Ask Away! with Jeff Smith--EFI Idle Speed and Cruising Gears

This month OnAllCylinder's tech guy Jeff Smith fields questions on tuning the idle speed on an EFI-equipped engine and explores choosing the proper rear end gears for highway cruising in a car with an overdrive automatic transmission.

Tuning Idle Speed on an EFI System
Reader B.L. has 468 cubic inch big block in his Chevelle. He swapped the 850 cfm Holley for a FiTech EFI system, and now the engine idles too high--around 1,025 RPM--even though B.L. specified an 850 RPM idle speed when he programmed the system.

Jeff explains that this can be an issue with 'self-learning' EFI systems like the FiTech. The first thing is to check for vacuum leaks. If that checks out OK, know that these EFI systems come with a basic tune that is guaranteed to make the engine start and run, but my not be optimal for a particular engine--in this case, a large cubic inch big block. That means some fine-tuning is needed. Jeff goes through the steps to set the desired idle speed for B.L.'s engine--it could help you too.

Choosing a Cruising Rear Axle Gear
S.C. has a 1972 Chevelle with a mild 454 cubic inch big block. He plans on installing a TH-700-R4 or 4L80E four-speed automatic, and wants to know how to choose the best rear axle gear ratio so the car will tach around 2,000 RPM at highway cruising speeds.

As you'll see, this involves math based on vehicle speed, rear tire diameter, overdrive gear ratio, and rear gear ratio, Jeff goes through all of the steps to solve for the rear gear ratio. If you can plug numbers into a formula, you can do this for your own car.