Can Too Much Octane Be A Bad Thing?

[AJ]

Ok so I am out an about thinking of differant stupid shit and started thinking about fuel. More specificly trying to find the whole trail leading to knocking and such. So we lay out the fact that we want the fuel to be "lit up" by the spark plug and not by compression as that leads to knocking. But when you bring it the issues of the differant levals of octane I am wondering how the compression ratio factors in more.

Is there a related formula for this? With the stock Compression Ratio we need 92/93 octane for best pump gas results and running lower octane results in poor performance. But adding 117 octane (C16) seems to allow us to run car for the most power. Is there anything that puts a limit to what octane would best suited out needs? Where too much could reduce power? Even though we see higher #s from the higher octane gas could we see anything higher at an octane of say 110? I guess I am just looking for more info for when it comes to raising compression and I think boost plays an effecting part as well.

Just a few thoughts and I think Im gonna go out and try and find my own answers and hope I don't sound like a moron. Yes I am trying to relate this to a Turbo DSM setup, but feel free to explain if it varies from car to car.

[JET]

You will get the most power out of the gas with the octane just above what your setup needs. This is assuming the burn qualities are the same.

C16 seems to have great burning qualities. You seem to get the high octane and also a very quick burn. I have done a lot of research on the fuels and tried to find info that would correlate the burn rates between them. There is not a ton of data available though. The specific gravity is almost always available, but does not seem to have a very good bearing on the power available. I am going with the tried and true, C16 from now on.

[AJ]

So C16 is tired and proven to give you power increases, but have you dyno'd on C16 (117 octane for those who don't know) and then drained the tank and tired to dyno on a lower octane with a differant burn rate? I was just talking to Mike as well about this. I did a search on NABR as well, but didn't come up with anything as far as trying 2 diff octane fuels in one session to keep variables as close as possible.

Jet what have you dyno'd on with your old system or did you not dyno the car at all?

[JET]

Unfortunately I didn't dyno my old setup. I was going to at the dynofest, but I found out about the crankwalk then. I always ran the 110 from Kath at the track. I was not overly impressed. 26 psi and 27 degrees of timing was all I could get out of it before I would start seeing some knock. I was on the stock headgasket, so I didn't want to boost over 26 psi. That is why I turned up the timing some.

I ran the same mph as steve hill, so I should have been around 450 whp too. I would think 440 - 450 whp at 26 psi. I am going for 600+ on the new setup at 30 psi. Hopefully over 650!

[LightningGSX]

Power is greatest when peak chamber pressures are just after TDC, when chamber volume is at its least.And higher the octane = slower the flame speed and greater delay in peak chamber pressure.So for greatest power, your octane is limited by the amount of ignition advance available or visa versa.Or for every amount of octane there is a certain amount of ignition advance needed for best power or vise versa.So after a certain point ,increasing either alone results in no power gains.And since you(or your ecu) can only advance your timing so much, there is a fine line when octane is actually bad.I personally think even c16 is too much for the average modded 4G63(no proof speculation only), but since you can't start at 100 octane and work your way up point by point I guess you have to go with what you can get.

[Shane@DBPerformance]

You are goign to lose power if go from a good non-knocking pump gas tune to a C16 without changing anything. To get the most out of C16 you usually need to raise the effect compression or run a lot more timing plus lower your fuel output.

C16 is a denser fuel than pump gas so more of it gets injected in the same pulse width time. A pump gas tune that is running around an 11.5:1 A/F will usually turn into an 10.8-11.0:1 mixture when you switch to pure C16. So you need to remove some fuel one way or another, like going farther negative on an AFC. You aren't really leaning out the mixture, your just trying to get it back to the same air/fuel you started with on pump gas. However, you can usually goto a leaner mixture than you were able to run on pump gas and start picking up some additional horespower.

