How to Build a Racing Lawn Mower – Engine Modifications

LMR-bannerHave you ever wondered how to build a racing lawn mower?  If so, keep reading.

Start first with the engine.  Prior to modifying your lawn mower racing engine, you must decide what class you will be competing in.  Once you determine the class, study the rulebook in detail to ensure your engine will comply with all of the rules.  It is equally important to take advantage of every modification allowed by the sanctioning body. These are important guidelines to follow when you are learning how to build a racing lawn mower.

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How often have you heard a driver say, “I need more horsepower?” I hear this all the time.  In fact, I have never heard a driver say, “I have too much horsepower.”  While there is a lot more to winning than horsepower, it is very important to take advantage of every improvement allowed in the mower racing rulebook.

If you are unfamiliar with the inner workings of mower racing engines, you will find many small-engine rebuild and repair shops located throughout the country. They are generally eager to offer advice and if contracted to perform your rebuild, they will take pride in generating more horsepower than their competition.  And they already know mower racing, saving you the time and learning curve.

If you plan to do your mower racing in an “open class” or “extreme modified”, the rulebook may look like this:
1) Any lawn mower racing engine is approved
2) All engine modifications are legal
3) Any carburetor is allowed
4) Any cubic inch displacement is allowed

This criterion is a racing engine-builder’s dream come true. The door is wide open for innovation. He will try new things and push the parts to the limit in his quest to find horsepower. As in any form of motorsports, attention to detail and the use of lightweight precision quality parts will take you to the mower racing victory lane.

The key components to figuring out how to build a racing lawn mower engine are: flywheels, pistons, crankshafts, carburetors, valves, and heads.

Purchase a high-quality billet flywheel like those manufactured by ARC. These are much stronger and safer than stock flywheels. Find the lightest flywheel available as well. With the inertia of the flywheel, the lower weight will allow the racing engine to rapidly increase in RPM and help you pass other racing lawn mowers as you exit the corners.

The crankshaft should be balanced for durability. Prior to balancing, you should consider placing the crankshaft on a diet. Like the flywheel, your crankshaft has a tremendous amount of mass. If you reduce the weight, the engine will accelerate rapidly, allowing the mower racing driver to make the pass.

Pistons and connecting rods are precision components. While overall weight is important, it is equally important to precisely measure each component for tolerance and fit.

Purchasing high-quality parts made of superior materials for durability is well worth the dollars during your first attempt at learning how to build a racing lawn mower.  After that you will have more experience and preference on which parts are worth the extra money.

If you recall your safety training on extinguishing a fire, you will remember that combustion requires 3 elements to occur: heat, fuel, and air. The more fuel and air the bigger the fire. Or in the case of a mower racing engines: the bigger the bang.

Your racing carburetor can be a limiting factor in the combustion process. To combat this restriction, you may spend countless hours porting and polishing the components. To save time, purchase an off-the-shelf racing carburetor prepared by a custom carburetor shop. Local go-kart racing dealers will have the part you need.

Now that you have modified your carburetor, the engine head itself may become the restricting factor. Port and polish the intake runners for improved air flow. Prepare your valves with a precision multi-angle valve job.

The camshaft is also crucial to engine performance in mower racing.  Aftermarket custom camshafts are available. These racing camshafts have increased lift and higher ramp speeds to open and close the valve quicker. This will allow more air to fuel the fire. You may find that you require a different camshaft for different lawn mower racetracks or for varying conditions.

To finish first, you must first finish, so take extreme care in learning how to build a racing lawn mower.

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