[pa-glazier]

Iwas looking at my take off heads (01 Road King) and decided to measure the intake opening size. I came up with very close to 40mm. So my question is what do you gain by getting a larger carb or throttle body if the intake port is 40mm. Seems to me if you get a larger carb or throttle body the air is still going to have to squeeze through the intake port so why not let the air squeeze through the smaller carb or throttle body? I know that Dan Baisley did some work on my intake port (didn't think to measure it before putting it on the bike) but he said that he doesn't do a lot to the intake size wise just work to make the air flow smoother. am I missing something here? :huh:

 

[hotshotfxdl]

It would be great if we had a carb that would perform well in the lower rpm band and also flow enough air to give us the power on top.....we don't have such a thing so we have to decide where we want to build for the power we are looking for. The 40mm carb will give us high velocity in the port and across the signal (venturi), the strong signal will flow better fuel on the lowend giving you better throttle responce. The problem with the smaller carb throat is it runs out of air in the higher rpm band...ie it has a restriction to where the air starts to stall from the friction going through the smaller hole. So we have a opening that will perform good on the lower end but stops breathing on top end.
A larger carb or throttle body will flow more air on top end which will keep the velocity up in the port on top end to fill the cylinder hence more power on top....but the bigger hole in the carb will suffer throttle responce down low because the velocity drops with a bigger volume in the carb.
This is a lot more scientific than this explanation but I think or hope this got my point across. Where do you want the power? If bigger was always better we should all strap a 4 barrel on the bike...lol
Drag racing all we want is a volume of air for high rpm power, for the street we want low end torque and its achieved with cylinder fill from VELOCITY. There is no free lunch, if you want high rpm you have to give some up on the bottom end...if you want lots of bottom end you have to give some up the top end.

Great explaination DOC !!! For drag racing there are carbs that do both and I am suprised that I havent come across ones for bikes that do the same. On my small block Ford for which ran 10.80 @ 124 mph. I ran a 650 cfm carb. It had been modified using a flow bench just like they do our heads etc..... to flow 1050 cfm. So off the line or the crack of the throttle I had the response of a 650 but on the top end I had a carb that flowed 1050 cfm. I know it worked too as I bought and tested a 750 cfm carb that flowed 1200 cfm and the 650 always ET'd better. So as you said DOC bigger isnt always better, but I got to think there is someone out there doing the same thing with the bike Carbs. It funny too standing in the stagging lanes waiting to race guys would always say what size carba are you running and I'd say a 650 and they'd say you should try a 750 it'll go faster.......................not
Maybe even Bob Woods ?? The guy who did my Holley lived in Texas and they dont come cheap but it worked big time. Wonder if anyone knows who might be doing that type of work on carbs ?

FYI http://www.chucknuytten.com/drag.htm

 

[barrybasinger]

...if the intake port is 40mm. Seems to me if you get a larger carb or throttle body the air is still going to have to squeeze through the intake port so why not let the air squeeze through the smaller carb or throttle body?...

Well, here's what I think:

Compared to a tract that was the same shape, but a constant 40mm throughout, you would get:
a). a larger plenum volume.
b). an additional low-pressure area in the port.
c). additional velocity in the port.
d). reduced total port wall friction.

Additionally, and related to #a above, 40mm is enough to feed a single cylinder in most cases, but may not be enough to feed two cylinders that fire that close together. A larger carburetor and manifold may reduce this effect. This is similar to why, all other things being equal, a single 45mm opening on a Y-manifold (for example) will breathe better than a pair of 38mm openings on a Individual Runner (like the MM throttle body) manifold, even though the total area on the 38s is way more than the single 45.

 

[ViennaHog]

Not a guru at all

We used a special type of dual barrel carbs on our souped up VW beetle flat fours in the seventies and the early eighties . At lower revs (=lower flows) only one barrel of the carb was active and above a certain threshold flow level the second barrel came on to provide additional flow. Some versions opened the second barrel on a vacuum signal from the intake manifold, others used a certain throttle position as a trigger. Most of us felt that the vacuum operated design was more responsive. Anyway, that design seem to have been lost in history.

 

[barrybasinger]

We used a special type of dual barrel carbs on our souped up VW beetle flat fours in the seventies and the early eighties . At lower revs (=lower flows) only one barrel of the carb was active and above a certain threshold flow level the second barrel came on to provide additional flow. Some versions opened the second barrel on a vacuum signal from the intake manifold, others used a certain throttle position as a trigger. Most of us felt that the vacuum operated design was more responsive. Anyway, that design seem to have been lost in history.

You mean lost to history like a Quadrajet? Vacuum secondary Holley? Progressive linkage Rochesters? (OOps! I'm showing my age there).

I had dual DelLortos on two of my VWs, and they were pretty responsive due to the short intake tracts.

Anyway, the idea is functionally equivalent to a CV, where more venturi is made available as the engine can use it.

 

[ViennaHog]

You mean lost to history like a Quadrajet? Vacuum secondary Holley? Progressive linkage Rochesters? (OOps! I'm showing my age there).

I had dual DelLortos on two of my VWs, and they were pretty responsive due to the short intake tracts.

Anyway, the idea is functionally equivalent to a CV, where more venturi is made available as the engine can use it.

Yep, Rochesters, Solex and the like, one dual for each side of the engine. We cranked 170 HP out of a 2.4 l beetle (well, rebuild every 6000-8000 miles).