what size throttle body for adequate air flow?

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Postby efahl » Wed Oct 05, 2005 8:07 pm

whittlebeast wrote:For fun I ran a few napkin calcs and got about 45 MPH airflow in a typical 1200 cc motorcycle pulling 13000 RPM with air going thru 4-42mm intakes and got about 50mph airflow in the throttles. (1200 GSXR stock)
Andy,

I get about 110 fps for that, assuming 7 mm throttle shafts and ignoring boundary layer flow... I think there is a lot more going on wrt turbulent flow in the tb than first meets the eye, especially considering how big the Reynolds number is in a typical TB at WOT.

Eric
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Postby whittlebeast » Thu Oct 06, 2005 4:13 am

I was ignoring the throttle shaft size and for that matter, the air sample tube in the MAF calc but it is amazing how large the throttles have to be to keep the air velosity down. I found out about this when the CRX went from four individule air horns to an airbox with a single 3" inlet air filter. The WOT map jumped from 97KPS to 92KPA with the air box. Things were even worse with a 3" dia 30" long "cold air intake" I found the air flow in the CAI of around 90MPH

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Postby efahl » Thu Oct 06, 2005 7:05 am

Yeah, that's why Scott got rid of the MAF on Jim's truck. If I remember right, he measured a 2 inch vacuum across the thing and immediately got rid of it...
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what size throttle body for adequate air flow?

Postby eliotmansfield » Thu Oct 06, 2005 12:01 pm

Fuel injected Rover V8' (3.5L & 3.9L and possibly the bigger ones too) came
with a 65mm throttle bodies I believe. (offline, cant verify)

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Postby whittlebeast » Thu Oct 06, 2005 12:21 pm

But the real question is what is the MAP compared to ambiant air pressure with that throttle body and at max RPM. I have seen pressure drops of from nearly 0 kpa to 8 kpa depending on the CAI/filter/MAF/throttle combination.

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Postby Thomas123 » Wed Nov 16, 2005 3:47 pm

chodjinn wrote:Hi,

I've just done the calculation for my engine. I'm running a 3.5litre V8 (215 cu) and used peak rpm as 6,000. I plan on using a single throttle body when I move over to Injection. According to the calculation I need a single throttle body with 20sq.in, which results in a 5" throttle body!

I was planning on using either a single 72mm or 90mm?

Anybody provide any advice?


Yeah, that sounds way huge!

BTW, is it a 1960s Buick 215 or the Rover one?

I have a 4300 series Buick 215 I'm rebuilding right now and have a selection of throttle bodies to choose from depending on how I choose to design the intake manifold.
Using the calculator here they list at.....

One @ 50mm is rated at 144 HP 5.0L V8 Mustang.
One @ 65mm is rated at 243 HP 4.6L V8 Thunderbird.
Two @ 45mm is rated at 233 HP 3.5L V6 Concord.

I'm thinking of two 180 deg (all 4 intake pulses 180 deg apart) plenums with a 45mm throttle body for each one.

In the Buick 215 V8 would be.
Cylinders 1 4 6 7 fire 180 deg apart in one plenum.
Cylinders 2 3 5 8 fire 180 deg apart in one plenum.
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Postby sime » Wed Apr 04, 2007 1:55 pm

I don't understan how you can calculate velocity...

I'm afraid that the ITB i purchased off a 98-99 gsx-r 750 (quad 46mm)
are way too big for my 1.3 dohc swift GT...

i want my peak tork around 7000 rpm....

THX FOR YOUR HELP
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Postby ronchinoy » Wed Oct 24, 2007 1:01 am

Im working on a 1.3 Swift engine also.
Was thinking of getting the R6 01-02 TBs.

Are you going NA or FI.
For FI I need to flow around 200 CFM at 10PSI and 85% Ve.
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Re: what size throttle body for adequate air flow?

