Unfortunately in California, the highest octane I can get at the pump is 91 octane. There is only one gas station in my city that has 100 octane at the pump for $9/gallon, but it is really out of my way. Given how sensitive our engines are to knock, I have been trying to find a higher octane option outside of additives. E85 is available in my area, and so I was wondering if mixing 91 octane with E85 during my fill ups is an option? I have read to only fill 2-3 gallons per tank as E85 burns faster, and the car can won't be able to adjust if you fill more.

I am currently running Cobb Stage 2 91ACN.

Thoughts?

 

This is great question and a topic that I've done quite a bit of research into and have done data logging on in conjunction with my tuner. I am in the same boat as you- I live in CA and can only get 91. My car is a 997.1TT with a built motor (3.8, Carillo rods, EVOMS head studs, 68mm turbos, GMG IC, Exhaust, etc.). The car has the stock fuel system, including the stock Bosch injectors, FPR, etc. It has a custom EVOMS tune. But it is my daily driver, and I didn't want to be running it on race gas, or straight e85. But I did want to get the most out of the car with pump gas.

Accordingly, I started mixing 91 pump and VP x98- which is 98% ethanol, about 115 octane. I get VP racing fuel x98 in 5 gallon pails, and its around 9$ per gallon. With each mixture I tried, I measured the ethanol content, then did pulls in 3d and 4th gear. I data logged these runs, then sent them to my tuner for more expert evaluation. This was all done in increments and in close consultation with my tuner.

Here's a summary of my findings:
1. With my 91 pump tune, the car was pulling around 6 degrees of timing with straight 91. I didn't feel or sense it, but the data logs confirmed it.
2. Chevron 91 here in the central valley of CA is 7% ethanol. It's not E10.
3. With 1.5 gallons of e98 mixed with 12.5 gallons of 91 pump, the octane ends up being 94 and it's 17% ethanol or E17. The car runs beautifully on this and the timing is completely cleared up, ie, no timing being pulled.
4. The car runs great all the way up to E25 mixture. We even did a custom tune for E25 which added more boost than the 91 tune. Again, the car ran great, although injector duty cycle is getting up there.

I ended up settling on running my 91 tune and adding 1.5 gallons of e98 to each 12.5 gallons Chevron 91 supreme. It's pretty easy to just dump in the 1.5 gallons of e98 with each fill up. It's not too expensive and is tolerable. The added benefit of the ethanol is that it also decreases the combustion temperatures in addition to bumping the octane.

You adding 2 gallons of e85 to 12 gallons of pump (assuming your 91 supreme is also e7) will make the ethanol content e18, which should be fine. Your octane won't be as high as my mixture though.... assuming your e85 is 100 octane, then your e18 mix would be 92-93 octane. That would still help though.

Keep in mind that my car is a 997.1TT- not sure if yours is a 997.2TT....fuel systems are different between the turbo 997.1 and .2's. Please let me know if you have any questions. Hopefully this all makes sense.

Best Regards and good luck!

 

Wouldn’t do it without a flex fuel setup, your best bet is methanol injection .... VP octanium is really good give it a try, 2 bottles with each full tank, costs half compared to mixing 5 gal of 100+ with every fill.

 

Wow, thank you for the very informative reply!

My car is a 997.1TT. I fill up with Chevron 91, and when datalogging, I would see as high as 9 degrees of timing pulled on a 80F evening. I didn't feel it while driving like you mentioned, but would like to clean it up.

Your data brings up a very good point, and that is measuring the actual ethanol content. I have not done this at my pump for both 91 as well as E85. I know there can be a huge variation of ethanol content at the pump, especially for E85. You are able to keep that value static by buying pails of high quality fuel from a reputable source. If I go the route of storing fuel (E85 or race fuel), then my options change. My hope was to go to my pump, fill up on 91 and splash 2-3 gallons of E85 from the same pump. It seemed the be the most convenient and cost effective way to raise the octane given our options here in CA.

Where are you buying your VP x98? There is a store in town that carries a range of Sunoco fuel in 5 gallon pails. If I went the stored fuel route, I could get 260 GT (105RON - $13/gal) or go to that one pump in town with 100oct ($9/gal) and fill up a gas can or two.

 

It works extremely well with zero issues. There is no need for flex fuel. A couple gallons of E85 to a tank of pump fuel will basically zero out any timing pull you have on your pump fuel map. I've run my 997TTs on pump gas this way for 6 years with zero issues. There is about 5% positive fuel trim needed, at most to do this, with the limit of the system at +25%...not an issue in the slightest. Have at it.

