Mazda's IMSA Racers

Sales of the RX-7 in the USA have been matched by popularity and success on the racing circuit, boosted by a remarkable first and second in class and fifth and sixth overall for a pair of factory-entered cars in the last February’s Daytona 24 Hour race.

For Mazda, Daytona was just one of 16 rounds in the IMSA (International Motor Sports Association) GTU Challenge, GTU being the up to 2.5-litre category in the overall Winston GT series. Mazda take their US racing very seriously, running two IMSA cars out of a Competitions Department managed by Damon Barnett within the Mazda Technical Centre at Los Angeles, and giving practical and technical assistance to several other RX-7 IMSA runners. The successful Daytona cars of Katayama/Terada/Yorino and Bohren/Downing/Mandeville were both Barnett’s responsibility. Bohren now has charge of his car and is currently lying second in the GTU Championship. Mandeville is down to drive the Katayama car for the rest of the season, though Formula 2 and March-Mazda sports car driver Katayama was brought in from Japan for the races at Sears Point and Portland. Currently lying third in the Championship is veteran racer Bob Bergstrom, with his privately prepared and entered, though factory assisted RX-7.

Our proposed track test of Tom Walkinshaw’s British Saloon Car Championship contending RX-7 was stymied by post Grand Prix damage, a lack of engines and a lack of willingness on Walkinshaw’s part. With Mazda’s very willing co-operation it seemed expedient to survey the US RX-7 racing scene instead, a plan which tied-in conveniently with the eleventh round of the GTUChampionship at Sears Point Raceway, California.

Sears Point nestles on the side of a bone-dry hillside in Sonoma County, thirty-five miles north of San Francisco. Most days this twisting, hilly circuit is occupied by the fleet of orange Datsun 280Zs and saloons of Bob Bondurant’s racing school. On the Wednesday leading up the Sprite Grand Prix meeting, of which the GTU race was a part, the dry and sun-browned hills echoed with the high-pitched bark of racing RX-7s, the most ear-splitting sound on the circuits.

My interests centred on the green and white Katayama car and the orange and white Bergstrom machine, prepared by two very different schools of thought. The IMSA regulations allow modifications more or less on a par with European Group 2, plus a bit more flexibility in some areas. But IMSA rules are nothing if not flexible on the part of the organisers and the RX-7s particularly have suffered chronically this season because of IMSA’s fears and lack of knowledge about the rotary engine. At the beginning of the season they gave the RX-7s the same weight handicap as everybody else: 0.9 lb./c.c. After Daytona, when IMSA found that the rotaries were running peripheral inlet ports instead of the standard side ports – quite legally – they upped the handicap to 1.1 lb/c.c., adding another 458lb. for Sebring. It was equivalent to racing with an extra three passengers and the cars were hopeless on handling and suspension reliability. IMSA relented, shifted the handicap to 1.0lb./c.c. and more recently have reverted to the original 0.9. Amidst all this shilly-shallying they are happily letting Datsun run 280ZXs with short-stroke engines to get them into the 2.5-litre class, and cross-flow heads. And British competitors complain about the RAC!

The Katayama and Bergstrom cars use similar factory built 12A engines rated at 270 b.h.p. (SAE) at 9,000 r.p.m. and 165.8 ft. lb. (SAE) at 7,500 r.p.m. The combustion spaces are fed from a single twin-choke 481DA downdraught Weber carburettor through peripheral ports in the rotor housings, the direct route instead of the tortuous route from carburettor through ports in the side housings. Exhaust ports are peripheral on both standard and racing engines. Inlet and exhaust ports are 43mm. The 9.4:1 compression ratio remains as standard.

There the resemblance between the two cars ends. Katayama’s engine stays where Mazda intended it, Bergstrom’s sits 4” further back (permissible so long as the bulkhead isn’t affected), necessitating a remote oil filter. The result is a weight distribution of 48/52 instead of 51/49, partly prompted too by use of a bigger rear axle assembly, using a Ford Fairlane differential with a limited slip and special axle tubes, to give a wider choice of final drive ratios and fully floating halfshafts. On this are mounted 11.97” x 1 ½” ventilated discs and Lockheed four piston calipers. Katayama’s car retains the Mazda axle, fitted with small discs and floating calipers and a pretty well standard suspension layout, whereas the other car has equal length and parallel locating rods, the top ones attached to the internal crash cage for rigidity, centrally mounted Watt linkage with equal length links, Bilstein coil spring/damper units mounted behind the axle and a 17-mm. adjustable anti-roll bar. Both have Bilstein inserts in the front struts, but whereas Bergstrom’s struts have rose-jointed, adjustable top mountings, the factory car has standard strut tops. Both have 25-mm. adjustable anti-roll bars. Again, the Bergstom car has the big discs.

Both cars are hampered by having to run standard gearbox ratios, except for a slightly lower fifth. They run 15” wheels, 10″ wide at the front, 12” rear, Bergstrom’s shod by Goodyear, Katayama’s by Bridgestone.

As Katayama’s car was running its race engine and in the midst of a strict test programme it was deemed prudent for me to restrict my driving to Bergstom’s exquisitely prepared and more sophisticated car, which was due for a pre-race engine change. First though a taste of action in the hot seat alongside Katayama, the little Japanese hurling the car over the blind crests and corners with the verve of a rally driver to overcome an inclination to understeer. The suspension felt too soft at the front to me and Katayama wasn’t very happy with it, though this didn’t stop him keeping the tachometer needle between 9,000 and 9,500 r.p.m. most of the time.

