when is it time to upgrade throttle body?

[Samsquanch]

Hey everyone, im riding a 2003 fatboy, about to install some 98 big bore jugs and 10-1 pistons, will probably have the heads done before putting them back on, and using the andrews 37 cam that is currently in the bike. exhausting through a white brothers e series pipe.

now my question is about the throttle body... my combination of parts should be a strong mid range build, not a high rpm screamer, is my throttle body and injectors going to hold me back? how much am i potentially leaving on the table if i stick with stock throttle body and injectors?

[Don D]

Maybe a little, the throttle body anyway.
So willing to spend 500+ and retune for 3 horsepower  plus or minus?

[Samsquanch]

Maybe a little, the throttle body anyway.
So willing to spend 500+ and retune for 3 horsepower  plus or minus?

Thank you! That's what I was assuming.

[Samsquanch]

This thread is getting alot of views, makes me think that many people could be curious about the same subject.

Might be a good idea if someone who knows (hd street performance and other pros) could chime in and lay out some general "rule of thumb" regarding time for new throttle bodies? Were some years bad, some better? Approaching X amount of power requires bigger throttle body? Operating over XXXX rpm requires bigger throttle body? Stuff like that.

[Pirsch Fire Wagon]

Throttle Body Up-Grades are necessary to take advantage of the additional Volumetric Efficiency [ the actual amount of air flowing ].

That being said, The Throttle Body itself is most often not the only parts change in and of itself. With more air come the need for additional fuel often. Larger Gram Injectors may also be necessary. Your Tuner should be able to describe in more detail the relationship.

IMO: You have to first determine if it's worth the additional expense [usually about $500 installed]. When adding this type of modification what you're really doing is getting the most Horsepower. But, that may only be 5 – 15 HP. Perhaps more with a "Good" Build.

You'll also need Tuning on a Dyno for best results, another $400 - $800. Even with a PV or TMax, it's necessary to Tune it initially. Especially with an "Auto-Tune" system. Otherwise, corrections tend to exhaust the RAM and cause unwanted problems as it is constantly receiving data and writing a new file time after time.. Additionally, Lower Torque with on/off throttle can be less than desirable and can be very problematic.

The entire system has to be matched, tuned, and built for best results.

IMO you build to Volumetric Efficiency from the Bottom Up. You may not need all but will need some: Crank, Cams, Head Work, Lifters, etc.

But, for what it's worth to the Rider, just replacing the Intake with smoothed Runners and Ports is all you would need to get the best Torque and operation in the 2,500 – 4,000 RPM for optimum riding.

Yeh, I said it.

[Bafflingbs]

You could probably move up to a 50mm, pretty comfortably. I wouldn't go beyond that. I'm pretty sure the 50 can support up to 120hp. The trouble with going too big, is you'll lose low-end torque. The stock tb's on the 2014-2016's is 50mm. I had mine ported for my 110" (probably had no effect) and ended up with 117hp/130tq. I'm very concerned about my low-end torque, jumping to a 55mm. I'd gain probably 5hp, but my torque curve will probably shift to the right. You'll also have to match your heads inlet ports (rails)

[04 SE Deuce]

   FI not same-same as carb.

[kd]

  FI not same-same as carb.

With a proper tuner and someone that knows how to tune the TB progressivity (is that a word?) and fuel shot can be dialled in on a dyno and give perfect results. My 62/64 HPI delivers over 130 tq at 2500 and 148 HP at 6000 with stock like manners in town and parking lots.  Can't (usually) get that with a carb.

[Hillside Motorcycle]

If the head/combination can't take advantage of the larger t/body, then no sense in using one.

[wolf_59]

I run a 57mm wild things TB on my 95" Fatboy is it needed Absolutely NOT!
is it cool HECK YEAH!
has it caused any running issues HECK NO!

[kd]

If the head/combination can't take advantage of the larger t/body, then no sense in using one.

  I discussed the recipe including the throttle body and goals at length with Mr Baisley and he reworked my MVA's to do exactly that. I wanted a beast around town that held on until the limiter which is a real tall order. Someone with experience needs to be involved and the owner MUST listen.  We are fortunate to have several builders of that quality frequenting this forum.    FWIW, I actually turned down an offer of much more rt side power to retain my touring and longevity.