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Reaching MBT at less than 85% ethanol

Flex Fuel Tuning

Discussion and questions related to the course Flex Fuel Tuning


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Hi there,

Recently had a facebook conversation with an individual that claims that most any road car will achieve MBT with 70% ethanol content. Care to chime in with some thoughts?

Amir Nazary You will hit mbt on e70. On pretty much every car on the road.

Jacques Dorey What about a 10:1 compression turbo Honda K series running 20+ lbs of boost?

Amir Nazary I'm running 10:1 vr38 with 45psi of boost

Jacques Dorey So are you saying that E85 is more ethanol than you need and it could be blended down to E70 and still make the same amount of power?

Amir Nazary Jacques Dorey there are a few things at play here. Firstly, the effective AKI of e70 is only marginally lower than e100. So you really aren't doing much in terms of knock resistance going with higher ethanol. You do get more volume of fuel with higher ethanol but not enough to make any appreciable difference in anything other than slightly quicker turbo spool (although I haven't seen this empirically)

The second thing at play is that you simply can't add any more timing to make any more power if you are already at mbt.

So yes, I'm saying e70 is all you need. Just make sure your tune reflects that. More than e70 is unnecessary but a proper flex fuel sensor and. Tune will make it so you don't ever have to worry about it.

Jacques Dorey But I could effectively make more power (turn up the boost a little) by mixing E85 with 94 octane for E70 if I were injector limited, right? More gasoline (30% instead of only 15%) means slightly less fuel volume is required. I do have a flex fuel setup.

Amir Nazary Hell yes. If you are injector limited, then definitely lower your ethanol content!! You will be able to run more boost if you have more boost left on the table.

About 7 years ago (damn, was it that long ago?), right before governments cut subsidies and research budgets for Ethanol, I had the opportunity to attend a presentation from a researcher who was studying E85 for the Sloan Automotive Laboratory at MIT. The work was actually split into two different SAE papers. What the guy did was use a GM LNF engine (Pontiac Solstice turbo, Cobalt SS) to try and understand

1. Chemical anti-knock resistance properties of E85

2. Charge cooling anti-knock properties of E85

Needless to say, his methods were rather involved. It involved a lot of combustion analysis on an engine dyno (looking at knocking cylinder pressures) and a lot of computer modeling. To summarize a lot of work, he found the following "effective octane number" based only on the chemical properties of ethanol.The attached graph is for a 97RON base (premium fuel) mixed with ethanol, where pump E85 is going to have crappier gasoline mixed in. Basically the chemical octane effect of Ethanol levels off at E70. In other parts of the research he demonstrated that on direct injection, the cooling benefits are amplified vs port injection.

So these numbers are going to be a little higher than real-world fuel, but it does demonstrate that traditional octane test underrates the anti-knock properties of alcohol fuels. Those tests are actually very old. During the presentation a representative from a major oil company we've all heard of was present. He basically said (off the record) that it's too much trouble to actually change around the official octane tests.The researcher also found that, surprisingly enough, Ethanol itself has negligible charge cooling improvement over gasoline unless used in direct injection. In short: there is good reason to believe that pure ethanol (versus a little bit lesser blend) doesn't actually cool your intake temps down enough to matter.

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So please correct me if I’m wrong, but you seem to agree with what the individual is saying as it applies to port injection? So if I do a proper flex fuel tune, tuning for E85 and 94 R/M octane and then extrapolate, I will achieve the same results when running E70 as I would achieve on E85? And I could in fact optimize the tune around E70 and possibly achieve slightly better results at that level of ethanol content (in my own injector limited scenario)?

E70 and E85 are only slightly different. Think of it like E0 and E10. So yes I agree.