What is the general consensus on why people suggest running on the track a mix of race fuel (100+) along with whatever octane (91 or 93) used on the street. Detonation prevention? Cooler temps? Burn slower/faster? Something else? How much race fuel mix is truly needed for the benefit (20/80, 30/70, 50/50, etc)?

This is in the case where the car is on a street (91 or 93 octane) tune, so the extra octane is not for added power.

Discuss.

 

Just run E85

 

Back when I ran race gas I would run my pump gas tune and simply top off with race gas eventually running a full tank of race gas. I did this to prevent knock from my heat soaked FMIC and over worked engine. Many people also bump up boost a bit to pick up some power.

Now that I'm on E85 I just run my normal tune (30PSI) all the time. The downside is E85 is high consumption and its hard to source in some areas .. practically not even available on site. Heck if I go to Gingerman or Grattan I have to switch back to race gas.

 

As Jeff said, I would start the weekend with a at least 50/50 in the tank, and continue topping off with race fuel. $100-$200 for race fuel is cheap insurance. Just make sure it's unleaded to keep the cat and O2 sensors happy. In Vegas, we have 100 octane readily available, and this is what I did before going to E85..

 

I've run a few track events and used the following method when I was on the stock 9 turbo at 25-26psi. 50/50 100oct/93oct . I'll check the AFR and make sure its between 11.5-11.6AFR on that tank before going to the track. I'll also bring another batch of the same mixture of fuel so the tune is close enough. If you have access to a dyno you can also add some timing to reduce EGT's somewhat on good fuel. My roadcourse tune had 1-2 more degrees of timing from 5000-7500 IIRC then my everyday pump gas tune. Even a few gallons of good fuel will smooth out the way the dyno graph looks and the engine performs. An engine needs a certain amount of timing to run well per octane level.
The stock ECU will still add fuel as temp increases to try and cool the engine so I like to start with an AFR on the high side as too rich of a mixture can cause knock. There is also fuel heating that will change the AFR also. I like to operate my engine 11.0-11.5 at the roadcourse on pump/race/e85.

A worst case example is your car was tuned for 11.2 ( safe pump gas AFR) and then when you added race gas it richened it up half a point . 11.2-.5 =10.7. Then your ecu added even more fuel when it got hotter and was 10.3-10.5. Most people including myself believe this to be too rich for the 4g63. Now, the engine is pretty tough so it may not care but I'd prefer to make it as right as you can before going out and hot lapping your car. Just my .02.

As stated by letsgetthisdone its cheap insurance to run good fuel at the roadcourse. Race gas is soooo much better then pump fuel.

 

Race fuel never change my AFR's...

 

im tuned for 93 pump gas. if i see a lot of knocks on hot day, i'll mix a gallon or two of 110 lead.

 

go with race fuel straight, very reliable fuel.

 

Has anyone played with alternate ethanol mixtures?
From looking at some research, it seems like the octane benefits stop/drop off significantly after E50, and it seems like even E30 has pretty decent anti-knock properties.

 

This one is much more difficult to pull off, because fueling must be adjusted at different Ethanol concentrations unless you have a flex-fuel sensor setup in place.

Furthermore: If you have access to E85 or higher ethanol and you're already retuning for it, why would you want to run less ethanol? Spraying all of that extra ethanol will help cool the intake charge as well, which is an added benefit.

 

Obviously need a flex sensor for this (or not, might only be one more map), but I wasn't thinking that was so rare.

You would want/need to run a lower eth mix to make race distance.

 

I put 1 gal of 98 in my jugs then 4 gal of 93 measured by watching the pump. It's a pita but this has been the only way to "consistently" keep the same ratio. Figure about 95 octane which is plenty for TTST3 levels. Things are happy in the mid 11's afr all day with 130deg track temps. Also a TON cheaper then running straight race gas.

 

I usually run 3-4 gallons of 104 per tank of 93, just for extra knock prevention.

 

I run straight E85 on the street in my Evo 10 but I didn't want to lug a billion 5 gallon jugs of fuel to the track so I did exactly what you are asking.

I set up my TephraV2 maps 1-8 for "manual" flex fuel. Basically each map slot has changes for 10% Ethanol fuel changes, from 10% (pump gas) to 80% (E85), with some head room for each so I can get away with the 5% differences in between map slot switches.

I also have an Ethanol sensor installed in the car. So it will tell my the Ethanol percentage that is currently in the car and I switch to the appropriate map. It's really easy to do on the 10 because you can switch via cruise control button in real time. I actually switched maps on track once at my last event.

It helps a ton. I only bring about three five gallon jugs of E85 to the track and usually end up running around an E50 to E60 mixture.

The car runs so much better on that mixture than 91 and even 100, which btw is crazy expensive at the track I go to. ($9.98 gallon compared to $2.50 gallon to bring E85) I save about $100 a track day running the E85 mixture over a 100 mixture.

 

I did a trackday today and ran about 25% race fuel 75% 93 which seemed to work good. However it still was almost $200/gas for the day w/ the 100 @ $8/gal. Think I'm gonna bump it down to about 15%/85% for the future.