Lock & Load

FOUND THIS VERY INTERESTING READING .



Mazda Engineering on RX-8 and Dynos
Source: Dan Mazzella - RotaryNews.com
Sep 24, 2003

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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.

[ posted on Sep 24, 2003 | Back Home | Comments on Article | Add a Comment ]

msydd

Just makes me think that all the crap about "power" and "dynos" is just that...crap.

Like we all know... this is a fast car and a fantastic car to drive.

End of story.

Lock & Load

Friendly fire has always been a forte of the americans ,this time instead of wasting its allies it has managed to create unnecessary contraversy about the very car that they have purchased the RX8(they love shooting themselves in the foot ).

However mazda should have acted earlier and stopped the dyno fiasco.

rpm_pwr

Makes you wonder though, Australia (and Europe where the Euro 3 emissions spec ECUs came from) don't have cat converter durability standards. Maybe OZ-spec cars put down more real world power than US spec cars?

-pete

labrat

Catalytic converter durability: Here's the usual qualification I make in these sorts of situations when I make a comment - I'm a chemist, but catalytic converters are not my field. However, I add what little knowledge I have to the debate:

Catalytic converters consist of a ceramic support material (called a "substrate") upon which is deposited a very finely divided catalyst material (platinum or a mixture of platinium and other precious metals).

The idea is that the substrate should have a very high surface area, lots of channels, nooks and crannies, right down to the sub-microscopic level (and sub-sub-microscopic level), so that the exhaust gas has the best chance of contacting the tiny Pt particles so that they catalyse the oxidation of all the nasties in the gas stream.

Unless ceramics are very highly compressed to eliminate all the cavities and have intimate contact between the crystal surfaces during the firing process, they are intrinsically weak - the ceramic disk brakes on the new super expensive mercedes sports are illustrations of ultra high strength ceramics which are virtually defect-free. They are the very antithesis of the ceramic in a catalytic converter. These high surface area ceramics are not fabulously resistant to thermal shock, and it is obvious that Mazda has acted responsibly in incoporating cat converter technology into its computer program.

rpm_pwr

yep I've cracked two factory mazda cats by running too lean (hot) for the cat. It's distressingly easy to do.

-pete

Wildcard

Awesome info L&L. If only Mazda had come out with this info 2 months ago. Think of all the heartache that it could have saved....

Hymee

iTrader: (1)

Great article L&L.

I'll have to show this to my "tuner". It looks like the Dyno Day I was going to organise will be a non-event.

But not all is lost. My trusted tuner (Power Torque Engines) has a Engine Dyno as well And no - my '8 has not been on his "rolling road", athough he has had a ride in it

Interesting comment about the DSC being "fully off", and it still intervenes.

Also - most Dyno's in Aus are not those "inertia" type ones that are very common in the US, but rather a computerised electro-mechanical "brake".

BTW - I don't think the published figures are that far out. I plugged some real world figures into a couple of 1/4 mile calculators and got pretty close.