When does adding ethanol make sense?
Thread Starter
Instructor
Joined: Jul 2025
Posts: 132
Likes: 4
From: San Marcos CA
When does adding ethanol make sense?
Hello I have a fairly stock a4 c5: intake, LT headers, X pipe, axel back, 3.42 gears, “stage 2” transmission, and a dyno tune making ~360 rwhp. Does adding a flex fuel kit to run e85 make sence at all at this point in time on the build? I’ve heard from some people that ethanol only really makes a difference with boosted cars and others saying that it’s the best “cheap” mod for easy horsepower regardless of being boosted or not. What’s the consensus here?
No Hostility Be Happy
Joined: Aug 2018
Posts: 10,172
Likes: 7,474
From: South Hill Wa
2022 C5 of the Year Finalist - Modified
2021 C5 of the Year Finalist - Modified
What most people don't tell you or neglect to tell you is how much MORE fuel it takes to make the same amount of power.
When you run E you will live at the gas station IMO.
There is a reason that you need more fuel system and injector to run E.
Do you even have pump E in commiefornia?
My opinion is, E really has no place in a street car that is a driver. Toy or racecar, sure.
When you run E you will live at the gas station IMO.
There is a reason that you need more fuel system and injector to run E.
Do you even have pump E in commiefornia?
My opinion is, E really has no place in a street car that is a driver. Toy or racecar, sure.
Tech Contributor
Joined: Jan 2007
Posts: 19,565
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From: Dyer, IN
Melting Slicks
Joined: Dec 2018
Posts: 2,685
Likes: 1,402
From: Orange County, CA
There's a lot of E85 in CA. On 2 different cars back in the day (7-10 years ago) I daily drove in SoCal on E85 and didn't have to go more than 2 miles out of my way to fill up.
If we were talking about a more fuel efficient car, I would disagree. E85 on my old 2001 Audi A4 1.8T 4-cyl with upgraded turbo was a great setup worth a decent bit more power than our crap 91 octane. But on a hefty V8... different story.
If we were talking about a more fuel efficient car, I would disagree. E85 on my old 2001 Audi A4 1.8T 4-cyl with upgraded turbo was a great setup worth a decent bit more power than our crap 91 octane. But on a hefty V8... different story.
Running Guns & Moonshine
Joined: Aug 2001
Posts: 12,819
Likes: 7,387
From: CT
I consider flex as a valid option and benefit for many who are supercharged if they have a strong tune and want to hit the track with more boost and timing. Absolutely valid, though other fuels are preferable for racing. E85 just offers convenience for those with nearby pumps.
On an NA engine I simply don't see the benefit. There is not as much power to be gained and the inconvenience washes that gain clear off the table. It isn't even much cheaper than gas around me now.
I ran one car for two weeks and maybe close to two thousand miles last year. My state having just ONE pump in the middle of it for E85 I was absolutely locked in on stopping there to and sometimes also on the way home from having fun. The only benefit I enjoyed was that on Friday and Saturday nights everyone running hot tunes and built cars was stopping at this pump which made it cool to see the Mexico builds heading out.
On an NA engine I simply don't see the benefit. There is not as much power to be gained and the inconvenience washes that gain clear off the table. It isn't even much cheaper than gas around me now.
I ran one car for two weeks and maybe close to two thousand miles last year. My state having just ONE pump in the middle of it for E85 I was absolutely locked in on stopping there to and sometimes also on the way home from having fun. The only benefit I enjoyed was that on Friday and Saturday nights everyone running hot tunes and built cars was stopping at this pump which made it cool to see the Mexico builds heading out.
Safety Car
Joined: Nov 2013
Posts: 3,592
Likes: 1,120
From: Ex DPRK, now just N of Medford, OR
And THAT sums it up quite succinctly!
Unless your combustion pressure requires higher octane, it just goes to waste.
Sure, you might see a small boost being able to run a touch more timing W/O detonation, but as for the economical part... you'll be pouring through it.
Unless your combustion pressure requires higher octane, it just goes to waste.
Sure, you might see a small boost being able to run a touch more timing W/O detonation, but as for the economical part... you'll be pouring through it.
Pro
Joined: Jan 2001
Posts: 679
Likes: 358
From: San Jose CA
With an N/A engine, the only real benefit to a flex-fuel tune is that E85 is cheaper than 91/2 Octane fuel. A few years back I did the math and and that cost delta would take almost a decade to pay for the conversation kit.
With a turbo/supercharger application, that's an entirely different conversation On my old 2004 STI, E85 was worth 45WHP with no other changes.
With a turbo/supercharger application, that's an entirely different conversation On my old 2004 STI, E85 was worth 45WHP with no other changes.
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