Throttle body sizing

[James Boughton]

Allan,
	The formula you wrote is correct if you want to calculate the
average flow rate that the engine is using at a constant rate.  The flow
rate calculation I used was to calculate average flow rate over the
intake event.  Thus, divide your equation by .25, or multiply by 4.  This
is the flow rate commonly used for individual runner calculations, and
would correspond with the 60 m/s number I gave.  If you would like to
calculate the values with your equation you must use 15m/s for your
target flow rate.
	Just FYI, if you calculate intsantaneous flow rate for the intake
event based on intake area and piston velocity you will find the equation
I gave to be close to the average over the 0 to 180 deg interval.  This is
how I checked my work :-)

Jim Boughton
boughton at bignet.net

----------
From: 	Allan Hines[SMTP:ahines at c3.telstra-mm.net.au]
Sent: 	Tuesday, July 22, 1997 1:51 AM
To: 	diy_efi at coulomb.eng.ohio-state.edu
Subject: 	Re: Throttle body sizing

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
> 
> 
> 


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