What size fuel injectors?

[kayakfisher]

I just completed the 103 stroker build on my 05' Road King and wanted some advice on fuel injectors. This build was completed to increase engine torque for pulling a lot of weight 2-up for touring. The whole top end is new at 10.5-1 compression with SE cylinders and heads, high compression SE pistons, roller tappets and rocker arms, 570 gear cam kit, compression releases and 51mm Horsepower throttle body plus other good stuff. The exhaust is a 2-1 V&H Propipe. Will the increased airflow through the motor require larger fuel injectors to achieve full torque and horsepower potential? I'm just breaking it in now but will have it dyno-tuned with my PCIII USB at probably 1,500 miles. I'd like it completely set up by then. Should I go up in injector size? Is so, what would you recommend?

Thanks!

[Mike]

The stock injectors would be fine with your combo. Larger injectors typically are required for applications exceeding 130 hp. In these cases we use the stock V-rod (VRSCA) injectors. These have proven capable of 170hp. I would be a little worried about an injector change and using the PC USB. It may work OK but Race Tuner has a facility to correct injector sizing which may make tuning more accurate. Along with larger injectors comes some difficulty with idle quality and control, cold start fuel, cranking fuel and warmup fuel. The PC will not be as user friendly in trying to correct these areas, back to SERT. This becomes more of an issue as the motor combo gets more aggressive. The formula for calculating injector size; HP x BSFC (brake specific fuel consumption, typ. .50) / 2 (number of injectors) x .8 (duty cycle). Your motor would calc. to 120 x .5 = 60 / 2 x .8 = 37.5 lbs/hr or 4.72gm/sec. This would suggest the larger injectors but experiance has shown the stock to be fine for this hp level.
Thanks, Mike