Carburetors Versus Fuel Injection

Carburetors on a Porsche Engine

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.

Mechanical Fuel Injection System on a 1970 2.2L Porsche engine
1970 911E engine with Mechanical Fuel Injection System

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.

2.7L CIS Fuel injected Engine
1976 2.7L Engine with CIS fuel injection installed

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.

Carburetors

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.

Weber 40 IDA Carburetor set
Set Of Weber Carburetors

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.

Engine Design

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.

Fuel Injection Piston
CIS Fuel-injected Piston and liner. Note the shape of the piston Dome

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.

Conclusion

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.

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18 thoughts on “Carburetors Versus Fuel Injection”

  1. Again, because fuel injection and modern electronic controls are more accurate, fuel delivery can be tuned to match driver demand. Carburetors are precise, but not accurate, in that they cannot account for changes in air or fuel temperature or atmospheric pressure.

  2. Amazing article full of facts. I own a 75 CIS Carrera and the car is great. Have you read the article on Pelican Parts about maximizing the HP of the engine by doing some easy upgrades ? Thanks

    1. No i have not seen the post you are referring to, please send me a link i would love to see it.

  3. Kurt, today I have seen a 911E engine with CIS. Is that possible? The engine number is 6231679, which is in my humble opinion a 911E engine, but it had the k-jetronic CIS. Could it have been retrofitted to replace the MFI, or will this be a hybrid engine made up of parts of an E and T engine?
    Thanks

    1. Peter,
      you are correct in that engine case was originally an MFI engine. but being that it is 47 years old it is most likely that someone used the case and transferred the top end of a 911T CIS set up to it.

      1. Kurt, thanks for your reply. I can send pictures. Would you turn away from this car? I.e., could this have been done properly or is this rather strange? Peter

  4. Adam Golightly

    My uncle has been thinking about making his car more effective, and efficient with some extra parts. He would really like to get better parts from a professional so that they will work a lot better. I’ll be sure to tell him about how different applications can alter its performance by changing the calibrated parts.

  5. Danny Bridgforth

    Kurt I have just restored a 1970 E. The car is not stock. Engine is the OEM 2.2 l that has E cams with S ( competition engineering ) pistons. Weber carbs that are running rich. It fouls a set of plugs in 800 – 1000 miles. OEM 3 pin CDI with 5K ohm rotor, Beru 1K ohm wires, through your coil. What jetting do you reccomend for this application? The venturies are 34 mm, with 135 mains 60 idle jets. Air correction 185, idle air 145. float level correct. I rebuilt the carbs putting them, jets venturies etc. back as they were on the car. Car runs great but plugs show rich (black soot) fouling.
    Thank you

    1. Danny,
      The Jetting that you have now is not that out of line, if anything I would say the 135 main jet and 185 main air jet would put the motor on the lean side. When using a 34 mm choke I use a 145 main jet and a 140 air corrector. My questions for you would be, what are the leak-down numbers on the engine? What spark plugs are you using? What are the cam timing specs you are using? and how well does the car run before fouling a plug? let me know when you have a chance and maybe we can figure out what’s going on.

  6. Danny Bridgforth

    Kurt, Thanks for your quick reply. One of the main reasons I like doing business with you. I will run a warm leak down test tomorrow. Cold compression numbers #1. 185, 2 182, 3 185 4 190 5 195 6 197. I have run several plugs. The NGK BP7ES did best. Tryed Bosch W4CS too cold and resistor ( I know its wrong ) but hotter champion RN9YC. Bosch lasted similar to NGK, champion less than 150 miles. Cam timing 0.129″ left 0.126″ right. The car runs great. No stalling or flat spots. Pulls strong from 2000 to red line. Has a blast at the track. Suspension and brakes not stock……
    Thanks
    Danny

    1. Danny, all that looks good. So is it fouling the same plug every time? or is just random or all cylinders? and lastly, does the fouling actually result in a misfire? or are the plugs just black?

  7. Danny Bridgforth

    Kurt
    Random plug fouling. one at a time. Can’t say if all six have failed but the last three culprits were #6, #2 and #3. All have pretty much the same black soot look. Fouled plug will be wet. I change them all at the same time. Under a loupe #4 shows the least ceramic discolorazation. Yes a misfire. The first clue is difficult starting. Cranking, it will start on less than all 6 cylinders. A few seconds later a POP and now all 6. Sometimes it will start on 4. A few seconds later POP now on 5, seconds later POP now all 6 are working. With that it will run ok but not great. Under a load like uphill acceleration a plug will fail and not recover. I get to a safe area let it cool a bit and change them.
    Thanks
    Danny

    1. Hi Rick, Porsche runs a system pressure of around 4.5 to 5.2 bar. The control pressure is regulated by the warm-up regulator and varies for each W.U.R, warm up reg.

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