RS5 and E85 official thread

I’m just loaded with a bunch of other stuff at the moment but I am looking hard at using E85 in the RS5. I’m going to deposit information here as I find it and maybe others can contribute and I can update the top thread.

First and foremost, understand that each and every car sold in the U.S. after a certain date has to be ethanol compatible. That means the internal components can withstand the corrosive nature of ethanol. This does not mean they can use E85 without any modifications. Far from it.

E85’s actual energy content, per gallon is about 66% less than a comparable amount of gasoline. That means you need 33% more E85 to meet stoich. Stoich on E85 is about 9.8:1 where gas is 14.7:1. What this means is your fuel system needs to flow far more fuel to meet the needs of the engine at wide open throttle.

So while your seals, hoses, and everything else can withstand ethanol, it doesn’t mean it can keep up with the demand for fuel at 8300rpm.

Where’s the weak link? We have the low pressure in-tank pump assembly, the high pressure fuel pumps (HPFP) which sit in the engine bay above the valve covers and the injectors themselves. At this point I don’t know if the injectors or the HPFP can keep up but based on reading from the S4 section, the low pressure in-tank pump is definitely a weak link.

Ok but I don’t own an S4 so how does this apply? Turns out the A4/S4/S5/RS5 in-tank low pressure fuel pump is the same part number, 8K0919051AS. That’s for the OEM RS5 pump.

Audi does make flex-fuel vehicles, the A4 being one of them. Shocker, the in-tank pump unit is exactly the same across various flex-fuel models and will work on a number of non-flex fuel vehicles including the RS5. The flex-fuel in-tank pump is 8K0919051AN.

The “AN” denotes flex fuel.

So what’s the difference between the two pumps? The “AN” flex fuel version flows more volume. Very important.

The HPFP’s and the injectors are still an unknown. The HPFP’s are only shared by the RS5 and the A8 although there’s another HPFP that bolts right in. I can’t remember if they’re from the RS6 V10 or the W12 A8.

The fuel injectors are the same ones used on the R8 V10, S6, S7 and a few other Audis. Unclear if the RS7 injectors will fit, plug in or even flow more.

So where do you start? First, figure out if you even have E85 stations near where you live. I’m fortunate in that I have one within a few miles of my home. Once you’ve located one or two stations which carry E85, you’ll need to get a sample and test it. This is a really simple thing to do. Just go on Amazon and get an E85 test vile which is nothing more than a plastic tube with measuring lines. Fill the vile up with the appropriate amount of water and ethanol based on said lines, shake, and within a minute, you’ll know how much alcohol it contains.

Here’s a link:

But wait, E85 is supposed to be 85% alcohol, no? E85 has summer and winter blends just like regular gas. Typically, the ethanol content in summer is higher, usually E70 or greater. It gets lower in winter so there’s less benefit in running it during the chilly part of the year.

Why run it at all? Well, if it’s 85% ethanol, a full E85, the octane rating is 108. See where I’m going with this? In addition, the exhaust gas temperatures are lower with ethanol than “gas” as it burns cooler and contains less BTU per unit measurement than pump gas.

Knowing the ethanol content can vary, we’ll need to monitor it on a regular basis and ensure the ethanol content meets the requirements of the ECU tune. Yes, you can take that vial purchased off of Amazon and test it each and every time at the station but that’s a bit tedious. Instead, there are a number of ways to monitor the content in-tank with an ethanol flex-fuel sensor.

There are inexpensive ways to monitor ethanol content and really expensive ways. The more convenient it is, the more expensive it tends to be. I’ll most likely go with something from which allows for bluetooth monitoring over your smart phone. You can source many of the parts, like the actual Continental sensor, separately and build up most of the kit for less and add the Bluetooth module.

There are hobbyist-built kits which have small LCD readouts for mounting somewhere in the cockpit. They tend to be much less expensive. Here’s one such kit which is functional with the Fuel-it kit;

If you have a P3 gauge, there’s a way to send the ethanol reading to the gauge;

Once the monitoring system is chosen, we’ll need to splice into the factory lines to install the sensor. More to come as I get to this point…

A few things I would suggest you REALLY Think about here.

E85 compatible cars and built motors have all of the things you listed but there are several more things to take into consideration.

