I am putting in my SE 103" BB kit, SE CNC heads, HPI 48mm throttle body and Andrews 57h cams on my Fatboy here as soon as the throttle body arrives. I am curious as to whether it makes sense to upgrade my injectors now or if I should just use the stock ones. I was looking at upgrading to #27654-06 the injectors from the SE 50mm throttle body but if its not necessary for the build, I'll just save the money. If it will help get the engine the correct amount of fuel needed, I'll do it.
Thx in advance.

 

http://www.hemrickperformance.com/Pa...injectors.aspx

Maybe this will help you out,depends on hp and % of duty cycle. I was 90% duty cycle and had the 3.9 injectors went to the 4.9 . I believe this mostly comes into play WOT at higher rpms. That where the hp is made.

 

A little above my pay grade but just as a reference point, the MoCo 103" upgrade with CNC heads and SE259E cams includes the 58mm TB with high flow injectors; not sure if the injectors are 4.9 or 5.3. Understand that comparing the HPI 48mm TB with the MoCo 58mm TB is not an even comparison but just based on the addtional 10mm bore of the MoCo TB, I would sure consider the 4.9 injectors but I would be calling HPI and following their recommendations as a starting point and see how she tunes out.

 

As Joe has mentioned, once the given engine is calling the on the duty cycle of the injector to push up past 85-90%, you need to move up.
Scott

 

I don't think you will but I would contact Eric at HPI in Erie

 

Scott, I am going to press you a little on this subject. How do you know when the engine is calling on the duty cycle to push past 85-90%? Is that something a tuner looks for during dyno testing?

 

Not Scott but you look at pulse with in the data. Then you can put the data into a calculator. 16ms@ 6000rpm will be80%. 16ms@3000rpm is 40%.

 

This is what I'm refering to.
Thanks, Russ!
Scott

 

So, if the rev limit is set at, say 6500 and the pulse width hits 80% at 5800; larger injectors are needed or can the pulse width be programmed? An EFI challenged guy like me needs more details.

 

For a direct answer to your question, Your stock injectors will handle that sort of build with those heads and cam with no issues. I have tuned dozens with similar setups and the stock injectors did just fine, the new Daytona twinscan III has duty cycle shown real time. DJL, pulse width in simple terms is the amount of time the computer holds the injector open based on many inputs which are adjustable in most tuning devices. Fule pressure itself will have some effect as more pressure will let more fuel through in the same amount of time.The smallest injector that will supply the motor with enough fuel at maximum demand will give the best idle, startup and driveablitly.