The easiest way for us to raise the effective compression ratio and make the C16 happy is to raise the boost. Along with the boost comes more horsepower. If you don't want to raise the boost for some reason, then your going to need to run a little more timing on the C16 to get the fuel to ignite at the right time to keep making the same old power. You can sometimes run a fair amount more timing and try to make some extra horsepower, if you aren't using up all the octane already in boost, lean A/F, poor intercooling, etc. Don't worry a lot about getting crazy high timing numbers though. The power gains are usually minimal once you reach a certain point.

Mara's power on her 1G compression usually stopped sees gains at all at 24 degrees. That is with the base timing advanced and the ECU running off of the final max flow timing map(the one that gives 22-23 max then goes down to 19 degrees around 7000). The advanced base would give a little power in the mid range and topend range past 7000 area where the ECU wasn't running as much timing. Peak HP wasn't really help at all, just in a few areas. On Andy Nash's car we went from a base of 0-1 degrees to an advanced base of around 8-10 degrees and saw a gain of about 5whp. Not a huge gain for a car running a retarded base timing going to advanced. It's usually better to just run more boost than try to push the timing to the limits.

If you want to run a lot of boost on pump gas and make decent power that way then run as little timing as you can get away with. With higher compression, you usually need even less timing. If you can get 20 degrees out of a 2G compression then I wouldn't spend too much time trying to go any farther. Stock without any AFC fooling going on the 2G ECU only wants to run 16 degrees of timing max on its highest airflow map versus the momentary 23 degree peak timing max on the highest airflow map in a 1G ECU.

On C16 you can usually push the limits of stupidity and still not knock. On Mara's car we did just about everything to the extreme edge of tuning and the C16 would take it. We ran high boost on a shitty intercooler, so a lot of the octane was used just to compensate for the high heatsoaked intake charge air temps. We shot for an airfuel close to 12.5:1 and we often upped the base timing from the stock of 5 to 8-10. We didn't always do the timing though because we knew that it did next to nothing and wasn't worth the effort and risk. Boost equals power, timing doesn't.

[niterydr]

Originally posted by ecoli@Nov 10 2003, 11:35 AM
You are goign to lose power if go from a good non-knocking pump gas tune to a C16 without changing anything.  To get the most out of C16 you usually need to raise the effect compression or run a lot more timing plus lower your fuel output. 

C16 is a denser fuel than pump gas so more of it gets injected in the same pulse width time.  A pump gas tune that is running around an 11.5:1 A/F will usually turn into an 10.8-11.0:1 mixture when you switch to pure C16.  So you need to remove some fuel one way or another, like going farther negative on an AFC.  You aren't really leaning out the mixture, your just trying to get it back to the same air/fuel you started with on pump gas.  However, you can usually goto a leaner mixture than you were able to run on pump gas and start picking up some additional horespower.

The easiest way for us to raise the effective compression ratio and make the C16 happy is to raise the boost.  Along with the boost comes more horsepower.  If you don't want to raise the boost for some reason, then your going to need to run a little more timing on the C16 to get the fuel to ignite at the right time to keep making the same old power.  You can sometimes run a fair amount more timing and try to make some extra horsepower, if you aren't using up all the octane already in boost, lean A/F, poor intercooling, etc.  Don't worry a lot about getting crazy high timing numbers though.  The power gains are usually minimal once you reach a certain point. 

Mara's power on her 1G compression usually stopped sees gains at all at 24 degrees.  That is with the base timing advanced and the ECU running off of the final max flow timing map(the one that gives 22-23 max then goes down to 19 degrees around 7000).  The advanced base would give a little power in the mid range and topend range past 7000 area where the ECU wasn't running as much timing.  Peak HP wasn't really help at all, just in a few areas.  On Andy Nash's car we went from a base of 0-1 degrees to an advanced base of around 8-10 degrees and saw a gain of about 5whp.  Not a huge gain for a car running a retarded base timing going to advanced.  It's usually better to just run more boost than try to push the timing to the limits. 

