40mm Throttle Body Upgrade Guide - Performance Mods for Motorcycles

Upgrading to a 40mm Throttle Body on Your Bike

Installing a larger 40mm throttle body can significantly boost the power and performance of your motorcycle or pits bike. Throttle response will be crisper with the increased airflow. But choosing and dialing in the right throttle body takes careful consideration.

How Throttle Bodies Work

The throttle body controls airflow by opening and closing the throttle valve via the throttle cable when you twist the throttle. A larger diameter throttle body is capable of delivering more air, allowing more fuel to be injected and burnt. This increases horsepower. A quality 40mm throttle body will optimize airflow while preventing turbulent flow.

Considerations for 40mm Throttle Bodies

Larger isn’t necessarily better for everyone. Going too big on the throttle body can negatively impact low end power. Also, fuel injection mapping and other supporting mods are crucial to get the most from a performance throttle body upgrade. You’ll want to upgrade to a high flow air filter and possibly larger fuel injectors. And don’t overlook tuning the ECU to match the new components.

DIY Installation of 40mm Throttle Bodies

Physically swapping the throttle body is straightforward. Start by removing connections and the cable from the old throttle body. Then install the new throttle body using the new gaskets and hardware included. This ensures an airtight seal critical for performance. Take care not to overtighten and damage the threads in the intake manifold. Finally, connect all hoses and the throttle cable to complete the installation.

Top 4 40mm Throttle Bodies for Motorcycles

Mikuni VM Round Slide Carburetors

The king for race oriented throttle bodies. Mikuni’s smooth bore carbs flow an enormous amount of air which translates to huge power gains. Their tunable nature lets you optimize the VM carbs for your specific setup. The round slide design is less prone to sticking when complicated by high heat environments. An excellent yet more expensive option.

Keihin FCR Flat Slide Carburetors

Another favorite especially for dirt and dual sport bikes. Keihin FCR carburetors are revered for crisp throttle response aided by the flat slide design. Lower cost than the Mikuni VM’s makes Keihin carbs attractive. Great for those wanting bolt on convenience versus jetting and tuning rebuildable VM carbs. Both Mikuni and Keihin 40mm carburetors require switchable ECU’s.

MNNTHBX Big Bores

A budget minded way to gain midrange torque and top end pull. These Chinese throttle bodies boast CNC machined billet aluminum construction. Oversized venturis direct more airflow resulting in measurably better performance. Install is simple being directly compatible with stock components and engine management. Provides up to a 15% gain introducing minimal extra cost.

ACN Advanced Throttle Bodies

The best fuel injected throttle body for many applications. Integrates perfectly with factory electronics. ACN throttles deliver quality comparable to Japanese brands with greater tuning potential thanks to the programmable ECU and bundled software. Dual butterflies allow a remarkable 30+ HP increase over stock. Compatible with many motorcycles via adapters.

Ecu Tuning and Supporting Mods

While the throttle body swaps mechanically in place, you'll need to tune the fuel and timing maps in the ECU to account for the added airflow. Standalone engine management systems with programmable chips offer the most flexibility for radical cam and head work. Or piggyback jetting systems can tweak signals going to the stock ECU on simpler bolt on installations.

Upgrading the air intake and filter removes another flow bottleneck. Larger fuel injectors may be needed to supply enough gas for big throttle bodies and heavily modified engines. All of this requires professional dyno tuning for maximum power gains and reliability.

With properly matched components, a quality 40mm throttle body yields excellent throttle response and top end horsepower. Take time selecting the ideal throttle body, tuning engine management accordingly, and addressing intake restrictions when installing larger 40mm throttle bodies.

FAQs

What are the benefits of a 40mm throttle body?

A 40mm throttle body flows more air allowing for more fuel and increased combustion. This boosts horsepower across the entire powerband, but especially top end power. Throttle response is also sharper.

What modifications are required when installing a 40mm throttle body?

You'll need to properly tune the fuel injection and ignition timing maps in the ECU. Switching to larger fuel injectors, free flowing intake components, and less restrictive air filter helps optimize performance.

Will a 40mm throttle body negatively impact low end power?

It can on smaller displacement engines. But slight throttle plate cutaways, proper ECU tuning, and torque biased cams can widen the powerband. There are some tradeoffs when gaining big top end power.

What is the best 40mm throttle body?

Mikuni VM and Keihin FCR round slide carburetors offer the biggest peak horsepower gains but usually require external ECUs over stock. For simpler bolt-on upgrades, the MNNTHBX and ACN throttle bodies integrate with stock electronics.