Octane requirements - Team Camaro Tech
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post #1 of 18 (permalink) Old Jan 12th, 01, 08:43 AM Thread Starter
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I know this has been discussed a lot in the past. I am trying to figure out an engines octane requirements so I can come up with the best mix of racing and pump gas. I am aware that cam lob center angle, compression ratio and combustion chamber design all have an effect. Are there any formulas that can be applied or can one of the professionals estimate it for me?

My application is a 406 with flat tops, Dart Iron Eagle 64cc heads and an Isky 292 cam that uses a 108 center line. Timing is at 36 degrees and I do not want to back anything off. Compression should be 11.65 to 1. My best guess is about 102 octane. Can anyone help me get this pined down any closer?
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post #2 of 18 (permalink) Old Jan 12th, 01, 02:24 PM
 
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Don't think so, at least nothing that a non-rocket scientist could understand. IMHO. pdq67



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post #3 of 18 (permalink) Old Jan 13th, 01, 03:46 AM
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Chris,

I would run 114 Octane with this combination. I don't think 102 is going to be enough. 114 is overkill. My thought is you really need a 106-108 octane number but I will try and dig up the actual formula if I can remeber it!!!.

Here is a link to making your own octane booster (no I have not used it myself!!)
http://www.vtr.org/maintain/gasoline-octane.html

UPDATE:

Chris, I found this later for you.

Octane requirements for a specific compression are generally perceived to be as follows:

Compressio Ratio: Octane Requirement:

8:1 92 9:1 96 10:1 100 11:1 104 12:1 108


[This message has been edited by sr71bb (edited 01-13-2001).]

[This message has been edited by sr71bb (edited 01-13-2001).]
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post #4 of 18 (permalink) Old Jan 13th, 01, 06:24 AM
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Previously Written by sr71bb

Compressio Ratio: Octane Requirement:

8:1 92 9:1 96 10:1 100 11:1 104 12:1 108

this ratios are a little off if you ask me. I have stand dynoed many engines with a setonation sensor to see exactly how far you could push octane with compression.

cheap old 87 will work fin for anything 8.0 - 9.0. With out any detonation or pre ingnition. 89 octane will work well for 9.0 - 10.2, 92 or 93 octane will work for anythng up too 11.0. 11.0 is pushing it, but if it is a daily driver it should be fine.
I personally do not like to mix gas to get an odd ball octane for any thing over 11.0. It is just not worth the risk. Differnt pump gases can have differnt specific gravity from brand to brand. personally my self i like to run either 108 race gas for any thing up to 12.0, 110 0r 111 anything up to 13.0 and 112-114 for anythin 14.0. If i have a motor 13.0-14.0 i really would rather run alcohol than gas. It produces much more power and torque
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post #5 of 18 (permalink) Old Jan 13th, 01, 07:26 AM
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I run 11/1 on 92 octane (aluminum heads)with no problem.Alcohol is a better choice for high comp. 12+ just because it's alot cheaper than race gas.
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post #6 of 18 (permalink) Old Jan 13th, 01, 10:42 AM
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Please note that CHRIS is running cast heads NOT Aluminum. That makes a huge difference boys!!!!

You can usually run 1 full point higher compression with aluminum heads compared to cast heads when it comes to being concerned about detonation.

That's why you can run 11.0 to 1 with Super Unleaded if you have aluminum heads.

[This message has been edited by sr71bb (edited 01-13-2001).]
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post #7 of 18 (permalink) Old Jan 13th, 01, 06:12 PM
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Just a curious note, what difference does material on the heads make compared to what kind of octane you must run?? I could see is if was open chamber aluminum then going to cast closed chambers. But i have never heard of material being a concern. Maybe i am wrong. Back in the summer I dynoed a 302 with double humps on it with 11.2 to 1 and it had no detonation at all. We also dynoed it with race gas, and all it did was make 2 more horespower, because, it didnt have enough compression to fully take advantage of the higher octane.
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post #8 of 18 (permalink) Old Jan 13th, 01, 06:33 PM
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I may be out of my league on this but I beleave it is heat disapation...

