Bigger is not better - Throttle Body that is.....

[FLTRI]

A lot of controversy over TB sizes and how a bigger than necessary TB is fine.
Consider this:
If a TB is 48mm feeds enough air to the engine and 58mm TB will only make the small throttle positions more sensitive, which in turn makes the "Drivability" of the bike less desireable.
Will it cause power? No
Will it take away throttle response? No
It just makes the bike harder to ride smoothly. In other words, with the 48mm TB it may take 1/8th turn to get the bike moving whereas it may only take a 58mm TB 1/16th turn to do the same thing.
Also that is the same for the TPS, which is basically a rheostat/potentiometer that has just so much resolution per movement.
Any others with experiences to share?

[Admiral Akbar]

Yeah,

But those with the 58 will say that the throttle response on the 58 is that that much better...    cuz it only takes 1/16 turn       Max

[hardyheadscom]

The days of finding out something like that are over. The days of carbs you could throw on a big "D"carb or straight pipes and go find out for your self. Now ya got to go and get a SERT FART ITT TTS ST PC TM or a BJ........... Now find somebody that will take your 400.00 and help ya find out.........it is what it is Just stick whith what`s proven.

[skyhook]

good post, bob...i ran my build 8k miles before adding t/body and injectors...power increased a good bit above 4k rpm...and it was harder to tune, especially the low throttle positions...so hard to make hp with 6 speed bikes, much cheaper to build for tq and keep the stock throttle body

[x52gnr]

Now ya got to go and get a SERT FART ITT TTS ST PC TM or a BJ...........

   If i had to choose I think I would much prefer the BJ.........

[nc-renegade]

   If i had to choose I think I would much prefer the BJ.........

Me too!

[IBARider]

Dang,  I hung in there with carbs until '06.  i would have switched in '99 if I had only known there were bj's involved.

Is this kinda like having too big carb?  You gotta have air flow 'velocity' in order to suck the fuel in.  Too big a throttle bore and there are low rpm issues due to not enough suck?  Hmmm  maybe there were bj's involved with carbs....

[Don D]

Interesting and I couldn't agree more.
Probably some sales and marketing interfering with technical facts cloudy up the information. One size fits all is a lot better scenario to make money with than multiple choices as we all know. Latus has it covered with the variable chokes that are easily changed on the large TB.
I get many calls from riders that have big bad motors that make 100tq at 2,000 and they often complain about how sensitive they are and hard to ride. This is related.
And the Mikuni throttle ratio is related.
I have suggested to some to pull out timing in the lowest speed cells and this helps a little, a band aid for sure. Just think it is ironic we get what we wish for then are not happy with the way the bike rides on the road.
Charts and graphs are all in order

[Bagger]

FWIW, here's a velocity approach by Johnson Engine Technology, interesting focus is on the manifold air speed:

http://www.johnsonenginetechnology.com/JETPAGES/parts/twin-cam-throttle-body.html

Our goal when we started our project was to offer our customer’s a high horsepower throttle body option for a lower cost than what is currently available. Right now, big throttle bodies are available from Screamin’ Eagle and other aftermarket manufacturers at costs between $400 and $800. They all produce good results, but the total cost of installation and tuning can be over $1000. For just a small increase in performance, this can seem a little expensive to many people. Instead of having to purchase an expensive new unit, we set out to see what we could accomplish by some simple inexpensive modifications to the stock unit.

All ’06 and later Twin Cams (except for ’08 FLH’s) come with a one piece 46 mm throttle body. Upon testing we found that this 46mm unit was limiting customers to about 92-98 HP depending on displacement and the level of performance work. We found the overall design of the part to be a little lacking (it was clearly designed without performance in mind). The port runner increases in diameter from 1.900” to over 2” after the butterfly and still gets larger in area until the port exit at the cylinder head which is 1.640.” Any intake manifold that is designed to increase engine performance has port runners that are smaller then the butterfly, which also has a constant taper to the cylinder head. This is done so that the air in the throttle-body is constantly increasing in speed (this reduces reversion and increased throttle response, it also helps prevent detonation by keeping the fuel in suspension (low velocity and reversion cause fuel to fall out of suspension which increases the likely hood of detonation).

