"They do sorda coincide though"

You are correct.

 

The flow of air is dictated by how fast it is travelling from the intake system (air box, filter, piping,etc), not by the TB. The amount of air entering the cylinders is dictated by the valve opening and the downward motion of the piston causing a vaccuum.

Sara Lou

 

"The flow of air is dictated by how fast it is travelling from the intake system (air box, filter, piping,etc), not by the TB."

Really? You mean the throttle body has no control over the airflow?

Actually where a bigger throttle body comes into play is close to wide open throttle. I think this is the main concern and the reason for switching to a larger throttle body.

 

Wonder if there's a way to 'convert' a MAF into an air speed sensor heh

 

I suggest you are misquoting Sara Lou. She correctly used the word "system". A tb is nothing more than a hole with a moveable slide.
Look at the big picture. An engine is nothing more than a glorified air pump. Air flows in, exhaust flows out. In between, the fun things happen, such as the rear wheels spin and we hoot and holler.
But any way, as an air pump, it will only flow as much as the biggest restriction. If the tb is too small, it could be the restriction. If the actuator (the piston/valvetrain) does not move as much air as the rest of the system is capable of, it is the biggest restriction. If the exhaust system will not allow enough exhaust to flow, it is the biggest restriction. So, your goal should be to optimize and match each piece of the system, all the way from the air intake through to the exhaust system.
AFA the stock tb, it has been shown to flow some good numbers. I noticed a slight increase in power when I went from the 48 to a 52, but I had already opened up the system with a CAI and a catback exhaust.
I am going to use a 58 mm tb on my new engine, but I am also aiming at 700 rwhp with F/I.

 

Could be regarding Sara. The throttle body should be the restriction in the air intake system to the plenum. If it is not then there are design problems.

Back to my point the air will be fastest through the throttle body as it is the most restrictive point going into the plenum. Some of you need to look up Bernoulli's theorem.

 

Sorry bud, but without O2 readings and DA for each run, that is a meaningless result. I would also suggest that an independant test would be better.

 

As for the "better throttle response" deal. I would suggest that you look into the theory of "lean is mean". If you are running the same tune you will get a lean condition at partial throttle compared to the stock throttle body (keeping in mind that the stock tune is a little rich).

 

I understand(I tried ), and I agree an independent test would be better. May'be someone could try and contact them to see if they have the other data. I would, but I got things to do lol.

 

No, I didn't say that. I said the speed at which the air is travelling is not controlled by the TB.
As example, if you hold a TB in your hand and open the blades, they will be no difference in air speed from one side to the other.

Sara Lou

 

Take your lips, and make a hole the size of a quarter and suck in. Air moves pretty quick right? Now open your mouth as wide as you can and suck in. Quite slow, correct? Same thing with the TB. The piston (your diaphragm) is still sucking the same amount of air into cylinder (your lungs), but the opening in your mouth (the TB) has changed the speed of air. The amount stays the same, it just slows down.

I'd also think that it would be able to get the amount of air it needs quicker, same as putting on headers in place of manifolds.

 

First, I cannot agree that the tb "should" be the restriction. Certainly its function is to restrict air flow at partial throttle, but keep in mind we are talking WOT here. At that point, it is two open holes, with the slide acting as a thin bar in the center of each hole.
AFAF Bernoulli is concerned, you probably meant Bernoulli's principle, which leads us to Bernoulli's equation. Although intended for liquids, it may also be applied to gases.
AFA a 58 tb being bolted to a 52 mm intake (stock), I suggest that moving the gas through a 58 mm opening into a 52 mm opening increases the amount of energy required, due to the restriction, rather than decreasing it, which would be the desired effect. This is an application of Bernoulli's equation, and can be found in many places on the web.
Furthermore, even if the intake were bored to accept the larger tb, the next restriction will be the engine itself, where again, Bernoulli's equation may be applied. Not designed or modified to move as much air as the new tb can flow, it becomes the restrictiion. Pretty simple stuff, and equations can be found on the web that will indicate how much air your engine can flow, given any set of modifications. You should then size your boltons (tb, intake, air cleaner system, camshaft, exhaust, accordingly.

 

Hmmmm.....let's see.....
The following formula is out of a book I have called "Auto Math Handbook" by John Lawlor.

