A thought on larger throttle bodies. Long

I've been thinking about larger TB's, I have a 62mm TB and it does make a difference. That along with a spacer changes my torque curve back to where with 31's I don't need to re-gear.

I was noticing that there are also larger TB 's available and that the stock TB's start out at like 60mm ( late model HO) and are necked down a bit and the intake on my 98 is 62mm so it appears that 62mm is a good fit.

Now for the question. According to Bernoulli's principle, as a fluid (or gas) is accelerated (via the venturi effect in the case of a TB) pressure decreases, Boyles gas law states that as pressure decreases, temperature decreases, so a necked down TB will cause a decrease in pressure as well as temperature.


Decrease in pressure is bad hence a larger TB flows more, but decrease in temperature is good so what we get with a bored out TB to 62mm is more flow but a slight rise in temp.

Given this theory, I would assume a TB bored to 62mm to match the intake opening but above the throttle plate bored to something larger would yield better results than a straight through bored TB.

Anybody have another theory?

Randy

Randre said:

Anybody have another theory?

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Yes, all that is insignificant. The engine has a given amount of air sucking power. A bigger throttle body makes it easier to suck more air = more horsepower. :dunno:

basalt51 said:

Yes, all that is insignificant. The engine has a given amount of air sucking power. A bigger throttle body makes it easier to suck more air = more horsepower. :dunno:

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I like the newly published "basalt51 Principle". Consider this a "Peer Review".

A carburetor uses this principle to provide a strong signal for fuel metering. I think the throttle body uses the pressure drop across the venturi to provide a strong vacuum signal for the MAP sensor. So, machining out the venturi would reduce the vacuum reading provided to the computer. This would be seen as a demand for more power so the injector pulse width would be increased. The result: richer mixture for more power and lower fuel economy.

I doubt the temp drop amounts to much when the intake manifold is at 175° on a summer day.

MaXJohnson said:

A carburetor uses this principle to provide a strong signal for fuel metering. I think the throttle body uses the pressure drop across the venturi to provide a strong vacuum signal for the MAP sensor. So, machining out the venturi would reduce the vacuum reading provided to the computer. This would be seen as a demand for more power so the injector pulse width would be increased. The result: richer mixture for more power and lower fuel economy.

Click to expand...

The vacuum signal for the MAP sensor is caused by the movement of the pistons down the bores, not by the TB. A larger TB will not adversely affect fuel economy. If anything, gas mileage will be slightly improved because the engine will be able to suck in more air and the A/F mixture will be slightly leaner.
I have a 62mm TB with 62mm TB spacer on my modified 4.0, and gas mileage is better than it was when my 4.0 was stock.