Flexfuel on MS2PNP - yay or nay?
 

So after reading just about every thread I could possibly find on the topic, I still feel like my question has not been answered. I am currently running an E85 only tune on my MS2PNP. I would like to be able to run 91 if need be, so I purchased a flex-fuel sensor. Megamanual says regarding Timing Addition setting, "In these cases, numbers entered indicated advance added, so the default of -13.0 for 100% ethanol represents 13 degrees of retard." Flex Fuel with MegaSquirt

However, in another thread someone said, "The MSPNP2 will multiply the pulse width by the Fuel Multiplier % number determined by the measured fuel compensation, and subtract the Timing Correction value. Since the Timing Correction is subtracted, negative numbers will advance the timing." (https://forum.miata.net/vb/showthread.php?t=578286)

My question then is which numbers do I want to put in place for adjusting timing with e85? Negative or positive? Does anyone have real world experience? I don't want to put - numbers and retard when I want to advance. My gut is to go with what Megamanual says but would like comments from those who have tuned on MS2 using this method. Thanks

E85 gives you the ability to run more timing without det. Use that information as you see fit.

the short answer is you're gonna have a very hard time running flex fuel on ms2
it will be very rough and nothing like ms3

With e85 you can advance timing more.

I run e85 on an MS2. I have on more than one occasion put 93 in it (we have that here) when I was low on fuel and couldn't find e85. I had no problems with it, but two caveats: 1) I'm naturally aspirated and 2) I never went over about 2/3 throttle 'til I got my E back up.

I am also running an MS2. I have one map for E85, and another for 93 octane. You may end up doing what I have to do, which is running the tank done most of the way, then switching to the other fuel and map at the same time. I have wondered about adding a FlexFuel sensor as well, but from the input we have here so far it sounds like that is not a great solution when running the MS2.

@zippysport so if I understand you run e85 without a flex fuel sensor on ms2? I have ms2 as well and I'd like to fully convert to e85 full time since it's readily available here in the corn capital Iowa. Do you have anything that monitors the content of E in your tank? This whole time I thought I. Needed a sensor but do I really for full time e?

@Hernj65 In theory, you don't need a sensor at all. All the MS2 can do is vary fueling in a very crude way based on E content. In practice, it's a bit less clear as what we call E85 can legally contain anywhere from 50% to 84% ethanol. Now...if you tune for 84% and run 50%...you'll be running rich. If you tune for 50% and run 84%, you'll be lean. How much does that matter? ¯\_(ツ)_/¯

If you are naturally aspirated, go for it. I don't think it really matters, just know that you might see some odd AFR readings. To minimize that, try to buy from the same chain of stations over time. If I were forced induction, I'd probably be a bit more worried, but I honestly don't know enough to know.

I'm naturally aspirated for now, and I think E makes a pretty obvious difference.

The other thing to keep in mind is that adding an E sensor, whether now or later, is actually (famous last words) pretty easy--just plumb it inline of the fuel, get it 12v switched and grounded and run one wire to the MS. Mine is screwed to the passenger side of the firewall. I think I spent more time trying to find the switch in MS to activate it than I did to install it.

@YourEconProf hey Thanks for the info. I'm running about 13psi of boost currently so I would not want to take any gamble and run lean up top. It does look simple to wire in the sensor. I'm going to ask a noob question but would an ms2 automatically adjust fueling and ignition timing based the e content or would I need separate tunes depending on the e content in my tank? I'm just trying to figure out if I can switch to gas station e85 (which varies in content) with a sensor and would I have to adjust based on content.

TL;DR: You won't need different tunes based upon E content. Timing doesn't need to change. Fuel should only change somewhat, and MS2 should be able to handle that.

Longer Reply:
MS2 doesn't adjust timing based on E content. That's the advantage of going to MS3 where you can blend tables. <EDIT: okay, OP points out that MS2 can change timing. sorry 'bout that>

All the MS2 does is give you a table that basically allows you to add/cut fuel by x% as a function of the sensor reading...imagine the Warm Up Enrichment table, but instead of temp on the x axis, you have E content.

The good news is that (from my understanding) timing won't vary between 50% and 85%--50% is enough to get MBT. The fueling is the difference. My other guess is that once you get the map dialed, the fueling adjustment based on E levels won't happen often or be really large, but personally, if I've put the time & $$$ into adding boost, I'd want the sensor just to make sure.

@YourEconProf I appreciate the explanation and I have learned more from your explanation than I have trying to decipher info on different threads and forums. I will definitely be getting a sensor before I go e85. I think it'd pair nice with a hotside supercharger setup that I have. I'd like to do e85 and maybe some meth for cooling the charger in warm days.

I'd suggest trying the E85 alone before adding meth--E85 also cools the charge much better than gasoline. Meth or water injection of course would be better...but E85 might get you more than you think.

Oh yes i have been back and forth on if i would even need the meth and i dont plan everything at once. The way i was thinking was using meth primarily to try and keep the actaul charger temp down a little since it gets very hot. Ive heard various things however about decreasing rotor life when exposing them to meth. For now i think e85 along with my fmic will be good.

FWIW E85 is awful for fuel systems. Very corrosive as it absorbs water.
Can really make a mess of a car if it sits a lot.

I specifically seek out E0 premium gas at Shell stations here (over the other guys who have E15 premium).

Yes, I'm running E85 without a flex fuel sensor. I currently don't have anything that monitors the content of E85 in the tank. It might be nice to have some idea of what that is, as it can change coming from different sources or in different seasons.

If you think ethanol is bad for engines, wait until you learn about forced induction.