If you look into the platforms that use E85 but are not built to specifically handle E85 you will see they tend to have issues with injector seals after a while due to them drying out. Increased injector failure from what some claim is the higher likely hood for E85 to contain water than standard petrol.

Also blackstome oil reports show 2x as much fuel in the oil over standard cars. In some cases it’s even higher.

You are correct in the fact that the A4 pump is similar and the A4 pump handles more flow but also remember the A4 pump flows more to handle the higher needed percentage of fuel BUT the A4 pump is doing that for a 4 cylinder making about 1/2 the hp with less rpm than something like the RS5 is…

What point am I making. Trying to run straight E85 on non E85 compatible cars in every platform not just Audi has ended up with issues for everyone if run long enough.

Also keep in mind the failure rate of the Audi FSI OEM injectors as it is. The FSI injectors have a pretty high failure replacement rate and it almost looks like the injectors have a life expectancy and should be considered to be replaced after a set amount of miles.

I think if the RS5 was a turbo car or a SC car then sure it makes more sense to run full E85. What I would suggest is running a mixed blend. Its more sustainable and HIGHTLY reduces the negative impact.

Also remember our NA 4.2 motors only need so much octane. While sure my motor is the older gen 4.2 after a while neither our gen or the RS4 gen 4.2 saw any benefit from more octane past a certain point. And that point was about 98 octane… Also the energy release of E85 is reduced per volume over standard petrol. So while your picking up some gain with E85 a lot of it is in its cooling properties after a certain octane point.

Here is some good reading on this. Now keep in mind if this is the wrong content for this thread. I will be happy to remove it. Just thought I would add to help the conversation.

There is some good data in this thread.

Yeah, good points and I’m definitely aware of most of the shortcomings. I did consider the potential injector failure issues and I just replaced all of my injectors at 62K miles. The plan would be to run Techron every few tanks and also switch back to “normal” gas in the winter months when there wouldn’t be as much benefit to running E85.

The A4 Allroad pump is literally the exact same pump, same flow rates and everything. Remember this is just the low pressure fuel pump. The only difference are the seals.

Loba makes higher flow HPFP’s which may or may not be needed. Maybe running an E40 mix would be best. Dunno. The S4 guys are fine on E40 but run out of fuel at lower ratios (higher ethanol content). Need to get with Kurt and start looking at fuel trims with the introduction of E85 in small amounts and see where it goes.

The increased octane and lower EGT’s would be a big help in the summer when it’s not uncommon to see temps well above 100 degrees for a month on end. Plus I have an E85 station right around the corner from my house tempting me.

A local Mustang runs E85 now and he gained about 30 wheel hp using it, naturally aspirated. Nothing but bolt on performance enhancers. He’s running 11.2’s in cold temps, drag radials and no cats. I recently ran against him in mild temps and he ran a 11.6 where I ran a 12.3. No cats, headers and E85, I would imagine, would really close that gap.

I did not consider the oil contamination issue. I’m already changing the oil every 5K miles but if I do go forward with this, I’ll plan on testing the oil.

This thread was originally posted over on the “other” forum, haven’t moved all the info to this one yet.

I think your spot on with E40 mix. That to me has been the magic bullet. The mustang guys that run E85 from the forums and guys I hang with don’t see any gain past E50. Most say E40 is the winning point.

The point with the A4 LPP is just that the pump is used for the A4 but keep in mind the A4 only has to supply enough fuel for 1/2 the motor size and 1/2 the hp.

I think E85 is a good thing to add and its a win if done right. I think this point and thread is a great idea good for you for putting this out there. I will stop clogging it with my jibber

Ha! No worries at all. Want to go in eyes wide open.

Just a clarification on the A4 pump though. It’s literally the same pump as the RS5 pump, just has different seals/bearings for use with ethanol. Both flow the exact same amount of fuel and it’s a drop in replacement. So you’re not loosing any flow by switching to this LPFP.

Granted, with ethanol I’ll need more fuel altogether but it sounds like it’ll work fine with E40 and won’t get eaten up by ethanol which has no lubricity. I guess I could also just run a bit of “premix” in there like I did during my rotary days!

Yes I suck at making the point. The point that I sucked at making was that. The A4 that has a pump that allows it to run E85 has enough head room to run a high powered high revving V8. If that’s the head room it takes to run a 4 cylinder. how much more head room would we need to add to the pump to give the V8 the same… Regardless E40 you won’t have any issues.

