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  #1  
Old May 8th, 09, 2:36 PM
DougA DougA is online now
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Default Octane Requirement vs Compression Ratio

Is there a hard and fast requirement about what racing gas octane is required for what C.R.? Such as for 12 to 1 you need 112 octane,for 11 to 1 you need 110,etc.?
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  #2  
Old May 8th, 09, 3:18 PM
Scotch Scotch is offline
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Default Re: Octane Requirement vs Compression Ratio

No- the fuel's resistance to detonation is dependent upon many factors beyond compression ratio. The design and efficiency of the chamber is a biggie, which is why the late-model LS stuff can run pump gas with higher compression.

Small heart-shaped chambers, and a matching-shape dish in the piston has been shown to be just about the most-efficient design. The addition of ceramic coatings to the piston tops, chambers, and valve heads has also been shown to keep detonation at bay. The finish on the chambers also contributes, as sharp edges (like those left around the circumference of the chamber after the head has been resurfaced) are also subject to getting hot. Think about it like when you watch a piece of wood burn, and the wind blows- the sharpest points glow the hottest. The same thing happens in your chamber.

You could always back the timing advance off until the detonation stops, but you'll lose a lot of power that way. That's counterproductive.

I designed my 383 to fight detonation, using all the techniques described above. It runs on 87 octane to 6,500 rpm with 9.7:1 compression and made 545 peak hp on the engine dyno. Detonation can be fought off, it's just up to you to decide whether the investment is worth the extra power. I think it is, but that's just me.
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  #3  
Old May 8th, 09, 3:22 PM
pdq67 pdq67 is offline
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Smile Re: Octane Requirement vs Compression Ratio

No hard and fast rule here b/c you can fudge several things that directly affect octane requirements.

Like cam intake valve closing point and Dynamic Compression Ratio stuff for starters.

Hunt up Pat Kelley's great DCR site and read it at least three times and I have!!

Next, a close quench dimension of like anywhere from just kissing the piston flats to to the head flats up to about .045". I figure from .035" to .045" here!!

Then fast-burn, compact chambers get involved.

Capacitive, multi-fire ignitions w/ timing pull back knock sensors too.

And "old-school" but now modernized water/alcohol injection too right down the throat under max load/boost to hold combustion temperatures down...

As silly as this sounds, E85 is the best fuel we have available if you do locally that will allow us to run up to 13.5 to 1 CR. engines once set up to do so!

And I thought I had read where guys are running it in dedicated engines up to 17 to 1?????

pdq67
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  #4  
Old May 8th, 09, 8:29 PM
bracketchev1221 bracketchev1221 is offline
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Default Re: Octane Requirement vs Compression Ratio

What compression do you have? For up to 12-13:1 I ran the 110. Over that I ran 112.
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  #5  
Old May 8th, 09, 9:14 PM
DougA DougA is online now
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Default Re: Octane Requirement vs Compression Ratio

We figured out at CR 11.9:1. Been runing 110,seems to like it just hard on the wallet.
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  #6  
Old May 8th, 09, 9:21 PM
pdq67 pdq67 is offline
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Default Re: Octane Requirement vs Compression Ratio

It's a shame it's not at 13.5 CR. so you could convert it to E85.

E85 costs about the same as the rest of our gas around the Midwest where its made ethanol. Like $2.00/gallon and it's generally right at 105 octane.

pdq67
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  #7  
Old May 8th, 09, 9:27 PM
bochnak bochnak is offline
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Default Re: Octane Requirement vs Compression Ratio

There are many factors, but I came across this:



http://www.popularhotrodding.com/tec...on_ratio_tech/
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  #8  
Old May 10th, 09, 10:14 AM
pdq67 pdq67 is offline
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Default Re: Octane Requirement vs Compression Ratio

E85 was $1.899/gallon yesterday here about a mile away.

I want to say we have ethanol plants in the state but I may be off???

Here you go.

http://www.dnr.mo.gov/ethanol/airper...forethanol.pdf

Off by one.

Kyra's Permits Section made the list up at work.

pdq67
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  #9  
Old May 10th, 09, 11:58 AM
rustbucket79 rustbucket79 is offline
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Default Re: Octane Requirement vs Compression Ratio

Quote:
Originally Posted by DougA View Post
We figured out at CR 11.9:1. Been runing 110,seems to like it just hard on the wallet.
I know what you're talking about, we sell race gas and it feels like robbery LOL. Looking back 4 yrs, we used to sell a drum for less that what our cost is now.

There's a simple solution, start mixing some premium with a plug check to look for signs of detonation. Back when I ran an 11.44:1 combo, I was running premium with retarded timing on the street, and mixed it 50/50 with 110 octane Sunoco for the track. I would start with 2 parts premium to 3 parts 110. Put in a fresh spark plug and make a pass, then pull the plug and look with a magnifying glass for black specs. Make some more passes and plug checks. If you don't see any, you can try 50/50 next time and do the same process. You may be able to get away with less 110, but be careful. Even 50/50 will be a big savings.
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  #10  
Old May 10th, 09, 12:10 PM
pdq67 pdq67 is offline
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Default Re: Octane Requirement vs Compression Ratio

Remember, I mentioned this too!

"And "old-school" but now modernized water/alcohol injection too right down the throat under max load/boost to hold combustion temperatures down..."

pdq67
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  #11  
Old May 11th, 09, 10:05 PM
boldm boldm is offline
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Default Re: Octane Requirement vs Compression Ratio

Something to think about;
SBC 406,
4.155, X 3.75;
6"rod,
AFR 210 eliminator heads , 66cc (checked),
SRP professional series FT pistons, 6cc reliefs (messaged and checked),
1014 fel-pro, .039 compressed,
0 deck.
Approx 11.45 - 1 compression,
UD Harold's 243/249/112LSA, solid roller,
160 thermostat, cold air intake.
34 degrees total timing,
94 octane fuel.
It works, no pinging, no signs of detonation.
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  #12  
Old May 12th, 09, 12:32 AM
Smokey1961 Smokey1961 is offline
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Default Re: Octane Requirement vs Compression Ratio

Haven't seen altitude mentioned.

Jeff
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  #13  
Old May 12th, 09, 12:38 AM
boldm boldm is offline
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Default Re: Octane Requirement vs Compression Ratio

2400 feet,
corrected = 4200.
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93 Camaro backhalf car, 3100lbs SBC406 (tow truck motor) AED 850DP,super victor, AFR210 elim, Flat tops, UD Harold SR 243/249/112LSA, 1-3/4 custom headers, T-400, 4500 stall 4.30, best ET/MPH 10.30 at 129 mph (corrected), 10.57 at 126 at 3100 foot air. 1.39 - 60 ft. 91 octane pump gas.
47 years old, into my 5th decade of drag racing.
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