I-NRD - Team Highball IMSA RX-7 GTU 1985

Team Highball

The Team Highball RX-7 Experiment

We started this project in 1985 with Amos and Bonnie Johnson of Team Highball. This team was running a first generation Mazda RX-7 initially with a 12A and later with a 13B Mazda peripheral port engine in the GTU Class of the International Motorsports Association of America (IMSA). This Team Highball RX-7 race car has more back-to-back 24 hours of Daytona victories than any car in history.

IMSA required that all cars running in its series in 1985 meet a 110 db-A limit on the track at a distance of 50 feet (~15 meters).

It is said that the main reason that the principals at IMSA decided on a noise limit mandate for their series was because they were trying to quiet the Mazda rotary engine race cars.  Even though these Mazda rotary race cars were very popular among racers at the time, they were incredibly noisy, especially when using a megaphone as an extension of the exhaust header. It is likely that the noise level experienced by the spectators and the racing track personnel was greater than 120 db-A when using the megaphone exhaust system.

A fellow racer commented that when the Team Highball RX-7 passed him on the driver side with the rotary exhaust pointing straight at him, that the pain in his left ear was so high that he placed his hand over the helmet around this area thinking that this would alleviate his discomfort.

Our first attempt at a solution to this challenge was made during the July 4, 1985 weekend at the famous Watkins Glen Grand Prix Circuit during the Six Hours of the Glen endurance race event. The unit that was being used by Team Highball at this time had dimensions of 4.0 in. (102 mm) outer diameter, a 3.0 in. (76 mm) inner diameter and a length of 16 in. (406 mm).

This first test proved to be a very humbling experience.  Our best effort in a thermal-acoustical solution for this unit lasted less than one lap before it was totally destroyed.

We want to thank all of the members of Team HighBall for their support and assistance with this project.



Several concepts were imagined, fabricated and tested before we were finally able to develop a design concept which could achieve a noise reduction from the exhaust with a minimum loss of power. This design idea was provided to an IMSA team which still uses this concept today.

Although this design concept addressed some of the primary goals established for the I-NRD, the unit still had to be refurbished after every race weekend.


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I-Noise Reduction Device History Introduction

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