TFX Engine Technology - Power Tuning

Power Tuning - Two Stroke Engines

TFX Engine Technology Inc. pressure analyzers provide a tremendous advantage over and above conventional methods of making power, including vast amounts of tuning experience. Everybody has a theory about why a particular modification did or did not work. The only way to really know is to see what is happening inside the cylinders.

One needs only to read through any of the Engine Masters summaries to realize how important it is to know what is going on inside the cylinders. This contest is for 4-stroke engines and we all know that 4-strokes are much easier to tune than 2-strokes. Some of the best engine builders in North America participate in this contest. More often than not the engine builder finds out that the setup that worked back at their facility just doesn't live up to expectations at the contest facility where the location and atmospheric conditions may be a little different. Even with many years experience most engine builders just cannot get the engine to work correctly after several attempts. Now if the engine builder has a hard time getting things right at a dyno facility, imagine what the chances are of a raceteam tuner being able to optimize the engine at the track where conditions are even less favorable and the tuner has only one attempt to get it right before the next run. Now if the engines were 2-strokes life would be even more difficult.

Our pressure analyzers record the pressures generated in the cylinder, intake, transfer and exhaust ports relative to piston position. Based on the pressures and engine specs the HP, torque and volumetric efficiency of each cylinder can be determined for every cycle. In addition, the burn rate of the air/fuel mixture can be plotted relative to piston position indicating whether the burn is fast or slow and how it is located relative to TDC. All data is displayed on a per cylinder basis and data from several cylinders can be plotted simultaneously vs. rpm and time.

TFX Engine Technology Inc. pressure analyzers provide the customer, whether a novice or experienced tuner, with access to critical power tuning information which cannot be had by any other means. A “window” is provided into the engine’s combustion chambers and intake/exhaust ports, allowing the tuner to see exactly what is happening inside each cylinder of the engine under all conditions of load and speed.

Over 20 data plot formats are provided including:

HP, torque, IMEP vs. rpm and time combustion pressure vs. piston position detonation intensity AF mixture burn rates vs. piston position volumetric efficiency vs. rpm and time combustion efficiency transfer port pressures vs. piston position intake port pressures vs. piston position exhaust port pressures vs. piston position individual cycle and overall data summary
plots of multiple cylinders overlayed

How to make more power using this technology

A brief outline of some of the graphs and a few basic ways to use this technology to further increase engine power are shown in the examples below. Many data displays and analysis techniques are not included on the website, please contact TFX Engine Technology Inc. for more detailed information. View more data...

Combustion Pressure Graph

The combustion pressure graph is only 1 of 20 different ways of displaying the data. The combustion pressure graph allows the tuner to determine in a single engine test the peak combustion pressure, location of peak combustion pressure and the combustion temperatures for every cycle of the entire test load/speed range. The correct ignition timing can be established using the combustion pressure graph, energy release graph and numerical data for the entire range of engine speeds.

Adjustments to the air/fuel mixture may also be required at certain engine speeds. As the dynamics inside the engine change with changing rpm and engine modifications, so to does the air/fuel mixture ratio. Minor changes in the exhaust pipe dimensions can cause significant changes in not only volumetric efficency and air/fuel mixture ratio, but also in combustion efficiency and total combustion energy released from the fuel. Powerband flat spots and detonation zones, related to the air/fuel mixture ratio and ignition timing, show up immediately on the combustion pressure trace.

The numerical data can be used to determine how well the cylinder was filled with the air/fuel mixture (volumetric efficiency). Everytime a modification is made to the engine which affects cylinder filling i.e. exhaust pipe, porting, carb, turbo, etc. the impact on how much air/fuel mixture is retained in the cylinder is indicated under the heading volumetric efficiency. The more air/fuel mixture that is retained in the cylinder the greater the potential for power. Sometimes an increase in cylinder filling does not result in an increase in power due to changes in combustion energy released and combustion efficiency. The pressure analyzer data indicates how well the cylinder was filled and if any other aspects of the engine need to be corrected to realize the power gain.

The combustion pressure trace can also be used to prevent engine damage. Detonation spikes will show up on the combustion pressure trace at the first sign of detonation. Sometimes detonation may start out by occurring mildly only every 3 or 4 cycles. Knowing when detonation is just barely begun allows the tuner to avoid making any changes which could lead to more severe detonation on the next test. The pressure analyzer data can also be used to effectively tune a commercial knock sensor so that the racer knows how much knock is OK and how much is too much.

Combustion Pressure

In the example above the scale on the left side shows the pressure (psi) in the cylinder. The scale along the bottom shows crank position in degrees. -90 indicates the piston is at 90 BTDC, 0 indicates the piston is at TDC and 80 indicates that the piston is at 80 ATDC. The blue curve shows the actual compression and combustion pressure in the cylinder. The lower pink curve shows what the pressure in the cylinder would have been if combustion had not occurred. The upper pink curve shows the combustion temperature in degrees Celsius (multiply by approximately 1.8 to convert to Fahrenheit). The software allows the user to scroll back and forth through the data in a manner similar to using a VCR.

