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[SBB]

If a TB is 58mm feeds sufficient air to your motor combination - a 62mm TB will only make the small throttle positions more sensitive - which in turn will make the "drivability" of the bike less desireable.

To large of throttle body will not result in less power....Having to large of thottle body will make the bike harder to ride smoothly at parking lot speeds.

But having to small of throttle body will result in less power...

Given those choices - I would prefer a slightly oversize throttle body vs. a undersized throttle body...  But the sizing of your throttle body should be based on your motors overall package - heads and cams being the most important factors - not your motor's cubic inch....

The throttle body on it now means I'm definately in the slightly oversize catagory.
Heads and cylinders are on the list.
Some day, I hope.

SBB

[Hoist!]

I tested both the 58 HPI and 62 HPI. The build was originally designed for the 58. It made basically the same top end power with the 62. But the lower and mid-range power, drivability, and mileage were all much better with the 62! That's with our drivetrain's flow characteristics.

Hoist!

[Black Diamond]

If a TB is 58mm feeds sufficient air to your motor combination - a 62mm TB will only make the small throttle positions more sensitive - which in turn will make the "drivability" of the bike less desireable.

To large of throttle body will not result in less power....Having to large of thottle body will make the bike harder to ride smoothly at parking lot speeds.

But having to small of throttle body will result in less power...

Given those choices - I would prefer a slightly oversize throttle body vs. a undersized throttle body...  But the sizing of your throttle body should be based on your motors overall package - heads and cams being the most important factors - not your motor's cubic inch....

Good info Scott, thanks. Heck I'm mostly doing this out of curiosity. When Good Howie asked if I could feel any difference. I was going to tell him the bike seems a little more responsive at take off. But I've also found sometimes when you make changes, you think there's a change but you can't put your finger on it. I can always re-install the SE 58 if we learn we can get more out of it. As I'm looking to change exhaust this winter, I have a lot of options to look over.

JW

[sadunbar]

Good info Scott, thanks. Heck I'm mostly doing this out of curiosity. When Good Howie asked if I could feel any difference. I was going to tell him the bike seems a little more responsive at take off. But I've also found sometimes when you make changes, you think there's a change but you can't put your finger on it. I can always re-install the SE 58 if we learn we can get more out of it. As I'm looking to change exhaust this winter, I have a lot of options to look over.

JW

The other thing to consider, Jeff, is the quality of the throttle body.  I would not be surprised to learn that a 54mm HPI throttle body outflowed a stock HD 58mm throttle body.  You made a good choice by sending your HD throttle body to HPI for modification...   

[StreetDog]

 I can't wait for HPI to come out with their throttlebodys (for the fly-by-wire 08' and up bikes) for my ordered SE StreetGuide. They have far superior flow than the HD throttlebody modified or not due to the Y shape manifold vs HD's T shape manifold. May not change it right away but that's the option I would want.

[Steve Cole]

To large of a throttle body can and does hurt low speed performance. It can be measure and shows as a loss in torque. While the displacement is not the end all be all for throttle body size it is a damn good rule of thumb as a motor can only consume the maximum size of the engine at a given RPM unless it is Supercharged or Turbocharged. 103 cu in. on the stock TB just seems to be the place in this case. If your looking to compare flow numbers you need to flow the TB with the intake and air filter attached to get real numbers. You would be quite surprised what we've seen when changing just an air cleaner backing plate on a HD.

[SBB]

Seems to be conflicting opinions here?

                       


SBB

[sadunbar]

To large of a throttle body can and does hurt low speed performance. It can be measure and shows as a loss in torque. While the displacement is not the end all be all for throttle body size it is a damn good rule of thumb as a motor can only consume the maximum size of the engine at a given RPM unless it is Supercharged or Turbocharged. 103 cu in. on the stock TB just seems to be the place in this case. If your looking to compare flow numbers you need to flow the TB with the intake and air filter attached to get real numbers. You would be quite surprised what we've seen when changing just an air cleaner backing plate on a HD.

A motor can only consume the maximum size of the engine at a given RPM but it does only consume what the air filter, throttle body, heads, cams and exhaust allows to flow thru it....i.e. a professionally ported set of heads (with the right air filter, TB, cams and exhaust attached) can flow more air thru a 103" motor than a stock set of heads can flow thru a 110" motor....

