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Spearco Turbocharged Mustang II V-6
by John Dinkel
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It's no big secret that in a head-to-head performance match between a V-6 Capri and a Mustang II with the same engine, the Mustang is the loser - and by a lot. Place identical engines in two identical cars but load one with 650 lb of ballast and in any test of acceleration the lighter car is going to win every time. That in essence is the millstone hanging around the V-6 Mustang II's neck; it's solid, well built, quiet, plush - and heavy.
What to do? Well, if you're an unhappy Mustang II owner you could replace all the V-6 emblems with 4-cylinder stickers and save yourself the embarrassment of explaining why your car is so slow. Or you could trade in your 1974 Mustang on a 1975 with the 302 V-8. This is not only a very expensive alternative (in first cost and fuel consumption), but initially the 302 will be available only with an automatic transmission.
What's the solution to this dilemma? One is turbocharging, or specifically the turbocharged V-6 Mustang II from Spearco Performance Products, Inc, 2054 Broadway, Santa Monica, Calif. 90404 (213 828-9552). George Spears, the man behind Spearco, has engineered a complete turbocharging kit for the Mustang II with V-6 and 4-speed transmission. The kit features a Rajay turbocharger and was designed to be a bolt-on installation; no welding is required, nor is it necessary to relocated the battery or battery box. Because the Mustang engine compartment is extremely crowded and space is limited with the turbo installation, Spears recommends that the kit be installed by an experienced mechanic with tools for working in confined quarters. Installation time under these conditions is approximately 10 hours; someone less familiar with mechanical things will need more time.
The Spearco kit (which costs $625) doesn't compromise the car or the engine the way some turbocharger installations do. It's designed to fit Mustangs with all the power equipment, such as factory air conditioning and power steering and brakes. In addition the kit is fully compatible with all emission control systems, including exhaust-gas recirculation, Thermactor air-injection piping and even the vacuum retard system. A few words of caution, however. Although turbocharging doesn't necessarily increase exhaust emissions and when done right can even help, it falls into the category of illegal engine modifications in some areas of the country, including Spears' own home state California. Spears and other turbocharger manufacturers such as Crown are currently having their kits tested by the California Air Resources Board in an attempt to gain an exemption from the prohibited modification list, but we suggest you check local regulations before proceeding with a turbo installation.
Spears' turbocharger design eliminates the complication of a wastegate or a pop-off valve for limiting boost pressure: the dual exhaust piping has been designed and sized so that with the removal of the stock muffler the exhaust system provides sufficient back pressure to limit boost to approximately 10 psi. The two stock resonators are retained, and these in conjunction with the muffling effect of the turbocharger quiet the engine. We found, however, that at a steady 50 and 70 mph the modified car is 3 dBA louder inside than the stock version.
Modifications to the engine to take the stressed and stains of turbocharging are few. Jet sizes have been increased slightly, from 0.056 to 0.0605 in. for the primary and from 0.071 to 0.073 in. for the secondary. The initial timing setting remains at the factory setting of 10° BTDC, but the highest grade premium fuel is required to prevent detonation. This indicates relatively high combustion temperatures, and another consequence of this is that colder-than-stock sparkplugs are required. After much experimentation Spears has settle on Champion N59G gold-palladium plugs, gapped at 0.028 in. in conjunction with a Delta Mark X capacitive-discharge ignition system (plugs and CD ignition are not include in the turbo kit) as the best compromise. The problem, Spears explained, is that spark plugs cold enough to survive temperatures with turbocharging have a tendency to foul in city-type driving. An easy way out would have been to enrich the mixture further, which would lower such combustion temperatures and let the plugs operate at less temperature differential between idle and maximum power. However, enriching the mixture reduces economy and probably would increase emissions; because Spears is trying to get this kit to pass for emissions he has decided to work with the lean mixtures. As it stands, with the colder plugs, after driving some distance in stop-and-go traffic there is a slight misfire for a few seconds the first time the boost pressure is kicked in. But our experience confirms what Spears has found: after this first burst of power,the plugs clean off and fire properly.
