Paul B. Anders
The question of whether to use the stock D-Jetronic injection or a carburetor setup is one of the most hotly-debated topics on the 914 discussion lists. I've owned both types of motors - I had a 2.2L with Weber 40IDF carbs before the 2.0L D-Jetronic motor I have today, both of my motors ran well and had their strengths and weaknesses. I make no bones about it - I'm pro fuel-injection, but I have constructed this page with as unbiased a view as I can, to provide good information about people considering converting to either system. I know it may come off as very pro-FI, but that's because Fuel Injection is hard to beat for most applications. Below, I've broken the topic into several areas that are commonly sources of debate.
Performance
For a stock motor, converting from D-Jetronic to any type of carb system will not result in any significant power or torque gain. Dyno tests have backed this up. The reason is pretty simple, D-Jetronic is optimized for the motor and they didn't leave any performance on the table. One counter-claim to this is that D-Jetronic is set to be much leaner at part-load than a carb can be set to, and isn't at the Air/Fuel ratio power peak point. True, but at constant part-load, this isn't an issue, as you simply open the throttle wider to match output. The acceleration circuits of D-Jetronic are superior to a carb (raw jet of liquid fuel) and you get better throttle response. And at full-load, D-Jetronic is tuned so that it is right on the A/F ratio power peak, so there's no advantage for the car. Additionally, at low engine speeds, fuel atomization of carbs is poor, especially for large venturi sizes (e.g. 44 mm).
Fuel Injection systems have excellent atomization at low engine speeds, so low-end torque is better, regardless of how much fuel you dump into the engine with a carb. Where carbs are superior to D-Jetronic comes with engine modifications. D-Jetronic can handle larger displacements up to 2.4L (for a stock 2.0L engine) without heroics - either readjust the overall mixture with the MPS, add head temperature sensor resistance ballast, or increase fuel pressure (or a combination of all three). Where D-Jetronic has problems is when a high performance cam with significant overlap is used. The low idle vacuum causes the motor to run very rich, leading to poor idle, high fuel consumption, etc. This is because cam alterations change the volumetric efficiency (Ve) curve of the motor, which is corrected by a hard-wired circuit in the ECU. While it is possible to modify this circuit to accommodate the change in the Ve curve, this is beyond the abilities of most owners and isn't really an option. A carb can handle a cam with overlap through careful tuning, but don't kid yourself. It's still going to lope at idle, and have low-rpm throttle transition issues. A high-performance cam is a compromise - you're giving up low end torque, drivability, emissions, and fuel economy for high rpm performance. Most street motors are actually worse after such modifications, because they have to be always at high rpm to perform smoothly.
Carbs can also make turbocharging or supercharging easier to some extent. Here, Fuel Injection can be made to work, too, but is quite limited. The best approach may be an aftermarket Fuel Injection system that can handle positive manifold pressures.
Drivability
D-Jetronic wins here, hands down. Cold and cold-cold start are superior to all carb configurations. Off-idle transition is smoother and you don't get "bogging". Hot start is about the same for both configurations due to a problem with the design of the D-Jetronic ECU that often provides too much fuel during a hot start. Low end torque is far better due to improved fuel delivery. Carbs can be made to work pretty well with careful tuning, but no amount of tuning will overcome the inherent design deficiencies of carbs that affect drivability. Case closed.
Ease of Maintenance
This is usually the big selling point of carbs. When problems arise with the Fuel Injection system, people have a hard time finding the cause, and they can't find mechanics who understand the system. Parts are expensive, and it's hard to tell which part is the cause of the problem. They hear that carbs are much more simple, so they decide to switch. What's wrong with that?
First, many problems attributed to the Fuel Injection system have nothing to do with the system. Low compression or low engine vacuum due to wear affect the operation of the system (and affect carb operation, too). Misadjusted valves affect performance and emissions. Ignition faults also are commonly the cause of problems blamed on the Fuel Injection system. None of these problems will be fixed by changing to carbs.
Real problems with the Fuel Injection system are usually due to a lack of knowledge or reliance on folklore and the advice of mechanics who have no idea of how to troubleshoot the system. I've created a set of web pages on D-Jetronic to educate owners on how it works and how to fix problems with it, that's a good place to start. And resources like Rennlist or the Pelican Parts BBS have many experts who can help you through just about any problem. And if that's not good enough, list members can steer you to mechanics experienced with D-Jetronic in your area.
