Throttle Body FAQ

[darkfox]

Throttle body flow measurements:

(flow ratings are at 7 in/HG of vacuum)

stock 2.4L (52mm) 240 CFM
stock 2.3L (56mm) 285 CFM*
bored 2.3L (60mm) 345 CFM*

* flow ratings based on Mantapart throttle body flow claims.

For comparison:

LT1 2x48mm @ 7" - 357 CFM
LS1 75MM @ 5" - 337 CFM
Ford 4.6L 2x57mm @ 3" - 357 CFM
2.0 Neon SOHC 52mm @ ?" - 283 cfm

engine CFM calculated for the 2.3L LG0

6800 rpm at 100% V.E. 276 CFM
6800 rpm at 110% V.E. 304 CFM

engine CFM calculated for the 2.4L LD9

6500 rpm at 100% V.E. 275 CFM
6500 rpm at 110% V.E. 303 CFM

engine CFM calculated for the 2.3L W41

7400 rpm at 100% V.E. 300 CFM
7400 rpm at 110% V.E. 330 CFM

[darkfox]

The following contains excerpts from a CarCraft magazine article on carbs. This information can also be applied to picking the correct sized throttle body for your engine.

article can be found here:

http://carcraft.com/techarticles/56919/

[darkfox]

The value of using CFM formulas.

"Textbooks tell us you can accurately select the proper carburetor size based on a relatively straightforward formula that takes into account engine displacement, max rpm, and volumetric efficiency (VE).

Plugging some actual numbers into the equation, what does it recommend for a 350 engine turning 6,000 rpm at 100 percent VE?

607.6 cfm?! Virtually no one—whether the original equipment manufacturer, aftermarket tuner, or racer—actually installs such a small carb on a high-performance 350. In the real world, everyone knows these engines make more power with larger carbs. Yet dyno-tests show the formula is an accurate reflection of an engine’s airflow needs. The equation breaks down as a realistic carburetor size selection tool because carburetor flow ratings (in cfm, or cubic feet/minute) are taken at an arbitrary vacuum drop—3.0-inches Hg for two-barrels; 1.5-inches Hg for four-barrels—and there’s no guarantee that, at max-rpm wide-open throttle (WOT), any given engine actually sees the theoretical vacuum drop for which the carburetor is rated!"

[darkfox]

The breakdown of what the flow ratings on carbs and throttle bodies really amount to.

"On the other hand, if a carb really does pull 1.5-inch-Hg (or more) vacuum at WOT, it has become a restriction. The carb is actually too small to let the engine realize its maximum power potential. That’s because the greater the pressure drop (the higher the vacuum reading) across or through the carburetor, the lower the air density is inside the intake manifold and combustion chamber. Racers like to see no greater than 0.5 to 0.75-inch-Hg vacuum on the top end. But race cars have high-stall converters and steep rearend gears, and most of them are lighter than the average street car. Racers don’t mind recalibrating the carb on the spot for changing track conditions. They are unconcerned about low-end driveability. That’s why all-out racers, when not restricted by the rules, run huge carburetors—the bigger the carburetor, the lower the pressure drop across it at any given airflow.

It’s a different story for street cars and dual-purpose street/strip packages. For these combos, the limiting factor in carburetor size selection is on the low-end side of the fuel curve. Will it have good part-throttle driveability in the engine’s normal operating range? How will it drive in the winter, desert, and mountains?

As in life, so it is with carbs: Compromises are called for—a fine balance. Not too big because you’ll lose driveability. Not too small or the carb becomes a major bottleneck. For most hot, dual-purpose cars, pulling about 1.0-inch-Hg manifold vacuum at WOT, max rpm on the dyno isn’t far off. But there are no guarantees. It’s impossible to know for sure how a carb will perform in a vehicle based on how it did on an engine dyno. Every combo behaves differently; what works in a featherweight car with a manual trans and 5-series rear-gears ain’t gonna cut it in a 3,700-pound ride with a mild converter and 3.23:1 rear gears. As Grant puts it, “You’ve got to do lots of empirical work and go through lots of trial and error. It really takes thousands of carbs before you get an intuitive feel for what’ll work on a given application.”"

[darkfox]

here are some general rules in the article that can be applied to throttle bodies:

• Higher rpm requires a bigger bore

• Higher horsepower requires a bigger bore

• Higher compression ratios require a bigger bore

• More ignition advance requires a bigger bore

• A manual-trans car can use a larger bore than an automatic-trans car

• Steeper FDR gears tolerate a bigger bore

• Lighter cars can use a bigger bore

• Heavy cars need a smaller bore

• Too large a cam for the application requires a smaller bore

• With an automatic-trans car, too low a torque-converter stall-speed for the application requires a smaller bore

• Mild FDR gears require a smaller bore

• Low compression requires a smaller bore

[darkfox]

The stock 2.4 and 2.3 have similar CFM requirements. If we calculate CFM needs for 100% V.E, we will need around 275 CFM of airflow.

Now, for the engine to use its max potential at WOT, we want the vacuum to be as low as possible. But, you you'd have a huge throttle body to flow enough air to get less than 3" vacuum at redline! even an LS1 throttle body, which is 75mm, will only flow 260CFM at 3" of vacuum! So to use the the 2.3's or 2.4's "maximum potential" we would need a gargantuan throttle body... and following common sense, we'd probably give up a lot of lowend. This might be acceptable for an all-out drag setup, but not for streetable or autox applications.

Instead, we can figure out CFM for the lowest RPM that is reasonable to use during all-out acceleration, and use this value to balance the throttle body's performance against the engine's maximum needs. Most Quads launch best at around 3500 and of course even the worse gearing will not upshift under this speed.

Max torque for the 2.4 is at 4400, the 2.3 at 5200.

For the 2.4, it will use no more than 150 CFM at 3500. the 2.3, no more than 142. For the 2.4, at max torque it will need 186 CFM. For the 2.3, at max torque it will need about 210 CFM.

So, a good throttle body for the 2.3/2.4 will need to have mininum vacuum at the range between 140 and 210 CFM. The engine V.E. is maximum at max torque, anyway.

SO doing some additional calculations, at 3" of vacuum, a 56mm Quad 4 throttle body will flow 188CFM. at 1.5", probably around 145. At 7" of vacuum at we see 288CFM of airflow. So, at all ranges it will flow well, and will have minimum vacuum at low end, midrange, and max torque but will still be able to adequately provide enough air for high-end operation. 56mms seems to be perfect for the 2.4L, and more than adequate for the 2.3L.

The W41, obviously because of its gargantuan redline, could benefit from a larger throttle body. Through similar arguments, 60mm seems a good option for this engine.

So i venture to say 56mm is probably the best streetable throttle body for the 2.4, and probably the best for the 2.3L as well. 2.3L engines could benefit from the use of an 87-91 Quad 4 throttle body. These will slightly outflow later models since they have a halve butterfly-shaft, whereas the later models have a full shaft. The 60mm seems the best option for a W41.