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Chevrolet Monza vs Ford Mustang II
Track comparison of the leading IMSA All-American GTs and their street counterparts.
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Remember the crash 'em, bash 'em, rock 'em, sock 'em heyday of the Trans-Am sedan series? Back in the late Sixties and early Seventies, U.S. automakers were overtly waging all-out war on the race tracks of the U.S. and Canada with the likes of Parnelli Jones and George Follmer in Mustangs, Jerry Titus in a Firebird, Mark Donohue in a Camaro and later a Javelin and other contructors/drivers such as Dan Gurney and Jim Hall also getting into the act. Those were the days of "ask no quarter, no quarter given" racing, where spectators drove to the races in cars that looked like the ones their heroes raced on the track. Thos onlookers stood and cheered at what was probably the best road racing ever seen in the U.S.
Everybody knows what happened next. Rough times in Detroit coupled with sagging interest in ponycars all but destroyed the Trans-Am series as we knew it. But coinciding with a resurgence of popularity of small cars came the SCCA's 2.5 Challenge in 1971. This series for sedans with engines displacing 2.5 liters or less, provided much of the excitement of the bigger cars as Datsun 510s, BMW 2002s and Alfa GTVs fought hammer and tong for small sedan supremacy. Datsun just eked out the championship in 1971 but overwhelmed the opposition in 1972. This dominance, couple with a lack of crowd enthusiasm for the small sedans, soured many track promoters on the series.
About this time IMSA was getting into the act with the Camel GT and the Goodrich Radial Challenge. Corvettes were big winners in GT competition in the early days but in the past few years not American car except John Greenwood's 221-mph super Corvette has been able to hold off the German hordes from Porsche and BMW. So in 1975 IMSA developed the All-American GT category to encourage the design, construction and competition of racing cars of a GT character built from volume-produced American components.
Last year several All-American GT Chevrolet Monzas made an inaupicious debut, showing tremendous potential but failing to finish because of several niggling problems. Following wins at Atlanta, where Monzas driven by Al Holbert and Mike Keyser finished 1st and 2nd, and at Laguna Seca, where these same drivers finished 1st and 3rd, we're probably safe in stating the Monzas have found their sought-after reliability.
Up until last December, Chevrolet was alone in battling the Porsches and BMWs. But at the Daytona IMSA race Charlie Kemp showed up with a radical All-American GT based on the Mustang II. Called the Kemp Cobra II, the car had IMSA tech inspectors tearing their hair out over what they considered various rule infractions. IMSA finally allowed the car to run, but only as a Le Mans GTX entry. Since that race Kemp has modified the Cobra II to IMSA's satifaction and, although the car has suffered the usual new car blues, it finished 5th at Ontario (Kemp would have been 3rd but the car ran out of gas on the last lap) and has been looked upon as serious competition for the Chevy-powered contigent. The good old Trans-Am lives again!
With Detroit-based cars once again in the thick of competition, what could be better than a head-to-head comparison test of two of the leading contenders for IMSA All-American GT honors, the Monza of Mike Keyser and the Cobra II of Charlie Kemp? And to add a little more interest we decided to include street versions of both cars so that Monza and Mustang II owners can see how their cars stack up against the all-out racing versions.
Up until this year Kemp and Keyser have been Porsche pushers in the IMSA Camel GT Challenge, but each could see that a properly prepared All-American GT could be highly competitive in the series and switched to American iron. Although both cars are built to meet the same set of rules, the designers took different approaches in coming up with the finished product.
In the case of the Cobra II, Kemp set down a couple of basic parameters and then gave Bob Riley pretty much a free hand in designing the car. Kemp knew that from an aerodynamic standpoint the Mustang II front end was like a brick compared to the Monza's nose. To negate this disadvantage as much as possible, the Cobra II was designed as low (almost 4-in. lower than Keyser's Monza) and as narrow (Keyser's car is 4.5-in. wider) as possible. "And we made the car as much like a watermelon in the front as we possibly could for better streamlining," Kemp explains. Kemp's other main concern was that the suspension be strong enough to withstand an off-course excursion because he knew that just as in the old Trans Am series and occasional fender bender was inevitable.
Basically, Kemp's car was built backward to the ordinary way you would construct a race car. Rather than take a "body in white" and strip off everything that wasn't needed, Riley obtained a set of blueprints of the Mustang II body and then ordered all the outer sheetmetal pieces that form the basic body shell. Under the rules the inner panels are removable but rather than remove them Riley never ordered them. The various sheet metal pieces were riveted and clamped together and then Riley designed a space frame to fit inside that package. The floor pan of the car is a stressed member that is epoxied and rivted to the space frame. The frame also ties into the roof, the dorrs and the pillers to form a rigid platform to attach the suspension and other chassis components too. The basic Cobra II styling is the result of aerodynamic tests Kemp had been involved with when he was racing a Porsche 917 for Bobby Rinzler, keeping in mind that these panels also had to fit a modified Mustang II called the Kemp GT Charlie is building for the street.
