Johnson Motors Service Promotion letterheadSPB #360

March 16, 1964

SUBJECT: BORE: 3-3/8", STROKE: 2½", CUBIC INCH DISPLACEMENT: 89.5, HORSEPOWER: 75
BORE: 3-3/8", STROKE: 2½", CUBIC INCH DISPLACEMENT: 89.5, HORSEPOWER: 90

HOW COME?

The subject article was first printed in the December 1963 issue of JOHNSON JOTTINGS. Since then we have had numerous requests for this information to be made available in bulletin form.

Since the introduction of the 1964 Johnson Golden Meteor 90 H.P. motor many questions have been asked by dealers, and by customers of how can two separate model motors with the same bore and stroke and the same cubic inch piston displacement develop in one case 75 horsepower at 4500 r.p.m. and 90 horsepower at 4500 r.p.m. in the other. This confusion is further compounded because both the 75 and 90 horsepower motors are shipped with a 10¼ x 10 inch propeller.

First of all, let's clear up the propeller aspect. Contrary to popular belief, there is no "Standard" propeller for either the 75 or 90 horsepower motors. As a matter of fact, there is actually no "Standard" propeller for any outboard motor, especially from 18 H.P. up. All of these models have a selection of propellers available to meet customer requirements. Four separate sizes for the 28's and 40's and five separate sizes for the 60, 75 and 90 horsepower models.

A propeller is shipped with the motor only to complete the package. Selection of the correct size for the specific application for which the motor is to be used is up to the dealer. This insures the customer of optimum performance when installed on the boat.

But let's go back to the 75 and 90 horsepower matter. Do not forget that these horsepowers are certified by the Outboard Industries Association of the Outboard Boating Club of America. Their certification is positive assurance to your customer that there is actually the advertised and certified horsepower at the crankshaft of the engine. The VM 90 H.P. motor gets its additional fifteen horsepower from a combination of several factors. The four barrel carburetor, the new crankcase and the new six seal-ring crankshaft all combine to make this additional horsepower possible from the same cubic inch piston displacement as the 75 horsepower motor. The straight-through or direct fuel induction system developed for the 90 horsepower motor makes possible maximum use of the power potential of the fuel-air mixture introduced into the individual cylinders. Having a crankcase and crankshaft whose design permits equal and identical and reduced (over the 75 H.P. motor) volumes along with equal pressures for each individual cylinder permits equal distribution of fuel to each of the four combustion chambers. This equal fuel distribution results not only in increased horsepower but also adds considerably to the fuel economy of the engine.

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By being able to introduce the correct fuel-air mixture into the combustion chambers at equal and increased pressures a greater volume of correctly atomized fuel can be used for combustion. This results in greater pressures on the piston head when the ignited fuel expands and pushes the piston down. This greater amount of compressed fuel-air mixture burns more completely and little, if any, unburned fuel is exhausted through the exhaust ports. Thus we get not only increased pressures on the piston head which is directly converted to increased horsepower, but also increased efficiency and economy.

Now, what does this all mean in regard to horsepower and value to the customer. It means that actually the 90 horsepower motor develops more torque at a given r.p.m. than the 75. Webster defines torque as follows: That which produces or tends to produce rotation or torsion; a moment (of forces); specifically, the turning moment of a tangential effort. We define torque as the ability to get a job done mechanically. The greater the torque, the greater the power which can be used to, in our case, propel a boat.

Actually the 75 horsepower motor in a certain application could turn a 10¼ x 10" propeller at 4500 r.p.m. The 90 horsepower motor would turn this same propeller on the same boat somewhere between 5000 r.p.m. and 5200 r.p.m. Or on the same boat, the 90 horsepower motor would be capable of turning a 10¼ x 11" propeller at the same r.p.m. that the 75 horsepower motor turned a propeller of one inch less pitch, (which actually means more speed and performance).

Basically the principal thing to remember is that because of a highly sophisticated fuel induction system which encompasses the carburetor, crankcase and crankshaft design, greater power and improved fuel economy can be derived from a given number of cubic inches of piston displacement. Translated to the simplest of terms, the 90 will permit you to turn the same propeller faster than a 75 or a higher pitch propeller at the same r.p.m. that the 75 turns a lower pitch.

JOHNSON MOTORSJack Penland handwritten signatureJack Penland
Service Promotion Manager

JP:eb