It's been 2 months...

[Circuit]

It's been 2 months and I still love this little black box. Three weeks ago (just after they were back from vacation) I got my custom tunes. I have been driving on my custom 93 octane tune with 94 in it leaving it on Bill's settings. Compared to the canned tune it is very much worth the $30. It is exactly what I was looking for. The truck drives smooth and fast. Downshifting is the way it should be from the factory. I can get 20+ mpg if I stay near the posted speed limits.

I am curious, since the tune is labled "93" performance should I kick the timing up 0.5 for the 94 octane? I tried it at +0.5 and +1.0 and it didn't really seem very different. With the canned 93 perf tune when I upped the timing +0.5 there was a noticable difference in power.

Just trying to have my truck be the best it can be. I am very happy with the way it is now.

[Jackpine]

Bill would have to confirm this, but if you told him you were using 94 octane gas, he would have written the tune for that, and it would include an adjusted timing curve. If it's already set where it belongs, then trying to advance it more will probably not do much since the spark knock sensing system will probably pull timing to keep detonation from occurring.

However, when you were running the canned tune, you were actually using a tune set up for 91-93 octane gas and it would probably do better with a bit of timing advance.

- Jack

[Circuit]

That's what I figured and makes sense to me.

[Power Hungry]

One other thing to consider is that to a certain point, higher octane isn't going to make any further improvements. The concept of octane ratings has more to do with compression and cylinder pressures.

Consider that timing will only get you so far because the combustion process needs to occur at a point where maximum pressure will begin to push the piston down. Any earlier and you end up fighting the piston on the way up. Once you reach a peak point, you can probably swing 2-3 degrees without much difference in performance which is why we will often tune for peak power and then back off 1-2 degrees for margin of safety. It just happens that on most vehicles, the peak timing point and cylinder pressure is just a bit more than the octane rating of the some fuels will allow. This is why we run higher octane fuels... to get the timing to the peak curve.

Now once your at the peak curve, the only way to get more performance is to begin increasing the compression ratio. Again, octane plays an important part in preventing detonation under higher cylinder pressures and higher octane fuels allow higher compression ratios.

So basically, you could tune for the highest timing that makes power and then run the lowest octane fuel possible while avoiding detonation. This, however, is not an ideal practice since detonation points can change in relation to many environmental influences, especially heat.

If timing changes aren't providing any further performance, then your timing curve is where it probably it needs to be and you might want to see how different fuels affect performance.

Hope this makes sense.