Ferrari 400 Bosch K injection basic principles

Discussion in '365 GT4 2+2/400/412' started by laperriere, Apr 4, 2019.

  1. laperriere

    laperriere Karting

    Mar 10, 2006
    Dear 400i owners,
    I bought a 79 400i in my 20’s and I have owned this car for the last 30 years. Lately I decided it was time to understand the way the Bosch K injection works, at least the general principle if not the details. The injection looks complex with all these hoses but after all it should not be rocket science.
    Following a forum member advice, I bought this book : Bosch Fuel Injection & Engine Management by Charles Probst.

    My initial belief was : the fuel pressure is assigned the unique role to feed the injectors with fuel.
    This was all wrong.
    I must say it took me at least two or three readings of the book’s chapter 5 to get it. The author knows what he’s talking about but he attempts to describe several flavours of the K : K-basics (mine), K-lambda, KE … at the same time and immediately digs into the details, whereas I think it would have been adequate to emphasize this simple statement on page 1 :
    On a K injection as fitted on our 400’s cars, fuel pressure is being assigned three distinct roles.

    1 – It feeds the injectors with pressurized fuel (this fuel pressure is constant).

    2 – Fuel pressure is also used to stabilize the airflow sensor.
    Indeed, there is a counterweight at the other side of the airflow sensor so it moves effortless. This is nice but the move could be too much effortless and too quick which would lead to variations in revs and power, making the car difficult to drive. To prevent this, there is a need for some form of energy used to stabilize the airflow sensor : this energy is supplied by fuel pressure.

    3 – Richness control
    The quantity of fuel can be adjusted through a variation in fuel pressure, this is the third role played by fuel pressure.
    Physically, despite all this fuel will eventually be burnt, there is a separate pressurized fuel circuit for 1 from the MCU to injectors and another pressurized fuel circuit for 2 and 3.

    Another component I was unable to describe was the warm-up regulator.
    I propose that description, correct me if I’m wrong.

    The warm-up regulators (one on each bank) are located at the front top of the engine, close to the cam covers.
    The book’s author says this name ‘warm-up regulator’ is obsolete, in the Bosch world this would be now ‘control-pressure regulator’.
    Anyway, the way the wup works is asymetric :
    - Cold : the thermo-time switch (a temp sensor) fitted on the block tells the wup the engine is cold : the WUP manages to increase fuel quantity which enriches the mixture.
    - Hot : this is not the thermos-time switch which tells the wup the engine is hot and would ask to stop enriching. In fact, the wup does not wait until the engine is hot : it takes 10 mns to the block to get hot whereas the wup only enriches the mixture for the next 40 seconds.
    So, there is inside the wup an electrical winding (this is the reason why there is this electrical connection on top of the wup) and a bimetal arm.
    As soon as the wup starts enriching, it takes 40 seconds to the bimetal arm to bend down. Once bended, it stops enrichment.

    The wup manages to adjust the fuel quantity : the increase (and sunsequent decrease back to normal) is being achieved within the MCU body. The MCUs are where the airflow sensors are located, in front to the windscreen, one on each side. The wup acts remotely thanks to fuel pressure variation. There is a hose to the wup with incoming fuel under pressure. There are two hoses for outgoing fuel : back to the MCU or back to the fuel tank. When the engine is cold, the wup redirects more fuel to the tank which in turn decreases the fuel pressure for the fuel going back to the MCU : this fuel pressure variation leads to the richness being increased. Vice versa.

    Last thing that was a bit confusing to me is that the cold start components are also used for … hot start. That’s why a hot start issue may involve a cold start component.

    Again, correct me if I’m wrong, I didn’t plan to be pedantic or a Mr know it all taking credit of Mr Probst’s excellent work, my purpose was only to share my beotian’s learning experience of the 400’s K-injection basic principles.
    Now I start to address again a hot start issue on my car.
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  3. Highmiler

    Highmiler Formula Junior

    Dec 8, 2010
    Full Name:
    One thing that book does not emphasize is that EACH pump requires 30 amps to deliver the required pressure. All this power runs through the circuit board under the floor. By pass this board by running appropriate sized wire from the battery to the pumps and turn the new wires on and off with the existing Ferrari wiring. This by pass cures the shut down of one or both cylinder banks and keeps a lot of heat out of that board.
  4. Al Campbell

    Al Campbell Formula Junior
    Silver Subscribed Owner

    Nov 22, 2013
    That looks like a good summary Olivier.

