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After reading a post on 100 octane available at the local 76 station...I waited till my tank was half empty of Chevron 91, I filled her up with the 100 which should yield somewhere in the 95-96 range. Will the Z make corrections to the timing to utilize the higher octane gas? Will slower burning gas improve performance?
 

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I think you actually get more like 94-95 octane with a 50-50 mix, but I could be mistaken - i might be thinking of a 2:1 mix. Someone here wrote an excel spread sheet to figure it out. I have it laying around, but its on a harddrive in a machine with no motherboard. Hopefully the kind soul that wrote it can repost (or someone can search for the thead).

The PCM will adjust to the increased detonation resistance of the new fuel. But it will take some driving time to do that. The easy way is to reset your PCM, but then you'll lose your long term fuel trims.

With my car, I have no mods, so I was pretty close to the fuel/spark tables already there, so it was no big deal. Resetting the PCM resulted in an immediate change. If you don't reset it, you'll probably not notice the change so much as its more gradual.

Also, if you run straight 100 octane race gas, you'll find that your gas milage will go up a bit. A result of all the alcohol in the street gas. However, with a blended fuel, you won't really notice.

Hope this helps.
 

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Discussion Starter #3
Thank you, very helpful.
 

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Per Steve Cole, everytime you refuel(at least 5 gallons), the computer resets to the max, which is about 94-95 octane. If it detects detonation, it retards the spark.

It only goes one way, and that's down. On the next refill of at least 5 gallons, it resets again.

Some cars, like my '93 TT Supra, will increase timing if it detects higher octane fuel.

Soooooo, if you go 100% of 100 or higher octane, you're wasting your money.

The Unical Mix chart is on their website.....

:cheers:
Barrie
 
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Z06_2002 said:
After reading a post on 100 octane available at the local 76 station...I waited till my tank was half empty of Chevron 91, I filled her up with the 100 which should yield somewhere in the 95-96 range. Will the Z make corrections to the timing to utilize the higher octane gas? Will slower burning gas improve performance?
That is actually a very good way to improve your Octane Rating. 91 is too low in the heat of the Summer, 93 is just right.

Your timing, however, is only negatively affected by poor Octane ratings. If the knock sensors detect even inaudible (to humans) knocking, it backs the timing off. We see this all the time on the dyno, and it is very hard to eliminate completely, even with better fuel.

The timing for maximum torque usually occurs about 16 o ATDC.

There are three timing curves built into the PCM. The IAT controls these tables. This is called "air retard compensation mapping". If the IAT sees 59-degrees ambient, it provides the most agressive timing.

Any more Octane than it takes to avoid pre-ignition (pinging) will not benefit your car in any way. Testing has been done (Not by me) by R&D Dyno, with Octane ratings, and they found that more Octane than you actually need will create a slight loss of power.

Low-octane fuel is the culprit because it burns faster than high-octane fuel and passes the flame front.

Bottom line, High Octane fuel actually burns slower than lower Octane fuel to avoid overtaking the flame front . This is how it prevents pinging, by slowing the burn rate.

Many folks confuse pre-ignition (premature ignition) with detonation (uncontrolled spontaneous combustion), which drives holes through pistons, ring lands, pitches rods and causes many other nasty deeds.


I believe the best number would be the 93 Spec, for max performance.

One more footnote. In turbocharged engines, years ago we would actually have a canister, and a small pump and jetting system to inject water directly into the intake. The water would turn to steam in the combustion chamber, but the effect was to allow higher boost pressure from the turbo before detonating.

Check out these articles on water injection-pretty cool: http://duramax.bizhosting.com/My_Truck/water_injection.htm
http://www.avatar.com/~kory/h2o-injection.htm

Today intercooling does this better, since it does not displace area in the combustion chamber that could be better filled with an intake charge.

Jim Hall
 

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Discussion Starter #6
Thanks you guys, saves 3.77 per unnecessary gallon. :D
 

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Z06_2002 said:
After reading a post on 100 octane available at the local 76 station...I waited till my tank was half empty of Chevron 91, I filled her up with the 100 which should yield somewhere in the 95-96 range. Will the Z make corrections to the timing to utilize the higher octane gas?
Only in that any detonation that was occurring before will now be eliminated.

Will slower burning gas improve performance?
No.
Actually, 76 Competition 100 is not a slower burning gas. It burns as fast or maybe even a little faster than 91-oct. pump gas.
 
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