im fortunate enough to have 100 unleaded out of a pump near my house. at 4.25 a gallon, its not cheap, but i drive the vette once a week, so i dont mind if its worth it. I dont guess full 100 is worth it, but what do you guys think of a 50/50 mix of 100 and 91? would a average octane of ~95 yield favorable results or be healthier for the car? car is an lt4 with about 40k on the clock.

 

Not worth it....Unless you are having knock issues. The car was made to run on premium so you will be wasting your money. :seeya

 

as i understand, the ecu advances the timing until it hears a knock and then cranks it back just a tad. so if i have higher octane gas, wouldnt it be able to advance the timing further getting me some more ponies? or am i missing something?

 

Not quite. The ECM will only advance the spark timing up to the values stored in the ECMs spark advance tables. The ECM will try to run at the values in those tables (there are different tables for different operations, such as part throttle, WOT, temperature dependent, etc, etc), but if it hears knock, it will pull timing out. It is an absolute myth that increasing spark advance increases horsepower. There is an optimum spark advance for different operating conditions. That optimum changes with engine speed, throttle position, EGR valve position, altitude, engine temperature, etc, etc. Any spark advance in excess of that optimum is wasting energy, and not improving power at all. Within a couple of degrees, you won't see a power change, but more than 2-3 degrees advanced from optimum, and power will begin to fall off. It is quite possible to have excessive timing advance and NOT hear knock. There is about a 4-6 degree window (+/- 2-3 degrees) around the optimum where the power change is negligible. The point is that if your engine is operating at the optimum already without knock, you will see no gains from higher octane fuel (there are exceptions, but I don't feel like talking about them right now). Advancing the timing more than you're at now might actually cost you power, and on an LT1/4, the only way to advance that timing anyway is inside the ECMs programming. In an L98 you can cheat and put a few more degrees of initial timing in the distributor, because the ECM has a setting which tells it what the base timing is. If you tell the ECM that base timing is at 6 degrees BTDC, and you set the distributor at 8 degrees, you just advanced EVERY timing table in the ECM by 2 degrees. Again though, you might not gain anything and could lose some power by doing this.

 

It s stock programming isnt agressive to boost the timing to the point where you are going to get knock in a stock engine with stock programming. Your money would be better spent on mods.Over $4 bucks a gallon is alot of money wasted. Im sure others will chime in. :p:


[Modified by ScottsLT1, 3:29 PM 5/30/2003]

 

Having too much octane can actually slow your car down if the requirement is not there for it. High octane fuels burn at a much slower rate than low octane fuels. If you don't have the compression that requires 100 octane there isn't anything to gain. It may benefit some in a 50/50 mix at times, if you had a knock problem. But generally these engines run just fine on pump 92 octane.

 

ok very good, i wont bother then. thanks for the writeups

 

This is actually a myth about octane. Octane does NOT slow down the burn of the fuel. It raises the "explosion" temperature and pressure of the fuel. In other words, it takes higher temperatures and pressures for higher octane fuels to explode instead of burn. Contrary to popular belief, the fuel and air mixture in an engine burns, not explodes. If it explodes, it's referred to as detonation. If it's a normal combustion cycle, it's burning, not exploding.

 

.I agree, I lost 1 mph and 1 tenth with Sunoco 104 unleaded vs 91 octane

 

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:iagree: I get this all the time and always go and find all the research to back it up to end the argument. Bottom line 93 octane is the most I would run in any Vette unless you are running something like a 500+hp motor. If you run 100 octane you are doing more damage then good.

 

I also have the 100 octane unleaded near my house. However at a much more reasonable price. Try $2.19 a gallon. I use it alot the car seems to like it better than the standard premium around here the best I can get is 92-93. I have done a few datamasters on this and knock count is nill with 100 octane and of course my new LT4 knock module.

 

Racing fuels are advantageous in that they are much more carefully formulated than standard pump gas. You get a more consistent mix of base stocks.

Other than that, if your car still has the stock compression ratio, it's not an advantage except in high summer heat. It'll help a hot engine.

Go to Union 76, Sunoco, etcs websites, they all have great technical articles about their racing fuels. And you can construct a dilution table for your C4 figuring in our 20 gallon gas tanks.

50/50 mix - 93 * (10/20) + 100 (10/20) = 96.5 octane

25/75 mix - 93 * (15/20) + 100 (5/20) = 94.8 octane

I think in some places you can get Sunoco 94 out of the pump, so you can see that there isn't big economics in running race fuel in cars with pump-gas compression and no forced induction.


[Modified by Matt Black, 7:22 PM 5/31/2003]

 

My LT4 is a daily driver and sees nothing but the 91. I have had it at the dyno and it shows no spark retard.

I have tried a mix of 100/91 when I went to the track and could find no difference. Bottom line is save your money.

 

Here's something else to throw in the ring, You know that 93 octane you had in the tank before you stored it? Well, after 3 months, it's not 93 octane anymore.

 

LOL too true on that one about storage. Plus usually storage gas you would have added a stabilizer to... :iagree:

 

This may only relate to forced induction, but I know a guy in the Bullittclub with a built 4.6L, supercharged, and he made 20 more rwhp on a dyno with 100 octane racing fuel versus his usual 91 octane.
:eek:

 

:iagree: Accidently put 87 octan in the Vette :nonod: Didn't like it.

I run 93

 

It does make a difference on any power adder. Super/turbo cars can run more boost with higher octane, and nitrous cars can run more nitrous.