    Greg I also found out this week that running on one bank may not be lack of fuel.
    I have a rusty 400i awaiting restoration that I start up regularly.
    This week when started it ran rough & didn't idle so I shut it down after about 15 seconds.
    Feeling the heads & the right bank was cold indicating it only ran on the left bank.
    I immediately thought fuel pump problems but then noticed unburnt fuel dripping from the right hand exhaust where it leaks.
    Turns out the metering piston of the right hand fuel distributor had stuck in the open throttle position & flooded all cylinders on the right bank when starting.
    Luckily it was a fairly easy fix & is running fine again now after leaving it for a day for the unburnt fuel to evaporate before attempting to restart it. :)

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  5. raemin

    raemin Formula Junior

    Jan 16, 2007
    Lyon (FR)
    Full Name:
    R. Emin
    Our Wur do also have a vacuum port that allows to enrich mixture when we hit the throttle. Not all wurs do have it. That's the type of WUR the porsche/audi guys are tuning (see Brian Leask).

    It is also important to understant that the lower the control pressure, the richer tge mixture.

    See my comment regarding UTCIS tip for aggressive camshafts on the columbo displacement discussion: lower control pressure allows the lever and disk to move even if the airflow is so-so (due to the camshafts design). As you said it will be less stable though, so presumably more poluting.
  6. spicedriver

    spicedriver F1 Rookie

    Feb 1, 2011
    You want a rich mixture when the motor is cold, and lean it out as it warms up. When the bimetallic strip bends, it allows a spring to put pressure on a diaphragm in the fuel cell section of the WUR. This increases control pressure. You probably have 20 - 30 PSI cold control pressure, and 45-55 PSI warm control pressure.

    Interestingly, these motors are setup for a low emission air/fuel ratio 14.7-1. An optimum performance ratio is closer to 12 -1. Some performance tweaking is possible.
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  8. raemin

    raemin Formula Junior

    Jan 16, 2007
    Lyon (FR)
    Full Name:
    R. Emin
    I think the lean spot is at mid throttle (based on my transparent spark plugs). The issue is the set screw enriches the mixture on the whole range, so in order to have normal mid-throttle, the engine has to be slightly rich at idle and high throttle. Definitively works better like this, but the car smells like a carbed 400, which defeats the interest of the injection somehow. Mine was back to close to normal for this reason. I'd rather have the WUR modified so as to enrich the mixture under high load.This would require a bextter expert (with more tools) than me as I cannot say for sure how much the WUR adds to the mixture under load: my transparent spark plug cannot be used under load. Professional modification of the WUR is roughly $1k for the two regulators...

    All in all adjusting for slightly rich at idle is the way to go for me, but your mileage may vary.
  9. SouthJersey400i

    SouthJersey400i Formula 3

    Mar 14, 2007
    Romulus, NY (Finger Lakes)
    Full Name:
    Ken Battle
    High Miller, as you know I rewired my fuel pumps several years ago with a #10 or 12 wire and high amp fuse back to place under rear seats and used two relays to feed the pumps. The running load of each pump is less than 15 Amps, they run with 15 amp fuses in original installation. The starting current of the pump however exceeds 15 amps for a short time on startup which causes the contacts to burn on the circuit board in original installation. That is part of the reason the pumps don't run until air flow is detected; they would compete with the starter motor for amps!

    Oliver, the pressure to the injectors must vary by load; it cannot be constant. I believe the pressure and therefor the flow to each injector is varied by the main control unit based on the air flow disc movement. One BASIC fact about Bosch K system is that flow to the injectors is continuous intake valve open or not (unlike modern pizeo injectors).

    If anyone is running on one bank (or none) and feel that fuel pump(s) is not running it is easy to check. With engine not running, remove the connector from the left side injection system and turn the key on; both pumps should be running and they are load enough to hear or feel to be sure both are running. Cold exhaust pipe when running will tell you which side is at fault. During my first two years of ownership I spent a lot of time on side of road sorting this out. Now fixed for good.
  10. Part Time

    Part Time Formula Junior
    Silver Subscribed

    Dec 16, 2013
    Port St. Lucie, Fl
    Full Name:
    Gary Shore
    Laperriere…..the only item I can add is that after production started on the 400i, a "cross/balance" tube was added from one WUR across to the other in an effort to equalize or average the working fuel pressure on both sides of the engine. The tube is one physical part connecting both sides of the FI system. Early cars did not have this evolution, however it can easily be added. Look at any engine photo of a later engine ….

    Here is my 1985, an early photo, (some parts have changed since then).


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  11. raemin

    raemin Formula Junior

    Jan 16, 2007
    Lyon (FR)
    Full Name:
    R. Emin
    I do have this cross-tube, on my 1982 400i, but I do not see it in 55421 (see thread "Very Last 400i Built, For Sale in CA").
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