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Old 12-12-2011, 03:06 PM   #1
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Default 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?
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Old 12-22-2011, 02:00 AM   #2
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Bump. Any way you could tune for 92 octane (if it's available)? I wouldn't think that'd be overkill.
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Old 12-26-2011, 03:59 AM   #3
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Get a good knock control system and stop worrying about it.
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Old 12-26-2011, 01:02 PM   #4
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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.
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Old 12-26-2011, 01:13 PM   #5
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run corn. forget about knock.
like an bawss
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Old 12-26-2011, 04:03 PM   #6
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Quote:
Originally Posted by hornetball View Post
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).
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.
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Old 12-26-2011, 04:07 PM   #7
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Then why do cars make more power on race fuel? Is it more then just high octane gas?
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Old 12-26-2011, 06:53 PM   #8
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Quote:
Originally Posted by Savington View Post
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.
I wanted to try to establish this. However, the references I found (by references, I mean, published scientific articles) indicated that octane and flame speed are truly independent characteristics. Chemistry changes can affect one at a time or both, in direct and inverse relationships. Two that come to mind are oxygenated race fuels for 9000RPM applications (your "happy gas" example) vs. highly leaded aviation fuels for 2700RPM applications. Both very high octane. Huge difference in flame speed.

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.
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Old 12-26-2011, 07:00 PM   #9
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Quote:
Originally Posted by Gearhead_318 View Post
Then why do cars make more power on race fuel? Is it more then just high octane gas?
The key to making power is fast combustion along with resistance to detonation so you can run at MBT timing. Under any given set of conditions, there are no gains to be had from additional detonation resistance if you can reach MBT timing. Using Hustler's example, if you can reach MBT on 91 Octane street gas, then there is no reason to run 93.

And, as Sav stated, many racing gasolines have high O2 content. Kind of like having a mini-shot of NOX.
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Old 12-26-2011, 07:03 PM   #10
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That makes sense. When I was younger I wondered why nobody ever thought to use liquid O2 as an additive, I guess they kinda do in a way.
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Old 12-26-2011, 07:17 PM   #11
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The only time this theory in OP will technically work is if you get a shitty tank of 93 or mix it up with 91.
So unless you want to step up to em that has decent knock detection/control capability the original plan sounds like an ok one to me.
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Old 12-30-2011, 10:12 AM   #12
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Quote:
Originally Posted by Savington View Post
Get a good knock control system and stop worrying about it.
STFU with your AEM peddling. I'm not spending $1200 on a computer and relying on a knock-sensor for tuning.

Quote:
Originally Posted by 18psi View Post
The only time this theory in OP will technically work is if you get a shitty tank of 93 or mix it up with 91.
So unless you want to step up to em that has decent knock detection/control capability the original plan sounds like an ok one to me.
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.
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Old 12-30-2011, 11:53 AM   #13
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ramp the timing up to like 35* at decel and high 20's+ in super light throttle areas, afr at like 14.7-15.5 in the super low throttle areas, and a overrun cut enabled and I think your mpg will be pretty sweet
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Old 12-30-2011, 02:16 PM   #14
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Quote:
Originally Posted by 18psi View Post
ramp the timing up to like 35* at decel and high 20's+ in super light throttle areas, afr at like 14.7-15.5 in the super low throttle areas, and a overrun cut enabled and I think your mpg will be pretty sweet
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.
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Old 12-30-2011, 02:18 PM   #15
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Quote:
Originally Posted by Gearhead_318 View Post
That makes sense. When I was younger I wondered why nobody ever thought to use liquid O2 as an additive, I guess they kinda do in a way.
F1 did something along these lines for a few seasons with crazy fuel chemistry in the turbo era.
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Old 12-30-2011, 02:20 PM   #16
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Quote:
Originally Posted by hustler View Post
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.
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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Old 12-30-2011, 02:49 PM   #17
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Are you experiencing shitty mileage in your DD or something? The last month or so I've been getting low 20's and my exhaust looks sooty, might have to replace O2 censers from what I've read
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Old 12-30-2011, 05:38 PM   #18
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Quote:
Originally Posted by shuiend View Post
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.
Werd... Just because you can run an engine leaner than that range (I've heard 15.2-15.5) doesn't mean that you aren't ingesting more air, and hence more fuel to keep it running there... for best mileage, tune timing for max vacuum @ 15.2-15.5 AFR. With e85 its a bit leaner @ 16.5 or so (in gas wideband terms, actual air:e85 is in the neighborhood of 12:1, lol)
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