How Critical are Control Pressures?
 

Those of us w/ CIS cars have all pored over the control pressure charts and the threads about WUR adjustment. Obviously the Bosch techs put forth a lot of effort to get this right.

One of the adjustments easily made is to the mixture. Since 1 complete rev of the screw yields a range of somewhere between 8 & 12%, and there are quite a few revs inherent in each control unit, there is obviously a lot of room to maneuver.

So, let's say your chart shows a range of from 1.5 to 2.0 bar cold to 3 - 3.5 warm cps and you want to set your mix to 3.5%. Say your readings show you are @ 1 bar cold and 2.5 bar warm, which would be .5 bar low across the board, or 2.5 bar cold & 4 bar warm, which would always be .5 bar high.

In either case, wouldn't there be more than enough adjustment available in the mixture control unit to compensate for this?

BTW I realize this is hypothetical. If someone has a too high ccp, which seems most typical w/ these cars, and an OK or too low wcp, that is another scenario.

I'd like to hear people's thoughts on this.

Hello Paul,

The mixture screw does not have the same effect on high speed mixture as it does on idle mixture when VE is lowest. The adjustment has a big effect at idle and a much smaller effect at high speed under load, which is all that's important outside emission testing. When you set the idle mixture at 3.5%, it does not mean that the high speed mixture is 3.5%. The high speed mixture is determined more by the mechanical design on the FD, fuel pressure and the injectors. When the FD reaches full flow, no amount of idle screw twiddling will increase the amount of fuel. Idle mixture is largely irrelevent when you are setting up the fuel curve for an engine. You need to measure the high speed mixture under load and adjust it. CIS is like most carburetors, the mfg decided to let amateurs play with idle mixture, but the high speed mixture is set by fixed jets for a good reason.

Hey Paul,

They do have similar affects, but they are different in that the WUR's effect is transient and dependent on temperature. I'm sure you knew that, but in order to achieve the same effect with the mixture control, you'd have to sit in the engine compartment and continuously lean it out as the engine warmed up. Perhaps you could hire a trained monkey to do this while you're driving. Hell fitting him under the engine lid though... :)

ianc

Agree with what's been said, and I like the idea for mammalian fuel injection. It brings a new meaning to CMA -- "Check, Monkey, Adjust."

You could also carry spare bananas in the tailpipe to keep the Bosch engineer happy. ;)

Brian

hey paul,
i have wondered the same thing, and like you, i have had the same reasoning. but now as i think about this some more, i may change my ideas.
if the CP is low, then the plate will rise higher for a given rpm. if it is too high, say at idle (even though the mixture has been compensated for) the plate will be in the regin of the "bowl" desigened for mid rpm range which means less plate movement for an increase in rpm's. that may give you poor throttle response off idle. so how critical, i dont know if 3.5 bar to 3.6 bar would make a difference. just another way to look at it i guess.

as for higher rpm's, i think the bosch book says it is a linear motion. for insatnce, if the plate is open 1 inch the plunger is say, 10mm (just for reference). if the plate moves up to 2 inches then the plunger moves up to 20mm and so on. so if at 1 inch (idle), you adjust the mixture screw so the plunger is at 11mm, then at 2 inches it would be at 21mm.