I think we are saying the same thing somewhat differently....I agree displacement is not the end all - IMO all the pieces of the package is the end all...  Give me the choice of a great set of heads and cams or displacement to hang a good TB on and I'll take the heads and cams every time...

[SBB]

The other thing to consider, Jeff, is the quality of the throttle body.  I would not be surprised to learn that a 54mm HPI throttle body outflowed a stock HD 58mm throttle body.  You made a good choice by sending your HD throttle body to HPI for modification...   

That's an apples and oranges comparison. HPI does not make a 54mm TBW TB. (yet)
I think maybe you mean Jeff's stock throttle body modified by HPI to 54mm will probably out flow a stock HD 58mm TBW TB.
That I would agree with after looking at the roughness of my HD58mm TBW TB.
Just note that Jeff's HD58mm TBW TB was modified by Cycle Rama so it can't be called stock either.
How about this, send a stock 50mm TBW TB and a stock 58mm TBW TB both to HPI, and then check the flow, that would be the telling difference there.
Then it would get back to the makeup and the ability of the engine components to handle the flow delivered by either of the two HPI modified TB's.
It's what I have learned from this site many years ago.
It's not the value of each individual component, it's the sum of of the combination that produces the best package.

SBB

[sadunbar]

That's an apples and oranges comparison. HPI does not make a 54mm TBW TB. (yet)
I think maybe you mean Jeff's stock throttle body modified by HPI to 54mm will probably out flow a stock HD 58mm TBW TB.
That I would agree with after looking at the roughness of my HD58mm TBW TB.
Just note that Jeff's HD58mm TBW TB was modified by Cycle Rama so it can't be called stock either.
How about this, send a stock 50mm TBW TB and a stock 58mm TBW TB both to HPI, and then check the flow, that would be the telling difference there.
Then it would get back to the makeup and the ability of the engine components to handle the flow delivered by either of the two HPI modified TB's.
It's what I have learned from this site many years ago.
It's not the value of each individual component, it's the sum of of the combination that produces the best package.

SBB

I think the real advantage of a HPI throttle body is their manifold as compared to the HD manifold...  Having HPI modify a HD throttle body will improve it over stock, just as having Wes modify a stock HD throttle body will improve it over stock.  And I suspect either modified 50mm throttle body would out perform a stock HD 58mm throttle body (no evidence, just speculation) - but when HPI comes out with a TBW manifold of their own, I suspect it would outpeform any of the current options...

[Hoist!]

Don't forget, the HPI manifolds compared to the HD, make a HUGE difference in the flow as part of this equation!

I think you're all saying the same thing. And like anything else, it's a properly matched combination of components that does the trick! In this case, matching the flow characteristics of the intake, exhaust, heads, cams, and volume will determine the optimum size TB. Mine and Scott's were designed for 58mm HPI. Thru testing I determined that the 62 performed better. Not necessarily up top, but better overall across the board. But no way would a 50mm SE have worked with our builds. The 58 wasn't available yet, but I believe the HPI TB/manifold setup WAAAY outperforms the HD 58mm! And with our setups, we needed all the flow we could get!

Hoist!

[sadunbar]

Don't forget, the HPI manifolds compared to the HD, make a HUGE difference in the flow as part of this equation!

I think you're all saying the same thing. And like anything else, it's a properly matched combination of components that does the trick! In this case, matching the flow characteristics of the intake, exhaust, heads, cams, and volume will determine the optimum size TB. Mine and Scott's were designed for 58mm HPI. Thru testing I determined that the 62 performed better. Not necessarily up top, but better overall across the board. But no way would a 50mm SE have worked with our builds. The 58 wasn't available yet, but I believe the HPI TB/manifold setup WAAAY outperforms the HD 58mm! And with our setups, we needed all the flow we could get!

Hoist!

yep.....

[Heatwave]

Does anyone have any data to support some of their claims? I'm reading alot of opinions and claims, however absolutely no data has been offered. Show me some flow data suggesting a "worked" 50mm TB is actually going to perform (flow) more air than a 58mm SE TB. The absence of data to support some of these claims makes them quite suspect. Keep in mind that the same 50mm TB that's been worked to 54mm still has to get the flow through a 50mm opening in the backing plate and A/C (unless that's being "worked" as well).