When the exhaust port opens the temperature in the cylinder is 1350 Celsius which is 2462 Fahrenheit ! Our pressure analyzers indicate the temperature the piston is exposed to just as the port opens. This temperature is critical to preventing piston damage and is very much higher than the average exhaust temperature (1200-1300F) which is normally recorded by an EGT. A conventional EGT measures the average temperature in the pipe several inches away from the piston. The pressure analyzer measures the highest temperatures the piston is exposed to right at the piston when the exhaust port opens.

Energy Release Graph

Some of the biggest power gains can be made by looking at the energy release graph. This graph indicates when combustion starts, how fast the mixture burns and when combustion finishes relative to piston position. It should be noted that the crankshaft rotates several degrees between the time the spark occurs and the time when any measurable increase in pressure occurs in the cylinder.

Energy Release

The pink curve shows shows how much of the mixture has combusted (left side scale in %) relative to crank position in degrees (bottom scale). In this example combustion starts at 14 BTDC. By the time the piston reaches TDC, 32% of the mixture is combusted. 83% of the mixture is combusted by 20 ATDC and combustion is completed by the time the exhaust port opens. The blue curve shows that combustion is occurring at the quickest rate at about TDC.

Significant power gains are made by burning the entire air/fuel mixture as quickly as possible and by positioning the combustion process appropriately around TDC. Almost every engine modification has an effect on the burn rate, but without using a pressure analyzer it is impossible to know how the burn rate is affected. Many fast-burn combustion chambers allow the tuner to reduce the ignition timing, suggesting that the burn rate is improved, but in most cases only the first 50% of the mixture burns more quickly, not the entire mixture.

Although the 440 cc engine in the energy release graph above is making 114 HP at 9000 rpm, a 10% increase in power at 9000 rpm is a realistic goal simply by getting the last 20% of the mixture to burn more quickly. Many tuners strive for a few % increase in power thinking that there may only ever be 2 or 3% more power available. Our pressure analyzers show the tuner where to look for more power and how much more power is realistically achievable.

Exhaust Port Pressure

Exhaust pressure wave tuning is the single most important factor in generating power in a two stroke engine. Small changes in the tuned pipe dimensions can generate significant changes in the pressure waves which are responsible for gains or losses in power. Exhaust port pressure analysis allows the tuner to see the magnitude of the exhaust pressure waves, and how they are timed relative to port opening and closing events. The exact effect of even small changes in exhaust pipe dimensions are displayed along with how well the cylinder was filled and how much power and torque was generated in the cylinder. The pressure waves in the exhaust port are displayed for every revolution of the crankshaft. Each plot displays the revolution of interest and the prior revolution so that exhaust resonance can be easily observed.

Exhaust Port Pressure (7143 RPM)

Starting at the lefthand side of the graph at –360 the piston is at TDC on cycle #169. Proceeding to the right the exhaust port opens at EPO near the top left of the graph. The transfer port opens at TPO and the piston reaches BDC at –180. Moving further to the right the transfer port closes at TPC, then the exhaust port closes at EPC. The piston continues to rise reaching TDC at 0 on the x-axis. 0 to 360 on the x-axis displays the exhaust port pressures for the next cycle. The test engine in this instance does not have an external exhaust port valve and as such the powerband width is narrow. At 7143 rpm volumetric efficiency is moderate at 80%, but peak combustion pressure and engine torque are low.

Starting at the left side of the graph, a pressure wave exists in the exhaust port just before the exhaust port opens. As the exhaust port opens the pressure wave becomes negative. Typically opening the exhaust port would generate a pressure in the exhaust port as the gases in the cylinder rush into the port. However the negative pressure wave resonating in the tuned exhaust is sufficient to overcome the exhaust flow and a suction is generated which tends to try to pull the exhaust out of the cylinder. A strong suction of about 5.5 psi is generated in the exhaust port around the transfer port opening by the divergent cones in the tuned exhaust and extends to about 20 ABDC (-160 on the graph). This suction helps pull air/fuel mixture into the cylinder, some of which is pulled down the exhaust pipe.

Starting at 20 ABDC (-160 on the graph) a pressure wave, generated by the convergent cone at the end of the tuned exhaust, pushes air/fuel mixture from the exhaust pipe back into the cylinder. This pressure wave peaks near closing of the transfer port. As the crankshaft continues to rotate the pressure in the exhaust port decreases reaching a suction of 5 psi by the time the exhaust port closes. As a result a large amount of the air/fuel mixture pushed into the cylinder by the pressure wave near transfer port closing, is blown out of the cylinder between transfer port closing and exhaust port closing. This results in a significant loss of volumetric efficiency and torque. On an engine equipped with external exhaust port valves, the valves can be easily adjusted to the correct position for every load and speed based on the pressure wave data.

