Calculating size of ITB?

Hi,
i'm trying to get my head wrapped around how to calculate a good size on the ITB's i want to install for my future 347 stroker (Ford smallblock stroked)
Aim is 600+BHP N/A tuned.
I want a fast responding throttle, and therefore i don't want to oversize so the respons feels slow.

I have seen some threads on this before, but i simply don't understand the math unfortunately...

Plan was to buy a good intake for ITB/Webers, then either buy Weber copies and gut them/install injectors in them, or just run ITB's to get the injection on the right side of the throttle plate. (But i can't find anything for me to select the appropriate size)
 

Terry Oxandale

Skinny Man
I cannot recall the source of this information, but I distilled this effort to determine how much HP is available for an x diameter of throttle body for ITBs. Per the following web site, this may be helpful:

The table below can be used to help estimate the correct size ITB

40mm 265cfm 61Hp 350-500 cc/cyl

42mm 304cfm 70Hp 450-600 cc/cyl

45mm 362cfm 83Hp 550-700 cc/cyl

48mm 408cfm 94Hp 650-800 cc/cyl

Link for the information: Article about ITB sizing
 
Very informative Terry, thanks.

Hmm, so according to the article i should choose 48mm when looking at displacement, but 42-45 when looking at power output.
 

Terry Oxandale

Skinny Man
Yeah, I noticed that as well. I'm using 48mm on a LS2 (6.0L), and my perception is the throttle response is great. I never felt I would every go beyond the 500HP range on the current goal of internal/external mods, which is why I felt the 48 was appropriate. Plenty of grunt from 2000 and up, but a lot of that is cam selection as well. I did calculate the desired max HP/RPM that I wanted in determining the entire length of the induction path, and then applied that when I cut the tubes that fit over the ITBs (top of tube, to the valve head). Hoping the article is accurate in it's information, because it just happened that everything I already did matched up well with it (tapered throttle body, tapered path until just prior to valve, proper intake path length, and moderation in sizing, which seems to work well for my set-up, but my car also only weighs about 2200 at most). Other articles I've read indicate the 48mm is good up to about 550HP (hence my decision on the 48s), so the table I supplied seems skewed from that, as you've noticed. I'm guessing his table shows additional HP from others I've seen because I'm guessing the author takes into consideration runner/intake length and configuration, rather than a simple "hole" of x size, flowing y amount of air.
 

Terry Oxandale

Skinny Man
One more point: Something a lot of articles neglect, which I feel is very important, especially in the modulating of power, and the "feel" of the throttle, is a highly progressive design in the throttle. I went to a lot of work designing my linkage so that the pedal travel/throttle plate movement ratio was as high as possible off idle, to being as low as possible at WOT. In other words, I want the throttle opening rate to start off slow (very slow relatively), and then toward the end of the opening window (increase rapidly relatively), in rate of opening. I feel this is a vastly overlooked designed-in feature that can really enhance the responsiveness of a ITB motor.
 
One more point: Something a lot of articles neglect, which I feel is very important, especially in the modulating of power, and the "feel" of the throttle, is a highly progressive design in the throttle. I went to a lot of work designing my linkage so that the pedal travel/throttle plate movement ratio was as high as possible off idle, to being as low as possible at WOT. In other words, I want the throttle opening rate to start off slow (very slow relatively), and then toward the end of the opening window (increase rapidly relatively), in rate of opening. I feel this is a vastly overlooked designed-in feature that can really enhance the responsiveness of a ITB motor.
This should be easily achived with an wire setup that gets closer to the centre of the pivot the closer it is turned to wot.
Seen solutions like that before, even as original on lots of cars.
And it would also be easy go replace if the feel isnt exactly right
 
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