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post #9 of 18 (permalink) Old Jan 13th, 01, 06:52 PM
 
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sr71bb,

Please post the formula for octane requirements if it isn't a "PITA" to type into the post. I'm not a rocket scientist, but would like a look.

I'm really interested as to what the variables are that need to be considered when looking at octane requirements for a particular engine.

I read an article at work about a company pushing approval from the EPA to sell a 75 octane liquid as a fuel. It's octane rating puts it out of the gasoline "fuel" catagory.

The article went on to say that it is not an oxygenated liquid (i.e. gas-a-hol) and that it works great as a fuel for cars.

I don't buy that because if I am not mistaken, it is the physical layout of the particular engine in question that dictates octane needs of the engine and not fuel.

I'm on this like "stink on sh-t"!!! pdq67




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post #10 of 18 (permalink) Old Jan 14th, 01, 12:30 AM
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Trial and error, that's the ONLY method to find out. Start out high and work down. Many variables influence octane requirements such as, compression ratio, cam duration and LSA, deck clearance, head material, piston top, timing, gearing, and .... The point is, your motor needs what it needs. So find out. I have a 406, Dart iron heads, flat top pistons, 290-300 duration solid roller, @11.1 compression........ It runs fine on 94 octane with 36 degrees timing and shoves my 73 Camaro through the quarter just fine. I originally ran 110 just to be safe and worked my way down from there. It never made a difference.Good luck.

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post #11 of 18 (permalink) Old Jan 14th, 01, 05:18 AM
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There are other factors also that affect octane requirements...camshaft lobe seperation angle, engine operating tempreture, air intake tempreture, and cylinder head combustion chamber shape. Tighter lsa's increase cranking compression and can make an engine more likely to detonate...higher operating temperatures greatly increase an engines demand for more octane, and combustion chamber shape probably makes more differeence than anything I have found. Also, a/f ratio is another consideration...leaner a/f ratios are more detonation sensitive than richer mixtures. When I switched from ported 882's to vortec heads, my compression increased 1 full point (from 8.4 to 9.4), and combined with the bigger cam I installed I have 170 pounds of cranking compression vs. 150 with the old setup. I still can run 87 octane in my new setup with no change in performance and no signs of detonation, although I do run 89 for a bit of safety margin. My engine also runs very cool (cooler than it should) because I installed a 4 core big block radiator (78 chevy truck, by the way) since I also tow my camping trailer with the truck...only during the summer does it run about 160...normally it runs about 140-150 degrees. Lower gears also tend to reduce an engines octane requirements as they dont load the engine as hard as higher gears do. What I am trying to say is that there is no set rule...like someone suggested in another post, start high and work your way down. Thats what I did, and quite by mistake found I could run 87 octane with no problems...I was a bit upset after using 92 octane for over a year and discovering I could have saved myself a lot of money.

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post #12 of 18 (permalink) Old Jan 14th, 01, 05:47 AM
 
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Thats why I run "Tech2000" oil and the "cheapest gas" I can find!!

If you aren't beating on it then why spend the money??? IMHO. pdq67

PS, Change the oil and filter religiously every 3/3500/4 miles and keep a good fuel filter in the fuel line.



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post #13 of 18 (permalink) Old Jan 14th, 01, 12:23 PM
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My (stock) 11:1 '69 Z28 runs just fine on 93 octane with factory spec initial and total timing - probably due to the overlap characteristics of the 30-30 cam. As outlined in the above posts, each engine has many variables that affect its octane requirement - no simple formula to determine it on paper.

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post #14 of 18 (permalink) Old Jan 15th, 01, 03:57 AM
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I have steel heads, 11.5:1 compression, and a very large solid cam. The cam has a bunch of overlap in it. The motor runs on pump gas. The only time it gets race fuel, is if I want to spray it. I also keep the engine temp down low, the car is set with a 160 stat, but normally runs right around 180.

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post #15 of 18 (permalink) Old Jan 15th, 01, 01:01 PM
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Hey pdq67 did you know that Tech 2000 oil from Wal Mart for 78 cents a quart is actually Quaker State oil you are buying? I buy it also.
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