How does throttle body size affect air tract turbulence?

[5.0stang]

A friend and I were discussing the possibilites of making turbuelence in an air tract where air is 'let in' by a throttle body.

Now let's say there is a 70mm opening for the actual intake (efi car here), and in front of this 70mm opening, we put a 65mm throttle body.

Would having a smaller throttle body in front of the 70mm opening 'hurt' potential horsepower/torque output, or create turbulence? I just can't see it happening since it is smaller.

Does it matter whether it is under part throttle or wide open throttle or is the result the same, either turbulence or no-turbulence.

I see how a 75mm throttle body in front of a 70mm intake tract could cause a problem because the 70mm wall (5mm less diameter) would allow air to hit it right?

I hope this made sense and any feedback is welcome:)

 

[5.0stang]

bump for those with an opinion.

 

[FredGarvin]

I am no authority on these things. It's tough to answer when one doesn't know the effects one thing will have on the end result.

I will say that having a small flow area and then suddenly dumping into a larger one is a sudden expansion and does induce some turbulence (some recirculation zones around the step change in diameter). This, in turn, causes a loss that must be accounted for in pressure drop calculations.

Whether this would hurt help or have no effect on the end HP output is really out of my field. Does a throttle body like to have laminar flow? I would think that turbulent flow would eventually help in the mixing prior to combustion. I really don't know.

 

[brewnog]

Combustion performance can be incredibly sensitive to turbulence in the inlet tract.

Stang, if you want more information, have a look at chapters seven and eight in Haywood, particularly with regard to swirl, tumble and squish.

 

[5.0stang]

Any more ideas, I haven't got a chance (money) to get the Haywood book yet but I'd like too!

 

[brewnog]

Loads of ideas, but I'm not prepared to copy the book chapters out for you. These aren't concepts that you can get an understanding of from a few simple explanations.

You'll find the book in any engineering faculty's library.


In short:

Would having a smaller throttle body in front of the 70mm opening 'hurt' potential horsepower/torque output, or create turbulence?

It may hurt performance due to a decrease in volumetric efficiency, and it may create turbulence. It may not.

Does it matter whether it is under part throttle or wide open throttle or is the result the same, either turbulence or no-turbulence.

Yes it matters, since at part throttle you're not interested in optimising breathing, but you may be interested in optimising swirl and tumble on the induction.

I see how a 75mm throttle body in front of a 70mm intake tract could cause a problem because the 70mm wall (5mm less diameter) would allow air to hit it right?

Possibly, it depends on how the flow is affected. You might also have a problem with control if the throttle body is too large.

Engine development teams wouldn't spend hundreds of hours optimising inlet/exhaust geometry of heads and manifolds on a flow bench if the answers to 'what works best' were so easy to obtain!