The Bazooka
As part of Jim's testing philosophy of reducing variables, he occasionally uses test gear to eliminate restrictions even though that gear is not workable on the street or perhaps even on the track. A perfect example is the "bazooka." Nothing more than a 4-inch aluminum tube with radiused ends to smooth airflow, the bazooka eliminates the air filter, any prefilter silencers, and associated tubing to provide a fail-safe supply of air. On the Cobra it was attached directly to the mass air meter and laid atop the radiator support where it would ingest "outside" air (the hood was open at all times anyway). Also, the shop's cooling fan was diverted slightly to ensure no ram effect--however slight--could take place.
When it's airflow you want,...
When it's airflow you want, the 4-inch bazooka delivers. It also screams like a fire-truck siren. Kenne Bell runs the bazooka to eliminate concerns about induction restrictions when testing blowers. The KB supercharger is more susceptible to induction restrictions than centrifugal blowers because it draws through both the mass air and throttle body.
Jim's idea with the bazooka is to sidestep issues of whose cold-air kit works best--he just wants to reduce inlet obstruction to the superchargers. This makes more accurate A-to-B compari-sons between blowers, and it shows exactly how much power is available to savvy tuners who can come up with ideal inlet air systems.
It's difficult to say which was more surprising--the shrieking scream from the bazooka when Brent matted the throttle (we were standing just in front of the car at the moment and nearly came out of our skins when this siren-like wail exploded in the shop), or the 35hp gain.
While the 451 peak horsepower reading was OK, nearly the rest of the power curve came with a dangerously lean air/fuel mixture. Up high, at the power peak, the stock electronics in the Cobra had the A/F ratio at a respect-able, power-producing 13.5 to 12.5:1. But in the lower rpm ranges, the A/F ratio was a scary 16:1. Obviously, we needed to richen the bottom of the fuel curve both to produce power and to avoid a melted piston.
On the other hand--wow--451 hp! Again, this is through stock cats and mufflers, stock cast-iron log-type exhaust manifolds, the stock air meter and throttle body, stock engine internals, even without a chip in the computer! Recall, however, that 103-octane unleaded was saving our skins. Had it not been there, the test would likely have been brutally interrupted right here.
Enter the Electronics
Because we could not continue testing with the air/fuel ratio dangerously lean, a new chip was burned by Ken. According to Ken, this was a scary, thrown-together calibration horsebacked from experience, but it came out fine on the dyno.
By increasing the A/F ratio from 15.6 to 12.8:1, power was increased substantially--50 hp--all the way from 2,000 to 5,000 rpm. At that point, the richer calibration ran evenly with the stock, lean calibration. After the power peak, the power dropped off slightly. Ken said this is because in the rush to produce the chip, it ended up with 2 degrees less ignition timing than the stock calibration. While we would have liked to have redone the chip, in the interests of time and adding a bit of a safety margin at the upcoming higher boost levels, we left the timing unchanged for the duration of the tests.
Well worth noting here--besides the big power a chip can provide--is that all you studs with the small pulleys and other breathing mods on an electronically stock '03 Cobra are running dangerously lean in the low- and midranges. The Four-Valve Cobra responds to airflow, sure, but without electronic help it doesn't have the fuel flow to match. Note also that at this power level, it isn't the dual fuel pump's capacity, 42-lb injector size, or other physical limits holding the Cobra back (hardly). It's the computer calibration that requires modification.