'73 450 Fuel Injection Sequence

[GaryF]

Hi All,

A question out of curiosity, the wiring diagram for my 73 450SLC shows the Fuel Injectors are wired in pairs meaning the injectors are operated two at a time. The cylinders are not synchronised and each operate in sequence as I understand it so why is fuel being injected at the same time into two cylinders at the same time or am I missing something ? Do two cylinders operate together on opossite sides of the block ?

Thanks in advance for any comments,
Gary

[Greg in Oz]

Each cylinder fires individually in sequence as you would expect. There is no need for the fuel injection to be timed precisely so the injectors are operated in pairs for cylinders that fire consecutively. This way the injection ECU needed only four outputs to operate the eight injectors. To demonstrate how unimportant the timing of the injectors is, the D-Jetronic system you referred to was replaced by K-Jetronic (CIS) continuous injection from 1976. In this system all injectors continuously spray fuel rather than operating in sequence. The sprayed fuel simply formed a mist that was drawn into the inlet port with the air when the inlet valve opened for the inlet stroke for that cylinder. Consider also that engines fitted with carburettors had the fuel mixed with air well before reaching the inlet port. Again in those situations the fuel was suspended in the air as a mist awaiting the opening of the inlet valve. The CIS injection system allowed for similar power and fuel efficiency to the earlier sequencial system. Only recent recent sequential systems with more precise electronic controls have been able to improve on these early injection system systems.

[GaryF]

Thanks Greg, that makes sense. It's interesting that Mercedes dropped electronic (well electical anyway) injection in favour of mechanical and most manufacturers are now back to electronic. I appreciate that the controlling systems are significantly more advanced now, do you know why they didnt stick with electonic at the time though ?
I find it amazing comparing against other cars on the road in '73, I know Mercedes cost significantly more but the same gap in technology between midrange cars and top-end cars is far smaller now.
I'll stop rambling now !

[psfred]

The D-jet is dirty -- you have to run it richer than optimum to prevent it going over lean at highway speed and burning the valve up.

The basic circuitry was used up until recently on all Bosch EFI systems, they've only gone digital in the last ten years or so.

As emissions requirements got tighter, Bosch discovered that the K and KE-Jet systems were eaiser and cheaper to make pass the emissions testing, and are much less trouble in operation. Not that the D-Jet is a problem, mine still runs perfectly (except for bad trigger points) after 33 years!

Peter

[Tomguy]

Indeed, the D-Jet was really only taken off due to the difficulty of getting it to pass emissions tests. If you look, you can see that when Benz switched from D-Jet to K-Jet, the engines lost ~40 HP (also a large contributor was the cooler cams, but the injection system switch cost a good deal). The D-Jet has a MUCH more involved feedback system which can make more adjustments on the fly than the K-Jet. One of the reasons the D-Jet was bad in emissions is that, under load, the system would put a LOT more fuel into the engine than the K-Jet to pump up the power to the maximum possible level, which caused much more CO release naturally. Also, on full throttle, the D-Jet enriches the system even further, and I believe K-Jet does not - good for at least 10-20 horses!

[karl]

In response to the original question:

Yes, the cylinders are paired.
The trigger points fire the injecters this way:

1 + 5
2 + 7
3 + 6
4 + 8

The ignition firing order is 15486372, just like early Ford V8's.
The cylinders are 1 to 4 on the pass side, 5 to 8 on the drivers side.

[j9fd3s]

Originally Posted by psfred

The D-jet is dirty -- you have to run it richer than optimum to prevent it going over lean at highway speed and burning the valve up.

The basic circuitry was used up until recently on all Bosch EFI systems, they've only gone digital in the last ten years or so.

As emissions requirements got tighter, Bosch discovered that the K and KE-Jet systems were eaiser and cheaper to make pass the emissions testing, and are much less trouble in operation. Not that the D-Jet is a problem, mine still runs perfectly (except for bad trigger points) after 33 years!

Peter

thats a programming issue maybe?

we've done sequential vs non sequential injection on the rotaries (never played with an aftermarket ecu on a piston), and i makes a huge difference mostly in smoothness.

peter, i just put new trigger points in mine, and it didnt fix the problem, you want my old ones?

mike

[psfred]

Mike:

Yes, I would be delighted to have your old trigger points -- mine are ancient and the rubbing blocks are badly worn, with the result that two (or four) cylinders don't get fuel until the dizzy gets warm. Makes cold running somewhat leisurly!

D-Jet doesn't measure airflow, it only measures manifold pressure. The result is inaccurate mixture control and higher emissions. Bosch determined that constant flow gave eaiser control (no trigger points, for instance), no electroncis, and better emissions. My 75 Audi Fox didn't have a cat, for instance, due to the better mixture. When air flow meters became more reliable, it was cheaper to use LH Jetronic (along with the necessity to shut off individual cylinders).