Sorry I sucked at making my point.

Just to clarify, are we saying that TYPICALLY we’re OK to run E40 without modifications? Have the S4 guys seen any problems long term with E40? I’m sure the A4 flex pump would be a peace of mind thing, but I’m curious none-the-less.

Maybe but with tuning. You can’t just run an E40 mix with no other tuning. The in-tank pump would be peace of mind. We can check in with the S4/S5 guys to see what, if any, long term problems they may be having as a result of using ethanol.

So the S4 guys seem to be able to run E40 but need upgraded HPFP’s (Autotech) for E85. Some years are better than others, pre and post facelift.

I’d say it definitely needs testing to see if the high pressure pumps can handle the additional flow. I think we could get by with E40 but it’s just a guess at this point.

Dear JHM.,
Please make upgraded internals for our high pressure fuel pumps.
RS5 people everywhere

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You know, thinking about E40, I’d have to look up a converter but the octane would probably be about the same as 93 premium here in the states.

Doing a bit of research on the flex fuel pump and it actually may have more headroom than the OEM pump. Not confirmed yet.

Well our standard fuel is E10. As most gas stations have 10 - 15% ethanol in them already at the 93 octane level.

Assuming your starting with 93 octane with a rating of E10 and then your adding another 30% ethanol to that base your going to be bumping the octane quite a bit.

Ok so thinking about this like this and using the math from the race gas thread…

Now check my math just to make sure and I’m padding the stats a little bit there. Since our standard 93 octane is E 10 already. I’m going to just add the difference of E content left to make E40. Here is how my math works out.

So E10 = 93 octane
To keep the math simple lets use 10 gallons of gas

So take 7 gallons of 93
Then since we already have E10 we want to get to E 40 we can add 30% more E content or 3 gallons out of 10 gallons.

So 3 gallons of E95 now keep in mind this is E95 not E100 but the octane content of E85 = 108

Doing the octane math works out like this.

7 gallons of 93 octane 7 x 93 = total of 651
3 gallons of 108 octane 3 x 108 = total of 324
651 + 324 = 975 / 10 = 97 octane… And since we used E85 not E100 we’re I don’t think even at a true E40 mix

Now that is crude math but you can see basically adding 3 gallons of E85 to 7 gallons of 93 bumps the octane up quite a bit enough so that its going to give you all the benefits your looking for while staying in a really safe zone for everything

Oooooor…use an online E85 calculator!

Basically, using that calculator, it’s right around 95-96 octane, E40. Not really worth it IMO unless runningE40 nets a nice drop in EGT’s (exhaust gas temperature). Lower EGT’s, cooler engine, ability to run more timing.

using the link you posted gave the same results as my math above. When started at the same point.

So my math is good at least…

But my math gives it a content of E33 = 97.5 octane

Here is almost E40 / Its E37 = 98.5 Octane

E40 is basically 100 octane.

So IMHO. E40 is basically 100 octane, Now keep in mind the mix ratio is based off mixing 93 octane and the fact that E85 is 108 octane not 100

You know what…when I did it, I used the default 100 octane for E85 hence the lower numbers.

I think it’s worth pointing out that on the other forum, there’s a Polish guy that states in Poland their standard is 95-98 octane (or at least that’s how I’m interpreting it). But when using Shell Racing fuel (100 octane), the ECU started giving him trouble with a EPC light.

I wonder if a simple tune would allow us to run E40 without much change in hardware. It’s be nice if this guy was tracking his results… wishful thinking.

He may not have a tune that can take advantage of 100 octane. The Count and I were just discussing octane and ethanol. Short version, the more octane there is, the slower the flame front (it ignites more slowly). So instead of using all the energy contained within the initial ignition to push the piston down, you’re actually pushing it out the exhaust or creating the big bang at a point where the combustion cycle can’t use the energy released. I’m sure this probably sends the knock sensors into a tizzy. Probably throws off the a/f ratio too.

Europe 95 - 98 = North America 90 = 93 So its not the same as in the states. It’s not our version of 95

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Tested the ethanol at the local station and it came back E73 which isn’t bad for winter. Was hoping they never let it drop below E85. I may end up waiting for spring so I’m not constantly having to recalculate and measure the ethanol content.