Once any kind of systemic problem has been fixed with the D-Jetronic system, regular maintenance is far simpler than carbs. The system automatically adjusts for altitude, air temperature, and engine temperature for all operating modes. There are only two external adjustments for maintenance - fuel pressure and idle mixture. Once these are set, they do not need to be checked or adjusted for at least 20,000 miles. In contrast, carbs need constant fiddling for changing temperatures and altitude.
Multi-throat carbs like Dellortos and Weber IDF's require careful synchronization and throttle adjustments that need to be checked every 5,000 miles or so. Single carb setups are so poorly matched to the engine, due to the long intake runner path, that adjustments cannot compensate for the basic problems with the system. and are a maintenance nightmare. Lastly, carbs require fairly frequent cleaning and secondary adjustments (e.g. float level, idle mixture, etc.).
Emissions
No debate here, Fuel Injection wins. Because of the principles they are based on, carbs have poorer control over the A/F mixture. If you try to run a carb lean for low emissions, you get wide performance variations because the slope of the performance/mixture curve is very high for lean mixtures. As a result, to prevent performance variations, carbs need to run quite rich, where variation in the curve is low (see the Probst Fuel Injection book for a nice diagram of this). The rich mixture is also needed to overcome the poor fuel atomization of carbs - Fuel Injection can run considerably leaner and produce the same torque and power, because more of the fuel is burned during combustion. Fuel Injection is also able to handle all of the various operation conditions without wasting fuel - such as cold and cold-cold start, overrun, acceleration, etc.
Another emission problem with carbs is that they enable the use of high performance cams with significant overlap. Valve overlap produces very high hydrocarbon emissions, even with your CO set as low as possible. To get cars to pass emission, the "solution" is to open the valve lash from 0.006 or 0.008 to as much as 0.030, to reduce the extent of overlap, then adjust back after you get home from the smog test. Add this one into the "ease of maintenance" section.
Conversion to carbs may also cause your car to immediately fail the visual portion of the smog inspection of your state. And lastly, for street cars, high emissions is simply bad for you, your community, and the environment.
Fuel Consumption
Fuel Injection wins again. Most of the reasons have been cited above. Fuel economy of well-tuned and mechanically-strong stock 1.7L and 2.0L 914 motors is really incredible. At 70 mph on flat roads with light winds, a 1.7L can get nearly 40 mpg! Not bad for a 30 year-old car. By contrast, when I had my 2.2L carbed motor with 40IDF Webers tuned to run the way I liked it, I got about 15 mpg, about the same as a 5000 lb SUV.
Cost
This is another motivating factor for carb conversions. Usually, the person who is considering the conversion has some problem with their Fuel Injection system that they can't fix, or that they know is going to require an expensive component to be replaced - MPS, ECU, injectors, etc. Or perhaps, the part that's failing is NLA and they're going to have to rebuild it themselves (e.g. the aux air regulator).
Nobody's going to deny that Fuel Injection components aren't cheap. But when you add up the cost of the conversion, you'll find that short of ripping out nearly every Fuel Injection component and replacing with new, a carb conversion will cost more. Twin Weber 40IDF's with all the trimmings for conversion will run you a minimum of $750. "Deals" on Ebay for $150 used carbs should be looked upon with skepticism. These used carbs are often trashed, they don't come with all the bits you need, and rebuilding can be extremely expensive - more than the cost of new carbs in some cases! You are almost always better off fixing the Fuel Injection system than tossing it.
Another factor is resale value of your car. While it varies, the prevailing wisdom is that if you're comparing essentially identical cars, one with Fuel Injection in good shape and one with carbs in good shape, that the carb car will go for at least $2000 less than the Fuel Injection car. In some areas, the difference is less, in others, more.
Conclusions
I'm sure those who are on the carb side of the fence will argue with some of the positions here, and I would really like to hear dissenting opinions. If they're based in fact and disprove what I'm saying here, I'll amend this page to reflect the changes. Let me know by Diese E-Mail-Adresse ist vor Spambots geschützt! Zur Anzeige muss JavaScript eingeschaltet sein!. But at the end of the day, for a stock or near-stock motor, Fuel Injection has such significant advantages over carbs that it's really not a contest. The issues covered here with the deficiencies of carbs is the reason that BOSCH and others developed modern Fuel Injection systems!
