Power Head
In my experience, the most misunderstood aspect of a proper engine setup is related to the top end. Whether it's a slightly modified unit or a full-on race engine, the results are very similar. Not a single day goes by without a phone call from someone with a serious engine problem: "Man, I had my motor modified and now it seizes/overheats/blows gaskets/melts pistons and just runs like junk." It's like an epidemic. It breaks my heart because of the financial losses that the consumers have experienced and the damage that's been inflicted upon a professional engine builder or aftermarket company's reputation. If engine modification is performed, it's mandatory to follow the proper break-in and setup procedures.
The responsibility for this is in the hands of the engine builder as well as the craft's owner. As builders, we must provide clear-cut information regarding the set-up of the carburetors, ignition, octane requirement, lubrication, and the safe operating speed (in rpm) of a given power plant. As owners, we are responsible for following to the letter each instruction given by the builder, as the slightest deviation may (and often does) have very serious consequences.
So the first step is to know exactly what needs to be done and the proper sequence for doing it. You don't need to have laboratory instruments to set up your machine, but nevertheless, any attempt to work without, say, a tachometer, water temperature gauge, or exhaust temperature gauge will result in less than optimum performance, as well as leaving you somewhat in the dark when it comes to understanding exactly what's going on inside your machine. This isn't to say that you can't operate without instruments, most people do. But when a high degree of accuracy in required, like in an all-out race motor, you better know what you're doing, because things heat up very quickly in there and there's not much room for mistakes. A race motor is designed to deliver hard-hitting horsepower in short bursts, and can not be run wide open for a long period of time.
A race motor requires the best fuel and lubrication, and the tightest tolerances when building the engine. For instance, a sport machine may go a season or two on the same set of rings and pistons. An all-out race motor will lose power drastically if the rings are worn beyond a tolerance point.
Oil is the life saver in every machine. The higher the degree of modification, the better the oil has to be. The operating range of an engine is grossly dependent on the engine block's temperature. As a rule, water coming out of a cylinder head should be between 120 and 150 degrees F. It's possible to run lower water temperatures if a long-chain synthetic oil is used. When operating in salt water, the engine block may become blocked by salt which begins to crystallize at approximately 165 degrees F - so you should stay around 140 F just to be on the safe side.
You don't need fancy instruments to set up a proper water flow. I use a very simple setup I make by inserting a "T" fitting into a water hose coming out from the cylinder head, which feeds into the head pipe. A 1/16" or 1/8" hose is rerouted toward the outside of a machine that feeds into a small cup. At full throttle, the cup is filled and I can record the temperature by using an ordinary round faced thermometer. Simple but effective.
If the water temperature is too low, I restrict the water going out. If the temperature is too high, I increase the flow accordingly. This is a simple yet important procedure that can save you a lot of headaches. You need a selection of restrictors, from 1/16" to 3/16" inside diameter in 1/64" increments.
Gasoline varies considerably, depending on brand and location. What you get in the South may be completely different from what's available up North. Alcohol in your gas tank spells disaster in a watercraft engine. It absorbs moisture from the atmosphere which may raise combustion temperatures beyond the safe levels, causing piston meltdown. Testing for alcohol content can be done quickly and easily by using a 50cc container. Fill it halfway with water and mark the level. Then fill it the rest of the way with gas and let it stand for 10 minutes. If the water level rises, it indicates the presence of alcohol.
When the engine is highly modified, its octane requirement can become critical. You must use a quality brand of gasoline, and if you can, use a higher octane than is absolutely necessary. Just in case.
Any modification of the cylinder requires precise matching of the combustion chamber compression ratio. The only sure way to do a proper job is by measuring the combustion chamber volume and using formulas to set up the combustion ratio (CR), squish clearance (SC), squish area (SA), and squish velocity (SV). A mistake of as little as 1/4mm in this area can cause combustion problems that may drive you to the psychiatrist's couch. Also, it's been my experience that you can't accurately determine an engine's CR with a compression gauge. I've seen gauges' readings of the same engine vary by as much as 30 psi. That's not acceptable.
Also, it's no longer safe to assume the stock engine has a lot of play in it. Back in the old days, working on Kawasaki 440s and 550s, it was possible to machine as, much as 0.125" from the cylinder head and still be in a safe operating range. Not any more. The Kawasaki 750 motor's tolerances have been tightened to a reasonably efficient level, and to just start machining away can be a disastrous mistake.
