Octane and performance tunes

Alabama-man · March 31, 2006

Having looked into getting a performance tune for my 2006 Sierra 4.8L, I found out that one tuner in particular offers up different levels of performance based on what octane of fuel used. In other words, if you use regular then performance gain is say 10HP (hypothetical) and if you use premium then performance gain is 20HP. Does anyone know how this works?

tlap · March 31, 2006

I would really like to know also. I was told by one of the computer flashers, (I wont say who) but she said that they are "smart computers" and did not elaborate any further.

Alabama-man · March 31, 2006

I also got a similar statement about smart computers. The whole thing about octane sounds fishy to me, unless these computers can somehow boost your engines compression based on the octane used. Its my understanding that low compression (i.e. my stock 4.8L) engines do not benefit from high octane.

Alabama-man · March 31, 2006

I would also like to add, that for me, a performance tune is worth the money if they can deliver a couple of simple things, such as richen up the air-fuel mixture, maybe advance the timing a couple of degrees, turn off the rear oxygen sensors, quicken throttle response and firm up the shifts. I know thats alot, but I believe they can deliver on these things and be well worth the money. My problem is when they go and add what may be fictitious statements to their sales pitch. This whole notion of improved HP based on octane makes me distrustful of what otherwise appears to be a very good product.

Mark_in_Tulsa · March 31, 2006

The higher the octane rating..............the more you can advance the timing and adjust the spark curve for performance.

Ross 650 · March 31, 2006

Howdy,

I just got my W4Me tune yesterday. I told the tuner that I wanted to stay with 87 octane gas. He said that I could expect about 20 h.p. and 2-3 mpg increase. I havent checked the mpg yet but after running it for a day, I can say that the h.p. increase is definately there. Mark is correct, you can get more h.p. by increasing the timing and modifying the curve. There is no need to increase compression. I dont think I need any more performance increase than what I got. I am one happy customer for Jesse at Wait4Me Performance.

Alabama-man · April 1, 2006

Not saying I don't agree with you and Mark, but it is my understanding that octane is related to compression in this way. Higher compression engines actually heat the gas charge higher than lower compression engines. Thus if you had a high compression engine and ran regular 87 octane fuel the engine will detonate. Detonation occurs when excessive heat and pressure in the combustion chamber cause the air/fuel mixture to autoignite. The octane rating of a given grade of gasoline is a measure of its detonation resistance.

Therefore, I'm still not sure how advancing the timing would require higher octane. Also if advanced timing did require high octane fuel, wouldn't the engine require high octane all the time?

Mark_in_Tulsa · April 1, 2006

Custom tuners, and hypertech, superchips all increase engine power by advancing the ignition timing, and by doing so they require you to run a higher octane fuel to avoid detonation. So the higher the octane you use, the more they can advance the timing.

And its really hard to explain but I will try anyways. When you advance the timing, you are increasing the explosion sooner as the piston is right before the TDC, so you are actually kinda increasing the compression. But when used with a higher octane you can spread out the explosion longer...... I hope that made sense.

Rammer · April 1, 2006

Having looked into getting a performance tune for my 2006 Sierra 4.8L, I found out that one tuner in particular offers up different levels of performance based on what octane of fuel used. In other words, if you use regular then performance gain is say 10HP (hypothetical) and if you use premium then performance gain is 20HP. Does anyone know how this works?

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There are 2 octane tables in the PCM tune, High and Low. The timing is increased for the higher octane fuel. If the knock sensors detect knock, timing is pulled by the PCM. The PCM "learns" this knock and it will blend the high octane table with the low octane table until the knock is reduced.

I have read that the PCM will relearn knock after about 5 gallons have been added to the fuel tank.

Alabama-man · April 1, 2006

It appears that the guys with the timing advance and octane requirements are correct . I found a couple of web sites describing the principles behind this, here is one of them:

www.se-r.net/engine/about_timing.html

Bottom line, to achieve the most HP gains in tuning these engines, the tuners adjust the ignition timing maps (advance the timing) requiring high octane fuel.

zippy · April 5, 2006

Howdy,
I just got my W4Me tune yesterday. I told the tuner that I wanted to stay with 87 octane gas. He said that I could expect about 20 h.p. and 2-3 mpg increase. I havent checked the mpg yet but after running it for a day, I can say that the h.p. increase is definately there. Mark is correct, you can get more h.p. by increasing the timing and modifying the curve. There is no need to increase compression. I dont think I need any more performance increase than what I got. I am one happy customer for Jesse at Wait4Me Performance.

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If you check the numbers on a dyno you'll find very little actuall gain in going to the 87 octane tune. A claim of 20hp gain is BS using 87 octane. The most gain you're going to get is from Jesse removing the torque management. Sticking with the 87 octane tune is going to allow for very little timing curve change, aside from the torque management that only leaves an actual hp gain to be had from leaning the a/f ratio closer to where it should be. I'd bet that on a chassis dyno you'd be lucky to see 5-7hp gain. If you take a 4.8L or 5.3L and do nothing more than lean it out, remove torque management, and change the shift points there is roughly a 3 tenth gain in quarter mile times. 3 tenths is extremly noticable even though your actual hp gain at peak changed very little. The truck will feel much stronger by keeping itself in the powerband from the shift points being changed and less power loss on the shifts due to the torque managment change.

Running more timing advance increases the time allowed for the spark to take it's effect in the cylinder prior to the cylinder actually firing. You will effectively burn more of the fuel by being able to start the fire sooner. The longer the time is between the spark occuring and the explosion caused when the cylinder firing actually occurs, the more flame travel will occur when the cylinder actually fires/explodes and you therefore will be getting more of the fuel burned per cylinder firing/explosion. This (as long as the explosion doesn't occur before the cylinder hits top dead center) in turn makes the explosion stronger for every time the cylinder fires creating more power. In the case of the newer engines the compression is higher and can benefit more from higher octane fuels. Higher compression create's a much stronger explosion caused by the combustion charge being mixed much better from the high cylinder pressures. The compression creates more heat as the pressure increases (piston moving upward on the compression stroke) bringing the fuel which has at this point turned into an actual gas (non-liquid) closer to firing. The higher octane fuels require more heat before they will ignite. This is where the extra timing advance comes in when running premium fuel. Premium fuel although it requires more heat to ignite, creates more power because the hotter the explosion is when it occurs, the more of the fuel particles will actually be burned per explosion creating more power.