Air Fuel Ratio

[jeffc280sl]

I have some pretty accurate before and after acceleration statistics I'd like to share.  The acceleration tests took place on the same stretch of road almost 3 weeks apart.  The test involved a full load acceleration in second gear from less than 2000 rpms to over 5000 rpms.  I decided to examine the time period which elapsed between  2500 to 5000 rpms.   I'ved converted the rpm numbers to mph.

Here are the results:

Nov 6      25 to 50 mph     5.16 seconds

Nov 25    25 to 50 mph      4.09 seconds

The improvement in acceleration is quite noticable as is the tone of the engine and exhaust.  In my case getting the afr mixture correct made for a big improvement in performance.

[jeffc280sl]

With my recent experience tuning I thought I'd take a crack at summarizing the purpose of internal adjustments of the fuel injection pump.  Please offer comments if something I've stated is incorrect or unclear.  It is a very difficult subject to write conclusively about.  It's maybe best to read Joe's fuel injection pump tour before trying to read my explanation of the internal adjustments.

I came to the realization concerning the roles of the three separate screws: they are anything but independent of one another. In other words, it is altogether too easy to over-adjust one set of screws, and overwhelm the adjustment oelsewhere. In general the white screws adjust high rpm partial load fuel delivery. Black screws adjust low to mid range partial load fuel delivery. The main rack adjusts fuel volume at all rpm's.   The white, black and idle screws set the preload on the governor flyweights by adjusting three springs of different weights and lengths. The longest and lightest spring being the idle, and the shortest and heaviest the high speed. Adjusting the fuel injection pump is a balancing act with a constant need to compromise.

From their CW stop the white and black screws can turn nine CCW turns before the screw heads are flush with the aft edges of the flat springs that provide the "detent" action. Therefore the white and black screws have an effective range of 54 clicks.

The further the flyweights are allowed to open, the leaner it will run. "Left is lean" on that externally adjustable center idle screw. Left backs it out, and allows the flyweights to open further and pull the space cam further back. It would stand to reason that holds true for the mid and high speed screws as well.  Space "cam" is actually a bit of a misnomer; it does not spin as one would expect a cam to spin. It looks nothing like a cam, either. More like a small metal hockey puck. It rotates about its axis for about 90 degrees or so when pulled up and down by an arm connected to the throttle lever. It moves fore and aft on its axis in response to the opening of the flyweights which is influenced by the three spring settings.  

[JimVillers]

Jeff ..... Excellent.  I have payed a lot with a wide band gauge but mine is a very basic system (read cheaper) without the logging and other inputs.  Your target numbers are also close to mine even though I have not been to adjust the carburetors on my 190SL as well as you have been able to tune your injection system.

We are currently working on a 230SL in my garage; the engine is out being rebuilt. We have a rebuilt injection pump ready to install and have tested the injectors (we found one bad injector that probably led to a lean cylinder engine failure).   I'll follow your tuning ideas when we get it running again.  My concern is that the fuel injection volume might need to be adjusted for an ethanol blended fuel.

[jeffc280sl]

To accomplish the MB road test you're going to need a manifold air pressure input of some sort.  One idea is to run a vacuum line from the intake manifold into the cockpit and tape a vacuum gauge up near the tach.  In that way you could see both inputs and when they meet the MB spec take your afr.  Another idea is to use a modern map sensor.  The sensor takes a vacuum input and outputs voltage.  As the vacuum changes the voltage changes from 0 to 5 volts.  With a multi meter again taped near the tach you can watch voltage and rpm.  When the voltage reading is about 2.4 volts you have met the MB 300mmHg load spec.  If you need the exact voltage reading for 300 mmHg/ 11.78inHg let me know and I'll get it for you.

[jeffc280sl]

I think I'm almost finsihed with my air fuel ratio experiments.  For $22.00 I added an accelerometer input to my LM-2 afr meter.  With this device I can measure acceleration and use a set of formulas included with the data loging software to estimate rear wheel horsepower and torque.  It uses data from the rpm and acceleration channels along with other data that you input.  Here is what the formula uses.

Veh wgt                                3000 lbs
Tire dia                                 24.7 inch
Gear ratio of run                     2.23
Final drive                              3.27
Rpm channel                          RPM
Acc channel                           accel
Frontal area                           25
Drag                                    .45
Atmosp                                 29.92
Temp                                   50



I performed the test in second gear under full accelration from 2000 to 6000 rpms.  At 5400 rpms my calculated rear wheel horsepower peaked at 129.3 hp.  Torque measured 114 ft lbs.  

Torque peaked earlier in the rpm range at 135 ftlbs.

How accurate is the calculation?

SAE Net- In 1972, American manufacturers phased in SAE net horsepower. This is the standard on which current American ratings are based. This rating is measured at the flywheel, on an engine dyno, but the engine is tested with all accessories installed, including a full exhaust system, all pumps, the alternator, the starter, and emissions controls.

SAE Gross-This is the old process that American manufacturers used as a guide for rating their cars. It was in place until 1971. SAE gross also measures horsepower at the flywheel, but with no accessories to bog it down. This is the bare engine with nothing but the absolute essentials attached to it; little more than a carb, fuel pump, oil pump, and water pump. Because the test equipment on the engine is not the same as in SAE net, it is impossible to provide a mathematical calculation between SAE net and SAE gross. As a general rule, however, SAE net tends to be approximately 80% of the value of SAE gross.

As a very general rule, rear-wheel horsepower on a manual-transmission car is about 15% less than SAE net, and rear-wheel horsepower on an automatic-transmission car is about 20% less than SAE net.

So

MB advertised Gross HP for the 280SL       195
Less for SAE net accessories         .80%
Less for power train            .85%

Adjusted rear wheel hp            132

The Innovate formula set rw hp at 129.3 for my car.

[awolff280sl]

Jeff, what you have done, to adapt modern technology to tune and spec our cars, is too cool. Given that no permanent modifications are needed, for a relatively modest investment one can avoid the vagueries of CO measurement/spark plug color, etc., and the hassle of a dynamometer(!?), and get accurate measurements. Hats off!

[jeffc280sl]

Thanks.  It was a fun and rewarding learning experience.  Setting up the equipment is one part of the effort.  Depending how far you're off can lead to a simple or in  my case very involved tuning effort.  But is is nice to be working with accurate data versus spark plug color charts.  I thought my plugs were perfect but the afr was off.