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Measure The Diameter of the Exhaust Throat - What's Your Problem?

Jeff Smith: I like your plan, Michael but I have a slightly different suggestion to approach the same level of control. Adapting the feedback Q-jet and the factory ECM to work with this much larger-cubic-inch engine would not be impossible, but it would certainly be problematic because it would require several attempts at part-throttle metering and timing, each requiring a new chip because your ECM predates the EE-prom erasable chips that late-model computers enjoy. I like the idea of the Q-jet and retaining the factory knock control. Keep in mind this won't be emissions legal, but it will work very efficiently.

Let's deal with the knock control first. A company called J&S Electronics sells a very slick electronic knock control device called the SafeGuard. It uses a factory-style detonation sensor that with J&S' electronic software efforts can identify the individual cylinder that rattled and retard the timing on that cylinder before it fires again allowing the other cylinders to continue with their normal timing settings and power levels. If the cylinder continues to detonate, the SafeGuard unit will pull up to 20 degrees out of that cylinder to prevent further detonation. We put this unit into action on a supercharged small-block Chevy on Ken Duttweiler's dyno and we were able to add timing to the engine until the SafeGuard began to set off the retard function on cylinders 5 and 7. Generally this happens around peak torque, where maximum cylinder pressure occurs. It's also possible to add a little more timing to make more power with the other cylinders while the one or two cylinders are retarded to prevent detonation damage. If you are interested, call John Pizzuto at J&S Electronics and talk over your application with him. In your case, you would disable the existing computer control of the ignition and use a stand-alone HEI or aftermarket ignition system to govern the spark curve. The SafeGuard system also includes a built-in rev control and a nitrous retard feature.

As for the carburetor side of things, I like the idea of using a standard, nonfeedback Q-jet for this application. The Quadrajet is an excellent street carburetor that continues to suffer from an undeserved poor reputation. I think the carb is ignored more because of its apparent complexity. The reality is that the combination of a primary metering rod inside the main jet produces a far more tunable combination if you take the time to come up with a lean, part-throttle tune-up. We would suggest investing in a quality air/fuel ratio meter such as the Innovate Motorsports unit or a similar piece to track your air/fuel tuning changes. It should be easy to get the main metering circuit to deliver 14.5:1 to even 15:1 at highway cruise at mild-throttle openings. Then you can adjust those huge secondaries to deliver 12.5:1 to around 13.0:1 air/fuel ratio at wide-open throttle (WOT) for best power. Pay particular attention to the idle feed restrictor size when choosing your carburetor. I'd suggest talking with a good Q-jet company such as Jet or Quick Fuel. They can dial in a carb with a relatively small idle-feed restrictor that is essential for not only a lean mixture at idle, but also for part-throttle cruise. If you want to be adventuresome, there is information in Doug Roe's Rochester Carburetors book on how to modify the idle circuit to produce the desired air/fuel ratio at low-throttle openings. It's a hassle, but well worth the effort.

More Info
Innovate Motorsports

Huntington Beach CA
714/372-5910
InnovateMotorsports.com

J&S Electronics
Garden Grove, CA
714/534-6975
andSSafeguard.com

Jet Performance Products
Huntington Beach, CA
800/535-1161
JetChip.com

Quick Fuel Technology
Bowling Green, KY
270/793-0900
QuickFuelTechnology.com

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