We flow tested the stock throttle body and found out that it flowed 219 CFM at 28.” We then bored it out to 48mm (.090” bigger than stock). The manifold was flow tested with just the bigger 48mm butterfly and we saw an increase of 13 CFM (232 @28”) We then did a little bit of clean up porting and re-flow tested the throttle body with results showing an increase of 22 CFM (241 CFM @ 28”). After flowing the throttle body we installed the unit on a 103 cu” TC with a mild camshaft and 9.8:1 compression ratio (baseline power of 92 HP and 104 ft lbs torque). Upon installation of the 48mm High-Flow throttle body and proper tuning; the HP increased to 96 HP and the torque increased to 106 ft lbs. SEE DYNO

Seeing as the 48mm worked so well, we thought we would push it a little further and try going to 51mm. After re-boring the throttle-body to 51mm (an increase of .196” over stock), we flowed the throttle body and that’s when the bad news started. The 51mm throttle body only flowed 243 CFM @28” an increase of only 2 CFM over the 48mm unit. Seeing as the flow bench is not always a tell-all in performance, we then installed the part on the same 103 cu” TC as before and proceeded to test and tune the 51mm throttle body. The results were not good; the HP and Torque both went down about 3-4 units in the low and midrange RPMs, but the power above 4750 RPM increased by about 4. SEE DYNO

All in all I would say that the 51mm ported stock throttle body was a failure, because not many people spend a lot of time riding above 4750 RPM, but we were determined to make the 51mm throttle-body work. We thought about what we could do to increase the velocity behind the butterfly. Increasing the velocity and the CFM would increase the power everywhere and make better use of the big butterfly. The port runners were then cleaned up and some very high-tech epoxy was used of fill the ports (to replicate the port runners of our J.E.T. intake manifolds). After the manifold was epoxied and ported, we tested it on the flow bench and it flowed a lot more with the epoxy, 257 CFM @28”. Now we were faced with a manifold that was about 15% smaller, had the proper taper that an intake should have, and it flowed 14 more CFM (38 more CFM than stock!!!!) This could only mean that the port velocity was much higher in the manifold with the epoxy. So we took the 51mm High-Velocity throttle-body to the dyno. 
Unfortunately, we were unable to retest the epoxied 51mm High-Velocity throttle body on the same 103 cu” TC, so we took advantage of a very mild 106 cu” TC with a very good baseline on it and a stock throttle body. The 51mm epoxy unit was installed and properly tuned in. This is where the results turned for the better… way better. SEE DYNO

The HP went from 100 HP to 107 HP and the torque went form 110 ftlbs to 115 ftlbs of torque. We also noticed the power increases were produced throughout the entire RPM range and not just in the higher RPM areas. These results were very encouraging find and could only mean one thing; we accomplish our original goal.

[noliners]

My large 58 HP inc. Throttle body and injectors are hard to cold start, and the gas mileage sucks. Oh well just the trade off for Power. I would build the most motor for stock throttle body next time....

[Don D]

I am not sure this is a "velocity approach" or just a fix of a known issue. The factory TB vortexes, apparently Johnson has fixed this. If it works as advertised then great, problem solved. The next TB may need a different approach and things need to be enlarged and enlarging and velocity usually don't play well in advertising literature from what I have seen. No put down on the product no experience with it, but Dan Baisley fixes the TBs too and his work as does HPInc.

[Admiral Akbar]

"The days of carbs you could throw on a big "D"carb or straight pipes and go find out for your self. "

Well gee,,, I got a D around here in a box some place.. If I can't get this darned fuel ejaculation working I'll pull the slip-ons and give her a shot.. 

FWIW,
I was picking on Bob a little and IMO he's correct.. It's just like running too big injectors.. The TPS better be working real well at small throttle openings and the injectors sized right or I'd bet there are slow speed drivability issues.. You really notice the difference between an E and G at slow speeds. No reason FI is any different. I don't buy the velocity thing unless you start playing with tuned length intake tracks. Unless you want to play with plenum size, lenght, TB size and cams go with what other people have found that works. Just because a Larger TB don't work on one build don't mean it won't work on different build.  Gonna try a 57 WT on a 95, if it don't work out either build the motor to 107 or try the D.. 

Max,
(just kinding about the D)

[hardyheadscom]

What`s really sad is the guys that want big 116,124 126 and B2 heads  270+ duration they want to stay away from race gas and then want to put this on an 07.09 with no gear change and run two up cross country and be able to run with the best of the best at the track. .I know I can`t give them clean drivability with a 63mm TB at 2000 to 3600 rpm range at 20 to 40 Throttle position with that set up.

Example My carbed 124`s flirted with 150 Te +/- a couple HP from 135 to 150 depending on cams. I have a guy in here right now no matter what we try with an 08 stock with stock gearing. 130Te 130hp using a S&S Kit  .640 cam good flowing S&S heads Rozella HPI 58 TB. Know it has  Knife edged balanced Dark Horse prepaired 4.625 crank after twisting the S&S crank.
Now I have him convinced to go to chain,,,,,,,,,, last night he informed  me he ordered in a Baker 7 or 8 speed He has changed from SE to TM to Twin Etc now he`s got a Master tuner he wants to use I Kid You not.I`ll keep you informed on the Baker 7 speed.

Out of all my performance packages the responce from my carb kits vs FI my carb kits are doing much better with less effort at the tuning stage.just my findings.

[hardyheadscom]

"The days of carbs you could throw on a big "D"carb or straight pipes and go find out for your self. "

Well gee,,, I got a D around here in a box some place.. If I can't get this darned fuel ejaculation working I'll pull the slip-ons and give her a shot.. 

