I-NRD - LMP-2 Mazda Courage 2005

Mazda B-K Motorsports 20B ALMS

Mazda 20B ALMS Courage LMP2

Several hundred more concepts of the acoustical/power design strategy were evaluated and tested in an attempt to improve the durability and the noise reduction capabilities of the I-NRD between 1994 and 2005.

In 2005, we were given the opportunity by the late Dennis Spencer of Dennis Spencer Motorsports and Fabworks to develop a custom I-NRD for a one race weekend.  This custom I-NRD was to be fitted to the Mazda Le Mans Prototype-Class 2 (LMP-2) race car, a Courage C65, campaigned by Robert Bach’s Team, B-K Motorsports for Mazda Motorsports. During this period, Dennis Spencer Motorsports and Fabworks were in charge of the preparation and the development of the Mazda LMP-2, Mazda 20B, 3 rotor engine which was running in the American Le Mans race series (AMLS).  A detailed description of this race car project is presented in a Racecar Engineering Article entitled “The Last Rotarian”, Volume 17, June 2007, pp. 22-26.

This LMP-2 race car had been using a single megaphone for the entire season with a typical sound limit of ~119-121+ db-A.  This megaphone was basically an extension of the exhaust header since there was no extra space available for the exhaust. Its next event venue, however, was going to be at the Portland International Raceway which had a very strict sound control city ordinance with a maximum noise limit of 113 db-A for all cars running on the track for the entire race weekend.


Mazda B-K Motorsports 20B ALMS LMP2 in the Paddock

Mazda 20B ALMS Courage LMP2 in the Paddock


The challenges for this I-NRD project were very difficult. These included:

A compressed time schedule

We only had less than two weeks to design and build the custom I-NRD.  We were initially contacted by Dennis Spencer on a Sunday, July 15, 2005. The I-NRD had to be completed by July 27, 2005 so that it could be installed on the car for the race event at the Portland International Raceway weekend between July 28-30, 2005 with no opportunity for testing.

The noise limit could not exceed 114 db-A

The race car with the I-NRD had to meet a required maximum sound limit of 113 dB-A for the entire race weekend at this venue with an allowance of 1 db-A to address any measuring discrepancies or environmental effect. The environmental agency for the City of Portland, Oregon USA was intolerant of any noise source from the race track venue which exceeded the mandated noise limit. This agency had its own sound measurement location at the track. This meant that there was no room for negotiation if the noise from a race car exceeded the established limit. The car would be black flagged, stopped in the pits and could not compete in the event.

Exhibit a small foot print

The I-NRD had to fit into a small space without protruding outside a flat plane at the farthest rear point of the car. The maximum dimensions available for the I-NRD were an outside diameter (O.D.) of 4.625 in. (117.5 mm) and a length of 12 in. (305 mm).

Display an extremely low outer skin temperature

The I-NRD had to fit between the rear air diffuser and the body panel at the rear of the race car. The air gap between the I-NRD and the body panels in its closest area near the body and structural panels was only 0.25 in. (6.4 mm).  All of these pieces were made of a graphite epoxy composite material which is prone to burning at relatively low temperatures even when covered by a thin insulation layer.

Exhibit a low weight

The I-NRD had to be made as light as possible so as not to hinder the handling performance of the race. Its weight could not exceed 5 lbs (2.3 kg).

Exceptionally high reliability for the entire race weekend

A failure of the I-NRD was unacceptable because it could result in a fire that would easily destroy the very expensive race car. Also, the I-NRD had to perform and survive the entire race weekend without rebuilding since there were no spare parts available to replace the sophisticated, custom thermal-acoustical package of this light weight component.

Any loss of engine power was unacceptable

The I-NRD had to exhibit the same performance as the open megaphone exhaust pipe it replaced for this event.

 Mazda ALMS Packaging


These Images Illustrate the Packaging and
Outer Skin Temperature Challenges of this Project.


Mazda ALMS Packaging 2
 Mazda ALMS Packaging 3
Mazda 20B ALMS Courage LMP2
I-NRD Packaging Constraints

During a test session before the start of the official race weekend on Thursday, July 27, 2005, the team decided to make a baseline test lap around the track with the exhaust system as it had been used throughout the season with the megaphone coming straight out of the rear of the car, as shown in the images. The sound level of the race car was recorded between 119 and 121+ db-A which exceeded the maximum noise level allowed of 114 db-A (113 + 1) .  The car was immediately black flagged after this one lap which meant that it had to go to the pits.

The team then decided to modify the exit of the megaphone with a 20 to 30 degree angle so that it pointed away from the sound meter. The exhaust sound pressure of the race car with the megaphone aligned at this angle was recorded between 116 to 118 db-A. Again, after one lap, the car was black flagged and stopped in the garages.

In the afternoon test session, the I-NRD was installed exiting in the standard or straight exhaust position, as it had been used with the megaphone up to that point during the season. The race car with the I-NRD in this position exhibit a noise level of 114-115 db-A and after one lap, the car was again sent to the pits and parked in the paddock.

On Friday, July 28, 2005, the first official practice of the race weekend, the I-NRD was modified to exit an angle so that its exit would point away from the sound meter. This angle was smaller (~10-15 degrees) than the one used with the megaphone in the earlier unofficial practice. The noise level of the race car recorded with the I-NRD exiting at this angle was between 111 and 113 db-A, with only one instance of reaching a level of 114 db-A. The race car with this acoustical attenuation performance was able to compete at its maximum capability throughout the entire race event without any issue related to noise. The Mazda race car finished second in the LMP-2 class at this race.

The average sound levels during the race weekend were as follows:

Official PracticesP1 @ 112.2 db-A and P2 @ 111.5 db-A

Qualifying @ 111.2 db-A

Warm Up @ 111.9 db-A

Race @ 112.0 db-A

This I-NRD was a great success, especially when it is considered that we had to overcome the many tough challenges of this project.

It should also be stated that several piston race cars exhibited noise levels that were on average higher than the Mazda 3 Rotor Courage race car. These cars included the famous General Motors factory Corvettes Nos. 3, 4 and 5, a premier team which competes regularly in the 24 hours of Le Mans, as well as several Porsche GT3 RSR.

Portland Sound Measurements ALMS

Mazda 20B ALMS Courage LMP2 Sound Measurements


This prototype race car later became the foundation of the beautiful design concept study performed by Mazda, North America Operations, the Furai Concept Car. The Furai Concept Car also had apex seals and an lightweight flywheel designed by the founders of I-Rotary.

 2008 Mazda Furai

 The Mazda Furai Concept was built on the Mazda 20B ALMS Courage LMP2


We want to thank Robert Davis, John Doonan and Steve Sanders of Mazda Motorsports, the late Dennis Spencer of Dennis Spencer Motorsports, Robert and Jamie Bach from B-K Motorsports for providing us with the opportunity to work on this project.  We also want to thank Rich Grupp of Fabworks, Peter Krause and Malcom Mangum for their assistance with the manufacture of this I-NRD.


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I-NRD Home

I-Noise Reduction Device History Introduction

Team Highball IMSA RX-7 GTU 1985

WSC IMSA Prototype Race Car 1994