Over the past few months I have tuning my -10 engine’s SDSEFI supplied electronic fuel injection / electronic ignition system to get the best possible performance for my needs.

This document details I how approached tuning the engine, an approach that may be applicable to others.

My -10 has an Aerosport Power IO-540 with 9:1 pistons, Barret Precision Engines cold air induction, Showplanes cowl, Vetterman custom exhaust, dual SDS ECU EFI and EI. Added to that was all the engine bling I could find – billet aluminum valve covers and induction tube flanges (both with O-ring seals). I expect that the engine is makes something north of 300 HP.

While this document is written from the perspective of my particular configuration, I expect that the concepts would apply to other SDS EFI/EI systems. This document assumes you have read, understood and complied with all SDSEFI installation documentation. That being said, use this at your own risk.

This document also assumes that you engine has been broken in in accordance with engine manufacturer instructions.

Post Break-In

This document starts post engine break-in as all my “tuning” occurred after the engine break-in per manufacturer’s instructions.

When changing ECU maps / data, be sure that the data is entered into both ECUs if you have a dual ECU system. The only data that differs between the two ECUs are the magnet positions and possibly the cylinder trim values. Initially, the trim values should be set to 0.

When changing ECU values in flight take great care that the correct values are entered. Review how data is entered on the ground before attempting this in flight. It is recommended that during flight all ECU changes be made by a non-flying person in the right seat. If care is taken and ECU changes are small, ECU changes can be made in flight.

In my case I decided that only the spark advance and mixture adjustments on the LOP switch and cylinder fuel trim values would be made in flight. Changes to the ECU map were made on the ground.

Data Collection

Most EFIS options that have been on the market for the past 10 years allow capture of engine data on a real time basis. In my case the Garmin G3X collects all relevant engine data.  I found post flight analysis of the G3X engine data to be most interesting. Although available, I didn’t use the SDS data capture option as the G3X data was sufficient for my purposes.

Step 1 – Define tuning objective(s)

 My objectives were to

  1. Tune engine so I could run Lean of Peak (LOP) whilst avoiding the “Red Box”. Mike Busch’s leaning video has lots of great info on this: https://www.youtube.com/watch?v=X-tKyiUZ3ts
  2. Tune engine so running LOP gave reasonable fuel savings whilst limiting TAS loss due to power reduction
  3. Tune the engine so “mixture fiddling” was kept to a minimum
  4. Tune engine so that Rich of Peak (ROP) was not too rich

I have no intention of running LOP at high power settings or lower altitudes. For me, I view running LOP more of a higher altitude / cruise performance option. Consequently I can avoid the “Red Box” simply by running LOP at lower power settings. In this environment, there is no “Red Box”.

Step 2 – Trim cylinder fuel flows so all cylinders peaked at same time

This was a very simple process. I performed this at WOT at 10,500’. I selected this altitude as it seemed representative of where I would typically cruise.

Initially I slowly leaned the engine allowing the EGTs to stabilize after each *slight* turn of the mixture knob. Once one cylinder peaked a waited 15-20 seconds to see if any more went peak. I then continued to lean, noting the fuel flows, as each cylinder went lean.

Once I identified the too lean cylinders, I adjusted the cylinder trim values by +/-1% as required.

In my case I found that one cylinder went lean sooner than the rest. I added +1% fuel trim for this cylinder.

I then went ROP and reperformed the leaning exercise. The leanest cylinder was still too lean so I adjusted the cylinder trim values by an additional  1%.

When I repeated the leaning exercise again I found that two other cylinders were peaking a little late. In this case I reduced the fuel trim for these cylinders by -1%.

After a couple more iterations of going rich and then leaning to peak EGT I was able to get all cylinders peaking with .2 GPH

Step 3 – Validate Cylinder Trim Settings art various altitudes

To validate the fuel trim settings were acceptable, I next leaned the engine to peak EGT at 10, 12, 14, 16, 18 and 20 thousand feet. I noted the fuel flow spread between when the first cylinder to peak and the last. In each case the range was acceptable and LOP operation was smooth.

