In this mod­ern world, ev­ery­one wants flex­i­bil­ity — in the way we work; the way we dress; and, most im­por­tant, in the cir­cles within which we nav­i­gate, the fuel we choose to power our dyno-killers.

Can you re­ally have your cake and eat it, too — high-oc­tane ethanol on the week­ends for rac­ing, then tra­di­tional pe­tro­leum-based 98-oc­tane fuel for the week­day shenani­gans? That is pre­cisely what a flex-fuel sys­tem al­lows, and it al­lows it without the need to run two sep­a­rate fuel tanks. It’s a com­mon fea­ture on many Amer­i­can and Aus­tralian OEM ma­chines, as ethanol (E85) is avail­able from the pump at the petrol sta­tion in those coun­tries — some­thing that’s also start­ing to hap­pen here. It’s be­cause of this that retrofitting such sys­tems to per­for­mance cars is a grow­ing trend here in New Zealand.

The key com­po­nent to a flex-fuel sys­tem is the sen­sor that is plumbed into the fuel line. It de­tects the ethanol con­tent in the fuel by test­ing the con­duc­tiv­ity of the fuel as it passes through. Ethanol is a more ef­fi­cient con­duc­tor than petrol, so the higher the con­duc­tiv­ity, the higher the read­ing. This in­for­ma­tion is then sent to the ECU, as with any other dig­i­tal in­put, so that it can ad­just the fuel trim ta­ble and re­tard the tim­ing; the lower the ethanol per­cent­age it reads, the less fuel and tim­ing.

In OEM ap­pli­ca­tions, this is ob­vi­ously all taken care of at the fac­tory. How­ever, in an af­ter­mar­ket ECU, it can mean a very long, and there­fore ex­pen­sive, stay on the dyno, as each and ev­ery per­cent­age would need to be con­fig­ured — un­less it is a high-end ECU that has a self-tun­ing func­tion. Some com­mon stand-alone ECUs don’t even have a spe­cific in­put for flex fuel, so it needs to be done via a dig­i­tal in­put, such as the fuel temp.

Run­ning a higher oc­tane al­lows for more tim­ing, more boost, and more com­pres­sion, but this is where the risk comes in. If your E85 tune is run­ning on that ragged edge and the con­tent drops, so does the oc­tane rat­ing, and that could spell disas­ter of the en­gine-blown kind

What we wanted to test was both the va­lid­ity of switch­ing fu­els in the sys­tem on the fly and the ac­cu­racy of do­ing so. Could you sim­ply dump your 98 on Fri­day night, fill up with E85, and go rac­ing for the week­end? To show­case this, we would first set up a bench test in­side the en­gine lab at Jtune. Jacky had set up a jury-rig con­sist­ing of a Zeitronix ethanol­con­tent anal­yser (ECA) with gauge, a Wal­bro in-tank pump, and 10mm lines. We would later real-world test with an Evo II on the dyno.

Our test day be­gins by fill­ing up two fuel cans at our lo­cal E85 pump — some­thing those in the greater Auck­land re­gion are spoilt in hav­ing. Us­ing our jury-rigged Zeitronix ECA with gauge, we set about test­ing the E85’s ac­cu­racy. This will typ­i­cally vary a small per­cent­age ei­ther side of 85 per cent, but this was a good batch, hit­ting its mark of 85-per-cent ethanol dis­played on the gauge. We then drop the same rig into a tub of 98 oc­tane, af­ter drain­ing it as well as pos­si­ble. Jacky tells us that the gauge would typ­i­cally read be­tween three and four per cent in 98 oc­tane, but we are sit­ting much higher. It takes us four rinses with fresh 98 to achieve the three–four per cent read­ing. Now, imag­ine that in your av­er­age full-size fuel sys­tem — that’s a se­ri­ous amount of residue. Even if you emp­tied the tank via a drain bung, achiev­ing ei­ther pure 98 or pure E85 would take a se­ri­ous amount of flush­ing and lots of wasted fuel.

To fur­ther push the point, we switch the rig back to the E85 tub and mix in 98 in 10ml in­cre­ments un­til we are read­ing E60. It takes only

The sen­sor is usu­ally mounted on the re­turn line to avoid re­stric­tion, as the sen­sor in­let is only -8

the small­est amount of petrol to bring the num­ber down dra­mat­i­cally. That’s when things get re­ally in­ter­est­ing — or, should we say, con­fus­ing. Jacky be­gins ad­ding wa­ter to the blended fuel, and, as more wa­ter is added, the gauge be­gins read­ing higher and higher num­bers. So, we ef­fec­tively end up with a low-oc­tane mix­ture that falsely tells the ECU we have 105- to 107-oc­tane ethanol in the lines. This oc­curs due to wa­ter’s con­duc­tiv­ity, and the fact that an ethanol sen­sor is a hy­gro­scopic sen­sor test­ing con­duc­tiv­ity. Con­fused? Yip, fair enough.

So, if you can’t rely on the sen­sor to read the true ethanol con­tent of the fuel, how can you rely on it for tun­ing pur­poses?

“Let­ting the ECU ad­just the tune en­tirely based on an ECA sen­sor when run­ning flex fuel is very risky. A good wide­band oxy­gen sen­sor should be used, and set up with closed-loop feed­back, as well as a knock sen­sor to de­tect any knock. I al­ways rec­om­mend run­ning two wide­band sen­sors in case of fail­ure and giv­ing out in­cor­rect in­put to the ECU for ST tun­ing [short-term fuel trim],” ex­plains Jacky.

Ethanol ab­sorbs wa­ter, so care must be taken when stor­ing the fuel, as the more wa­ter it ab­sorbs, the lower the oc­tane of the fuel, while the sen­sor will tell the op­po­site story. Petrol, on the other hand, does not ab­sorb wa­ter. A good in­di­ca­tion that your E85 has ab­sorbed wa­ter is if it has turned cloudy

Our fi­nal test is a real-world ex­am­ple of switch­ing a car from 98 to E85, as the owner wants to have tunes for both fu­els. With the fuel tank on empty, we fill the Evo II with 40 litres of pre-tested ethanol. When we fire the Evo up, the gauge shows E15; then, as it sits idling and cir­cu­lat­ing the fuel, the fig­ure rises and the idle be­comes rough. Af­ter a quick fuel-map ad­just­ment, it is left to idle for 10 min­utes, achiev­ing a best read­ing of E70.

So, you can see that a sim­ple swap-over, even with an empty tank, is never go­ing to achieve a true E85 mix. To achieve that, the fuel would need to be dumped, the tank and surge tank drained, the fil­ters and pumps dried out, and all the lines and rails flushed with E85. A big ex­er­cise no doubt, and some­thing to be con­sid­ered by all the week­end war­riors out there swap­ping fu­els on the reg­u­lar without an ECA set-up. If your tune is on the ragged edge, it could spell disas­ter as that oc­tane rat­ing fluc­tu­ates.

Even this DC2 race car, which had an all-new sys­tem, fresh E85 from the pump, and run time on the dyno, is not yet able to reach E85

Typ­i­cally, E85 re­quires 30 per cent more vol­ume to pro­duce the same en­ergy as 98 oc­tane, as it con­tains 30 per cent less en­ergy per litre. How­ever, the added us­age is far out­weighed by the ben­e­fits, such as the higher oc­tane rat­ing of 105–107 for pump...

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