Although the rules require the original type of suspension principle (MacPherson strut, A-arms, etc) be retained, suspension design is free. On the Cobra II that means unequal-length A-arms up front and a solid axle at the rear. To aid in sorting out the car, as many adjustable components as possible are used. The Gabriel shocks are adjustable for jounce and rebound and have threaded collars to allow ride-height changes; the front and rear anti-roll bars are adjustable for roll stiffness. Kemp's Cobra II has a Frankland NASCAR-type rear end, located by upper and lower parallel trailing links and a Watt linkage. Rear suspension travel is limited to about 2 in. to prevent rear steering effects.
Brakes are typically race-car fare: huge vented Hurst-Airheart NASCAR-type disc fornt and rear with a balance bar for adjusting bias. The steering box is a production Mustang II power rack-and-pinion unit (minus any assist) requiring 3.3 turns lock-to-lock. BBS alloy wheels are fitted all around, 12 x 15 in. up front and huge 17 x 15 in. rims at the rear. The racing tires are Goodyear Blue Streaks, 25.0 x 10.0-15 in the front and 25.0 x 13.0-15 rear, though this is one area where continual experimentation is occuring.
The engine in Kemp's Cobra II is a Gapp and Roush-built Ford 351 Cleveland. It has a steel crank and 4-bolt main bearing caps, special pistons, Mister Rod connecting rods, a General Kinetics camshaft and uses Gapp and Roush ported heads equipped with titanium valves, springs and retainers. In addition, a Jomar stud girdle if fitted to the top of the valve train to increase rigidity. The induction system is Kinsler-designed fuel injection. Kemp has gone to this more expensive system because he feels the engine is more responsive and picks up more smoothly in the corners with the injection than it would with carburetors. The oiling system is a dry-sump designe, and for cooling a Harrison Corvette radiator is currently being used with Stewart-Warner oil and differential coolers.
Backing up the engine is a Schiefer clutch and aluminum flywheel that lead the power into a Ford NASCAR road racing 4-speed transmission. This gearbox has a cast iron case and tailshaft and once Kemp has all the car's little problems sorted out he weill probably switch to an alloy box that weighs 3-40 lb less.
DeKon Engineering (consisting of Horst Kwech of Trans-Am Mustang, Alfa and Capri fame, plus some stellar Detroit engineering talent) designed and constructed Mike Keyser's Monza. Chevrolet was responsible for the intial concept and shape of the Monza, but in the two years since the first mockup appeared at Chevrolet new-car preview the car has undergone numerous detail changes.
According to DeKon the Monza was essentially packaged around the engine. The rules state that the back face of the block can be moved rearward to the front edge of the windshield and for purposes of weight distribution it obviously behooves a constructor to move the engine as far back as possible. Several other things are locked in before you start: The location of the body relative to the wheelbase and the width of the tires which affects where and how the suspension is located and the geometery. Like the Cobra II the Monza uses a space frame tied into the production roof structure for rigidity. The windshield pillars and side pillars along the roof panels and rocker panels are stressed members that strengthen the driver's compartment. The floorpan, however, is not a stressed member.
The suspension was computer designed and is standard racecar practice. The front system consists of wide-base A-arms that spread the loads over two bulkheads, adjustable Monroe shocks and an anti-roll bar. The rear suspension is virtually identical to the one on Kemp's car (or vice versa) with a solid rear axle located by trailing arms and a Watt linkage, coil-over-spring units and an anti-roll bar. The full-floating locking differential is a Ford unit similar to the ones used in NASCAR. The center seciton is easily removable ("We can change ratios in about 20 min," Kwech boasts.), and numerous alternate ratios are available.
The front brakes are Lockheed units originally developed for a Camaro Frank Gardener campaigned in England. Each front caliper has eight pistons with varying diameters that compensate for taper wear on the pads. There are four pads per caliper. The rear discs are a more conventional Lockheed design with equal piston diameters and two pads per caliper.
The Monza, like the Cobra II, uses a Mustang II power rack-and-pinion steering gear, chosen, Kwech says, because it's a beefy unit and requires minimal modifications. The Monza races on BBS wheels, identical to those on the Cobra II. The racing tires are again Goodyear Blue Streaks, 24.5-15 at the front and 25.0 x 13-15 in the rear.
Ron Neal of Prototype Engineering builds the 35-cu-in. Chevrolet engines. His basic philosophy is to use as many production parts as possible because, Neal says, "Chevrolet has spent thousands of hours and millions of dollars engineering those parts. About the only things we buy from outside sources are the Carillo rods, the Weaver dry-sump systems and the camshaft and valve springs." For engine cooling, the Monza has a Harrison aluminum radiator with a remote header tank. For short races the only other cooler fitted is a Modine oil cooler; for races longer than an hour, a rear-end cooler and possibly a transmission cooler are installed. Neal builds engines with either fuel injection or carburetors and on Keyser's car the induction system consists of four twin-throat Webers.
