update on tbi setup

[wstrass at eastman.com]

To: DIY     --INTERNET DIY

From: Wayne Strasser (CED)
_______________________________________________________________________
Subject: update on tbi setup

This is response to Bruno's question about 15m/s being a goal to keep the
flow laminar.

I am not sure the reasoning behind this, but I do know two things:

1.  15m/s in that flow regime produces a mean stream Reynolds number of
around 35,000 (with a gut-feel estimate for the viscosity of air at that
temperature and pressure of 0.016 cps).  This is well above the laminar
to turbulent transition.

2.  The fact that the fluid makes many twists and turns entering the system
(this includes the turning around elbows that creats twin helical vortices
due to coriolis effect) will enhance the local reynolds number due to all the
rotational and recirculating kinetic energy that the air has.

In short, you would have to get the flow down to below 1 m/s to get it laminar.

_______________________________________________________________________
        Wayne Strasser
        Polymer Development
        wstrass at eastman.com
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Date: Fri, 8 Aug 1997 12:10:42 +1000 (EST)
From: Bruno! <b.marzano at student.canberra.edu.au>
To: diy_efi at coulomb.eng.ohio-state.edu
Subject: update on tbi setup
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Reply-To: diy_efi at coulomb.eng.ohio-state.edu

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.