Throttle body sizing

[Allan Hines]

James Boughton wrote :

> 	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

The above formula is incorrect

Assuming 100% volumetric efficiency 

	(6000rpm x 500cc) / ( 2 x 60 ) = 25,000 cc/sec = 25 Litre/sec


Allan Hines

----------
> From: James Boughton <boughton at bignet.net>
> To: 'diy_efi at coulomb.eng.ohio-state.edu'
> Subject: RE: Throttle body sizing
> Date: Tuesday, 22 July 1997 12:43 PM
> 
> 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
> 
> 
>