Bergstrom too had complained about understeer and poor front brakes early in the day, but by the time I had strapped myself in the seat and helmet of this so very helpful sports shop proprietor, he had sorted the car to his satisfaction. There was no mistaking that this was an American specification racing car, with stout crash tubes down either side of the cockpit and a multiplicity of backing bars in the roll cage, the whole designed into the structure of the car between front and rear suspension so that it almost constitutes a space frame chassis. The mandatory NASCAR safety net in the side window and twin bracing bars down the windscreen area gave a claustrophobic feeling.

Cockpit drill included switching on the electric pump which circulates gearbox oil through a cooler, and starting the fuel pumps. “Pumps” is very much in the plural – there are four of them! Two pump fuel from the main tank into a 2-gallon surge tank within the main tank and another two take fuel from the surge tank to the carburettor. A fail-safe one-way valve system ensures that the failure of one pump won’t halt fuel flow. A fuel gauge connected to the surge tank shows full until the 2-gallons begins to drain off after the main tank has run dry, to give Bergstrom plenty of warning to head for the pits in a long distance race. All these US racing RX-7s run with oil mixed in the fuel, a la two-stroke, instead of metering a supply from the sump into the carburettor inlet tracts. The little metering pump usually fixed to the front of the engine is dispensed with and its drive used to run a mechanical tachometer.

Sears Point is a difficult circuit to learn, a challenging series of blind brows and corners and strange cambers as the tarmac weaves its tortuous path through cuttings in the hillside. I found roughly where the corners went in a few laps in a very beautiful road-going Mazda RX-7 fitted with the similar glass-fibre wings (sorry, fenders) and spoilers to the racer’s, a road pack which Bergstrom’s Doell Racing concern will be selling through the Mazda US Competitions Department. But to get to grips with the racer in only a handful of laps was an impossibility, especially as the wrap-around side supports on the seat made for the taller Bergstrom trapped my shoulders and restricted arm movement. And Sears Point is all about arm movement. Thankfully, this was a much more finely balanced, more neutral handling car than the Katayama car had felt, and within a high-ratio steering box.

Like the road engine the racing rotary revved with a dynamo smoothness, but accelerated the tachometer needle so much faster. Race tuning hadn’t spoilt that flat torque curve nor cost flexibility, which was fortunate on this circuit where the gap between second and third gears was a hindrance. Where a conventional engine would have fallen “off the cam” in third, this kept going, but it still cost time, for the rotary’s performance didn’t become urgent until at least 7,000 r.p.m. was registered on the Jones tachometer.

Good traction is especially vital at Sears Point out of the two tight hairpins and round the uphill Turn 4, which suddenly drops away so that the car goes light. The racing Mazda scored in this respect, reflecting the good traction of the road car. The heavy, unservoed brakes hadn’t as much feel as I would have liked, but their stopping power was to best racing standards.

I wasn’t able to drive this Mazda as hard as I would have liked to get a real idea of its potential, but there was no doubt that here was a very good handling car with a lot of performance in spite of a weight of 2,100 lb. The RX-7 will be homologated in Group 2 for European touring racing; with a 2,029 lb. minimum weight limit it could spring a few surprises.

From a journalist playing at “racers” over to the serious business. Practice and the early parts of the race could almost have been laid on by Mazda to impress this visiting UK journalist. Katayama shattered the lap record to make pole position, with Sam Posey in the Bob Sharpe Racing Datsun 280ZX alongside and Don Devendorf’s factory-backed 280ZX partnering Bergstrom’s Mazda on the second row. Bohren’s Mazda was a non-starter, stuck 3,000 miles away with a transporter problem. Devendorf’s Datsun, with a rumoured 320-330 b.h.p. SAE, had been expected to dominate practice, but a major rebuild following a heavy crash in testing looked to have taken the edge off this quickest GTU car. The rest of the field weren’t in the same league as these four, not even the potent-looking 2.4 Porsche 911s or White’s 16-valve Schnitzer-engined BMW 320i. Katayama and Bergstrom made a magnificent start, chased by Devendorf and Posey into Turn 1. Bergstrom parried every Devendorf manoeuvre, letting the Japanese Mazda driver break clear. First Posey then Bergstrom smashed the lap record, then Bergstrom again as these fast four set a tremendous pace, well clear of Jeff Kline’s fifth-placed Racing Beat RX-7. Even the experienced Posey fell out of contention as Katayama turned on all the taps and Devendorf hounded Bergstrom. Then suddenly Bergstrom was past Katayama out of Turn 11 on the eighth lap as the leaders moved through slow traffic. Bergstrom pulled out a 3 sec. lead as Devendorf made furious efforts to pass Katayama and break up the two Mazdas. His break came on lap 22 when Katayama’s Bridgestone tyres began to go off and then the Datsun hared after Bergstrom. The Mazda domination finally waned on the 25th lap as Devendorf took the lead as slow traffic bogged down Bergstrom. As the flag fell at 30 laps, the Datsun had a 10 sec. lead over Bergstrom, Katayama and Posey, with Kline 5th.

Devendorf’s win gave him a 52 point lead over Bohren in the Winston GTU Championship with five races to go. Datsun still have the whip hand, but as Katayama says, the Mazdas are still in their first season and only 80% developed. One thing everbody agrees is that the RX-7s have put new life into the IMSA GTU Championship and whilst they and the Datsuns are able to make good racing, nobody is going to say too much about short-stroke 2.5-litre engines in 2.8-litre cars . . . – C.R.