Exhaust Port Pressure (9000 RPM)

As the engine speed is increased to 9000 rpm exhaust resonance continues to occur and the amplitude of the pressure and suction waves is further increased. High volumetric efficiency (104 %) and torque is generated. The 440 cc engine is generating 114 IHP at 9000 rpm.

Although the pressure waves are well timed there is room for further improvement. The reflected pressure wave peaks right at transfer port closing (TPC), drops off a little as the crankshaft continues to rotate. The pressure drops quickly in the last 10 degrees just before the exhaust port closes (EPC). Clearly air/fuel mixture in the cylinder is being blown out of the cylinder into the exhaust port between transfer port closing (TPC) and exhaust port closing (EPC). The convergent cone used in this test was of typical dimensions. A better convergent cone design could reduce or eliminate the loss of air/fuel mixture from the cylinder, resulting in even more power and torque, particularly when combined with the combustion process shortcomings indicated above.

Exhaust Port Pressure (9278 RPM)

As the engine speed is increased to 9278 rpm the exhaust pressure waves are only slightly different from that at 9000 rpm. The waves are slightly later (shifted to the right). Volumetric efficiency is still high (102 % at 9278 rpm vs. 104% at 9000 rpm). The tuned exhaust is still within its powerband, but the power and torque have dropped off substantially (17% reduction in HP, 19% reduction in torque) between 9000 rpm and 9278 rpm. Without being able to see what was going on inside the cylinder and exhaust port the engine tuner would assume the engine had revved beyond the powerband generated by the tuned exhaust and port dimensions. Clearly this is not the case and combustion modifications are in order, not exhaust pipe and porting changes. Substantially more power could be made at 9278 rpm without changing engine torque at lower speeds.

Power vs. RPM Graph

As previously indicated engine power and torque can be plotted vs. RPM for each instrumented cylinder. This is not possible with conventional dynamometer outputs. Each cylinder can be tuned to provide equal power as well as equal cylinder pressure.

An often asked question is how many cylinders need to be instrumented? The answer is 1 cylinder. Most customers instrument no more than 2 cylinders. One of the benefits of using a pressure analyzer is that all modifications can be carried out on 1 cylinder since power etc. is determined for each cylinder. Once the cylinder is optimized within time constraints, the modifications can then be transferred to the other cylinders. In some instances physical limitations dictate that each cylinder be tuned differently. For these cases each cylinder can be optimized independently either by transferring the sensor from cylinder to cylinder or instrumenting most or all of the cylinders.

All instrumented cylinders can be displayed simultaneously so that the power vs. rpm of each cylinder can be compared. Plots of torque vs. rpm and IMEP vs. rpm are also included in the software. Please refer to the four stroke tuning section for an example plot of power vs. rpm.

Maximum Combustion Pressure Graph

Maximum combustion pressure of each engine cycle vs. RPM can be plotted for the entire engine test either as a function of time or cycle number. Fluctuations in maximum combustion pressure and fluctuations in the location of maximum combustion pressure relative to TDC, need to be minimized, allowing each cylinder to put out as much power as possible on every cycle.

Significant fluctuations in maximum combustion pressure from cycle to cycle are a sign of poor air/fuel mixing, insufficient ignition energy and sometimes a poor combustion chamber design. Significant fluctuations in maximum combustion pressure and location relative to TDC can indicate that the mixture is not burning consistently from cycle to cycle. Substantial increases in power are available by making modifications to the engine which eliminate the lower pressure cycles.

Reducing the higher pressure cycles a little can help the engine stay away from detonation. As an example, if the ideal cycle for this engine combination has a maximum pressure of 1800 psi which occurs at 7 degrees ATDC then all cycles which generate maximum combustion pressure values which are lower than 1800 psi or positioned earlier or later than 7 ATDC result in a loss in power. Please refer to the four stroke tuning section for an example plot of maximum pressure vs. cycle number and rpm.

Intake/Crankcase Pressure

Pressure analyzer data is used to determine intake and crankcase pressure relative to piston position. The goal on the intake side is to optimize the reed valve or rotary valve, along with the carb and intake dimensions so that the pressure in the crankcase is maximized when the transfer port opens. In some instances the reed valve will re-open a little after BDC to let even more air/fuel mixture into the crankcase if the intake system is designed just right. This second reed opening is easily recorded by the pressure analyzer.

One of the goals on the crankcase side is to maximize the transfer port timing without having exhaust gases flowing back into the transfer ports as the ports open. The transfer port timing can be optimized knowing the pressure in the crankcase and in the cylinder to ensure that the pressure in the cylinder is about the same as the crankcase when the transfer opens. If the transfer port opens too soon the exhaust gases will be blown into the crankcase when the transfer opens causing a loss in power. Our pressure analyzers show the tuner exactly how soon the transfer can be opened without generating backflow, for any engine combination.

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