RSX Throttle Body: How Big is Big Enough?

The throttle body can be a deceptively simple system. Its main responsibility is to let air into the engine in proportion to how much you press the pedal. While other cars, such as the 8th generation Civic Si have computer controlled servo motor "drive by wire" systems, the RSX has a cable-actuated throttle body. That means whenever you press the throttle pedal, the cable it pulls is directly opening the throttle plate inside the throttle body with no computer adjustment or delay. It's incredibly important in both street driven cars as well as track and drag cars to have a correctly sized throttle body, for reasons I'll disclose below.

Stock RSX Throttle Body

When Honda designs a throttle body, there's a common theme across them all. For all Honda 2.0 liter naturally aspirated engines, they use 60-64mm throttle bodies. This is true for the VTEC B20 and the K20 in addition to other manufacturer's 2.0 liter engines. In fact, all of the following engines have OEM 62mm throttle bodies:

• K20A (JDM RSX Type-R)
• K20A2 (RSX Type-S 02-04)
• K20A3 (RSX Base)
• K20Z1 (RSX Type-S 05-06)
• K20Z3 (Civic Si 06-11)

And the list goes on. Honda must have some reason to make these throttle bodies all the same size, right? Well, they do. On a 2.0 liter naturally aspirated platform, a 62mm throttle body provides the best balance of air volume, linear throttle response, and air column intertia. Also the OEM throttle body has provisions for cruise control, and all currently produced throttle bodies in the aftermarket do not.

What can happen as a result of using too large of a thottle body is lack of responsiveness. Larger throttle bodies don't allow the stock engine to draw air in at the correct rate due to a lower pressure in the throttle body when the throttle plate is opened, and turbulence from the intake manifold not being port matched to the throttle body diameter. A lack of response manifests itself in delays in power when you put your foot down.

Another problem with too large of a throttle body in higher horsepower applications is bad drivability. Track cars and drag cars require a high level of throttle control. Too much power at once can result in spinning the tires, upsetting the car in a turn, or jerky handling. Even the smallest opening of the throttle plate will result in letting a massive amount of air in. This is also relevant to street driven cars. If your slight throttle application causes the car to jerk forward, it can result in accidents, loss of control, and many bad situations.

While it may initially looks like only a 10mm difference between a 62mm and 72mm throttle body, consider this calculation of the gross area of the throttle plate:

A

=

π

r

2

=

π

·

31

2

≈

3019.1mm

A

=

π

r

2

=

π

·

36

2

≈

4071.5mm

Increasing throttle body diameter 10mm is actually increasing the net effective full throttle opening to be 1000mm^2 larger. This doesn't bode well for low throttle drivability and low-midrange power.

If you want proof, iBUILT, member on k20a.org did a side by side comparison of a stock 62mm throttle body versus a 72mm Blox throttle body. Unfortunately the pictures on the forum showing the dyno graphs are lost to the sands of time (aka bad image hosting), but iBUILT also posted results on his website showing power was identical with both throttle bodies, despite port matching the intake manifold to the larger throttle body. 

IPGparts.com's Dyno Data, 62mm OEM TB vs Blox 72mm TB

Notably, Hybrid Racing offered a 76 - 70mm tapered throttle body some time ago. They are discontinued now, but their sales page contains one of the only documented dyno sheets claiming power increase attributed to a throttle body alone. However after a closer look, the RSX they tested is equipped with full bolt-ons, as well as high performance camshafts. Camshafts can increase the amount of airflow through an engine and can necessitate a larger throttle body.

Hybrid Racing's Dyno Graph - With High Performance Camshafts

With no noticeable gains and the loss of cruise control, it's hard to rationalize a big purchase like a large bore throttle body. So what are some applicable situations to use a larger throttle body than the OEM one?

Supercharging is definitely one of the best reasons to increase your throttle body size. Adding a supercharger to your engine basically makes it act like a larger normally aspirated engine due to its pulley being driven by the crank, linearly stuffing more air into the engine. Due to needing more air in the engine under all conditions, this requires a larger throttle body. There is still such a thing as too large and too small, but it will definitely increase the optimal throttle body diameter.

Turbocharging and adding more aggressive cams can also benefit from a larger throttle body, but maybe not as large as the ones you might use for a supercharger. These also benefit from more air coming into the engine, but with the caveat that at some parts of the power band, they still behave like a stock size engine and have more linear throttle response with a smaller throttle body.

The reason why I personally switched from an OEM 62mm throttle body to the larger 70mm Skunk2 throttle body was simple; I couldn't run my intake of choice on anything smaller and I thought that might be the manufacturer indicating there was a benefit from it. I ended up purchasing a Skunk2 Alpha Series 70mm throttle body to mate to my Hybrid Racing Cold Air Intake. It is a bit jerky, but I would mainly attribute that to the weight of the flywheel and lash in the drivetrain. 

In conclusion, the stock throttle body is often the correct size for full bolt on cars, even with an RBC intake. The only time I would consider a larger throttle body is if you absolutely can't fit the stock one, or if you have a supercharged, turbocharged, or higher compression / better flowing naturally aspirated engine.