If you want to run a lot of boost on pump gas and make decent power that way then run as little timing as you can get away with.  With higher compression, you usually need even less timing.  If you can get 20 degrees out of a 2G compression then I wouldn't spend too much time trying to go any farther.  Stock without any AFC fooling going on the 2G ECU only wants to run 16 degrees of timing max on its highest airflow map versus the momentary 23 degree peak timing max on the highest airflow map in a 1G ECU.

On C16 you can usually push the limits of stupidity and still not knock.  On Mara's car we did just about everything to the extreme edge of tuning and the C16 would take it.  We ran high boost on a shitty intercooler, so a lot of the octane was used just to compensate for the high heatsoaked intake charge air temps.  We shot for an airfuel close to 12.5:1 and we often upped the base timing from the stock of 5 to 8-10.  We didn't always do the timing though because we knew that it did next to nothing and wasn't worth the effort and risk.  Boost equals power, timing doesn't.
my thoughts exactly.
C16 and higher leaded gases lets you go farther on the 'stupid meter' without blowing stuff up. By simply just switching gasoline, you do not and probably will not gain power. In turbocharged and supercharged setups, the only reason to go to a 'higher grade' gasoline is to allow you to turn up the boost and/or add timing to get back to the a/f ratio you were at, hince how 'c16' makes power.
As shane mentioned, leaded gasoline, especially c16 burns much slower than standard 92 octane. But in order to gain from such things you need to take fuel out to get back to where you were at, or add cylinder pressure to 'consume' the 'extra' fuel.
Another point is that leaded gasoline is usually more stable than unleaded. The gasoline is more constant as well as the fact that it is easier to control (burns slower), this allows you to run a different and slightly leaner a/f ratio without the worries of burning stuff up.
Hope this helps.

I also agree with lighteninggsx, IMO c16 is overkill in alot of applications used on the dyno. Most of the setups out there dynoing on 'race gas' really don't need 'race gas', they are just looking for that added 'kick' that there particular setup is missing. The only way I see it useful is when you have the cylinder pressure and temperatures so high that you need the more stable gasoline to 'cool' things down, or you have the timing so jacked up that you need it to avoid such things as detonation. The only other reason I think it should be used is when you simply run out of fuel on your setup with 92 octane and you need all the help you can get (too small of injectors and pump come to mind).
sorry for the rant.

[JustROLLIN]

The speed at which gasoline will burn in the combustion chamber is effected by which type of gasoline it is (ie- leaded or non) but also the octane rating itself. An octance rating IS the speed at which the gasoline will burn. Hence running 87 octance will cause knock. This is due to the flammability or psuedo-flashpoint of the gas. The 87 will have the fastest burn in the chamber, and the speed at which the gas burns will decrease as we move up the ladder. The top unleaded fuel being 110.

Then the lead comes into the equation. I would agree with most of the points made above. The lead gas will burn the slowest out of all the gases due to its nature. Hence some the best tuning will come from the use of c16 leaded race fuel.

Joe

[rick shindley]

No, too much octane is not a bad, per se. If your engine runs happily on a lower octane fuel then feeding it higher octane fuel is just wasteful.

There is more to "octane" then a number, though. Generally, fuel octane rating represents the average value between "research" octane and "motor" octane (the (R+M)/2 method). Research octane is greater than motor octane (if I remember that correctly). How far apart the two numbers are determines some of the characteristics of the fuel. A brand blending 50/50 82 (M) octane with 102 ® octane has an overall 92 octane rating. The disparity between the 82 and 102 figures can create some goofy performance issues. A brand with less disparate blending may perform more consistently in your car. The only way for us plebes to discover what works is to try different brands. In my experience, BP Amoco is consistent over time and provides good performance. On the other hand, the 93 octane from Holiday stations seems to be more problematic (in my experience).

Rick

[LightningGSX]

I disagree, higher octane than what is necessary will result in less power, as it delays peak combustion pressures.However ,I agree with you that 93 octane sucks and the use of high octane fuel is usually wastefull.