The 58mm TB from HD uses a matching larger back plate to maintain that 58mm opening all the way through to the engine.

Without the performance data, some of these claims start to sound somewhat like commercials and merely a slam of HD (which I understand and do at times myself). I just think before making a claim that a 50mm TB that's been ported/polished is better than a the MoCo's 58mm TB, you should provide some supporting evidence because the math of surface area would suggest otherwise.

Show a dyno for the same engine with both size TBs or share some flow-bench data, otherwise I'll stick with the bigger number and the MoCo over smaller non-oem options (this all assumes a larger TB is the best approach for a particular engine build).

[sadunbar]

Does anyone have any data to support some of their claims? I'm reading alot of opinions and claims, however absolutely no data has been offered. Show me some flow data suggesting a "worked" 50mm TB is actually going to perform (flow) more air than a 58mm SE TB. The absence of data to support some of these claims makes them quite suspect. Keep in mind that the same 50mm TB that's been worked to 54mm still has to get the flow through a 50mm opening in the backing plate and A/C (unless that's being "worked" as well).

The 58mm TB from HD uses a matching larger back plate to maintain that 58mm opening all the way through to the engine.

Without the performance data, some of these claims start to sound somewhat like commercials and merely a slam of HD (which I understand and do at times myself). I just think before making a claim that a 50mm TB that's been ported/polished is better than a the MoCo's 58mm TB, you should provide some supporting evidence because the math of surface area would suggest otherwise.

Show a dyno for the same engine with both size TBs or share some flow-bench data, otherwise I'll stick with the bigger number and the MoCo over smaller non-oem options (this all assumes a larger TB is the best approach for a particular engine build).

I've clearly stated my opinions are just that - my opinions - and also clearly stated they are somewhat speculative - albeit somewhat educated speculation (again, IMO).  No one should take anything they read here as gospel fact - especially anything I state...  With that....carry on....

[Hoist!]

Does anyone have any data to support some of their claims? I'm reading alot of opinions and claims, however absolutely no data has been offered. Show me some flow data suggesting a "worked" 50mm TB is actually going to perform (flow) more air than a 58mm SE TB. The absence of data to support some of these claims makes them quite suspect. Keep in mind that the same 50mm TB that's been worked to 54mm still has to get the flow through a 50mm opening in the backing plate and A/C (unless that's being "worked" as well).

The 58mm TB from HD uses a matching larger back plate to maintain that 58mm opening all the way through to the engine.

Without the performance data, some of these claims start to sound somewhat like commercials and merely a slam of HD (which I understand and do at times myself). I just think before making a claim that a 50mm TB that's been ported/polished is better than a the MoCo's 58mm TB, you should provide some supporting evidence because the math of surface area would suggest otherwise.

Show a dyno for the same engine with both size TBs or share some flow-bench data, otherwise I'll stick with the bigger number and the MoCo over smaller non-oem options (this all assumes a larger TB is the best approach for a particular engine build).

Sorry dude, no dyno charts. But I spent 2 days on the dyno after my build was done, trying exhausts, intakes, and TB's! I used the 58mm HPI to start, as that what was what my build was designed around, flow wise. I assume you must agree that the 58mm HPI out flows the SE 50mm. I tested duals, then FatCats w/different baffles. I tried the S&S single bore intake and the DPP. Everything bigger performed better. Then we thought of trying the unheard of 62mm on a 110"!!! Well lo and behold, the motor loved that setup. Wasn't even rooting for the combo I wound up with. But I wasn't giving up the best performance combo we found either! I wound up keeping the 62mm w/HPI A/C assembly, and the FatCats with the Performance Big Bore muffler in the end. Even though the 62mm sticks out further and you MUST use their A/C assembly with it (it uses the stock cover at least). And I liked my duals, or would have preferred the FatCat Quite muffler. That Performance Big Bore is LOUD too! But you can't argue with the curve, the numbers, and the way the bike feels! In the end, I proved the best combo for my setup, and that the motor wanted the 62mm TB. But I guess you'll have to trust my word on all of this, since I can't prove it to you with the charts! So take it as you must!

Hoist!