TB CFM math questions.

[2802NR]

Hi everyone, New user here. I'm knee deep in the Intake manifold design process and am going through the formulas for proper throttle body size in relation to CFM requirments. I found some very good info at http://msefi.com/viewtopic.php?t=9731 and Im just looking for someone to double check my use of the forumla found there. PLEASE READ THE LINK. It explains all the formulas I use here. If any one else has used this method and find any thing wrong, please corect me.

I will be fabricating a dual-plenum design and using a dual bore throttle body so I figured this for half my displacement.

 

84.5cid * 7,000rpm / 2 = 295,750 cubic inches air per minute @ 100%VE

 

295,750 / 1728 = 171.15 cubic feet per minute

 

171.15 / 900 * 9.62 = 1.83 square inches

 

Now I'm unsure on this part, I am suplying 3 cylinders per throttle valve so should I figure an intake stroke 16.6% of the time or an intake stroke 50% of the time? I will be supplying #1,2,3 with one valve and #4,5,6 with one.

 

Just to get an Idea, I figured the rest for 50% with a -5% margin for blow back. (same as the example from msefi.com)

 

1.83 square in. /.45 = 4.07 square in.

 

Now to convert area to diameter:

 

Area = pi radius squared = 3.1416 r squared = .7854 diameter squared

 

4.07 / .7854 = 5.18 d squared

 

Square route of 5.18 = 2.28 diameter

 

Now convert inches to mm

 

2.28 * 25.4 = 58mm Throttle body per 3 cylinders.

 

Now I know My engine will never run 100% VE but I just needed a ball park figure.

All suggestions welcome and appreciated!

[Tony D]

Curiously, if you figure the VE at 80%, you end up with a twin 46mm throttle body fromthose calculations. (58mmX80%=46mm)

 

Hmmm, what size were the stock SU's? 46mm?

 

I guess the engineers knew what they were doing.

 

BTW, putting twin stock throttle bodies on the SU manifolds is a very easy thing to do, just some angular milling on the bolt holes to let the SU studs fit the back of the T/B...

[2802NR]

Thanks for the info about the SU manifolds. I have access to the equip. to make my own manifold so I am going to give it a try. I think I am going to use the dual 48mm??? TB off of an early TPI chevy. I will have an advantage over the SU manifolds though, Longer runners with a 1.5 degree taper.