the changes affected the ratio of pounds of air to pounds of fuel. Even with all of our experience with this combination, the determination of nozzles & jetting for the different rotating assembly required a lot of time on the calculator as well as the test track. The calculator allowed us to easily determine a new jetting combination with the same fuel pressure as before (for good engine response) by holding the total jet area the same. It allowed us to stagger the nozzle sizes, holding the total jet area the same.

We converted to port nozzles soon after the first test runs. The calculator allowed us to determine fuel split (illustrated in our fuel injection books) between the hat & port nozzles and the new jetting sizes to achieve that fuel split. It allowed us to determine the nozzle and main bypass sizes to adjust for air density changes from day to day and from hot afternoons to cool evenings.