I am finishing up my winter mods to my 2014 103" FXDF. I have replaced the stock cams with Andrews 57h and had the heads ported.

The guy that did my heads did some flow bench work on the heads, but also measured the throttle body itself while he was there. It's the stock 46mm one.

The heads flow 275 cfm @ 28" @ .500" lift.

The stock throttle body flows about 242 cfm at the same.

So now the stock throttle body is the restriction in the airflow. I want to replace it with a larger one, but want to get it sized correctly.

My dealer say to go with the 58mm SE TB. I don't think this is the correct answer as i think it would actually reduce velocity and not be the ideal solution.

I did some basic math....

The basic cross sectional area (not taking into account of restrictions) of a 46mm throttle body is 1662 mm2

The same for a 50mm throttle body is 1963mm2

So a 50mm TB has 18% more cross sectional area than a 46mm one.

All else being equal (which it isn't), if a 46mm stock throttle body can flow 242 cfm, then a 50mm unit should flow 18% more or 285 cfm. Probably more, as either the SE 50mm unit or the HPI 51mm almost certainly have better flow characteristics than the stock 46mm.

Theoretically, a 58mm TB would have an area of 2624 cfm. Potentially 1.57 times that of the stock 46mm - or around 379 cfm. Surely way too much, right?

Anyway...I know it's not that simple and this post has been quite long. But the basic question is.....55/58mm TB, or 50/51mm TB. And then SE or HPI? Pros and Cons?

As one last input, I will probably never add a big bore to this bike. Happy with 103 as opposed to 110 and if I want bigger I will probably build another bike with an S&S 124 or similar.

Thanks in advance!

 

A 50mm Throttle body will be more than sufficient for your needs.

 

Thanks - that's kind of what I thought.

Dealer says buy a 58mm, but I see no reason to do so, and think it would actually hurt performance in the low to mid range....

 

What MAP reading are you getting at WOT?

What throttle position gives you 100 KPA in the RPM range you actually ride?

How much riding time do you spend at say <10% throttle compared to the amount of time you spend at >90% throttle. I bet you would be shocked if you were to look at the data.

Big throttles tend make driving at the range you really ride very difficult to live with. Erring on the small side is almost never an issue in the real world of street riding.

Andy

 

Well....not sure about the MAP readings as the bike is still in bits right now. But I'll certainly check that out when I take it out for a ride. I have a Power Vision, so I can just set it to show MAP.

But - for sure most of my riding is in the 25% to 85% throttle range. I don't scream around at WOT all that much.

I have also read that too big a throttle body will make the bike "twitchy" at low throttle ranges, and that's exactly why I want to use the correct TB rather than just go "bigger is better", as I don't believe for my bike it will be!

 

I bet that when we plot your riding data in MegaLogViewer, you will be shocked at what you see. Most people almost never get over about 20-25% throttle in real world riding.

Over sized drive by cable throttles get very twitchy to ride at low speed. The DBW ones tend to be able to fix with the throttle map.

 

You are almost certainly correct. For sure all my riding around town and in traffic is at low throttle - and I probably only ever get to WOT on open roads and even then only for a short time while going through the gears.

I do prefer riding in a lower gear at around 3K or so RPM rather than lugging in the low teens or 2K range - but nonetheless, that's still only partial throttle.

It will be interesting to really look at it in the logs....

Anyway. Conclusion here is 50 or 51mm TB for sure. Now to find one at a good price....

 

Oh - and one more stupid question.....

Does anyone know if the A/C backing plate mounting holes on the 50mm SE TB are the same as the stock 46mm one, or will I also need a new A/C assembly? I am currently running the DK Customs Outlaw 636v and like it a lot.

 

I have a 2000 Road King frame & title (e-Bay'd the rest) and purchased a new 124CI & Spike 6 speed. It is a 2006 + Model design so I have been told. When asking other members on this great forum about throttle bodies in the real world, I was told this same thing as the other members here have stated by whittlebeast & OldEnuf2NoBtr. A member has the exact same motor on the same bagger application. He runs a 50mm also. Taking his advice of what he already did, I purchased a HPI 50mm TB. This member also gave me his results on injectors. He tried larger ones and, while from start to the end of the 1/4 mile was better, fuel millage suffered in regular day to day driving.

I know absolutely nothing about EFI as I am from the carb days, but I am sure as I am pissing money away, I will learn. This forum and the great members whose prior experiences are shared from those who have already been there and done that, THANK YOU for making my life much easier.

In my business (suspension), I have found that the MOCO & dealerships know absolutely nothing other what caveman practices they are programmed with and to get the maximum pay (fixed rate) by reducing time and cutting whatever corners they can while not caring or even learning what a good job is. That is why I tossed everything other than the frame & title, the only thing good on a HD. When I sell suspension I suggest purchasing your own tools and motorcycle lift & doing the job yourself. The job of a first timer will be superior to that of a dealership (in most cases) with a knowledgeable person supporting you.

The Dealership is the best place to buy officially sanctioned Chinese T-Shirts which I would never own.

 

The mounting holes will be the same, you'll need a new backing plate for the new throttle body if you can't get someone to modify the existing one.