Blowing It Wide Open
The 1993 world finals will go down in history as one of the most difficult times ever for the professional racer. Increased pressure from the new generation of racers, coupled with unrealistic demands for speed and horsepower from the watercraft, has created serious problems for many racers, especially those not receiving full support from their sponsors. I don't recall ever seeing as many racers fail to finish their heats as I did at the '93 World Finals.
Judging by the number of scattered engine parts throughout the pits, it seems painfully obvious to this tuner that we're in big trouble in the engine setup department, not to mention the areas of race preparation, traction, and handling. As far as I'm concerned, the most serious problems are related to the engines' top end, especially the combustion chamber designs, induction/carburetion, squish areas, and ignition timing. As the demand for horsepower increases, so does the failure rate, which is related to big-bore cylinders, out-of-square stroke-to-bore ratios, and abnormally short connecting rod lengths. It seems we've gone overboard and become horsepower dependent, putting more stock in building super-powerful equipment than in developing super-talented riders whose abilities match their craft. It's interesting to note that in the past few years the Europeans have made notable progress using less powerful machines than the Americans, simply by paying more attention to the rider's skill, properly setting up the motors, and providing old-fashioned maintenance.
(Below) You don't need a CNC machining center to produce a limited number of quality heads. If you're careful, you can get excellent results using homemade cutting tools and a conventional vertical milling machine.
(Below) With the right formulas programmed, computers can deliver fast, precise head designs which can be followed by sing conventional machining or by direct link-up with a CNC machining center. just a few years ago, it took designers hours to create a proper head, and now it can be done on a home PC in a matter of minutes. This is the printout of a properly designed head for a Limited WaveBlaster.
What happens all too often in big race situations is that too much pressure is put on the craft to perform beyond its ability. Here's what I mean: every engine needs a certain amount of proper break-in time. Period. You simply can't expect full power with newly installed pistons, or by dropping a just-rebuilt engine into your boat. I would guess that in cases like these, you're looking at a failure rate of nearly 50%.
You can cut this failure rate radically if you try. First, remember that the money you save by doing a mediocre job will be spent (with interest) before long on repairing the damage that mediocre job caused - not to mention the frustration and disappointment of having your ride and joy break down every other race. Second, bear in mind at every part has +/- machine tolerances. For example, connecting rods may be off 0.2 mm, pistons 0.3 mm, cylinders 0.1 m, etc. Combine a couple of these and suddenly the combustion chamber is affected, with messed up squish areas and fouled timing.
If you ask me, the biggest mistake made in setting up an engine is using a pressure gauge to measure the compression ratio. I've watched in disbelief as racers asked their tuners whether 250 pounds of cranking pressure is okay for their engines, only to have the tuners answer with, "Not bad." Wrong. It could be very, very bad.
An engine's usable power is directly related to the amount of fresh charge trapped in the cylinder, how tightly this charge is compressed prior to ignition, and how efficiently it's burned. This trapping, compression, and burning is a result of a number of factors, including: the flow map (pattern) of gases entering the cylinder; the velocity of the gases in the combustion chamber; the gas turbulence affected by the chamber shape; the scavenging efficiency of the exhaust system; the trapping efficiency of the pipe and porting; the octane of the fuel; the lubricity of the pre-mixed oil; and the design of the combustion chamber. The proper matching of combustion chambers, port shape and timing, and exhaust system can yield major (and economical) gains in performance. Proper redesign and machining of top-end components is most critical to attaining a dependable power assembly. Undetected errors in your calculation methods, going on hearsay, and/or inaccurate machining will lead to unpredictable results, ranging from power loss all the way to engine self-destruction.
SQUISH THICKNESS AND AREA
Improperly set up combustion chambers are responsible for many blown engines. You must fully understand the importance of this critical area, for only then can you build engines with reasonable reliability. The manufacturers know that parts vary from one to the other when they come off the production line. For that reason, they build engines with squish thickness as much as twice the required size. This procedure works fine in a Stock Motor, but when modification takes place, the results can be devastating. An experienced tuner knows that squish that's not working is worse than no squish at all. Why is it so important to select proper parameters? ... because squish serves as a cooling source for the top of the piston, especially around the edges. If the squish thickness is too big, the engine may overheat and seize. If the squish is too small, the trapped mixture may ignite prematurely, creating a second combustion front, which acts as a big hammer blow to the top of the piston, causing detonation. Beware of white, speckled spots on top of a piston around the squish area. Also, squish thickness and area controls the velocity at which the fuel mixture is moved toward the center of the cylinder head. This velocity is measured in meters per second (m/s), or Mean Squish Velocity (MSV). The central head type works best from 15- 25 m/s for all-out race motors. There are a number of variables that relate to and control MSV. For examples, see Chart A.
COMPRESSION RATIO
Compression ratio is of considerable importance in any performance application. The most common method used for CR adjustment is shaving the cylinder head. Builders and tuners use two methods to calculate CR. Method One, referred to as the Two-Stroke or Effective Method, divides the volume of the combustion chamber and cylinder at the point of exhaust port closing (EPC) by the trapped volume of the combustion chamber with the piston at top dead center (TDC). Method Two, known as the Geometric or Four-Stroke Method, divides the calculated volume of the cylinder and combustion chamber at bottom dead center (BDC). NOTE: Neither method is 100% accurate, because there are losses due to the fresh charge trapped in less than optimum exhaust systems. This means lowered CR. Conversely, if a tuned exhaust system operates more efficiently than we expect, the CR may become much higher than predicted, due to the supercharging-like effect of I the combustion chamber. That's why we design, argue, re-design, blow up engines, test, test, test, test, test. In the end, we race to show that we've figured something out, that we've got the right stuff, and then we keep our mouths shut about the number of wasted engines we have hidden behind the workshop. Realistically, racing sucks, but without it, we'd still be making wheels out of stones and we wouldn't be having nearly as much fun. The best thing I can tell you to do is to keep in mind that the new power plants must be modified very care fully. For the best results, let the experienced shops and builders do the dirty work and spend the money on R&D, and once they have something that works, try to get your hands on it. It's a lot safer, easier, and cheaper to copy something than to invent it.
FINAL TIPS
Redesigning the cylinder head for greater performance and reliability is a critical task that shouldn't be attempted carelessly. If you're considering sending your cylinder head to a performance company for modification, provide the technician with complete information regarding the bore size, squish clearance, the type of fuel you plan to use, and so forth. Tell him about any other modifications you have made, or plan to make, to the engine. His complete understanding and your choice of matched components will ensure peak performance and reliability from your engine. For best results, each motor should be considered as a separate unit, due to the variations in manufacturing tolerances this is the only way to assure precision and accuracy.
George Grabowski HPT Sport USA