Ask our Experts, we're here to help!
For most semi-savvy car enthusiasts, unless you're a died-in-the-wool carburetor guy, installing and setting up an EFI system can be one of the simplest and most rewarding upgrades to a classic car. In the past, this 1955 Chevy has had a mild 350 small block with a single plane, four-barrel intake topped by a Quadrajet that we were always trying to dial in properly, followed by a 327ci with Tri-Power that looked straight out of the early 1960s, but with similar situations. We eventually replaced the 327 with a freshly-built stroker motor topped by an early throttle body EFI with forced induction. It ran like a scalded cat, until it hit the wall due to the limits of the small injector size of that early unit. It remained that way for years, until we decided that it was time to tear down the wall and join the modern world.
Since our 377ci engine was dressed to appear as though it was built that way in 1955, a tuned port EFI system was out of the question. We needed an option that would fit under a stock-style air cleaner and not look completely out of place, given our early aesthetic. But while it needed to look the part, we also wanted to cure our fuel limitations.
To get down to brass tacks without over burdening the topic, our original throttle body was equipped with four 80lb/hr fuel injectors and hit maximum duty cycle (100%) at just under 5,000rpm, as witnessed on the dyno. That meant that running the engine over 4,000rpm or so would put our engine in a precarious position where it could suffer from a lean condition. On a naturally aspirated setup, that would be enough to warrant your attention, but in a forced induction scenario, the idea of encountering a lean condition, under boost and acceleration, was a situation we wanted to desperately avoid.
While we could sit down and calculate the fuel load necessary for our particular application to yield an appropriate duty cycle, the engineers at Holley have made selecting an appropriate system for our engine easy, thanks to the extensive Holley product line that covers a wide range of power levels. Given that we hit the proverbial fuel wall at 578hp / 620lb-ft., we knew we needed a system that could eclipse those numbers, with some room to spare.
With all that said, it made the most sense to choose one of Holley's newest EFI kits, the Sniper EFI Stealth 4150. Based around the original architecture of a 4150 4-barrel carb, the Stealth contains EIGHT 100lb/hr fuel injectors (that's over three times our original throttle body!), capable of supporting up to 1,250hp. And yet while the Stealth can support some big numbers, it doesn't take any special equipment, tuning, or capabilities to get the system installed, and up and running. Using the provided handheld controller, an intuitive wizard aids the end user through a series of questions to get the proper information loaded into the computer (number of cylinders, engine displacement, target idle speed, camshaft type, etc.) before it then loads a base map tune automatically, preprogrammed by Holley's engineers. From there, the system can be operated in a "closed loop" configuration, resulting in a self-learning / self-tuning situation that adjusts on the fly, without the need for expensive dyno tuning time.
The result truly speaks to the state of the hobby when it comes to the technology that's finding its way into our classic cars. Twenty years ago, the thought of a DIY EFI installation without the need of a laptop or expensive tuning was truly a pipe dream. Today, not only is it a reality, it's becoming commonplace. Upon completion of our Sniper EFI Stealth kit, checking fuel pressure, and ensuring that we didn't have any fuel leaks, we ran through the Sniper's Initial Wizard and had our small block fired up faster than you could say "main jet swap"! And with the system up and running, just like the song, you know who came out and said, "honey, take me for a spin". As it turns out, that's exactly what we needed to do!
At first glance, the Sniper EFI Stealth 4150 bears a strong resemblance to the venerable Holley 4150 carburetor, and that's no accident. What better way to design a carb-replacement throttle body than to mimic said carb's basic architecture? But while the Sniper Stealth appears to have bolt-on, side-mounted float bowls, fuel level adjusting nuts, and identical fuel inlet locations as its carbureted cousin, internally, things change drastically.
Hidden inside the clandestine carburetor sits eight 100 lb/hr fuel injectors capable of supporting up to 650hp naturally aspirated or a whopping 1250hp on forced induction applications. At the heart of the throttle body sits a self-learning ECU that is also capable of ignition control. All wiring for the internally housed ECU as well as the integrated Manifold Air Pressure and Throttle Position Sensors exits below the rear of the throttle body, further lending to the stealth characteristics of the kit.
To allow for a near plug-and-play installation, the Sniper Stealth also features traditional throttle and transmission kick down linkage attachment points.
One other item worth mention is the throttle blade adjustment screw. This will be used once the engine is running at operating temp to adjust the IAC position to a percentage between two and ten percent as noted on the handheld monitor.
Three vacuum ports are available on the throttle body. The first one is a 3/16-inch ported manifold vacuum at the front, perfect for a vacuum advanced distributor connection.
The other two are at the rear: a 3/16-inch manifold vacuum port and a 3/8 manifold vacuum port that works perfectly as a brake booster source. You can also see the location where the wiring harness exits the throttle body.
At the end of that harness are six connectors. From left is the connection for the handheld (or laptop for tuning using the provided adapter), an 8-, 7-, and 10-pin connector, coolant temp sensor, and O2 sensor connector.