Postby aerowerks » Thu Jul 31, 2008 9:30 am

Throttle body area is only one of the things you have to consider. The diameter is the maximum airflow limiting factor. The max any pipe or TB can flow is when the velocity through it reaches sonic i.e. M=1. To tune for max torque (max VE) you have to change the length of the intake or change the volume of the plenum. Velocity through the TB's is never constant, the 300 ft/s is an average that seems to work quite well all around. If you measure intake pressure less than 7psi at wide open throttle, there is a good chance that you need bigger TB since the velocity through the TB just hit sonic.
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Re: what size throttle body for adequate air flow?

Postby joedokes » Fri Jun 18, 2010 10:23 pm

I didn't carefully read all the previous posts so if I'm repeating some information forgive me.

1. There is a big difference in sizing between Individual Throttle Bodies (ITBs) and a throttle body that feeds a plenum for all or at least more than one cylinder. For example, a race 360 Cubic inch engine can run 600 hp on as little as 750 to 850 CFM of airflow through a single Holley type carb the single carb would probably feed into a open plenum that would feed all eight cylinder. While a 200 hp VW motor with 4 48 IDA Webers in which each barrel has an individual throttle plate for each cylinder has over 1200 CFM of flow potential for a mere 120 Cubic Inches. Thus, throttle body sizing for ITBs is SIGNIFICANTLY different than throttle body sizing for a single throttle body that feeds a common plenum.

2. One way to calculate ITBs is to figure out what the heads are going to flow and then add 20% for a maximum power effort. For example if the intake port will flow 300 CFM at max lift (Keep in mind that if your head will flow 250 CFM at .500 lift and 300 CFM at .650 but you only use a Cam with .500 lift you should use the lower number.) You would want your ITB to flow around 360 CFM. Now keep in mind that this is for a Max effort motor and thus under-sizing it a bit might help with low speed drivability. Further, the over-all runner length can have a dramatic effect on torque and horse power. For example, the ram effect can be designed to maximize torque and thus hp at a particular RPM. If the runner length is too short, the ram effect would happen at an RPM outside that used by motor, thus you'd never get the benefit. On the other hand if the runner length is too long, you'd get the ram effect at too low an RPM and the longer length would limit top end power to some degree.

3. Keep in mind that runner length is a compromise. Engine bays and your ability to lengthen and shorten the length may be determined by your physical layout. Also understand that the ram effect really only occurs at a relatively small point and while it does have a real and significant impact on the ability of the engine to maximize power, other factors in intake design, camshaft design, compression ratio, and transmission gearing will probably have a greater impact on the performance of the motor.

Hope this helps.

Regards

Joe Dokes
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Re: what size throttle body for adequate air flow?

Postby LenCaverly » Fri Feb 03, 2012 6:48 am

I don't know if this helps but it takes about 1.4cfm/per hp on a naturally aspirated engine, and would think max hp should be the determining factor.
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Re: what size throttle body for adequate air flow?

Postby 357supermagnum » Mon Dec 30, 2013 8:57 am

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Re: what size throttle body for adequate air flow?

Postby Healey3000 » Tue Mar 18, 2014 2:48 pm

Hi,

Here's a link to one I found. A little exploring on his site offers an explanation of the equation.

http://www.hipermath.com/math_center/ra ... lculations

Regards.
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Re: what size throttle body for adequate air flow?

Postby schilpr » Mon Dec 08, 2014 3:50 am

I've done this a few times and tend to just assume that OEM engine builders do extensive testing on their TB's, so their OEM use is probably close to (if not) optimal.

So the earlier example of 4 TB of a 1200cc engine with a 13,000rpm redline move enough air to satisfy that motor, the simple calculation is:
(displacement * rpm) / cylinders = (1,2 x 13)/4 = 3.9

Now take a Ford mustang 302, 5,000cc (close enough) and about a 6,500rpm redline for an 8 cylinder, the same calculation
(displacement * rpm) /cylinders = (5,0 x 6,5)/8 = 4,0625

Those 2 numbers are close enough.. so if you take the TB's of that bike and apply them to the mustang engine they will work.. just make sure you use 1 TB per cyllinder just as in the original application.

Just keep in mind that this simple method works for air, NOT for fuel, you'll have to size your injectors and fuel pressure to the charachteristics of your situation.

and yes, I did displacement in liters and rpm in 1,000's to make the calculations easier (less 0000 to deal with)
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Re: what size throttle body for adequate air flow?

