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A turbo modification compared to a larger displacement engine for a working mower, not a toy hot rod project is simple. No, a turbo will not be viable.

First, most all turbocharged engines are designed for the additional stress of a power adder. A turbocharger, supercharger or nitrous are all considered power adders. Let's take the Ford Eco-Boost engines for an example. Ford didn't simply throw a set of turbos on a 3.5 and call it a day. These engines have had many modifications internally to handle the additional stress a turbo puts on the engine when it's making boost. Boost is simply higher compression regardless of what is producing it. Higher compression needs more fuel to produce more power. It further usually requires an ignition timing re-tard to maximize power and reliability. The size of the turbo needed varies depending on the purpose and size of the engine (displacement). One could theoretically produce enough boost and power from a turbocharged 25hp engine to match the power of a 31hp or maybe a 37hp naturally aspirated engine. The point being, added stress requires added reinforcing of the components and that is quite costly.

But why would you want to? Cost wise it makes no sense, not to mention the extreme amount of R&D time to get it dialed in.

Then the added fuel consumption to make this additional power...nothing is for free! To make the same power as the 31hp or 37hp engine you will be using the same amount of fuel as the 31hp or 37hp naturally aspirated engine. You will also likely be making more heat to the point of damaging your air-cooled engine. Car & Driver said about the 2.7L twin turbo F-150 Eco-Boost pickup..."BIG BLOCK POWER! BIG BLOCK FUEL ECONOMY!!" When you work (boost) a turbo 6 (or 4) there's no savings in economy over a comparable output V8. Only off-boost will the smaller engine provide any economic benefit.

The internal mods needed for a turbocharged engine to survive is many. Stronger crankshafts, rods, wristpins and pistons, copper head gaskets, premium ignition systems etc. Ford, GM and major manufacturers can afford the R&D to make it work for an application they have deemed viable...you cannot.

Now in spite of all this could it be done? Sure! At great cost...would it work very well? HIGHLY unlikely for a working mower...let's just say no, it won't. If you want more power, get a bigger engine/mower for mowing. If you want a toy?

For reference: I have turbocharged many motorcycle engines since 1977 to current. I have run blowers on various BBC engines in both boats and cars. I'm not a fan of spray. These are all for high performance applications. None of them is for a constant load/boost situation. I have considerable experience in blown high performance engines.
 
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