FWIW,
I was picking on Bob a little and IMO he's correct.. It's just like running too big injectors.. The TPS better be working real well at small throttle openings and the injectors sized right or I'd bet there are slow speed drivability issues.. You really notice the difference between an E and G at slow speeds. No reason FI is any different. I don't buy the velocity thing unless you start playing with tuned length intake tracks. Unless you want to play with plenum size, lenght, TB size and cams go with what other people have found that works. Just because a Larger TB don't work on one build don't mean it won't work on different build.  Gonna try a 57 WT on a 95, if it don't work out either build the motor to 107 or try the D.. 

Max,
(just kinding about the D)

i know............ .the point i was making those two components combined little alone by them selves  are the worst, combine them and set the air fuel ratio

it`s just not that easy to try suttle changes and get honest results without going back to the dyno and set the AFR and compare. takes allot of time and money for Mr public.

[redmtrckl]

On a carbed bike switch from a S&S 'E'[ to a 'G' and to a 'D' on same motor and you will quickly find out how hard it is to tune for low rpm/speed.
Even boring a G .100 over will be a challenge. Seems to me the TB's would be that much different, just software vs jetting and screws. Both a PITA.

[GoFast.....]

This thread should say bigger is not always better. I have about 350 miles on the 107 WT build. Everything about this build turned out perfect. The 55 hpi with Se 4.89 injectors works flawless, Starts easy, Idles great and make that Wild Thing build flat get it on all the way up to the 6200 revlimiter.

[Shadowdog]

Howdy Folks,

Thanks Bob et al for this post. I'll bet the folks that need to read it won't.

Best to all,

Shadowdog

[se]

i have  a 58 and you guys know  i like it altho i did not have to tune it and i also have access to a dyno when ever i want. picked up a few ponies and tq over on the right of the chart thats it. also the 9b i am running runs out of steam way before the heads do
Dan baisley wants us to run a tw68 for $hits and giggles probley do it over the summer .

[roadglide65]

A lot of controversy over TB sizes and how a bigger than necessary TB is fine.
Consider this:
If a TB is 48mm feeds enough air to the engine and 58mm TB will only make the small throttle positions more sensitive, which in turn makes the "Drivability" of the bike less desireable.
Will it cause power? No
Will it take away throttle response? No
It just makes the bike harder to ride smoothly. In other words, with the 48mm TB it may take 1/8th turn to get the bike moving whereas it may only take a 58mm TB 1/16th turn to do the same thing.
Also that is the same for the TPS, which is basically a rheostat/potentiometer that has just so much resolution per movement.
Any others with experiences to share?

Well Bob Iam going from a stock T/B to a 48mm HPI that I got on ebay we will see what the difference is. Any guess what it might gain.

[GoFast.....]

This thread should say bigger is not always better. I have about 350 miles on the 107 WT build. Everything about this build turned out perfect. The 55 hpi with Se 4.89 injectors works flawless, Starts easy, Idles great and make that Wild Thing build flat get it on all the way up to the 6200 revlimiter.

Thanks for the correction, but the issue isn't how much power you get from your TB it is the rideability in town as you roll over railroad tracks and the throttle gets a slight bump and jerks the bike. It's the ability to putt through a parking lot @ 1/32-1/16 throttle and have the engine be smooth as silk. It's the very small throttle openings that make it difficult to slow-putt when the TB is 51mm-62mm on a 95-103ci engine.
Sorry if I didnt make that clear enough, but it seems there are some that got it.

What makes you think the 55 hpi will not do that, I must have a more than usual perfect setup with the TMAX on this 107" 

[GoFast.....]

What makes you think the 55 hpi will not do that, I must have a more than usual perfect setup with the TMAX on this 107"

Still insist on baiting?

It's strange Bob that out of 5,900 members you are the only one that keeps bring this Baiting thing up.You need to give it a rest.

[txtech]

What makes you think the 55 hpi will not do that, I must have a more than usual perfect setup with the TMAX on this 107"

Still insist on baiting?

use your ignore button bob. 

[Showdog75]

I think ya'll two need to meet up and put the boxing gloves on and get it outa your systems.Lol  
 

[nc-renegade]

My thoughts are you size the TB for the max. CFM the engine is capable of.  This allows the TB to be nonrestrictive, yet keep the velocity maximized for the engine size.  This assumes you use a well designed TB and not a hogged out stock one done without consideration for the flow and velocity behavior of the TB

[wfolarry]

They buy cams too big because they want to make the big #'s so it just stands to reason that they wll do the same when it comes to throttle bodies. Always overbuild 'em then try to get them to be drivable AFTER they have that dyno sheet in their pocket. I've been trying to steer people away from the 'bigger is better' principle since the early 70's when I was working on cars. Even then they would buy the 850 to try & make their car go faster when a 650 was all it could handle. The years roll on but the thinking still hasn't changed: If it's bigger it must make more power.