Postby lutorm » Tue Apr 14, 2015 6:01 pm

I was just thinking about throttle body sizing and came across this thread. It makes sense that an ITB would have to be proportionally larger than a manifold throttle body, because of the uneven flow through an ITB.

For an engine with at least 4 cylinders, one cylinder is basically on the intake stroke all the time and the flow through a manifold throttle body should be pretty even. (Even more so because of the large damping volume.) Thus, a calculation of air flow velocity based on 0.5*total displacement per revolution should be pretty good. For an ITB, though, the cylinder only needs 1/N_cyl as much air, but that cylinder is also only pumping 1/4 of the time. The end result seems like it should be that for a 4-cylinder engine, an ITB would actually need to be about the same size as the correponding manifold TB to keep "peak airflow velocity" through the throttle body the same.

Then there are of course details like resonant airflow in the runner etc that make reality more complicated... :)
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Re: what size throttle body for adequate air flow?

Postby m3ltd0wn » Mon Apr 18, 2016 1:02 am

Here is something that might help you with ITB sizing and other intake related stuff.

intake_design_v1_8.zip
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Re: what size throttle body for adequate air flow?

Postby CMac89 » Thu Apr 21, 2016 9:00 am

I think the best way to do it is figure out how much HP you want to support.

1000hp = 100 lbs/min

volumetric flow (cfm) = mass flow (100 lb/min) / .069

Flow becomes compressible when the mach number exceeds .3 (turbulence sets in). Speed of sound is reached when mach number is 1, meaning no more air can flow. So we set the mach number equal to .3 and that would allow us to calculate the smallest TB diameter.

Mach Number = velocity of air / speed of sound (1125 ft/s)
velocity of air = .3 (mach number) * 1125 (speed of sound)
= 434 ft/s = 26086 ft/min (to get the numbers to cancel out nicely)

Diameter of TB (in) = 24 * square root of (1450cfm/(pi*26086 ft/min)) = ~3.2in

~3.2 in = ~80mm

Then this depends on your application. If you're naturally aspirated or draw-through with forced induction, you'll need to go higher than that. If you're blow-through turbo/supercharged and you aren't maxing the units out, then you'll be able to use this value as-is.

Just use this equation since i doubt anybody cares about the gibberish above:

Diameter of TB in millimeters = square root of (6.572 * Horsepower)
Last edited by CMac89 on Thu Apr 21, 2016 9:11 am, edited 1 time in total.
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Re: what size throttle body for adequate air flow?

Postby whittlebeast » Thu Apr 21, 2016 9:10 am

My motor is about 150 hp

(6.572*150)^.5 works out to 31.4 mm

Are you saying 1 - 31.4mm throttle body is plenty for 150 HP? That sounds small.

As a side note: I happen to run 4 - 1 3/4" ITBs
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Re: what size throttle body for adequate air flow?

Postby CMac89 » Thu Apr 21, 2016 9:20 am

whittlebeast wrote:My motor is about 150 hp

(6.572*150)^.5 works out to 31.4 mm

Are you saying 1 - 31.4mm throttle body is plenty for 150 HP? That sounds small.

As a side note: I happen to run 4 - 1 3/4" ITBs


You would be subject to density altitude. Above works at sea level. I'd have to sit and think about adjusting that for density altitude. It'd come from a document like this, for example. The higher the DA, the more TB you need. Also, that equation assumes constant flow. NA engines take air in pulses, which means that the HP per cylinder number would be average over the period of each intake stroke. That does not reflect what the peak air velocity is. Peak air velocity would need to be measured or either calculated, then you'd have to apply that to the equation above the general one i have in red.

Now that I think about it, the equation i gave really only works well for forced inducted, blow through setups.

http://www.tscm.com/mach-as.pdf
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Re: what size throttle body for adequate air flow?

Postby whittlebeast » Thu Apr 21, 2016 9:39 am

I use this spread sheet. I like to keep the air velocity under about 40 MPH as displayed in cell D38.

http://www.nbs-stl.com/fuel/ReqInjectors03.xls

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