STS-V Fuel Injectors Added more air, Need more fuel

My 2008 Cadillac STS-V is lightly modded — Spectre Cold Air intake, and Corsa axle back exhaust; STS-V fuel injectors may be next as a supporting mod.

Each mod so far has proven out on the dyno to improve the performance of the STS-V. The STS-V was factory rated at 469 hp at the crank, and mine is just over 500 hp at the crank now.

STS-V Fuel Injectors

An issue to keep up with as you tune is how the fuel injectors are doing.  Mine are running at a duty cycle of 85-86% as we saw on my HP Tuner gauge charts done during the Virtual Dyno baseline runs.

HPTuners Gauges Cadillac STS-V First Run Peak HP

HPTuners Gauges Cadillac STS-V First Run Peak HP

On a cooler day, with the air more dense, it would run even higher.  Above 85% duty cycle fuel injectors can tend to have less precise control of the amount of fuel injected from what I have read.

There are several different conventional wisdom STS-V fuel injector solutions.  Tim C added a Boost A Pump to his V; Jamie went to 60# Ford Racing (Siemens) injectors.

The Boost a Pump increases and regulates or stabilizes the voltage to the fuel pump to make sure that it provides a steady pressure at all RPMs and to increase the voltage to the pump which increases its output.  By increasing the pressure output by 30%, the injectors would then flow more at the higher pressure.

The fuel system in the STS-V is not boost indexed.  The STS-V uses a 58 psi fuel pressure with 42 lb injectors.  Using the index tools at Fuel Connector Connection for a 550 hp STS-V we would want 50 psi injectors. The Stock 42 lb injectors likely flow 42 lbs at 43.5 psi; at 58 psi they should flow 48.5 psi.   Increase the pump voltage from 12-14 to 17 volts and the pressure goes up 20-40% to perhaps a steady 70 psi.  At 70 psi the 42 lb injectors would act as 53.2 lb injectors.

Jamie went to 60 lb injectors in his STS-V.  This reduced his injector duty cycle from 90% at high RPM to 55% at high RPM.  Adding larger injectors requires that the new injector characteristics be set in the tune.

Siemens 60 lb injectors matched set

Siemens 60 lb injectors matched set [click to go to Injector Connection page]

I am considering adding STS-V fuel injectors, the Siemens 60 lb injectors as my next mod.  Then I could add the boost a pump with the injectors if needed, but I think the injectors will give me margin for my build.

Speed costs money; how fast do you want to go?

Have better advice?  See a problem with my logic?  Am I all wet? Do you agree?  Join the conversation in the comments!

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