((rpm x displacement)/3456)(volumetric efficiency) = cfm

So the factors are:
1. What is the maximum rpm of your engine?
2. What is the displacement (cid) of your engine?
3. What is the V.E (volumetric efficiency) of your engine?

The first two are pretty self explanatory. Number three is a bit more complicated. Here's a exerpt from the book:

According to Mike Urich, former engineering vice president of Holley Carburetors and author of HPBooks' Holley Carburetors and Manifolds:

"An ordinary low performance engine has a V.E. of about 75% at maximum speed; about 80% at maximum torque. A high performance engine has a V.E. of about 80% at maximum speed; about 85% at maximum torque. An all-out racing engine has a V.E. of about 90% at maximum speed; about 95% at maximum torque. A highly tuned intake and exhaust system with efficient cylinder head porting and a camshaft ground to take full advantage of the engine's other equipment can provide such complete cylinder filling that a V.E. of 100% - or slightly higher - is obtained at the speed for which the system is tuned."

That obviously doesn't account for forced induction which can easily have a V.E. of 110% or higher, but I digress.

Anyway, back to the formula.....

I'm going to use a 350 with a 6500rpm redline, and 100% V.E. (basically a typical cam/heads LT1) as an example.

((350 x 6500)/3456)(1.00) = 658 cfm

From what I've read, here are some approximate flow figures for various size throttle bodies:

48mm: 600cfm (stock)
52mm: 750cfm
58mm: 1000cfm
monoblade: 1300cfm

Therefore, a 52mm throttle body should flow enough air not to be a restriction for that application.

Now, let's try a supercharged 396 with a 7500rpm redline, and a 110% V.E.

((396 x 7500)/3456)(1.10) = 945 cfm

As you can see, a 58mm throttle body would probably work well for that engine.

 

I stand by my statement. You guys are way over complicating things. You are also forgetting the loses associated with each piece of the air intake system. The total flow will less than the most restricted part.

Back to what I said. The slower the air entering the plenum(larger throttle body) the more it will be able to make the turns into the front ports.

 

To be perfectly honest, I doubt a H/C LT1 breaks 95% (if it even breaks 92%). My personal guess would be more around 90%.

 

Well, I suggest you are oversimpliifying things. Physics are physics, the whole world runs on them.
There are always losses, one should consider them the price of playing the game. Of course the total flow will be less than an ideal system, that is a given. It not only applies to the intake system, but also the heads, the piston shape, the exhaust manifolds and the rest of the system.
I further suggest that your air speed theory (assuming the manifold is bored so there is no turbulence at the manifold entrance) has little to no meaning in the overall performance of the system.
I don't mean for, nor intend to participate in, a forum argument. But the OP asked an honest question, and deserves the best information possible.
Peace.

 

I cannot disagree. I believe he was trying to show most optimal performance, rather than an average.

 

True, but a "typical H/C engine" won't do that, like he was implying. Either way, water under the bridge.

Also, intake TB ports are apparently 54mm, not 52mm. And while 2mm on each side is still a lot to go w/o porting, I think I'm actually going to go for it.

Otherwise, It's stuffing the intake with 80w90 soaked rags, and buying a small-engine 2 or 3 stone cylinder hone and porting it to 58mm. The one site I have says use a dremel to port, but I think a hone would work much better. Slower, but better. And I don't have a dremel lol

 

From memory, it is actually 52 mm. But my stock intake is currently on the car, so I can't verify with my caliper. The main thing is to avoid going from a larger opening to a smaller opening. This would cause turbulence issues. Going from a smaller opening to a larger opening will cause much smaller and less important issues.
A dremel tool with the appropiate metal cutting tool would be much, much faster. A hone will do it, certainly, although probably taking more time. But only you can judge how much your time is worth.
Best of luck.

 

V.E = actual cfm/theoretical cfm

The only way to be completely accurate would be to have actual measurements. Since you're saying that a typical heads/cam LT1 would have a V.E. of around 90%:

((350 x 6500)/(3456))(.9) = 592 cfm

Anyway, the point I was trying to illustrate was that the airflow needs of the engine determines whether or not a particular piece of the intake tract is a restriction. Most of us know that the most restrictive part of a stock LT1 intake tract is from the airbox/filter up to but not including the MAF (if installed).