Calculating size of ITB?

[Daniel Lorenz]

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]

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

 

[Daniel Lorenz]

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]

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]

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.

 

[Daniel Lorenz]

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

 

[Daniel Lorenz]

Something like this.
Left is standard straight opening curve.
Right is an slow to aggresive opening curve.

 

[Terry Oxandale]

Update for anybody looking at this thread. My 48mm ITBs, when dynoed, resulted in 481 HP (chassis dyno at the wheel), and 517 lb/ft of torque. This is an OEM based 6.0L motor, with only changes being a stage II cam, 20" intake stack (runner) length, and 4-1 headers. Torque was right at 500 at 3500 rpm, and stayed nearly at 500 until about 5000 rpm, and then tapered off. Max HP was at 5400, and stayed within 30 HP until 6800 rpm (end of run). Avg power was 415 for the run, avg torque was 440 ft/lbs (3000 to 7000 rpm).

 

[Ryan L]

If you go too big in either the throttle body or the inlet port area, then you will lose air velocity/momentum which will result in a lowering of the volumetric efficiency and lower overall power figure. In theory you want the smallest dimension (combination of head and intake cross section) you can get away with that still makes the power.

 

[Ryan L]

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

from that link, they put 40mm throttles on a 2.5L (2500cc 6-cylinder engine, 416cc per cylinder). If it was a 4 cylinder of that capcity it would likely need a 45mm

A 302CI = 5000cc engine would be 625cc/cylinder V8 => would probably work best with 45mm
A 347ci = 5700cc = 712cc/cylinder V8 => tough one, could be better with 45mm depending on head properties and RPM range. If it is not going to spin high RPM and you want better drivability it might actually be better to try a smaller diameter.

6000cc = 750cc/ cylinder V8 probably does need the 48mm.

I would expect a 10 or 12 cylinder to need a comparatively smaller diameter for the same total engine capacity.