According to Neal, the 350 is somewhat of an evolution of the 305 Trans-Am and Formula 5000 engines. "They're pretty much the same except for the stroke. I've built 350 V-8s with as much as 590 bhp, but I try to build in only enough horsepower to do the job because the reliability naturally goes down with increasing horsepower. A lot of torque is more important than ultimate horsepower. I'd rather build and engine with a broad torque curve from about 4500-7200 rpm. I'm not overtly concerned with high horsepower at high rpm, because such an engine is usually peaky and doesn't have much torque at the low end."
Neal uses Accel ignition but unlike the Accel breakerless system Kemp uses, Neal prefers contact breaker points because "If a problem occurs it's usually easier to diagnose and correct."
The engine is mounted in the Monza in an ingenious fashion using aluminum plates that attach to the front and rear of the block. The plates act as structural members, stiffening the chassis and also allowing the engine to be mounted closer to the ground than is possible with the usual side engine mounts. Behind the engine is a hydraulically actuated production high-performance clutch that feeds power to a Muncie 4-speed transmission, known as the Rock Crusher in racing circles.
For the production side of this test Ford supplied us with a Mustang II equipped with the 302 V-8 and 4-speed; Chevrolet countered with a Monza fitted out with the 305 V-8 and a 4-speed. At the moment the 305 is available only with an automatic transmission, but the 4-speed will be offered in 1977 and Chevrolet made a version of that power team available to us for this test.
For the timed-lap portion of the test we gathered all four cars together at Laguna Seca Raceway. We used Ontario Motor Speedway and Riverside International Raceway for the acceleration and braking tests. We ran the street cars around the Laguna Seca track two days before the IMSA race and used the times achieved by Keyser and Kemp during practice, qualifying and warmup for the GT race comparison.
The track performance of the two production cars was as close as our subjective street impressions had led us to believe they'd be, Kemp horsing the Mustang around the 1.9-mile Laguna Seca circuit in 1 min 39.31 sec and Keyser trailing only slightly with a best lap of 1:40.11.
Kemp found the Mustang very predictable in its handling and a lot more neutral than he expected. "High speed cornering posed no problem," Charlie said. "The main thing I found was that the engine wouldn't pull any more than about 4500 rpm so going up the hill you sat there at about 3800 rpm all the way to the top. The car feels like it could use a little more shock control because when I put on the brakes it wanted to lift up and down a lot. But the steering and handling were very predictable. I would go wherever I pointed it."
Keyser had similar comments about the Monza. "It could be stiffer as it rolls quite a bit," Mike said, "but it has very neutral handling. I thought it would have a little more pep up the hill than it did but the brakes and tires both felt good. It's an easy car to drive because the steering wheel, gear shift and instruments are all in the right places."
When we queried Mike and Charlie as to how they liked the lightness of the steering and braking compared to their race cars both said they adjusted to the power assists very quickly.
Keyser turned the tables on Kemp when it came to the race cars, turning a 1:09.42 lap vs 1:11.00 for Kemp's Cobra II, which was suffering form brake problems. And in the race Mike finished 3rd while Kemp dnf'd with a broken throttle cable. So now we knew. Around Laguna Seca the race cars were about 30 sec a lap faster than the street cars!
The street Monza and Mustang were also evenly matched in our normal acceleration, braking and handling tests. The Monza was slightly quicker in the quarter mile, stopping the clocks in 17.3 sec vs 17.7 sec for the Mustang, but in the 0-60 mph acceleration the cars ended in a dead heat. The Mustang was 0.4 sec faster to 80 ph but then the Monza's better aerodynamics took over and the Chevy got to 100 mph almost 3 sec sooner. The Mustang won the skidpad test but the Monza stopped in shorter distances and suffered no fade vs 35 percent fade for the Mustang. The Monza's lack of fade was a welcome surprise compared to its horrendous performance when the car was first introduced. In the noise tests the Mustang was 2 dBA quieter at 50 mph but the cars were equal at 70 mph.
How did the racing Monza and Cobra II accelerate? Very quickly. Quicker, in fact, than Greenwood's Corvette, although the latter car was hampered by very tall gearing that caused it to bog off the line. Like the production Monza and Mustang, the racing cars' performance was evenly matched, both turning the quarter mile in 12.1 sec, but the Cobra II reaching 120 mph in 10.8 sec vs 11.3 sec for the Monza. We don't have a slalom speed for either race car but both corner better than 1.2g on the skidpad.
Neither car perfomed up to expectations in our braking tests. There was not fade from either car but stopping distances were long. The reasons? Kemp is still sorting out the brakes on his car and with Keyser's Monza it's possible than the pads were either too hot or too cold to generate their maximum coefficient of friction. Horst Kwech says the Monza can stop as well as any car on the track and can do it lap after lap, something Keyser's racing performance confirms. We'd speculate both cars should be capable of stopping from 60 mph in less than 120 ft and from 80 mph in under 220 ft. performance comparable to Greenwood's Corvette.
What about noise? Both are equally loud and we'd suggest ear plugs for anyone who gets within 10 ft of either car.
Both the swoopy Monza and the knee-high Cobra II represent the current state of the art in GT race car design. With crowd pleasing cars such as these tearing up the road courses of the U.S. and Canada we wouldn't be a bit surprised to see an American car win IMSA's 1976 Camel GT Challenge series.
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