Old guy new to bike EFI. Just when I think I have a pretty good grasp of basic performance, I keep stumbling across references to HPI 51 and HPI 55 throttle bodies I presume. Jeez! What do you have to build that needs to breath through a 55mm TB?

BTW, what are the spec's on the stock HD TB anyway? Size? CFM? Injector rates?

Also, I thought that by moving up to EFI I would get out of the Mikuni big carb rat race. Yeah. Right. Just a migration up to fancy electronic carbs now I guess.

 

What year bike. No sure the 99 sporty in your sig. If FI

 

Uh no. I know when you build a Sporty, the CV40 turn into a Mikuni 45 after about 100hp.

Up on the table is a 2007 Dyna TC96. What size is stock and at what point does that need to grow? Worked heads with larger valves over a big cam? Is stock sized pretty good and how far can it go? When to go larger bore and/or larger injectors? X hp or Y displacement?

 

Stock intake/injectors will support 110hp fine. Many go higher but reaching limits??

 

the stock tb is 46mm with 3.9 injectors, on my 107 build with stage 2 heads and 555 cams the duty cycle of the injectors was maxed out. ran it for the summer until I could afford to buy a hpi 51mm with 4.9 injectors. more power, fuel economy came up over all performance was a lot better hp and tq went from 101/103 to 108/112.

 

What you get with EFI is the ability to fine tune the fuelling to a much closer degree than is possible with a carb. More engine needs more air as well as fuel, hence bigger throttle bodies! If you are actually making mods to your bike are you doing so with the help of a local tuning shop? Id so they should be able to advise on the best combination of parts, including throttle, to get the best from your build.