Souping up surface drives

The power plant of choice for surface drive motors is the 35-horsepower Briggs & Stratton engine. These stock engines can be modified to improve their performance, something done every day by Andy Johnston and his crew at The Boat Doctor Inc. in Metairie.

“What we are trying to do to maximize horsepower is to maximize the engine’s efficiency — we want it to ‘breathe’ as freely as possible,” Johnston said.

Johnston explained that, using Backwater Performance Systems technology, four big areas of modification exist: carburetors, cylinder heads, internal parts and exhaust.

Carburetors are provided that have larger throats. More air and more gas equal more power. The carburetor also has an accelerator pump, providing an instant shot of power that is really useful in digging out of mud.

The Johnstons have found that one of the weak links in standard Briggs engines is weak valves. They install larger one-piece stainless valves instead of two-piece steel valves, They do porting and polishing work on the valves and seats to increase the volume of air to the engine. They also “deck” or shave off part of the cylinder head to increase engine compression.

Internal parts that can be changed to performance parts include pistons, cams, rocker arms, push rods and connecting rods. These maximize air flow in and out of the engine.

The exhaust system, including the muffler can be changed to a free-flow stainless steel exhaust to allow for a freer flow of exhaust gases. These parts are also stronger and last longer because they are stainless steel.

Johnston estimated that between a third and half of the engines that leave The Boat Doctor’s shop have at least one of the modifications.

If every modification is put on a motor, the complete package will cost about $4,500 and increase an engine’s horsepower from 35 to nearly 60 horses.

He stressed that not every hunter needs every modification; much depends on what the hunter (or fisherman) wants to do with the boat.

Johnston offers free consultations at 504-737-1203.

About Jerald Horst 959 Articles
Jerald Horst is a retired Louisiana State University professor of fisheries. He is an active writer, book author and outdoorsman.