How does it go? If you drive the car normally - accelerating gently and shifting at 3000-3500 rpm - you'll never know there's a turbo under the hood. Disappointed? Don't be. Because if you floor the loud pedal, the tone of the engine changes to a mellow roar as the turbo cuts in at approximately 1900 rpm. Then events start happening quickly: the needles on the boost gauge and tachometer start a fast upward climb and the Mustang is propelled forward at a smooth but rapid rate. This Jekyll-Hyde personality is one of the biggest advantages a turbocharger has over radical camshafts, big ports and valves, high-flow carburetors and the other more usual means of increasing horsepower: it's there on demand when you want it but doesn't intrude when its presence isn't required. You don't have to put up with a lumpy idle, poor low-end response or any of the other driveability problems associated with the more common horsepower boosters. To save you the trouble of searching through your back issues of R&T for the pertinent figures here are the performance numbers for a 1974 Mustang II Mach I, a 1974 V-6 Capri and the Spearco turbo Mustang:
| Mustang II V-6 | Capri 2800 V-6 | Spearco Mustang II |
| 0-¼ mi, sec | 19.4 | 18.0 | 17.5 |
| ¼ mi speed, mph | 70.5 | 76.5 | 83.5 |
| 0-30 mph, sec | 4.5 | 3.7 | 3.7 |
| 0-60 mph, sec | 13.8 | 10.8 | 9.4 |
| 0-80 mph, sec | 27.0 | 20.0 | 15.8 |
Very impressive. The Spearco Mustang not only walks all over a stock Mach 1 but runs away from a V-6 Capri as well.
The turbocharger was not the only modification to the Spearco Mustang we drove. This particular car also had Spearco's front and rear spoilers; a padded, small-diameter steering wheel; 1-in-diameter front and 7/8-in. rear anti-roll bars; heavy-duty front coil springs; a lowering kit for the rear leaf springs; and Koni shocks front and rear. All these goodies and numerous others are available from Spears. The nicely integrated spoilers give the Mustang a sporty appearance, but the steering wheel and suspension drew only mixed reviews. The 13-in. padded wheel is nice to hold but too small: a compromise size, perhaps 1 in. larger in diameter, would be better. Overall handling is much better than that of a standard Mustang, but even with the $378 worth of chassis modifications the car still doesn't handle as well as a stock Capri:
| Mustang II V-6 | Capri 2800 V-6 | Spearco Mustang II |
| Lateral acceleration, g | 0.683 | 0.726 | 0.714 |
| Slalom speed, mph | 51.7 | 54.9 | 54.2 |
The skidpad improvement is less than expected because the turbocharger accentuates the understeer problem. If you bring on the boost as the car is scrubbing around the skidpad in its normal understeering attitude, the rear tires lose their grip momentarily and the attitude approaches neutral. But once the rear tires regain their grip, which they inevitably do, the front tires continue their outward course even more strongly than before. So the car goes around the pad in a jerky series of grips and slides. Very untidy, both to look at and to drive. But on a road course, a situation approaching real-world driving more than the steady-state skidpad test, the handling is considerably better than stock. Here the turbo is an asset; you can come into a turn harder and one gear higher than normal, brake smoothly for about the first third of the corner, and then start applying power. The boost comes in and kicks the tail out smoothly, so the car corners neutrally and the understeer which was so oppressive on the skidpad isn't a problem. The improvement in transient response also shows up in our slalom: here the nose heavy Mustang is only slightly slower than the better-balanced Capri. And as you'd expect from a car with marginal wheel travel to being with, lowering causes the car to bottom more easily and also introduces harshness and mild pitching. The Koni shocks, however, hold these last two ride irregularities to a minimum.
So with the Spearco modifications the Mustang owner has a car that is not only a match for all its sporting coupe competition in any acceleration contest but one that won't embarrass the driver on twisty roads either. But Mustang II owners had better not get smug or complacent, though, because Spears is making the necessary modifications to adapt his turbocharger kit to the V-6 Capri (the alternator is on the opposite side of the engine and there are a few other interference problems) and we don't think Capri owners are going to stand idly by while they get their doors blown off by an upstart newcomer like the Mustang II.
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