Hot VWs - November 2008 - Project Mileage Motor - Part 14
Testing a single throttle body EFI system

Project Mileage Motor: Part 14
Testing a single throttle body EFI system


Back to the December 2007 issue we installed CB Performance's "Quick Tune" Electronic Fuel Injection system onto our Project Mileage Motor's 1745cc engine. That particular system, which allows you to easily tune your engine with a laptop computer, was really aimed at the high performance crowd, with dual 48mm throttle bodies designed with mega flow in mind. But even with its big bore throttle bodies were able to tune it to perfection, and ended up with an impressive 36,56 miles-per-gallon in the February 2008 issue. Not bad for a quad of beer can sized butterflies on a 1745cc VW engine!

Besides the great fuel mileage from this sytem, we also experienced a driveability that was truly unique for a VW engine. Because of its ability to maintain the correct air/fuel ratio and ignition timing through a wide range of operating conditions, we could lug this engine down to 1,500 rpm in fourth gear, then stab the throttle and have it pull smoothly up without a hiccup. The torque of this engine is impressive.

But there are a lot of VW enthusiasts who, for one reason or another, do not like to run dual carburetor or throttle body systems (usually the linkage). For them, a properly running single carb/throttle body is the way to go. And for our purpose - an engine designed for maximum mileage and not maximum performance - a single throttle body seemed like a more appropriate system. Too bad it hasn't been available - until now!

When the staff of CB Performance showed us their new single throttle body "Quick Tune" EFI system, we jumped at the chance to change our dual throttle bodies over. Luckily, most of the components, such as the fuel pump, most of the fuel line, Electronic Control Module, MAP sensor, ignition, and wiring are the same, but the twin throttle bodies and manifolds are replaced with a single 40mm throttle body and center section manifold. The injectors, which are different that the ones used in the dual setup and designed to atomize the fuel better for increased mileage, are neatly installed in special end castins for a clean, no hassle installation. In fact, the hardest part of the swap was removing the old manifolds, due to the tight quarters in the engine compartment. A different throttle cable is also required, which uses a "Morse-style" end to run the cable from the stock location coming out the rear of the pan, then up to the air cleaner and finally the throttle body arm. We found the cable supplied a bit long, and perhaps CB will make a shorter version available to make installation a bit cleaner.

There is another advantage of the single TB system, and that's the ability to use an IAC (Idle Air Control) valve, which automatically controls idle speed. That's right - no more adjusting, and no more wondering idle! The IAC valve is easily plumbed into the Electronic Control Module (instructions provided).

Interestingly, once converted our engine fired within seconds, even though we had not yet installed the new engine profile program for this system, which is provided by CB Performance to fit a particular engine's configuration. A new program is needed because the single throttle body system has considerably different vacuum levels than the duals, which is an important measurement the computer uses to set air/fuel ratios.

Before long we were up and running with the new system, eager to see just what the mileage would be. We should note that there was slight difference in performance - the duals obviously pulled a bit harder than the single, but the engine was still no slouch. Running through the gears it still pulled strong, and we were able to lug it down to near 1,000 rpm without a problem. But we're looking for mileage here, so after setting the timing and air/fuel ratios to match that of our previous best runs, we hit the freeway.

Just as a reminder, all tests to this point were run using Shell premium pump gas, and all runs were at 65mph on the same approximately 100-mile course. On our initial run with the new throttle body we really weren't expecting big numbers, as we simply used many of the setting from the dual throttle body program that netted our best mileage numbers to date. And we were in for a present surprise. Would you believe 36.75 miles-per gallon, our best yet with the fuel injection system? Very impressive!

At this point the best mileage from our Project Mileage motor 1745cc engine came in the October 2007 issue when we ran dual Weber ICT carbs with the unique Jet-A-Vator device fit between the carb and manifold, which netted 38.46 mpg. But the CB Performance Quick Tune fuel injection has come close, and provides much more performance in the proces. Next month we'll try fine-tuning the system to see if we can up those numbers, as well as doing a few rather unique and hopefully interesting test, such as driving at different speeds and filling up with some good old fashioned "Texas" gas. Should be interesting!