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Many owners regard their
modern outboard motor as a sealed and sacred piece of machinery which
should never be violated. These individuals think that even turning a
single screw on an outboard
power plant will cause it to malfunction.
On the other end of the spectrum, there are owners who believe that no matter how many carbs they add, how much they “cut” the heads, how badly they butcher the porting, or how high they bring the rpm, power will always be better than it was stock. Of course, any resulting problems are then promptly blamed on those “stupid factory engineers!” Obviously, neither of these notions is correct. In the following article, I’ll explain some of the engineering “facts-of-life” that apply to all 2 cycle outboards. These fundamental principles should help outboard hot-rodders understand what can and cannot be expected from their engine modification projects, and hopefully prevent disappointing and expensive mistakes.
DISPLACEMENT (in ci) x BMEP (in psi) x RPM
NET BRAKE HP = -------------------------------------------------------------------
INTERNAL DRAG (in)
This ominous looking equation represents the theoretical power output capability of any 2 cycle outboard engine. Since four important categories of variables control the results of this formula, their affect on the engine’s power curve should be clearly understood before you attack your engine with wrenches and grinder.
INTERNAL DRAG: Friction and pumping losses inside any reciprocating engine are obviously undesirable power reducing parasites. Unfortunately it is very unlikely that there can be significant reductions in either of these drag factors. Internal friction results primarily from the pistons rings which are, of course, necessary for cylinder sealing. Lower tension rings or single ring pistons can reduce the drag problem but there is a point of diminishing return if cylinder sealing is lost. Pumping losses in the engine are an unavoidable result of moving air in through the carburetors, pumping it up to pressure, and exhausting it out the ports. Even more unfortunate to the performance minded engine builder, is that all these losses grow with any increase in rpm, BMEP, or displacement. (Bad news, already!)
DISPLACEMENT: Total cylinder displacement is probably the most powerful variable in the formula. Because increasing displacement (either by bore or stroke enlargement) yields an almost directly proportional increase in power over the entire rpm band of the engine, it is typically the outboard design engineer’s favorite method of increasing the power of his engine. Since a broad power curve is essential for a general purpose production outboard, modification like displacement increases, which improve power over the entire rpm range of the engine, are the most suitable.
Unfortunately, significant increases in cylinder displacement (over 5%) are usually possible only at the drawing board stage of motor development. For example, the backyard engine builder will find it virtually impossible to increase the stroke of his outboard. Normally, there is no extra room in the crankcase for any additional crank swing, the porting layout will not lend itself to any stroke change, pistons and/or rods will not be available, and local crank shops normally will not be equipped to properly stroke a hardened roller bearing outboard crankshaft.
Likewise, a major increase in bore size is also usually unfeasible. Again, the lack of cylinder wall liner thickness, and piston and ring availability thwarts most big bore plans. Although some independent companies do offer sleeve kits or services, in most cases, increasing the displacement should only be done by a factory. Also, even though a displacement increase almost always results in some power increase, it must be remembered that as cylinder volume is enlarged, internal drag increases, while BMEP and rpm capability are reduced. (There is no free lunch!)
BMEP: Brake Mean Effective Pressure in psi, is a fancy sounding expression for the average pressure existing in the engine’s cylinder during a power stroke. It is an excellent indicator of the engine’s effectiveness in making power from its displacement at any given rpm. The total effects of volumetric efficiency (how close the engine comes to pumping its actual displacement with each stroke), combustion chamber design, compression ratio, ring seal, mixture dilution, air/fuel ratio, ignition timing, heat loss, fuel, etc. are all contributing factors towards the BMEP developed in the cylinder. While it is not mandatory to understand the engineering methods for calculating the BMEP of an engine, it is important to realize the importance these figures will play when determining that engine’s power curve.
Find a way to increase the BMEP of an engine and you’ll have increased its power. But since factory engineers have already worked long and hard to develop porting and tuning specifications that will yield high BMEP figures over most production engines designed power band, it will be quite unlikely that any appreciable improvements can be made by the independent hot-rodder over the entire stock power curve (compression ratio increases aside).
It then becomes the job of the engine modifier to effectively alter the stock specifications to increase the BMEP in a selected, narrow portion of the rpm band. Successfully accomplishing this typical requires a good deal of experimentation using airflow test equipment, engine dynamometers, etc., and actual on water testing – or at least a clear set of instructions from someone who has already done such testing on your particular model engine. Boaters that approach this task armed with only high hopes and “intuition” have ruined countless engines.
