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Chevy Big Block EFI - The Coolest Rat

A 615hp Pump Gas Rat For Your Everyday Ride

By Mike Petralia

It's about time we catch up with, and then surpass, the OEMs when it comes to power and driveability in our hot rods. While carburetors are still a fun and cost-effective way to make power, electronic fuel injection (EFI) is really the best way to cruise. If you want to build an engine that truly demonstrates the best of all worlds, good starting, smooth idle, decent economy, low maintenance, and big power, then a stroker EFI big-block is the only way to go. When we set out to build this EFI big-block, which we've since named "Cool Rat," we had no idea just how cool a motor like this could be.

What makes this Rat so cool is that it idles around 700 rpm, makes over 600 hp on pump gas, requires practically no maintenance, and can be driven anywhere at any time by just hopping in the car and turning the key. There's no carb tuning to fuss with and no pumping the gas pedal a couple times before startup. You're probably thinking right now, "All that can't be true," but it is. That's because this stroker Rat make gobs of power, and the EFI, combined with the hydraulic roller cam, makes it suitable for everyday use on the street. And we didn't have to take out a second mortgage to build it, either.

Although no EFI motor can truly be considered a budget buildup, this one counts as an affordable alternative. This whole engine, top to bottom, would probably cost around $10,000 to build, meaning it would be about $16.25 per hp, which isn't too bad when you consider that this could be a 615hp daily driver! Where else could you get that much power in a package that is not only self-tuning but doesn't require any valve lash adjustments, starts and runs great on cold winter mornings, and won't vapor lock on a hot summer day? Also, consider the fact that this Rat idled like a kitten, making over 14 inches of vacuum. Can you say "stealth fighter on the boulevard?"

Tuning The Coolest Rat
One of the things we learned while tuning this Rat on the Vrbancic Brothers DTS dyno was that with the proper EFI system and a well built motor, very little tuning is actually needed to make optimum power. And the right combination can also give you peak power figures at a reasonably low rpm. One of the reasons this Rat is so streetable is because its horsepower peaked below 6,000 rpm and it had no trouble running on 91-octane pump gas with 36 degrees of total ignition advance.

With our laptop computer connected to the Holley Commander 950 ECU we fine-tuned the fuel map to get the best performance at all rpm. Unlike tuning with a carburetor, the EFI system gives you the ability to tune on the fly, with the engine running. Also, the Holley tuning software is able to pinpoint exactly where in the fuel map your engine runs, telling you precisely where to tune it. After getting the engine to start and idle easily by tuning the start-up enrichment and then using the DTS dyno's wide-band O2 sensor to get the best idle quality, we stepped the engine up in 500-rpm increments, then tuned each increment specifically.

The dyno held the engine at a steady rpm while we increased or decreased the fuel injector pulse width, watching the A/F ratio and listening to the sound of the engine. As we neared the perfect setting for each rpm point, the engine responded with higher vacuum, higher revs, and a smooth tone. While it'd be nice to maintain a consistent A/F ratio throughout the entire rpm band, real world testing proves that sometimes an engine likes to be rich in some areas and lean in others. An engine never runs best at just one A/F ratio. In fact, ours ran up and down the A/F scale from 10:1 to about 13:1. Once we figured out how a minor fuel map adjustment could make a big difference, we simply chased the computer cursor around the map as the engine made a sweep and tuned it as if we were driving down the road. By doing so, we were able to increase torque between 3,000 and 3,700 rpm by more than 50 lb-ft! Try doing that with a carburetor while you're driving. It can't be done.

By Mike Petralia
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