How to Replace a GM Quadrajet Carb with EFI

10/21/2022

How to Replace a GM Quadrajet Carb with EFI

10/21/2022

Not long after the original muscle car era ended in the early 1970s, a lot of young hot rodders began to take matters into their own hands.


There was an abundance of Chevelles, Novas, Camaros, Cutlasses, Firebirds, GTOs ripe for the picking. They could be had for normal used car prices and often already had the basic equipment necessary to build on performance.


Sniper Quadrajet carb shot

GM put millions of Rochester Quadrajet carburetors on cars all across their lines for decades. Properly tuned, Quadrajets actually work very well for the intended application. Get too far outside the box though and there can be trouble. This owner is opting to swap his out for a Sniper EFI Quadrajet so he can have new car drivability and efficiency in his 55-year-old Camaro. Except for an upgraded fuel system and an O2 sensor, it will hook up just like the original Q-Jet.


A lot of attention usually went first into the engines to up the power levels, which almost universally started with the addition of headers and bigger cams. Improved induction was also high on the list, especially for those V8s that came with 2-barrel carbs. But a lot of these cars came equipped with 4-barrels from the factory and a good lot of those were Rochester Quadrajets.


The Q-Jet was used across the General Motors lines from 1965 to 1990, including some of the high performance muscle cars, while the later ones were adapted to early computer controls. Your typical hot-rodder was very likely to run into one. They were actually a great design that worked very well for their intended applications. That’s why GM used them for so long and made a zillion of them.


Sniper Quadrajet overall kit

These are the key components that come in the box. Don’t be concerned by the wiring. The instructions are very thorough and the install is actually pretty simple. Only 4 flying leads are required for hookup! There is no separate computer box to install. Everything needed is contained within the Sniper itself.


Their small primary throttle valves provided great drivability, throttle response and even fuel economy, while opening the secondary plates delivered 750 ( and later 800) CFM of air flow which was more than enough for most builds. The problem that most ran into trying to use them on modified engines was that few really understood how to tune them for non-stock applications. The Q-Jet would do the job if it was tweaked correctly but far too often the actual result was a terrible hesitation or “bog” as the throttle was floored.


The frustration was real and got to the point that the most common advice to someone struggling with a Quadrajet was to “just put a Holley on it.” There was just a lot more good information out there at the time about how to tune a Holley and frankly it was easier to do with the abundant aftermarket support available.

Muscle Car Restorations sees a good number of these and they regularly recommend installing a Sniper in place of an original carb provided the project is not a pure stock resto. It’s not that a properly tuned carb doesn’t provide good drivability, it’s just that they’ve found that a Sniper does it better.


Now Q-Jet cars have a direct EFI replacement for their carbs that will run hassle free on anything from a stone stocker up to a 500 HP sleeper; no manifold swap needed. MCR simply plugs in the basic engine parameters and starts driving. The system learns remarkably fast exactly what the engine needs; much, much faster than fussing around with jets and metering rods to get a Quadrajet to flow correctly.


Sniper Quadrajet on engine

The Sniper really is a direct bolt on replacement for the carb. No adapter plates of any kind are needed. All the vacuum lines and linkages are also direct connections. The liquid fuel pressure gauge behind the unit is not required but MCR routinely uses these to monitor fuel pressure at the Sniper to confirm that it remains constant during the set up process. Note the original distributor and coil will be retained.


While the Sniper EFI Quadrajet is a direct bolt on replacement for an original Quadrajet carb and is very much plug and play, it will still need some additional items installed to support the unit.


First off, a fuel system capable of operating at a reliable 60 psi is mandatory. An external pump can be used with the original fuel tank but MCR strongly recommends avoiding that and going with Holley’s fuel tank kit which includes the pump and sending unit. Besides, who knows what settled into the bottom of a 50-year-old tank.


Sniper Quadrajet Touch Screen 1

The touch screen acts like a mini dashboard that can be used to show real time data. The system can also log data for later review.


An O2 sensor and coolant sensor will need to be added, but everything else is already built into the Sniper itself.


The Sniper has the ability to control ignition timing through multiple distributor types but since this is a mostly stock build, MCR will be using the stock coil ignition system to manage the timing curve. Except for the couple items above, a 50-year-old Q-Jet really can simply be replaced with a Sniper EFI Quadrajet and get all the benefits of modern EFI while leaving the rest of the car as is.

As with previous installs, once everything was hooked up and confirmed, the engine fired right up and settled into a smooth idle. Once warmed up, a few parameters were checked and the car was ready for some easy cruising on MCR’s Mustang dyno. Learning takes place very quickly and before long the techs felt comfortable making a few full power pulls.

In this case, the max power levels were only very slightly higher with the Sniper but the real benefit isn’t in max power but in overall drivability. There were no cold or hot start issues and no flat spots in the power curve. Staying in the primaries yielded a crisp throttle response and stomping it hard made it go without hesitation. There was even that classic Q-Jet wide open roar when those huge secondaries popped open.


The small primary, large secondary throttle plate design has always been sound. It’s just that now with the Sniper EFI Quadrajet it also has optimum trouble free fuel metering built in.


Sniper Quadrajet Dyno Driving

The learning process begins with some easy driving taking the engine through different speeds and loads. MCR does this on their Mustang chassis dyno so they can monitor everything up close and eventually do some full power runs.


Sniper Quadrajet Air Cleaner

The Quadrajet Sniper accepts all the usual air cleaners but if you run with the stock one it’s likely no one will notice that you are running EFI on your classic muscle car.


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