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Vintage Mercedes-Benz Fuel Injection 1954-1971
By slmarket | August 21, 2007
Source: SL Market Letter #223
Author: John Olson
THE FIRST CARS SOLD to the public with fuel injection were Mercedes-Benz 260D (diesels) in 1936. Gasoline applications were more difficult, though M-B's Racing Department was planning its introduction for the 1940 Grand Prix racing season. The outbreak of WWII. circumvented M-B's 1940 participation. Within three years the technology surfaced in both German and U.S. combat aircraft.
The idea of precisely metered gas/air ratios at each cylinder was attractive, as carburetors were infamous for uneven distribution. A big reason for multiple carburetors (six of them on some V-12 Ferraris) was not so much for volume of output (though it helped) as to bring the delivery systems closer to their assigned cylinders. For cars Bosch had to smooth out low R.P.M. fuel delivery (not so critical in airplanes where 2/3 throttle or more was the norm). Automatic adjustments for altitude, humidity & temperature changes were a bonus inherited from planes.
Germany's Goliath and Gutbrod minicars were the first to sell the Bosch units on gas engines, (2 cylinder, 2 stroke) in 1951, though public interest in two stoke engines was on the wane. Both cars were short lived. The M-B racing department encouraged its development over supercharging as the power boost was from more precise fuel distribution not just more air & fuel. The ideal air/gas ratio, 13.7:1, was produced without wasting fuel or the opposite risk, too much air, which can quickly burn valves. Fast race cars seldom exceeded 3 to 4 M.P.G. & fuel injection double these figures! In street form, production 300SL Gullwings frequently hit 20 M.P.G., while Ferrari of the same period give 10 M.P.G. Throttle response was also faster, as injectors were in or closer to each cylinder than possible with carburetors.Variations were tested, including a combination of fuel injection and supercharging (which M-B reincarnated for the 1996 SLK). One factory prototype 190SL was built with fuel injection, but the cost/benefit ratio was not justified given the inherent breathing limitations of that engine.
No less than 57 changes occurred during 17 years. Three big ones were:
- moving the injection nozzles into the intake portion of the cylinder head (some had tentacles protruding way into the head from the intake manifold).
- switching from vacuum to mechanically activated fuel metering.
- reducing plungers to 2, 1 for each 3 cylinders. This may have been inspired by GM's Rochester fuel injection of 1957 that only had one plunger for eight cylinders. Lance Reventlow's Scarabs won many races with this simple system, but when it clogged from any foreign particles the whole motor stumbled, rather than just one (or three) cylinders. Mercedes did not make this change on all models, and returned (retreated?) to a plunger for each cylinder in 1963 & 64. The 230/250/280SL always had six plunger mechanic pumps, and the 6.3 V-8s through 1972 retained eight plunger mechanical pumps. Both used manifold mounted injectors.
Experiments with electronic fuel injection did not satisfy M-B engineers enough for it to reach production until the 3.5 V8's. This required new tuning and rebuild equipment & knowledge. By 1975 catalytic convertors added more demands. In 1976 a mechanically driven continuous injection system (CIS) was introduced. These models will be discussed at another time.
CARE AND FEEDING OF MECHANICAL FUEL INJECTION PUMPS
Considering what they do (see shaded area on page 5) Bosch pumps are extremely reliable. We asked our favorite authorized rebuilder, Robert Fairchield (Jerry Fairchield Industries, Redding, CA 916-241-1592) what advice he had for owners:FAIRCHIELD: "These mechanical pumps are vastly superior to carburetors. 200,000 miles without repairs is not unusual, & when pumps are rebuilt they often require little more than a good cleaning and new "0" rings. If they get proper lubrication wear is minimal."
SLML: What can Mercedes-Benz owners do to prolong the life of their pumps?
FAIRCHIELD: "Monitoring the correct level of oil is important. There are two portions of the pumps that receive lubrication… the lower portion has its own fillable oil reservoir. All except the 280SL have a dipstick that can be checked. Too much oil is as bad as too little. Lack of lubrication is obvious too much inhibits proper advancement of the flywheels that are in the lower portion… and this allows the pump to run too rich. Overfilling on 220, 230 & 250 engines can be caused by careless servicing, or by leaky "0" rings from the plunger chambers up above. The upper parts receive lubrication separately. Leaky "0" rings will cause flooding of the lower pump chamber (in 280SLs overflow drains back into the engine). In addition to poor M.P.G. running too rich washes oil off cylinder walls (maximizing wear) and diluting oil (destroying main bearings). Maximum permissible oil dilution is 5 percent."
SLML: What advice do you have for people that store their favorite cars in the winter or longer?
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