All too often, we bolt on a new part, expecting to be immediately-and effortlessly-rewarded with more horsepower and faster e.t.'s. What's more likely is that we claw and scrape for any appreciable gain, expose the flaws in our plans, and find all the weak spots in our machine-then we take it to the track, only to have the hood fly right off the car and soar over the roof as we cross the line. You've been there, right? This month, that's exactly where we find ourselves. In short, we managed to squeeze more power from our project '84 Z28, but we also managed to break a few things in the process.
We got after this project in earnest during our June issue, outfitting our 75,000-mile, L69-powered '84 Z with a set of Trick Flow heads and a matching hydraulic flat-tappet cam. To this we added an intake, headers, and an after-cat exhaust system from Edelbrock, along with a new catalytic converter from Random Technology. Our efforts were rewarded with 53 added horsepower, 55 more lb-ft of grunt, and a nice 1.29-second gain in the quarter. These gains were especially encouraging, given that we hadn't yet done any tuning on our Z's computer, ignition, or carburetor to maximize the benefits of our previous work. That was the task we laid out for ourselves.
Of course, this bright outlook was tempered by the fact that we had a tough nut to crack. Our goal was to extract as much power as possible from our Camaro's original 305 H.O. powerplant, all while remaining nominally smog legal. It's a frustrating goal, especially if you live in a state with less draconian smog laws than we have to deal with on the Left Coast, but it's a fact of life here in California. According to the ground rules, then, we were stuck with our L69's computer-controlled Quadrajet carb. The problem here isn't with the carburetor itself; a good Q-jet could easily support our power goals. It's the "computer controlled" part of the equation that's the problem.
We'll talk more of this unholy union of carburetor and computer elsewhere, but for now, suffice it to say that we tapped an expert, Sean Murphy of Sean Murphy Induction, to make the most of our Q-jet situation. We addressed the other half of the equation through GMCOPO, which not only burned us a new chip for our combo, but also sent along a later-model computer. Looking to add a hotter spark to the mix, we added a replacement computer-controlled distributor from Performance Distributors.
According to Sean Murphy,...
According to Sean Murphy, our compu Q-jet's mixture solenoid can handle part-throttle operations, but some work is needed on the idle side. With our bigger heads and cam package, the computer can't cycle the mixture solenoid in an appropriate range to provide enough fuel. Enlarging the idle downtubes from 0.033 to 0.036 inch helps provide the needed fuel.
Murphy also pays attention...
Murphy also pays attention to transition areas when building a computer-controlled Q-jet. Since off-idle engine vacuum drops harder with our cam in place, he enlarged the accelerator-pump discharge orifice from 0.026 to 0.031 inch. Compensating here instead of via a jetting change has less of an effect on emissions. It also lessens tip-in hesitation, according to Murphy, which gives the car better throttle response.