As an example of the gains (and problems) associated with raising the BMEP an outboard engine, let’s take a crack modifying a hypothetical 145 ci production outboard developing 200 Hp at 5,000 rpm. I’ve drawn a solid line in Graph #1 to represent this engine’s stock power curve. The engine has a BMEP of approximately 105 psi at 5,000 rpm. We’ll start our modifications of this imaginary engine with a substantial increase in compression ratio, say from 9.0:1 to 12.5:1 (theoretical full volume compression ratio). We could reasonably expect the power to rise along the dashed line’s curve in Graph #1. You see that we have increased power at all rpm’s of the engine for a peak power of 213 Hp at 5,000 rpm and a BMEP of 112 psi at that rpm. That’s great you say, let’s raise the compression some more!
Unfortunately, every modification has limitations and accompanying side effects. Although increasing compression ratio can often improve power over a wide rpm range (like a displacement increase), it creates serious piston and combustion chamber overheating problems if carried to extremes. We have already modified our motor from an easy going pump gas model, to a mill that will promptly melt a piston with less than 100-octane aviation gas in the tank. Raising, the compression ratio much higher would only result in power losses from detonation, and possibly a holed piston. In fact, if future modifications are made which further increase the BMEP, the compression ratio may actually have to be reduced just to stay within the thermal limits of currently available piston and ring materials. (There’s still no free lunch!)
Now let’s try a group of the standard racer’s tricks to improve volumetric efficiency (breathing of the engine). We’ll avoid the possibility of porting the engine incorrectly by allowing the factory racing team to port this imaginary motor to their racing engine specifications. While it’s there, we’ll have them hang a few extra carbs as well, since it’s just like their race engine, now it will really scream! Right?
Back at the dyno, we see that this modification has raised the power to 225 Hp at 5,500 rpm with a BMEP of 108 psi (see the dash dot dash line in Graph #1). Better, but not what we expected. Look at the low rpm end of our engine’s horsepower graph! What happened?
Almost any major improvement made in volumetric efficiency at high rpm will be accompanied by a loss of volumetric efficiency at lower rpm’s and of course a proportional loss of BMEP and horsepower. There are some exceptions but you can be pretty sure that factory engineers know most of these tricks and have already incorporated them into your engine’s stock design.
Well you say, “I still have an extra 25 Hp over the stock engine, so who cares about the low rpm range anyway?” You will, the first time you try to out accelerate a buddy’s stock engine! With only a small improvement in peak power to compensate for power loss at lower rpm’s this motor will still be getting on plane when most races are over. (Don’t worry though, things get better!)
RPM: It seems everyone has their own notion about what constitutes peak power rpm. Some believe it is the point where the connecting rods start poking expensive holes in the side of their “Wonder Motor,” while others think that it is the original rpm recommended by their owner’s manual, even though the motor has been ported, fuel injected, etc. Actually, peak power rpm is the engine speed at which the BMEP (which eventually starts going down as rpm continues to go up), multiplied by the rpm, gives the highest results, and therefore the highest horsepower. Revving the engine beyond that point produces less power, not more. (Another dead end in our power search? No!)
Here is where all those neat little race tricks, which only improved the BMEP in the upper rpm range, can be put to good use. Let’s go back to our imaginary engine in Graph #1. By raising the rpm of the stock engine, which had a power peak at 5,000 rpm, to 7,600 rpm as shown in Graph #2 (solid line) we see the BMEP has fallen to 48 psi and the power has fallen to 140 Hp. We indicated before, that this would happen past peak power rpm. But assuming we made the correct porting modifications, as we did with the “race version” of the imaginary engine, to keep the BMEP at about 100 psi of 7,600 rpm (well within reasonable expectations) we would have increased the power to 288 Hp at 7,600 rpm (see dashed line in Graph #2). A further increase in BMEP or rpm would raise peak power even more. Although such improvements would result in a decrease in the BMEP (and Hp) at lower rpm’s as we previously noted, the increased new upper rpm range would allow for a lower unit gear reduction or propeller pitch reduction to “hide” such loss of low rpm power. This is how most of the outboard manufacturers have managed to increase usable power in increasingly smaller displacement engines over the last few years. They have raised the BMEP of the engine to increase high rpm power and then moved the operating rpm band upward to regain the lost low end torque.
Many of the recent advances in power output, displacement increases aside, have come from developments in piston and ring design (allowing higher BMEP figure before thermal failure) and improvements in crank-train components (allowing higher continuous operating rpm). High rpm coupled with good BMEP is the “trick” to factory racing engine performance. This same power formula can be used by individual outboard hot-rodders to carefully plan their modifications to fully utilize the thermal and rpm capabilities of any production power-head. Power outputs of over 2 Hp per cubic inch are possible from some of the present production V6 outboards. Next time around I will cover the nuts and bolts of extracting some of that power from your outboard.