E50 to E85 and everything in between

[Spamby]

Hello everyone. New here and this is my first post. Be gentle .

I am getting tuned on 9-8, at Cobb/Plano, for corn. I have been doing some more research and stumbled on this scientific paper discussing ethanol content of various blended fuels in respect to knock, etc.

My question is: Is there any validity to this in regards to, say, tuning for E70 and then filling with E77, etc? Is the scare of knock, due to ethanol blend changes, on our cars being overly hyped?

Basically I only have E70 available to me year 'round and thus this is what I will be tuned with. But what if I get some higher ethanol later on down the road without knowing?

Thanks for all of your help!

http://delphi.com/pdf/techpapers/2010-01-0619.pdf

 

[HolyCrapItsFast]

This is a very good question Spamby!

I can only speak from my experience using and tuning E85 and other blends of ethanol and here are a few observations I have made over the years...

First it is true that ethanol offers a greater resistance to knock but more over when it does knock, it does not do it at the same rate as gasoline and it may not even register as knock because the event happens to quickly. Also because of the combusted volume that ethanol makes over gasoline, the more damaging a knock event will be because of the increase in inherent cylinder pressure over that of gasoline. This is a problem because you could be tuning beyond the threshold and not even know you are causing damage because you may not even register knock. This is why I tend to tune using the same timing I would use for gasoline and then play upon ethanol's resistance for knock by being able to increase boost while using the same timing. The key is to focus on the correct AFR and exhaust temperature. Which kinda plays into the answer to your question.

(Now keep in mind that the results listed in the paper are for the appropriate AFR for each of the individual mixtures. You will see where I am going...)

The difference from E70 to E85, with regards to knock, is not as grand as people lead it to be IMO. Here is why I think this... Lets say you tune your car to E70 today. Your AFR's will be appropriate for that mixture. Now you go ahead and fill it up one day with E85. You will now be running leaner because your mixture contains more ethanol now and you are tuned for a fuel having more gasoline. For closed loop operation, where the ECU is compensating for the difference and correcting the AFR, this is of no concern. But for open loop you may not be rich enough and there will be no compensation. But here is where the contradiction comes in. Because of it's resistance to knock the added ethanol over gasoline will suppress any knock that other wise would have been if it were just gasoline and regardless of the fact it is leaner. A similar analogy can be said for the opposite. Lets say you are tuned for E85 and you put in E70. Your AFR's are richer because of the increase in gasoline over ethanol and any knock is being suppressed as a result of this richness even though the mixture has a lesser resistance to knock.

Now if you are going from extreme differences in mixture (say E50 or E100) then it is imperative that you have a tune specific to the mixture being used. If you really want to be safe you should have different tunes for each E70 and E85. One strategy I have used with great success is I will tune somewhere in the middle. This is especially helpful in the northern regions of the country where you might see any blend from E70 to E85 depending on the season. If you tune in the middle the differences from one to the other become more inconsequential.

Again these are only my observations and I have no real empirical data to back it up but this is what I'm seeing.

 

[Spamby]

So, basically it's better to be safe than sorry. AFR, as you stated, should not vary too much because in open loop your ECU is not compensating as it is in closed loop. Basically running on one set parameter? An increase in ethanol content will change the stoich level and if the parameters in open loop are set at one level, the ECU will not compensate for the change therefore, detonation could occur? It is becoming all more clear now. As clear as ethanol. lol
As I understand it, E70 to say E85 may not be that large of a jump to warrant high concern over engine damage but nevertheless does warrant a note of caution.

I don't believe I will need multiple ethanol maps as it seems my gas stations are hell bent on only serving E70 and I don't use the STi as a road trip vehicle. At the very least, I can splash blend, with petrol, to a higher content of ethanol to match my 70% tune if they ever increase the percentage. The converse would require more ethanol, by volume, and thus be more difficult to achieve.

Thanks for the confidence! Can't wait to see how she feels over the crummy OTS stage 2 map; both the 91 map and ethanol map should be a considerable difference.

 

[HolyCrapItsFast]

So, basically it's better to be safe than sorry. AFR, as you stated, should not vary too much because in open loop your ECU is not compensating as it is in closed loop. Basically running on one set parameter? An increase in ethanol content will change the stoich level and if the parameters in open loop are set at one level, the ECU will not compensate for the change therefore, detonation could occur? It is becoming all more clear now. As clear as ethanol. lol
As I understand it, E70 to say E85 may not be that large of a jump to warrant high concern over engine damage but nevertheless does warrant a note of caution.

I don't believe I will need multiple ethanol maps as it seems my gas stations are hell bent on only serving E70 and I don't use the STi as a road trip vehicle. At the very least, I can splash blend, with petrol, to a higher content of ethanol to match my 70% tune if they ever increase the percentage. The converse would require more ethanol, by volume, and thus be more difficult to achieve.

Thanks for the confidence! Can't wait to see how she feels over the crummy OTS stage 2 map; both the 91 map and ethanol map should be a considerable difference.

AFR "will" vary in Open Loop according to the stoich level of the mix because there is no compensation. However from E70 to E85 the difference is about 6-10% as observed. If your desired AFR is 11:1 it will actually be about 11.6:1-12.1:1, Which is still very reasonable and should not cause knock if the timing is properly defined. Additionally the resistance of the extra ethanol acts as a buffer and will likely suppress any knock seen due to leaning out. IMO this will only be a concern if the tuner defines his timing to the ragged edge. Cobb of Plano does very safe work so I don't think you should have any issues. If you have any concerns you should speak to them in detail about it.

Also if the tuner sets up the cal properly, the ecu will take care of any knock that is seen way before it becomes damaging. Again I feel Cobb is very knowledgeable in this area and I think you are in good hands..

 

[Spamby]

Excellent. I really appreciate your knowledge and patience. You have soothed any worries I might have had and broadened my think tank a bit. Yes, Cobb definitely has a good reputation. This is why I have chosen to use them. Plus, they seem to be one of very few within four hours of me that tune Subarus. oklahoma does not have a huge market for Subaru tuning... though there are more subies than one might think, here.

Thanks again!

 

[HolyCrapItsFast]

My pleasure. I enjoy having conversations like this :tup:

 

[damascius]

If you tune as for E85, there is a timing table the tuner uses. The higher the ethanol percentage automatically increases the timing accordingly. Flex fuel is the way to go. You can?t have too high of an ethanol percentage. The higher the better.