Someone please explain this to me

I'm sure someone that knows airflow dynamics can explain it better than I can but basically the larger you go the lower the velocity of the air coming in. You go too big and it can't get as much air into the combustion chamber because the incoming air has slowed down. There is going to be that sweet spot where you don't have a restriction but you don't go too big either. Stock 5.0's had like a 55mm and by opening it up to 65mm they picked up power but go too big and it does nothing but lose power.

A turbo or supercharger changes this because the air is being forced in and a bigger opening means more air can be pushed in but now you have the turbo or supercharger doing the work.

Bill

I've heard numerous debates on this topic, I personally got the Accufab 75mm TB(with a C&L plenum) because I plan on going FI somewhere down the road when I'm not poor and in college.

Its all about moderation. I think the best gains for a bolt on car are from an accufab 70mm TB and their plenum. You also want to get the exhaust gas out, but using 1 7/8" X 3" headers w/ 3" exhaust on a stock car is robbing you of HP... The same principle applies with the TB. Bigger doesnt always mean more HP.

you guys are talking about a venturi effect? the bigger the less venturi, i guess that makes sence..

It would be better for higher flowing heads, etc

Mustang92 said:

There is going to be that sweet spot where you don't have a restriction but you don't go too big either.

Click to expand...

Well said.

It might help to think in terms of the mass rate of flow using some very simplistic, many-assumptions-made calculations to demonstrate the point.

At, say, 5300RPM, assuming 85% volumetric efficiency, your 4.6L engine will be flowing about 366 cubic feet of air a minute (cfm). All of that air has to pass through the throttle body bore.

A 55mm (~2.17") throttle body has an area of about 0.0257 ft^2 (ignoring the obstruction of the plate and shaft.) This means that to get 366cfm through this opening you need an airflow speed of about 14250.6ft/min or 161.0mph. You can imagine the momentum such a column of air will have as it speeds through the throttle body into the intake...

If you up now to a 90mm throttle body with an area of 0.0685 ft^2 -- roughly 2.7 times the area of the 55mm TB -- you'll see that the same mass rate of flow -- 366 cfm -- results in an airspeed through the TB of only 5343 ft/min or about 60.7MPH. Imagine how lazy its flow velocity will be at just 2500RPM...

So that 55mm TB might be too small for high RPM. Too much air trying to cram into a small area might result in turbulence and pressure drops. The 90mm will flow like gangbusters but if your engine isn't moving that much air, you gain nothing up top and because you lose flow velocity down low, your torque may suffer. Somewhere in between these extremes is, as Mustang92 pointed out, a "sweet spot" where the high-RPM losses and the low-RPM momentum are compromised just so to deliver the best overall performance.

You see this sort of concern even when porting heads: hogging out a head for maximum port volume might look good in a print ad but in the real world, such volumes slow port velocities and hurt street-driven cars' performance.

Calculations not checked over...corrections welcome

I have looked into this topic pretty extensively...

First off, a throttle body should not be compared to a carb

Ever heard of the Bernoulli effect?

Dynamic (moving) fluid = Lower pressure
Static (no movement) fluid = Higher pressure

In other words, faster moving air creates less pressure than air with no movement. Take an airplane wing with the flat bottom, and the top part of the wing is curved.

In order for the air to get from the front of the wing to the back of the wing at the same time as the air on the bottom flat part of the wing, the top air needs to move faster. This creates low pressure on the top part of the wing, which creates a higher differential pressure on the bottom. This causes lift, and makes airplanes fly

Ever been in a house with the windows open? As the wind starts blowing outside, your drapery will start "sucking" towards the outside. This is because the air is moving faster on the outside, than inside. It creates a higher pressure on the inside of the house and pushes outwards until the wind dies down and creates "even pressure."

So what is this getting at?

Air gets into the combustion chamber due to atmospheric pressure (14.7 psi). The more pressure (less air speed in the throttle body runner/plenum), the possibility of more power.

A large throttle body is not going to hurt your performance.

It will give you one thing, that can be described in two different ways:

- More throttle response
- Touchy throttle response

They are the same thing, just worded differently.

Remember, that not all throttle bodies are created equal. One 70mm throttle body, may not flow like another brand of throttle body of the same size.

The design, shaft thickness, and screw protrusion are all pretty big factors in this even though the blade itself may be 70mm.

As far as area, just multiply PIE x Radius (squared).

60mm - 4.4" ^ 2
70mm - 5.9" ^ 2
80mm - 7.8" ^ 2
90mm - 9.8" ^ 2

For every degree the throttle is pressed under wide open throttle, the larger blade has more surface area to pass air. This is what gives the extra "throttle response."

I believe the term "overkill" is correct, if you use it correctly. It is not overkill in the sense that it will hurt power, but it may be not needed.

Anyways, not lets look at the CFM the cubic inches can get you at a certain RPM and efficieny level.

Displacement * RPM * VE / (1728*2) = Volumetric flow of the engine in CFM

The 1728 converts from in^3 to ft^3, and the 2 comes from dividing RPM by 2 in order to get the actual number of intake strokes. Many dynos will configure the VE for you.

So take 281 * 6000 * 1.0 / (1728*2) = 488 CFM

Now take a look at the throttle body flow rates from Accufab:

708 CFM at 28" of vaccum.

Now an engine does not see 28" of vacuum 99% of the time, but you can get an idea that the flow is typically not an issue.

Most "gains" are seen by the different variables that can occur.

Anyways, a large throttle body is not going to hurt your straightline performance one bit

How can having to much air hurt you at WOT?

If you want to throw a 75mm on, do it. If you want to throw on a 70mm, do it.

If you've got a throttle body that delivers 100% of the peak air requirements of your engine when the throttle plate is fully open, you have control of the air throughout 100% of the throttle position range. If you go to an oversized TB that delivers 100% of the air that your engine can consume while the throttle plate is only 60% open, you have given up usable throttle-control range for no advantage.

It is your money

Just keep in mind, a larger throttle body is not going to hurt you at the track.

Good Luck.

5spd GT said:

Anyways, a large throttle body is not going to hurt your straightline performance one bit

How can having to much air hurt you at WOT?

If you want to throw a 75mm on, do it. If you want to throw on a 70mm, do it.

Click to expand...

I will just opine that there is more to this than flow area. Velocity is crucial, whether it's through an intake runner, a throttle body or a port. Balance and component matching is important. As with exhaust, camshafts and porting, I think it is possible to say that "bigger is not always better."

Mustang92 said:

Stock 5.0's had like a 55mm
Bill

Click to expand...

'87-'93 5.0's had 60mm TB's stock. I had mine upgraded to a 70mm TB but I had all the parts to go with it. Better Intake, Aluminum Heads, everything but an aftermarket cam.

"Velocity" can be effected at part throttle, but not at full throttle.

I wonder what that plenum is for

I do not have one, but it is an interesting question. I need to look into that though...

Have you had your car dynoed? Did you find out the Volumetric efficiency of the engine? Likely above 100%...

http://www.accufabracing.com/article 3.htm

http://www.accufabracing.com/Flow Data.htm

The ideal throttle body can be different for one person or another.

The deal with a blower is that the larger one you get, the more exaggerated the "throttle response" will be.

Honestly, a 75mm would be good for you in my opinion.