Green Carb Power
If you read the previous E85 story, you know that from a fuel-delivery standpoint, ethanol carburetors must be capable of supplying a much larger volume of fuel to make up for the fuel's lack of heat output. In our first story, Quick Fuel Technology supplied us with a pair of metering blocks hoping they alone would make a regular gasoline carburetor work with E85. While we got by with this, the whole gasoline carburetor was right on the edge of what it was capable of flowing. This is why we had to go so big with the jetting.
The owners of Quick Fuel, Marv Benoit and Marty Brown, agreed this next step would require a specific carburetor, since we would be using this carb on a blower making even more power. What they sent us was a 750-cfm carb with more traditional drop-leg boosters whereas the original E85 carb used on the normally aspirated engine was of annular-discharge design. And to call attention to this custom E85 setup, they colored the throttle plate and metering blocks green and attached an E85 decal to the float bowl just in case the bright-green color wasn't enough of a clue. Both the idle and main metering circuits were enlarged to compensate for the additional E85 fuel demand.
Out of the box, Quick Fuel spec'd this carburetor with 80-primary and 90-secondary jetting with a power valve only on the primary side. We added five jet sizes to both ends and a rear power valve to bring up the fuel curve. We also experimented with high-speed air bleeds in an attempt to tailor the fuel curve to what the engine wanted. From our previous experience with E85, we knew the max power air/fuel ratio window can be slightly rich and still make max power, so to be safe, we stayed with the 7.25:1 air/fuel ratio throughout most of the testing.
E85's Special Spark-Plug Demands
One of the things we learned the hard way while testing our first E85-fed small-block was that cold spark plugs are absolute necessities. In our normally aspirated tests, we melted a plug. Later, Bosch Product and Technical Support Manager, Matthew Hallis, told us ethanol is particularly sensitive to pre-ignition. In other words, it doesn't take much of a heat source to initiate combustion. Pre-ignition is caused by a heat source in the combustion space that begins the combustion process before the spark plug fires. Any kind of hot spot, like a glowing spark-plug ground strap for example, can light off the intake charge during the compression cycle long before the spark plug is supposed to fire. This creates dangerous cylinder-pressure spikes and can cause massive engine damage in the form of melted pistons. In our first test, we were lucky the only damage was a bunch of nuked spark plugs and two warped intake valves.
The solution to this E85 pre-ignition problem is to run a minimum of three heat ranges colder than the heat-range that spark plugs would normally survive on gasoline. Cold plugs are designed to transfer as much heat as possible away from the spark plugs so they won't melt. Supercharged, turbo, or nitrous'd engines demand a nonprojected-nose spark plug, which minimizes the length of the spark-plug ground strap. While platinum and iridium plugs are very popular for mild street cruising, they should be avoided when pushing an E85 engine hard, such as on the dyno, since these materials tend to retain heat and could cause pre-ignition problems.
Also, because the supercharger boosts cylinder pressures, we knew a smaller plug gap of 0.030 inch would help to get the fire lit. Very cold plugs tend to not want to start as well as hotter plugs on a cold engine, so if you decide to build a similar package, you will need to experiment with heat ranges by starting out cold and going warmer until you see excessive heat evidence in the plug.