The Sniper Stealth system is configured using a handheld, touch screen 3.5-inch LED controller. A tuning wizard is used to input various parameters that the ECU then uses to create a custom, base tune for the EFI system. From there, the ECU operates in a closed loop configuration while the system "self learns" using the various sensors to achieve an ideal fuel map. The handheld controller also functions as a great mini-dash giving the end-user access to a vast amount of information from coolant temp to inlet air temps, air/fuel ratio, etc.
One of the most important sensors in the system is the wide-band O2 sensor. This sensor provides accurate air/fuel mixture readings to the ECU by measuring and averaging the reading from one bank of cylinders.
This will be installed towards the top of the exhaust pipe, approximately 6-10-inches after the collector.
While our project features a previously installed weld-in bung, Holley provides a Clamp-On O2 Bung Kit as a unique solution for those without access to a welder. Also pictured is the provided lap top cable adapter that can be plugged into the same connector as the hand held.
Our second sensor is this coolant temperature sensor. This sensor measures the coolant temp and relays that info to the ECU, which can then determine the operating temperature of the engine and what fuel tune to use. For example, there are different parameters built into the fuel map when the engine coolant is cold versus operating temperature, cold start fuel mixtures, etc.
The simplest example of a Sniper Stealth installation requires only four wires to be connected: the red and black Battery POS and NEG, the pink 12v switched, and the yellow Coil input wires. Most installations however will require a handful of additional wires to be terminated in order to take full advantage of the system and its capabilities (ignition control for example). Here, aside from the four main connections, is a 12v source to power a fuel pump (blue wire) and the purple/green crank signal connector.
A separate 10-pin connector provides additional inputs to take further advantage of the Sniper Stealth's capabilities.
From left, these extra connectors provide the following: ground side trigger of a/c shutdown relay (grey), points output used to trigger a CD ignition box or the included Coil Driver Module (white), tachometer gauge output (dark brown), electric fan relay #2 ground trigger (light green), Holley EFI digital gauge outlet (tan), electric fan relay #1 ground trigger (light blue), programmable ground input (yellow), and ground triggered A/C relay (orange).
The Sniper Stealth 4150 comes equipped with another 8-pin connector providing even more additional input/outputs for further capabilities. These can be configured using Holley's EFI tuning software using a laptop for Nitrous control or additional fan relay triggers, for example.
Our application will not require the additional 8-pin connector and will only be using five of the eight wires on the 10-pin connector. A Holley Pin Removal Tool (p/n 567-101) is used to remove the unnecessary wires and the remaining cavity in the connector plugged.
Here's our Sniper Stealth kit with the wiring harness cleaned up and ready to install in our vehicle. Note the relay for the fuel pump feed as well as the main fuse, both securely fastened to the harness.
Our homemade linkage bolts up to the stock location on the Sniper Stealth, and since we're running a 5-speed manual transmission in the vehicle, our installation does not require a kickdown cable.
We're going to be using a combination of hose and hardline between the fuel tank-mounted electric pump and the Sniper Stealth. Between the fuel pump and the fuel filter, we'll be using Earl's Black Nylon Braided AN-6 Hose with a variety of Earl's Swivel Seal hose ends.
A Holley In-Tank Retrofit Fuel Module (p/n 12-132) will be used to provide a steady stream of cool, pressurized fuel.
An Earl's 100 GPH Billet Aluminum Fuel Filter (p/n 230606ERL) will ensure nothing gets passed the 10-micron screen and into the throttle body. Note the transition from hose to hardline.
Up near the firewall of the vehicle, we mounted a Holley HP Billet EFI By Pass Fuel Pressure Regulator (p/n 12-846) on the frame to control the fuel pressure. From here, the inlet will be plumbed towards the throttle body, while the return will be plumbed back towards the tank.
While we appreciate all the positive traits of electronic fuel injection, we also wanted to maintain the stock-ish appearance of our "283" build. One of the things we wanted to retain was the hard line routing from the stock mechanical fuel pump to the throttle body. To accomplish this, we modified a stock fuel pump and made a dummy, bypass only lower housing that looks the part but is simply a housing with an inlet on the bottom and an outlet on the side.
Using Earl's Annealed Stainless Steel Tubing (p/n 661696ERL), AN-6 Stainless Steel Tube Nuts (p/n SS581806ERL), and AN-6 Stainless Steel Tube Sleeves (p/n SS581906ERL) we fabricated a hard line that ran up the front of the motor and into an Earl's AN-6 Tee- Female Swivel On Run (p/n AT926106ERL) that attaches to the front most fuel inlet on the throttle body.
Another hard line runs from the Tee into the rear fuel inlet (both inlets require connection). That AC Delco fuel filter further adds to our vintage aesthetic.
With the fuel line installation and wiring complete, a quick check for leaks and a pressure test at the throttle body to ensure the regulator is set to 65lbs is completed before the rest of our induction setup is assembled. From there, a quick run through of the setup wizard using the handheld controller yields a calibration that will get our small block Chevy project up and running.
