update on tbi setup

[Bruno!]

Hi All,

Firstly, thanks to those who replied to my question on throttle bodies 
and sizing.

I used the following specs:

engine capacity = 2000 ccs
number of cylinders = 4
maximum rpm = 6600 
tb id = 40mm = 0.04m
throttle shaft area = 8mm x 40mm

I also got my hands on an efi plenum for the engine, which came on the 
later model twin cams, and this has a cross-sectional diameter of 36mm. 
As the plenum was damaged, and able to split into two parts, i decided i 
would use the bottom half, which holds the fuel rail and injectors 
(thereby getting out of having to build one from scratch), and put the 
throttles on this.

So,

vol of air for each cyl = (500cc/2) * (6600/60) = 27.5 l/s = 0.0275 m^3/min
area of tb = (area of tb id) - (area of shaft) = 9.366 x 10^-4 m^2

air speed = volume / area = 0.0275 / (9.366 x 10^-4) = 29.36 m/s

I also found that the runner cross-sectional area is about 8.4% larger 
(average) than the tb effective area, though the throttle shaft has been 
machined so that there is less exposed metal at wot.

It was mentioned in a previous post by James Boughton that a target air 
speed of 15 m/s is ideal over the intake air event (or so i read, please 
correct me if i'm wrong). The figure above is about twice this, and 
assuming the valve is open for 180 degress (half of intake cycle) , is this 
about right?

What is the reasoning behind the 15m/s figure above? am i trying to keep 
the air laminar in the intake?

I am yet to measure the length of the whole system, though as a side note 
the twin carb setup un the car right now has a very peaky intake 
resonance at about 4100 rpm. I had noticed a slightly louder "gargle" at 
about 2000 rpm when running the engine in (still does - just that i 
didn't rev past 3000 for a while), the noise at 4000 took me 
completely by surprise when i first heard it. Good prompt for shifting 
while street driving :). The intake runner on the efi manifold will 
ultimately be longer, as it is about the same length  without the tbs & 
trumpets bolted on.

Any ideas and comments most welcome. Thanks.

		 Bruno. (b.marzano at student.canberra.edu.au)

                On the internet, no-one can hear you scream.