For years I have seen many people taking off the factory delivered fuel injection systems and install carburetors. Most of these people believe that they are going to make more power with the carburetors. But is this really true? In this article, I look at the pros and cons of this popular swap.
Introduction of Fuel injection
Porsche used carburetors on all their cars up until the end of the 1960s. in 1969 with the introduction of the 911E and 911S models they began using fuel injection. The fuel injection system used on the 911E and 911S were the Mechanical Fuel injection, (MFI), style. These systems were largely based on the Diesel fuel injection systems and were complex to tune. The only model that carried a carburetor into the 1970s was the 911T, and that only lasted until 1972.
By 1973 all Porsche models were fuel-injected and would be going forward. In the 1973 ½ models we saw the introduction of CIS fuel injection. CIS, (Continuous Injection System), is a form of mechanical fuel injection, though less complicated than the MFI systems. Instead of using a mechanical injection pump to atomize and inject fuel into the engine, it uses a high-pressure electric pump to deliver fuel to the engine. The CIS system was much easy to tune without all of the moving parts of an MFI system.
Advantages and disadvantages of fuel injection
Fuel injection has a number of advantages over carburetors. The main one being that it’s method of controlling the amount of fuel injected into the engine is far superior than carburetors. This along with temperature and load responsive controls make it far more efficient and drivable than carburetors.
The dis-advantage of a fuel injection system is that it cannot be easily modified. It also requires more specialized tools to be able to work on the system effectively. This usually means for the home mechanic a limited amount service that can be done themselves. Also, if the engine is modified to increase displacement, compression and or cam profiles that require more fuel it can be difficult to change the fuel curve.
As a lot of these cars got older and the value of the cars dropped, we started see more people installing carburetors. This was for a number of reasons, the cost of repairing the CIS system can be high, not having the skills or the tools to repair fuel injection, and perceived performance gains. As time went on, I started seeing more people calling carburetors a “Performance Up-grade” rather just something they were more familiar with.
Unfortunately, though most of the so-called performance upgrades turned out to be a performance downgrade. Everything in the engine has been designed around using the fuel injection system. The piston design, cam profiles, camshaft timing, and the ignition system with the use of ported and manifold vacuum systems were all designed around using fuel injection. When someone installs a set of carburetors onto the engine and makes no other changes to it, they are taking a huge back step in performance.
A carburetor just cannot keep up with all the changing fuel demands on an engine designed for carburetors let alone an engine that is designed for fuel injection. The shape and design of the piston domes are specifically designed to push atomized fuel and air around the spark plug. Carburetors just cannot atomize and mix fuel and air as well as the fuel injected systems.
Instead they are relying on vacuum to draw in fuel and hope that it will mix with the incoming air as it moves through the manifold and ports. The camshafts are designed for fuel injection and have a larger over-lap between the intake and exhaust lobes. This makes for better cylinder scavenging and filling, but also results in low intake manifold vacuum signals at idle. These signals are somewhat smoothed out when using a single intake plenum but when a carburetor is installed these pulses will adversely affect fuel delivery and low-end performance.
The ignition timing curve in the distributor has also been designed around the engine mechanical requirements as well as the using vacuum signals from the intake system. When switching to carburetors usually the only thing that remains is the mechanical potion of the curve. This means modifying the distributor to try and compensate for all the missing vacuum inputs. When modifying these distributors there are never quite the same as using all the vacuum controls.
The only perceived advantage of using carburetors, is the ease of adjustment for different operating ranges. It is easy for an individual to change the calibrated parts of a carburetor to alter its performance for different applications. However, to do this well also takes , skill, experience and patience to get everything right. There are also more moving parts to the carburetor systems, this means more maintenance and adjustments are required.
Just because a fuel injected system is harder to modify doesn’t mean that it cannot be modified. If the base engine configuration is changed to a point that it requires a different fuel curve, this still can be done. It just requires a specialist in that area to set up correctly. In my opinion sticking with the fuel injection system is a much better choice than using carburetors. Yes, the upfront cost can be a little more than carburetors, however the end result is so much better. Once the system is setup it will maintain its tune for much longer time. The engine will produce more power consistently through out all the operating ranges. This leads to better fuel economy and drivability.