NoPistons -Mazda Rx7 & Rx8 Rotary Forum - Mazda Makes Another Statement About Hp Loss

Statement:

There is no true way to generate flywheel horsepower from a chassis dynamometer because of frictional losses in the driveline, clutch, transmission, differential and tires. Also, variations in testing procedures will cause highly disparate readings: open/closed hood, high/low humidity, high/low ambient temperature, tire pressure, how tightly the car is tied down, which gear the car is tested in, etc.

In addition, we have determined that, in order to prevent damage to the catalytic converter and the entire driveline, when the PCM determines unusual operating parameters such as excessive slip in the drivetrain from the front to the rear wheels, it causes a rich high-RPM mixture and retardation of the timing. All these items combine to cause apparent considerable horsepower loss.

BACKGROUND:

Horsepower Measurement

There is only one true method for measuring engine horsepower: on an engine dynamometer at constant speed and utilizing variable load.

The engine should use the same intake and exhaust system as in the car. HP results must be corrected to SAE J1349 standards as listed below:

77 degrees Fahrenheit

Sea level

0% humidity

Correction factors must be applied to reference the measurements to SAE J1349 standards. Any correction factor beyond 7% is considered invalid.

Chassis Dynamometers

SAE has produced a technical paper (SAE Technical Paper Series 2002-01-0887) that attempts to address the ongoing debate about inertia dyno horsepower versus OEM net horsepower. You can order a copy by visiting their website at www.sae.org -- we have no intentions of getting mired in the middle of this discussion.

Bottom line: If used properly, chassis dynos are great tools to assist with tuning and modifying vehicles. It is impossible to measure the actual flywheel horsepower because there are simply too many variables.

Other issues that are unique to the RX-8:

The RX-8 uses a very advanced engine management system. Besides precisely controlling the operating parameters of the engine, self-preservation (of both the engine and the catalytic converter) is also considered.

The engine management system continuously monitors all engine functions and adjusts accordingly. For example:

Under heavy load acceleration, the timing is retarded and the fuel mixture richened to reduce the likelihood of pre-ignition or spark knock. If spark knock is encountered, a knock sensor senses the condition and further retards the timing. Gradually timing is advanced and fuel mixture leaned after the load is reduced.

A second reason for fuel enrichment is that when timing is retarded, exhaust temperatures increase; a richer mixture lowers the exhaust temperatures and reduces the chances of damaging the catalytic converter.

In real world driving, this all goes unnoticed to the driver and appears seamless with no disruptions to the performance of the engine. The car encounters a load under acceleration but the load quickly diminishes as the car accelerates in each gear.

Operating on a chassis dynamometer, however, creates a completely different environment. Inertia dynos use a known mass that is accelerated to measure torque at the wheels. This is usually done in one gear under heavy load conditions:

Only the rear wheels are turning while the front tires remain stationary.

On cars equipped with DSC with traction control, the difference in speed between the front and rear wheels is sensed and the power is reduced immediately to compensate for what the car senses as excess wheel spin.

If the DSC is turned off, the traction control is disabled but the brake functions of the DSC are still operational.

If the DSC system is completely disabled, this removes the brake functions from the equation, however it does not fully remove the engine management system functions.

The ABS hydraulic unit/control module (HU/CM), or the DSC HU/CM for cars with DSC, determines vehicle speed by comparing the speed of all four wheels. If two are turning and two are stationary, it will still compute a speed but senses that the car is experiencing excessive wheel spin. To protect against engine or catalyst damage:

The engine management system compares the throttle opening, gear selection (determined by engine speed and road speed) charging efficiency and engine coolant temperature to determine the driving condition.

Since the car is under heavy load, in a tall gear (testing is usually performed in third or fourth gear), with a wide throttle position angle (wide open), spark timing is reduced and the fuel mixture is richened to reduce the occurrence of spark knock and to reduce catalytic converter temperatures.