91 octane tune, run 93 octane for safety
#1
Tour de Franzia
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91 octane tune, run 93 octane for safety
I'm going to put my car on the dyno again soon because I previously tuned on 60% leaky valves and gutted my intake plenum. I've always considered tuning on 91 and running 93 at the track for more headroom in regards to safety...and when I run Hallett only 91 octane is readily available. Previously I hit MBT in every cell up to 16psi without detonation. If I'm hitting MBT and beyond without detonation, then it makes sense that I'd see no output decrease from 93-91 octane if I continue hitting MBT...unless I experience detonation next time.
Is this too much headroom and is it overkill? I will admit that although my engine looked great when I pulled the head off, after nearly 100 hours of track time, I'm always afraid of hurting the engine at the track. This might help more with paranoia than anything else.
Thoughts?
Is this too much headroom and is it overkill? I will admit that although my engine looked great when I pulled the head off, after nearly 100 hours of track time, I'm always afraid of hurting the engine at the track. This might help more with paranoia than anything else.
Thoughts?
#4
Only thing I would add is that MBT is all about flame speed. Flame speeds can vary quite a bit. The one control that we normally use to vary our flame speed is mixture. Fastest flame speed (gasoline) is at about 12.5-13.5:1, with flame speed slowing by going richer or leaner. Flame speed is also affected by chemistry, with many racing gasolines specially developed for fast flame speed. So, if you tune for MBT on street gas, you would be giving up some torque if you use race gas (you should reduce timing for the race gas).
Thus, assuming the 91 and 93 in your example have the same flame speed and, therefore, hit MBT at the same timing without det, then power would be nearly equivalent.
How's that for a barely readable post?
Rambling.
Thus, assuming the 91 and 93 in your example have the same flame speed and, therefore, hit MBT at the same timing without det, then power would be nearly equivalent.
How's that for a barely readable post?
Rambling.
#6
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Half wrong. Unless you are using oxygenated race fuel or other happy gas-style stuff, the flame speed will decrease as the octane rises. The car would make less power on 93 octane than it would on 91 octane (an imperceptible amount, well within the margin of error for ambient temps, piston heat soak, etc) because the 93 octane will burn a touch slower.
#8
There does seem to be a more direct relationship between flame speed and a fuel's volatility (how readily it phase changes from liquid to gas). So, Winter gas vs. Summer gas? High altitude gas vs. Sea Level gas? So many variables.
#9
And, as Sav stated, many racing gasolines have high O2 content. Kind of like having a mini-shot of NOX.
#12
Tour de Franzia
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STFU with your AEM peddling. I'm not spending $1200 on a computer and relying on a knock-sensor for tuning.
I think I'll keep tuning on 93 and pull 3* for OK's 91-octane.
I'm also interested in this for running 87-octane in my daily driver, while tuning on 93-octane for power numbers. It's a highway car which makes a 500-mile trip three times per month. I'm going to do a lot of experimenting with cruise tuning.
I'm also interested in this for running 87-octane in my daily driver, while tuning on 93-octane for power numbers. It's a highway car which makes a 500-mile trip three times per month. I'm going to do a lot of experimenting with cruise tuning.
#14
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I'm starring to think that leanness is not the answer. Hitting LPP with a more powerful AFR may be the answer. Going from 15.1 to 16.5 with spark advance had a negative effect on MPG. I'm going to work on a tune that minimizes throttle position on the dyno and see what it gets me.
#16
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I'm starring to think that leanness is not the answer. Hitting LPP with a more powerful AFR may be the answer. Going from 15.1 to 16.5 with spark advance had a negative effect on MPG. I'm going to work on a tune that minimizes throttle position on the dyno and see what it gets me.
#18
I think TravisR posted a while back about how one of his fancy PHD level text books talked about 15.5 being the best AFR for getting good mileage. Something about leaner then that and it does not burn right, or some other fancy science **** that is way above my pay grade.
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