The K-Jet is a derivative of turbine engine control systems -- it was easy to meter just a portion of the airflow through a venturi while controlling the whole engine's fuel flow. Much cheaper.

Peter

[j9fd3s]

hmm maybe its the nature of the djet, but theres several modern cars that use manifold pressure (hondas), and they seem to do fine.

as an aside the trigger points will run an aftermarket ecu just fine....

mike

[Tomguy]

Originally Posted by psfred

My 75 Audi Fox didn't have a cat, for instance, due to the better mixture.

That's another good point - DJet ran rich, very rich at times (Full throttle!), and would literally MELT a catalytic converter.

Mike: As far as I am aware, ALL modern EFI cars have a manifold pressure sensor, but they also have oxygen sensors and/or air flow meters to properly tune the mixture.

[j9fd3s]

lots of cars use airflow meters.

i thought about this, and it might be a sensor resolution thing. when we put aftermarket ecu's on the rx7's and use a "3 bar" map (manifold absolute pressure sensor, like the djet, but solid state), there isnt enough sensor resolution to get everything nice. its a 0-5 volt sensor, that reads from -15psi to +30psi, and so you really dont have much resolution in the -10 to 0 range that you spend most of your time driving in.

another thing is that load varies with the gear, given the same map, the same car will be rich in 1st gear and lean in 5th. most modern oem ecu's have a speed input so they know what gear you're.

also they have an oxygen sensor, which will compensate for things like small hills and whatnot, that vary the load but not really the speed or the throttle input.

can anyone tell i've just about sold myself an ecu?

[GaryF]

Having a software/electronics background with mechanics as a hobby makes this an interesting area for me.
I think most modern cars now use an air mass sensor rather than an air flow meter and this is able to compensate for driving conditions at differenent altitudes. It also doesnt introduce as much air resistance as an air flow meter as there is no moving flap just a hot wire or film being cooled by the air moving across it.
Given that these old mercs have the basics in place to control the fuel going in then a couple of extra sensors and a bit of work on getting the ECU mapping right should reduce fuel consumption/emissions ?
Are the 30+ year old injectors as good as modern ones ? Can they deliver the fuel as accurately ?
A couple of knock sensors on the block, a lambda probe, an air temperature sensor and an air mass sensor should be all the additional inputs covered for an ECU ?
Are there any companies that do these as kits or is the marketplace just too small ?

[Tomguy]

I've thought about grabbing the entire ECU off of a modern V8 (Like a Chrysler/Jeep 4.7L) or even a modern V8 Benz next time I go to the junkyard, just for the sake of testing! I just worry that it might be an expensive test if it doesnt work

[psfred]

Man, you DO like to tinker, eh?

That ECU on the D-Jet is all discrete components (can you say "dumb servo mechanism?) so I guess you could monkey something in, but it's gonna be a real PITA. Probably end up with something completely new.

Best upgrade is to get the heads and manifold off a post '76 and convert to K-Jet with feedback (the stuff from the 420 or 500 will fit perfectly).

There is already an intake air temp sensor, but the MAP is an induction type sensor -- I'm not an electronics expert, but messing about with that particular circuit could be rather tricky. It does NOT generate a voltage reference signal directly unless I'm very seriously mistaken. It works by moving an iron core back and forth in pair of coils.

Adding a mixture control feedback circuit with an oxygen sensor would fix most of the problems not directly related to pressure change lag.

Peter

[Tomguy]

Indeed Peter, you're right about the MAP - I took mine apart so I could make sure I could hammer a torx bit into the stripped-out allen socket without damaging it. There was, if I recall correctly, 2 sets of coils. One stopped at one point where another would keep going. There is metal spring somewhat like a bladder between the outer housing and inner (right by where the mix screw is - changing the mixture screw increases or decreases the pressure on this springlike bladder). The bladder's hole goes through the mix screw (so it's IMPORTANT that this hole/socket is clean) to get the outside pressure. The higher the pressure is outside, the more it pushes the coil inward and the more gas it gives the engine (Changing the screw is simply adjusting the air pressure curve).

When I say the entire system, I mean injectors, fuel rails, sensors (anything that feeds into the ECU on that vehicle) and the ECU itself from that vehicle. I'm looking to see if replacing the DJet with a more modern equivalent of the same thing would reduce gas consumption and yet provide the proper power when needed, instead of being rich at 50MPH in 3rd gear and "lean" (just right, I'd say) at 80.

Edit: While I'm at it, I'd use that vehicle's ignition system setup as well, in order to advance the timing as far as possible and have it back off or make the mixture richer when it detects knocking.