 

Glad to hear your car is also a 997.1TT. I get my VP x98 from a local VP dealer here in Fresno (Red Triangle Oil). It looks like you are in Sacramento- Looks like Hunt and Sons is a VP supplier there. I have to have my local dealer special order the x98, but it is free shipping, and only takes a week or so for them to get it. It would be more convenient to just splash in a couple gallons of e85 at the pump while filling with 91. But what I am doing is not really too big a hassle......I have a 2 gallon fuel jug and an electric hand pump- I just prefill the jug with 1.5 gallons the night or morning before, put it in the front trunk, then pump it in with my hand pump at the gas station right before I fill with 91. It's pretty easy. I buy 6 pales of x98 at a time and keep them in the garage. They are sealed and easy to store.

Since you are obviously knowledgeable about your car and can datalog, I would just splash in 2 gallons of e85 at the pump next time you fill up, and see if that's enough to clean up the timing. If not, go 3 gallons, then recheck your timing. That would be the easiest and cheapest way to go. As A418t81 accurately said, the fuel trims totally accommodate for this. I ran my mixture up to a bit more than e25 and trims, etc, were all still fine. Or you could do the slightly more expensive and logistically annoying route and buy e98 and add it like I do.

I did look into methanol injection recently. Very tempting. But I just spent $35K on an engine rebuild and to set up meth properly, I'd be upgrading injectors ($3000 plus engine drop labor), going with a whole new tune ($$), cobb accessport ($1800), and the cost of the methanol system itself and labor (maybe $5K). It would be $10K or more total. I can't justify the cost right now. I wish I had looked into methanol at the time I did the rebuild.

 

Thanks for the feedback!

That was my exact plan. Add 2 gallons of E85, datalog and evaluate. Thanks for confirming my thoughts.

On the topic of the fuel trims, is this something I can expect my ECU to do on its own? Or is this a parameter I will need a tuner to change in a tune/map?

Sounds like you are in deep on your engine. I was considering going full E85, but the thought of R&R the motor to install the injectors was a bit much for me at this point and what I am looking to get out of the car. It snowballs because if I have the motor out already, I'll have coolant pipes done, and other maintenance items.. the list goes on LOL

 

The ECU runs closed loop fueling at all times. It does it all on its own. You need do nothing other than add the little bit of ethanol. No need to test the concentration either given how little you will be running. .1 Turbos flat out run better with a couple gallons of E added to pump gas even if on the factory tune given how sensitive the knock sensors and how aggressive the timing pull routines are. I add a couple gallons if at all possible. It adds a little bit of power and a lot more consistency.

 

On the topic of the fuel trims, is this something I can expect my ECU to do on its own? Or is this a parameter I will need a tuner to change in a tune/map?

The ECU will adjust on its own. The ECU is pretty good at compensating for the ethanol (at least up to a point..... somewhere above E25...) Let us know what the datalogs looks like with 2 gallons of e85. Hopefully it will clean up the timing. If so, I'll think about going to my local gas station with e85 and do the same, rather than continuing with the VP product.

 

I'll keep you both posted. I appreciate the input!

 

I'll have to give this a try. Thanks for the research fellas.

 

What was the injector duty cycle on E25? I chatted with Sam @ EvoMS and he said the same thing. I'm assuming he helped you tune?

 

Indeed I was working with Sam at Evoms, The injector duty cycle was roughly 90% as I recall on the e25 tune. As an aside, I also considered using Torco octane booster rather than ethanol to clean up my timing. Apparently the stuff works pretty well. I opted not to use it due to concerns about long term use with catalytic converters (MMT may potentially clog up the cats long term.... depends on who you talk to and who you believe!)

Again, let us know if your timing is clean with 2 gallons of e85. It sounds like A418t81 has had success with doing that. That's great...easy and cheap!

A418t81- are you running your 09 997TT on e85 all the time? I'll bet your car is a monster.

 

So far, yes exclusively. No issues to report. Car has ID1300s so plenty of fueling headroom left there. I keep tinkering with the idea of getting some Alpha28s. We will see.

 

A418t81,
Sounds awesome. How's your day-to-day driveability on e85 with the ID1300's? I'm still contemplating just going e85 and ID 1300's. Also- I'm running GMG 3.5" core intercoolers. I am thinking about upsizing to do88 big-pack or equivalent. Not sure if that will be a hugely noticeable improvement over my current IC's though. I think I saw a prior thread that indicated that you had do88 big-packs.