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Ignition Timing for Mogas VS 100LL

R

Randy

Well Known Member
I am about ready to fly again after installing everything new firewall forward. My new engine is IO375 w low, 8:1 compression and I am running dual Pmags along with the EI Commander.

The EI Commander gives me the opportunity to have different ignition timing curves that are selectable.

I would like to have one timing curve for 100LL, which I will certainly use throughout engine break in, and another curve for running auto fuel which I may choose to use as I have alcohol free auto fuel available locally:).

So my question for anyone that knows:

How many degrees of ignition timing difference should there be between use of 100LL and 91 octane pure auto fuel?

I know the auto fuel will burn faster and thus will not need as much timing advance, but how much less?

Randall in Sedona
RV7A
 
Randy,

After your engine is broken in, you can fill one tank with 100LL and one with auto fuel. Take off on 100LL and switch to the auto fuel. Once you have cruised on that for five minutes to make sure you have used all the 100LL in the fuel lines you can use the EIC to adjust the timing in small increments while monitoring your CHT's. Do some high power, full rich climbs, from low altitude to get the worst case scenario. Stop making adjustments, when you are comfortable with what your EMS is telling you.

I also recommend turning off one ignition and make the timing changes to the other. That way, if there is an issue, you can simply turn on the unchanged ignition and turn off the one you have been messing with.

Once you have a timing configuration you like, you can save it and send it to both ignitions.

Feel free to call me, if you have any questions.

Also, don't forget to send the proper engine timing to your P-mags before your first engine start. With the EIC installed, the P-mags default to the B curve and you need to send them the correct timing configuration before your first start.
 
Thanks Bill,

I guess to use this method I have to assume that higher CHTs are a good indication of when the timing is advanced far enough? Most people seem to experience higher CHTs with the PMag advance curves anyway so I am wondering high much higher in CHT temps is OK?

Sure wish the GAMI folks or someone would do some research on 4 cyl Lycomings on well instrumented Dyno. Having the data of when peak pressure occurs on auto fuel versus 100 LL would be great.

I may just give you a call, I appreciate the offer.

Randall in Sedona
 
High CHT's depend on many things. They could be the result of bad/leaking baffles, too much advance in your timing, bad fuel, etc.

Make sure you understand what "high" CHT's mean for you engine before playing with your timing.

Start with the timing that is recommended for your engine. If that is 25 degrees, then you want to send the P-mags the "A" curve. If it is 20 degrees, then you want to make a custom configuration with a negative offset to get the timing down to that number. (The "B" curve starts around 30* BTC and this is the default, "no jumper" configuration.)

For a new engine or a newly overhauled engine, I typically recommend a custom configuration that starts at the timing recommended for the engine and caps the advance at that same value. Then after 10 to 15 hours, change the advance to match what P-mag typically does but leave the initial timing at what is recommend for your engine. Then when you get up to around 50 hours and your CHT's are under control, start playing with the timing to get something that you are comfortable with.
 
Sure wish the GAMI folks or someone would do some research on 4 cyl Lycomings on well instrumented Dyno. Having the data of when peak pressure occurs on auto fuel versus 100 LL would be great.
Click to expand...

Why on earth would they do that? :roll eyes: That being said they do run a really good demo at the beginning of every APS class in ADA OK, running 100LL from the FBO, premium mogas from down the street and G100UL.

You could watch this for yourself, and learn a heap. If you are willing to invest in your education that is.

PS it will be a TCM engine but that matters not. The fuel does not know which engine it is in.
 
N941WR said:
Randy,

After your engine is broken in, you can fill one tank with 100LL and one with auto fuel. Take off on 100LL and switch to the auto fuel. Once you have cruised on that for five minutes to make sure you have used all the 100LL in the fuel lines you can use the EIC to adjust the timing in small increments while monitoring your CHT's. Do some high power, full rich climbs, from low altitude to get the worst case scenario. Stop making adjustments, when you are comfortable with what your EMS is telling you.

I also recommend turning off one ignition and make the timing changes to the other. That way, if there is an issue, you can simply turn on the unchanged ignition and turn off the one you have been messing with.

Once you have a timing configuration you like, you can save it and send it to both ignitions.

Feel free to call me, if you have any questions.

Also, don't forget to send the proper engine timing to your P-mags before your first engine start. With the EIC installed, the P-mags default to the B curve and you need to send them the correct timing configuration before your first start.
Click to expand...

Hello Bill, Thank you for your explanations. Can you tell me if the EI commander is necessary to burn 100LL and auto fuel ? Is it necessary to change the setting of the P mags each time I change of the fuel ?
Sylvain
 
Sylvainsting said:
Hello Bill, Thank you for your explanations. Can you tell me if the EI commander is necessary to burn 100LL and auto fuel ? Is it necessary to change the setting of the P mags each time I change of the fuel ?
Sylvain
Click to expand...

Sylvain,

I cannot answer this with any authority because I have only run 100LL in my RV. However, I don't believe the auto fuel STC's for certified airplanes require a change to their mag timing when switching back and forth between auto fuel and 100LL. Hopefully someone with experience using auto fuel in a certified airplane will speak up.

To be clear, the EICommander is not required for any safe P-mag operation, the same as we don't need an Electronic Engine Monitor (EMS) to keep track of our engines. However, it is essentially an EMS for your ignition system that also lets you adjust the ignition to match your engine, fuel, and/or flight conditions.
 
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