Throttle body sizing

Tue Jul 22 02:44:38 GMT 1997

Bruno,
	To determine the proper size for an intake system you need to
know what rpm you intend to run to.  Typically the intake system is sized
to create a theoretical flow velocity of ~60m/s.  Not perfect, but a good
starting point.  I would use the area of the throttle body throats minus the
area taken up by the throttle shaft (probably ~10mm dia.)  So if you are
using a 40mm t/b you should have 1257mm^2 area.  If you remove the area
of a 10mm shaft you should be left with approx. 860mm^2.

	Next, take your cylinder displacement of 500cc/cyl and the rpm
(I'll use 6000rpm) and calculate the airflow rate for an intake event.  This 
should be 

	6000rpm x 500cc x 2 / (60sec/min) = 100000 cc/sec

divide 100000cc/s by 8.6cm^2 and you get ~116m/s.  This would be a little
high which would likely cause power to drop off early.  If you want to run
a lower peak power speed then you would have to recalculate accordingly.
By the way, the mysterious 2 in the equation is because the intake event only
gets a half of a rev to occur.

Also, the value of the area used should be the average intake area and can be
made higher by using trumpets (air horns, stacks, whatever!)  that have a large
taper.

All of this also depends on cams, port flow coefficients, and so on.  When you
get to the point of tuning for length let me know :-)

Jim Boughton
boughton at bignet.net

----------
From: 	Bruno![SMTP:b.marzano at student.canberra.edu.au]
Sent: 	Monday, July 21, 1997 10:52 AM
To: 	diy_efi at coulomb.eng.ohio-state.edu
Subject: 	Throttle body sizing

Hi everyone,

I have a 2lt twin cam and am considering rigging up a quad throttle efi
manifold for it. Firstly, I will be using the throttle bodies from
stromberg carburettors to save the pain of trying to fabricate a tb from
scratch. I can get these in sizes up to 40mm (i think 1 9/16") at a
reasonable price. 

The intake runners which will mount on the head, and hold the tbs, will be
made using aluminium tube, the internal diameter of which i can get in
different sizes quite readily. again, i could get this in 40mm, or
slightly smaller, or even larger (i think 50mm od & 3mm wall gives 44mm).
The 44 mm option would be a little silly, as the tb being 40mm would be a
restriction (compared to the runner). 

My question is whether 40mm is too large for this type of engine. It 
currently gives about 130hp, and is my mild-mannered street car. 
Driveability is my main criterion here.

Any ideas would be greatly appreciated.

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

	Early to bed, Early to rise
	Makes a man or woman miss out on the night life
						-Morphine

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