Wayne Cook
September 14, 2007
Photos By: The Manufacturers, John Dickey
Here, Jay Brockwell of AED puts in the hours on the Superflow engine dynamometer. The flow bench tells only so much, and driveability matters such as throttle response and part throttle operation have to be sorted out on a real engine. Many hours of combined effort using both the flow bench and a real engine on the dyno got results.

Those using fuel injection need to adjust control systems. For example, filling the combustion chamber with the same explosive equivalent as gasoline requires a wider pulse width allowing for the injection of more fuel. Pro Max Carbs and Performance Parts of Indianapolis, Indiana, provided the table below, which shows the disparities in air-fuel ratios for four different types of automobile fuel.

When the required fuel flows were figured out for the 850-cfm test carb, the AED engineers went to their in-house Superflow 901 engine dyno. Initial tests showed the mixture running way too rich across the fuel curve. The big-block test engine sputtered and backfired, so they started going progressively leaner until operation smoothed out and the power came around.

If the carburetor is even a little too rich, the engine will miss. The tune-up must be targeted to a small operational window for good performance. Once satisfied with wide-open throttle performance, they moved on to general driveability issues such as idle, throttle tip-in, and cruise.

With gasoline, when the idle circuit is adjusted correctly-including the idle feed restriction and air bleed-then the transition circuit will usually follow suit and function properly. Not so with E85 fuel. A lot of experimentation with power-valve channel restrictors was required to completely solve all the driveability issues and get a good balance between part-throttle cruise and wide-open throttle. For E85, the stock transition circuit runs too rich, so adjustments to both the main jet and power-valve channels were necessary. To get all the circuits working together, extra restrictors were added for the carburetor main body to control the transition of fuel.

This is a billet metering block, where much of the E85 tuning adjustment takes place. The power valve resides in the center port, and the power-valve channels are on either side leading to the center. For home tuners or otherwise confident types, these metering blocks are available separately at AED and cost between $79.99 and $109.99.

In drag-race applications, some of the fine-tuning isn't as essential because part-throttle driveability isn't really a concern. Most of those interested in performance street use of E85 will be drawn to it because of the high octane rating. The higher-octane fuel supports higher compression, and higher compression means more power.

Street enthusiasts will definitely be interested in not just power but also driveability. Many people figure they can buy a set of metering blocks and convert their existing carburetor to E85 use, but it's not that simple. All of the factors we've looked at are integral to producing a reliable E85 carburetor. Remember also that instead of running a 14.7:1 air/fuel ratio for gasoline, we now need to run a 9.8:1 A/F ratio with E85.

As you can see, there's a fair amount of work involved in providing a smoothly functioning E85 carburetor. Just keep in mind that the engine will also need a fuel system that can support an additional 50-60 percent of volume of fuel to the carburetor.

Air/Fuel Ratios
 IdealRich LimitLean Limit

Air/fuel ratios for E85 show the biggest difference when compared to other fuels. Because E85 is so different from other fuels, careful attention must be paid to tuning the carb.