what size throttle body for adequate air flow?

[efahl]

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

[whittlebeast]

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

AW

[efahl]

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...

[eliotmansfield]

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)

Eliot.



Posted by email.

[whittlebeast]

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.

AW

[Thomas123]

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.

[bigworm]

very interesting

im very new to this but always thought about the speed of the airflow as a limiting factor

ive looked at the calcs and cant get my head around them
maybe if someone could do the calcs for me and then ill be able to understand a little bit better

250 cc twin/twostroke

at 11,500 rpm

one tb per cylinder 38mm cv

please help as im not sure if im going to get the right airspeed

matt

[whittlebeast]

see

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

[bigworm]

thanks
if im doing it right velocity at 11500 is 66mph

that should be just right for a ram air system to work well
me thinks

[panic]

I have an Excel spreadsheet (originally written by Stefan Vince, my mods to make it easier to use) that inputs engine data and gives TB size both N/A and boosted. E-mail me for a copy.