Step 4 – Adjust spark advance when LOP

The default spark advance for my engine was set to 24 degrees before top dead center (BTDC). Other engines may have different default values – be sure to set in accordance with engine manufacturer recommendations.  When running lean of peak, it is advantageous to advance the spark somewhat more as the leaner fuel charge burns slower in the cylinder. A greater advance allows more fuel to burn generating more power at the optimal crank angle.

Best economy occurs 30-50 degrees LOP. However the fuel savings will result in a noticeable power loss / decrease in TAS.

With the SDS EFI/EI system, the LOP switch can be used to add an additional spark advance and/or mixture reduction when turned on. Initially, I left the mixture adjustment set to zero and only used the switch to add additional spark advance.

At 16,000 ft the engine was manually leaned to about 100 degrees LOP. Then various spark advance values were tested using the LOP. It was found that advancing the spark by an additional 6 degrees increased TAS by 2-3 kts. Parenthetically, EGT values dropped as expected as more of the fuel was burned before the exhaust valve opened. Increasing the spark advance beyond 6 degrees yielded no discernable benefit.

Step 5 – Find how far LOP to go.

Leaving the spark advance set to 6 degrees, the mixture was adjusted to see the TAS result / fuel savings at various LOP points. Once I found a “sweet spot” that worked for me, I noted the mixture adjustment value (% shown on the SDS programmer display).  

Step 6 – Adjust ROP fuel settings

The final in-flight test was to see how Rich of Peak then engine was running when the mixture knob was in a neutral position. In flight I found that my engine was running considerably richer than I desired and so I had to manually lean the engine. I wanted to “tune out” the need for this manual adjustment.

Inflight, I noted the mixture adjustment (% shown on the SDS programmer display) required to lean the engine to a setting between peak and best power. In my case this was a 10% reduction.

Step 7 – Adjust maps in ECU

The data collected above was used to adjust the ECU maps while on the ground.

When changing ECU maps / data, be sure that the data is entered into both ECUs. The maps must be identical.

In my case I reduced the fuel factor settings in the ECU map by 10% for all RPM settings at or below 2500 RPM.   

When I did my LOP testing I found that an 19% decrease in fuel would cause me to be 30 degrees LOP. This was the “sweet spot” I wanted.  As I had changed my base map in the ECU by -10% I set the LOP switch to further reduce my fuel flow by an additional 9%.

Then end result was that when I flip the LOP switch, my spark advances by an additional 5 degrees and my fuel flow to the engine is reduced by 9%. This is about a 1 GPH savings in cruise.

There is a very interesting interaction between the spark advance and the fuel flow. Normally when the engine goes LOP there is a sudden reduction in fuel and a corresponding reduction in power. This in turn causes a noticeable deceleration of the aircraft. However, I found that when I go LOP by only 30 degrees or so, the spark advance seems to compensate for what would otherwise be a power reduction from reduced fuel flow to the engine. Consequently, my TAS remains relatively constant and there is no discernable deceleration. Ultimately I see a 2-4 TKAS speed reduction with a nice drop in fuel burn. How great is that!

All that being said, if I went 100 degrees LOP, I would see a much larger fuel savings and would see a drop in TAS. While that may be more cost effective, I want to go fast(er).

Step 8 – Adjust maps for take-off power

Typically in normally aspirated engines full rich is used when take-off power is applied. The extra fuel cools the engine. Without the extra fuel CHT temperatures would rapidly climb.

During my break-in, I manually adjusted the mixture, usually adding an additional 25% of fuel when taking off or climbing. Once I reached cruise I would manually reduce the mixture to remove this extra fuel.

Post break-in, I have eliminated the need to manually increase the mixture on take-off by changing the fuel factor in the ECU map by +25% for 2600 and 2700 RPM. As I only use these RPM settings for take-off & climbs, the engine receives the extra fuel required whenever these RPM settings are selected.

So as not to run the engine too lean with a high power setting, I make a point of never using 2500 – 2550 RPM and typically cruise at 2400 RPM or less.

Additional Notes

While the tuning I have done has minimized my need to manually tweak my mixture knob, I do make a point of keeping my CHT temps below 390 and prefer to keep them below 380. Therefore I will adjust the mixture, as required, if these limits come into play.

It remains to be seen if the ECU maps will need additional tweaking when hopefully warmer summer weather arrives.