A two-stroke engine appears to be simple, but as any engine builder will tell You, the more they work with this little beauty, the more they respect it. Experience is priceless. It teaches you the importance of attention to detail. And there's more detail than many people realize. There's the CR, the squish velocity in the combustion chamber, the proper fuel octane, lubrication, ignition timing, carburetion, exhaust system, and engine block cooling. All of which play an important role in your set up and should be standard procedure to ensure trouble-free operation at the races or just tooling around the lake.
Let's take a look at how some modifications affect our power plant. For example, a stock Kawasaki 750 appears to have plenty of room for modification. The cranking pressure is around 140 lbs., so the first thing most people do is mill the head and get that sucker moving.
Not so fast. This may be setting the engine up for disaster. This may work great on a highly compressed 125cc or 250cc motocross engine, but it may not work so well on a watercraft engine that runs at a constant speed under heavy load. In my experience, your motor knows the difference between load variations of as little as 50 lbs. In the quest for top speed, it's easy to make the deadly mistake of going overboard in an attempt to beat everybody. How good is a high-speed, high-revving contraption if it spends more time at the shop than in the water? None. Remember, the combustion chamber setup is governed by strict rules which must be followed closely. Squish velocity is first and foremost. The proper setup of squish is related and determined by the engine's speed in rpm, trapped compression ratio, squish area ratio in %, and squish thickness (or clearance).
Let's take a closer look at a stock Kawasaki 550, and at the same engine set up to race. We must know where we are with the stock motor in order to successfully modify the machine to its upper limits. The first step in any motor modification is to gather all the necessary data related to that unit. Some of it is given in the factory instruction manual, the rest must be recorded from engine measurements. The stock engine has a 75mm bore, 60mm stroke, 7.0:1 compression ratio, and delivers around 45 horsepower at 6000 rpm. Now we need to dig inside to get the port specs and the rest of the needed dimensions.
In illustration A, the combustion cham- ber volume is 27.9cc. The deck height is 1.4mm. The squish is 2mm. The squish area is 50%. We have 45.62 hp at 6000 rpm. Fuel is minimum 85 octane, the minimum BMEP is 91.4 psi at 6000 rpm. The average top speed is around 44 mph.
Now let's build a respectable, race worthy machine capable of track speeds of 50 mph or more.
Step 1 is the selection of a tuned exhaust system which will operate at or above 8000 rpm.
Step 2 is to use a high quality race fuel. High horsepower is only possible when using the upper limit of the CR at a reasonably safe engine speed. Computer programs help enormously in designing a power plant, but will NOT take the place of experience and expertise.
Step 3 is establishing the proper compression and porting configuration.
Step 4 is to determine the proper port time areas, very important for efficient power delivery.
Step 5, the final step, is selecting the proper MSV.
To start, we must determine the running parameters of the engine. After, selecting the exhaust system and the proper rpm range, the exhaust port duration is selected to complement the horsepower of the tuned pipe. The next step is to select a reasonable compression ratio which will work with the available fuel. I use a computer program from Two-Stroke Racing which will print out the outputs, indicating the possible horsepower, the estimated minimum octane required by the selected CR, and the indicated BMEP at 8000 rpm. (As a note: Due to the notorious failure of cranks in the 550 engines, I use a n conservative BMEP than the program indicates, just to be safe.)
Machining the transfer ports to complement the engine's rpm range is achieved by precisely machining the main transfers, the fifth port, and the bust port to provide the optimum power band range. As a rule, an engine that runs in a lower power band has more area, while a motor running at high horsepower will have a lower TA value. The process of selecting the exhaust port's TA value must be approached very carefully and precisely. The idea is to match the target TA values (which the computer tells me is 13.57024 for 8000 rpm) by changing the exhaust port size. Note that the top and bottom corner radii play very important roles in finalizing the port TA value.
Squish velocity is the final step in setting up the engine. The program I use is one of the most popular, again from Two-Stroke Racing. Many mistakes can be made in this area, as any change made to the trapped compression ratio, squish area ratio, squish clearance, rpm range, exhaust port duration, and even the bore size diameter can show up as a partial piston seizure or total engine failure if the motor is set up with a dangerously high MSV to begin with.
It's my sincere hope that at least a small part of this information will become useful to you in building, setting up, or simply learning a thing or two about two-stroke engines. If nothing else, I hope you get a sense of the complexity involved within the combustion chamber. Engine building requires knowledge, and you can't get that from someone who's keeping "secrets". You should always know exactly what's going on with your engine, and you should always know what affect a certain product will (or at least should) have regarding your engine's performance. There are plenty of sharp people in this industry you just have to find them.
George Grabowski HPT Sport USA