Octane Question

I know there's some sharp people in the group so I'd like to get some feedback on something I've been thinking about. The T/A is driving me nuts as it picks up power at the track the hotter the engine gets. I'm looking at
timeslips and the car consistently picks up 2.5 to 3 mph if I hot lap the car. If the car sits for an hour or so it slows down and I go through the process again.
It's an aluminum headed 462 Pontiac with the deck height of -0.025. I run a 100 octane blend to reduce the chance of detonation with the cast pistons so far down in the cylinders. The static compression ratio is around 10.0:1. I pulled the plugs last week and they all seem to be running dark to me. I just had the car tuned on a dyno with a wideband O2 sensor so the engine shouldn't be running rich. Can too much octane cool the combustion chamber to the point of fouling the plugs? The car sees street traffic as well as time at the strip and runs well in both settings, idle vacuum is around 13.5 to 14.0 @ 950 rpm idle with a 255/265 @ 0.050 duration cam. I'm thinking of running the octane down to 95 and seeing what happens. I ran the car with new plugs at the track Saturday but haven't had a chance to pull one yet. If they are fouled again, I'll be leaning towards lowering the octane. Also, oil consumption isn't an issue, the rings are still tight, no smoke or blowby noticed yet.
Dave
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poncho462 wrote:

I wouldn't think higher octane would be the problem, but it's happened before, so maybe. If it likes running hotter, why not just put a higher thermostat in? Many people like to stick colder thermostats in even though it makes them go backwards. I can't see you needing 100 octane blend with only 10.0:1 compression anyway.
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Lower the octane, the slower the burn.. slower the burn, more fuel that is needed to create same HP...
Try dropping it a notch and see what happens...go to 91, and if you get no knock, and you probably wont, even go to 89..you might be VERY surprised..
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*CBHVAC* wrote:

    Brad and Steve both have the right idea. With a 10.0:1 static ratio, you probably don't need more'n 94-95 octane. Keep in mind that the hotter the engine gets, the quicker the air fuel mix expands and that ratio will change +/- a 100th...not much. That shouldn't really affect anything, but who knows, eh?
    And as Steve says, the burn will be slower with a lower octane mix however, your spark curve will be different as well. I believe THIS is what's affecting your performance Dave. The flash point isn't optimal for 100 octane until your current setup is running hotter. It might be worth a couple of runs with a lower octane rating and hence a lower flash point. Keep in mind that other hydrocarbon additives affect spark curve and flash point as well, Xylene, Heptane just to name a few. (I'm not sure if unleaded gasoline contains Dacane, Hexadecane, or Undecane...getting off topic.) The point is that Octane is one of the heavier additives; heavier additives have a higher flash point.
I dunno...play with it. Just the fact that you have the vehicle and a Desert Eagle makes me envious in my book! =) (Two forms of brute force with an elegance all their own.)
    Martin     '01 Formula -    MTI Air Box Lid, K&N Filter, Hurst-6,             SLP Cold Air Induction & Smooth Intake Bellow             Corsa Catback w/Premium Tips     '83 V45 Magna
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Everything I've ever read in the last 40 years says exactly the opposite, that a higher octane rating burns slower.
http://www.handymanusa.com/articles/octane.html
...scroll down to, "...using the right octane level", first paragraph.
http://theserviceadvisor.com/octane.htm
..."What is octane rating?".
Besides, this has nothing to do with a rich mixture.
nb
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well a higher octane fuel does burn more completely then a lower.
All aboard the Yankee express, next stop October.....
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notbob wrote:

I would submit that you're reading what I wrote wrong. Maybe I screwed up quoting Steve, but that doesn't change what I added to the conversation. Now that I think about it, I should have double checked what I was supporting rather than blindly quoting it; anyhoo, re-read what's important. I didn't address the length of the burn at all but rather to bring to light the difference in flash points.
I don't think I ever addressed "a rich mixture" at all.
BTW, that first link is perhaps the worst reference material I've ever seen on Octane. (2nd one is pretty good though.)
Point in fact, octane is nothing more than a hydrocarbon with eight carbon atoms chained together. Since octane can handle higher compression and a has a higher flash point than say....methane, a greater amount of octane mix should be used in higher compression vehicles.
Dave's 10.0:1 compression can probably get away with 94-95 octane rating. And I'm willing to bet that my theory is correct, that his block needed to heat up enough so that the ignition sequence and the compression were in step with each other...just enough not to cause knock. (Obviously, if the fuel is igniting under compression and causing knock, he's going to need to slowly bring up the octane rating till he finds a happy equilibrium.)
    Martin     '01 Formula -    MTI Air Box Lid, K&N Filter, Hurst-6,             SLP Cold Air Induction & Smooth Intake Bellow             Corsa Catback w/Premium Tips     '83 V45 Magna
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wrote:

Sorry I haven't replied but some minor work on the house has turned into a major job so I've taken this and possibly next week off to take care of things. I have been checking for replies every morning though and I appreciate all of the input. Before I try running lower octane I have to pull the intake to repair a blown valley pan seal and plan on cutting the water crossover from the intake at the same time. I'm also going to replace the mechanical fan with a 4600 cfm electric unit at the same time.
It will be a few days before I get back with some results on lower octane testing at the track. ;^)
Dave
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I run 93 pump gas with 11.25:1 compression and haven't had any problems.
All aboard the Yankee express, next stop October.....
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Is that the LS1 engine? I thought I remembered reading the compression was high on those. numbers like 11.25:1, 10.9:1 or 10.1:1 come to mind but I don't remember which (if any) is correct.
In any case, the manual for my LS1 equipped Z28 recommends 92 octane or better but says even 87 can be used (with slightly less acceleration power). Apparently, such high octanes as 95 to a 100 aren't required on all higher compression engines.
On 28 Apr 2004 21:13:23 GMT, snipped-for-privacy@aol.com (Hitman0187) wrote:

** To email a reply, please remove everything up to and including the underscore in my email reply header.
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no i built a 355 carburated motor with sportman II heads and 11:25 dome pistons. Hums pretty sweet.
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Yeah, I'll go along with that. :)
nb
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DAMMIT....I had a brain fart..
Ever have one of those moments...the HIGHER the octane, the slower the burn...
I should know that...I mean...running 110 in the Hemi from time to time..:)

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Also, please trim your posts. Some ppl have to pay for content. Thank you.
nb
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