Originally Posted by 383vett

Not sure where you came up with the arbitrary 400 ci. When I was running a 383, I ran low 11's with a 58mm throttle body and a SuperRam. I couldn't run this number using a 48mm throttle body. By the way, Lingenfelter's SuperRam assembley which I ran was suited for a 350 or a 383 and came with an Accel 58mm throttle body.

Originally Posted by AgentEran

Like blaster said, you will prob spend alot of money to make your car slower if mounting a large TB on a stock motor. Heads / Cam / Intake are way better areas to improve.

Rear gear ratio change is another good mod.

Originally Posted by TheBlaster9001

Has to do with laminar flow. I can only speak from a theory standpoint, although Aklim has verified in practice that a stock TB on a stock engine runs faster than a bigger one.

From an assumption that airflow is always laminar, then yes, a bigger TB wouldn't hurt. But if you assume that there is not a laminar flow, you will find that a bigger TB will actually end up creating more turbulence than a small TB, until around 400 cubes (or potentially lower cubes with very high volumetric efficiency).

Because a larger TB lets more air through quicker (10% throttle on a bigger TB is more CFM than 10% on a stock TB), you will reach the point that your engine cannot suck any more air quicker than the stock TB. That is, the total CFM that your engine can suck down is lower than the max CFM the TB can flow. Because of this, there is a point on each TB where no more air is passing it even if you open it further. Assuming a laminar flow, and 100% VE, my 383 still won't ever suck more air than the stock TB wide open will allow through, unless it somehow revs above 6k (which it can't, and won't).

So while the TB therefore is not a bottleneck, a bigger TB can introduce more turbulence because it reaches the point where no more air can get sucked down quicker than the stock TB. The difference the turbulence makes will be rather little, maybe a 5% decrease in VE. But the larger opening that a larger TB creates at full throttle ends up decreasing air velocity, and introduces eddy currents because the engine cannot suck enough air through such a large opening to keep velocity high enough to keep a near-laminar flow.

Originally Posted by bjankuski

This post has many issues, I don't want to get too far off the OP question so I will just say I agree to disagree. I will also say i have never seen a larger throttle body on a TPI style engine make less power on the dyno. TB are not carbs and most of the items you list above are incorrect.

Originally Posted by TheBlaster9001

I was trying to deal in theoretic as much as possible in that post - governed around Bernoulli's equation: bigger diameter = decreased air velocity, and a decrease in air pressure. Air will behave more laminar when at high velocities (unless you get close to or above the speed of sound). A 58mm TB won't decrease air velocity by much, and if tuned right, would make no measurable difference in power vs a 48mm TB. But from a conceptual standpoint, high air velocity leads to high VE, which is good for performance. The closer matched air velocity is from the intake to the cylinders, the higher VE will be.

So Tl;Dr: There's no way a huge TB helps a stock engine, and in theory it could hamper air velocity some. My cousin did his thesis paper for Mechanical Engineering about this same debate, only on Fox Body Mustangs. Different application, same physics.

But sure, we can agree to disagree. It doesn't really matter what either of us think with regards to the other. And for OP's purpose, the important takeaway is a bigger TB is not a good choice.

Originally Posted by vader86

Rear gear change would be a terrible choice for an L98, it is an LT1 mod.

If he has stick, he has 3.07, which is perfect for the engine.

Originally Posted by bjankuski

-See the attached sheet, air velocity decreases laminar flow not increases it.
-Air is pushed through a throttle body not sucked through it
-Throttle body CFM are determined by a how much air is flowing through the throttle body at a pressure differentail of 1.5" HG, which means at the rated flow the pressure in the plenium of the engine is 1.5 inches lower then atmosphere. So if the atmospher is 29.92 the pressure in the plenium at rated flow is 28.42 or 5% lower then atmosphere which means you are lossing at least 5% of you engine power to that restriction. Does not sound ideal to me! That formula you use to determine throttle body size was based on carb data where you need velocity through the venturi to cause a pressure differential that will cause fuel to flow through the main jets. The fuel is pushed out of the main jet not sucked. This does not apply to a dry flow throttle body system on a TPI car.
-Air flow speed through the throttle body is not important what is important is air flow through the runners on the engine. The highest pressure in the plenium will give the highest flow through the runners or highest VE for the given situation.
-I agree that a larger throttle body is not needed for this application but I want to kill that old wise tail that keeps getting passed around that larger throttle bodies will decrease power. They will not.
-Just think about this, would GM put 90mm and larger throttle bodies on there new cars if they killed power? The air flow of one 90mm throttle body is 17% more then twin 58mm throttle bodies. These are dry flow systems and larger does not hurt.