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Tweaking hot control pressure for performance?

Started by raueda1, 10 November 2022, 09:34 PM

raueda1

Between summer and ski season I get sucked into various rabbit holes.  At the moment it's more thinking about CIS and WUR stuff.  Here's the question.  Control pressure, as set by the WUR, is always a range, say 3.4-3.8 bar.  Hence the midpoint is 3.8 bar,  Well, that range is about +/- 6% from the midpoint. 

Adding to the mystery is the High Altitude WUR (supposedly for over 4000 ft).  The range there is 3.6-4 bar or about 5.5% higher pressure.  So I guess that's the pressure that Bosch determined to get the right richness needed for altitude?  And the lower range with the altitude WUR overlaps the regular version.  So is the ideal to set the pressure based on your local range of altitudes?

I'm not an engineer but this all seems like a pretty big range to me.  If there's an ideal mixture under "normal" conditions it seems like this leaves a lot of range for over- or under-shooting the ideal.  And if so, what would be the effect on performance and economy?  Or maybe, given all the other variables, this range is "good enough?"  Or maybe you'd need to tweak it with an exhaust analyser? 

So, should we all be trying to hit the exact middle of the range?  Or, at the end of the day, is this all splitting hairs?  Or, if it were easier to set more precise pressures, would doing so have any real world benefits?  Obviously I'm tortured by this stuff.  Comments and conjectures welcome!  Cheers,
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

ramiro

I am fighting my control pressure right now with an afr gauge and i can say that 3,4 or 3,8 makes a difference.
With 3,2 - 3,4 i get an afr of around 12.8 to 13.2 at full throttle at 3,8 it is around 14 - 14.7.

The main problem for me is that it is not steady , after 50 km of driving i suddenly i have an afr of around 14 at full load, not 13.0 that i have for the first 20 km (the Engine was already warm before i started), but maybe that is a problem with my car.

So if you really want to be sure you can use an AFR gauge, but i would just set it to 3.4 and hope that it stays there and if not it will usually go more up(atleast for my wur) so setting it to 3.4 will give it some headroom.

raueda1

I don't follow. What is an afr gauge?

Ignore, just googled.
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

gurrier

1973 350SE

raueda1

Quote from: ramiro on 11 November 2022, 04:44 AMI am fighting my control pressure right now with an afr gauge and i can say that 3,4 or 3,8 makes a difference.
With 3,2 - 3,4 i get an afr of around 12.8 to 13.2 at full throttle at 3,8 it is around 14 - 14.7.

The main problem for me is that it is not steady , after 50 km of driving i suddenly i have an afr of around 14 at full load, not 13.0 that i have for the first 20 km (the Engine was already warm before i started), but maybe that is a problem with my car.

So if you really want to be sure you can use an AFR gauge, but i would just set it to 3.4 and hope that it stays there and if not it will usually go more up(atleast for my wur) so setting it to 3.4 will give it some headroom.
Quote from: ramiro on 11 November 2022, 04:44 AMI am fighting my control pressure right now with an afr gauge and i can say that 3,4 or 3,8 makes a difference.
With 3,2 - 3,4 i get an afr of around 12.8 to 13.2 at full throttle at 3,8 it is around 14 - 14.7.

The main problem for me is that it is not steady , after 50 km of driving i suddenly i have an afr of around 14 at full load, not 13.0 that i have for the first 20 km (the Engine was already warm before i started), but maybe that is a problem with my car.

So if you really want to be sure you can use an AFR gauge, but i would just set it to 3.4 and hope that it stays there and if not it will usually go more up(atleast for my wur) so setting it to 3.4 will give it some headroom.
Thanks much!  Are you saying that control pressure actually changes after driving 50km or just the AFR?  This has me wondering about the effect of full load enrichment. What should the AFR be under load?  Have you noted effect of full load vs steady state on afr?  In any case, it's not clear to me how the injection system would change abruptly even after engine is fully warm.  Seems like after warm-up it should be steady state apart from throttle position and altitude.  All very interesting.  Cheers,
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

raueda1

Quote from: gurrier on 11 November 2022, 10:08 AMAll is explained herein;-

Perfect!  Thanks much.  I guess I know what my next project will be.  If anybody has done this on a 6.9 any tips would be welcome.  Cheers,
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

ProfessorExperimental

Quote from: raueda1 on 11 November 2022, 12:56 PMWhat should the AFR be under load?  Have you noted effect of full load vs steady state on afr?  In any case, it's not clear to me how the injection system would change abruptly even after engine is fully warm.

Depends a bit on the engine but most piston engines like no leaner than 13:1 at full load (better to talk load than throttle position, this is more obvious when considering forced induction tho). I'd consider 14.7:1 at full load dangerous, as that's averaged across all cylinders and the whole combustion cycle, so the chance of detonation as the flame front reaches the hot exhaust valve is higher than the nominally stoichiometric reading would suggest. In any case torque will suffer.

There are many more factors than "warmed up" affecting burn completion (remember all a lambda sensor measures is the proportion of free oxygen in the exhaust) and that's why modern EFI engine management measures so much. Jetronic basically just measures airflow and that's it, with a low temp cutoff and a preset enrichment slope below that cutoff. If it were a programmable ecu it'd have a 3x3 table in its fuel map and that's it. To answer "why does it change" the question is "what changes". There are many things that could be different about the driven car vs the merely "warmed up" car: think under bonnet temperature, heat soak in things like ignition system and distributor, even vacuum lines that read differently when the brake booster is being used regularly or fuel pumps that deliver different pressure when the fuel in the tank starts sloshing around (there's a reason surge tanks exist!).

But also, what could cause the afr gauge itself to read differently after driving. Recall that the sensor measures free oxygen. It will give a richer reading if there is unburnt fuel or other hydrocarbons in the exhaust. Like oil smoke from blowby. Which will burn off the sensor once egt gets high enough.

So ramiro if you're getting the car warm with no load you might have valve stem seals allowing oil past  when the head is (comparitively) puddling or oil rings that are leaving enough film on the cylinders for the unloaded compression rings to allow more past than they will once the crankshaft is resisting and leaving more pressure to force them onto the cylinder wall (the oft noted "blue smoke at idle" that certain BMW's are known for, just not as obvious).

There's other things too, like what's the voltage being delivered to the sensor, and also the gauge. Heaps of cars have voltage sag under certain types of accessory and engine load, and conversely voltage boosts once the alternator spins above its self-excitation speed (around 2-3000 engine rpm in a lot of cars). My first car had a voltmeter in the dash and that sucker only showed 12v at idle, 14v at steady cruise in the middle of the rev range and bounced around to 16v most of the rest of the time. If you're running an external sniffer from a lighter socket that's the least regulated circuit on most cars and I doubt there's much of a "sensor supply" in our benzes anyway. This is why modern EFI uses injector latency maps to compensate for voltage changes affecting the time it takes for injectors to open too, btw. Get it wrong and you'll never have stable afrs (I learned that the hard way).

And another thing. Heated or unheated lambda sensor?
-Mark

ramiro

The changing AFR with a warm engine was my own mistake , i tried adjusting the hot control pressure by adjusting the heigt of the rod that holds the bimetall strip, offcourse that makes the hot control pressure unstable and that leads to a leaner engine when the engine bay is heatsoaked.

Now adjusted the warmup regulator according to the manuals i found online by adjusting the heigt of the part where the lines connect to and now it seems to be stable at 3.4 , but i didn't do a testrive yet because our roads ar salty.