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Old 02-11-2012, 03:16 AM
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Fuel octane for comp ratio.

Hello fellow petrolheads. Ive got a 408ci sbc with 10.4/1 comp ratio and running a set of 195 AFR eliminator heads on it. I've recently discovered that it seems to run perfectly fine on 91 octane, even with a good thrashng. Our fuel here in New Zealand is crap and everybody tells me there is no way it should run without detonation on that octane. My cam is reasonably agressive, a lunati 60105 flat tappet hydraulic cam with .525 and .546 lift - 241 and 249 degrees at 50 thou. So I am wondering whether the cam is letting me get away with a little more compression by leaking a bit past the valves. Would I be right in thinking this? My engine has so many 'so called' experts bewildered because they dont think a cam like that should'nt be streetable, yet it idles at 600rpm in gear with a 2900 stall and pulls from way down low. A real nice behaved performance street engine :-) I know that for best performance you should run as low an octane you can without detonation, so should I stick with the 91 or be on the safe side and run the more expensive 98 octane?

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Old 02-11-2012, 06:06 AM
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Here's a link that might help. It's got a dynamic compression ratio calculator at the end that I find very helpful. Can't say that I have verified the results personally, but I am building my current engine on some of the data I've got from using it.
FWIW
ssmonty
http://cochise.uia.net/pkelley2/DynamicCR.html
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Old 02-11-2012, 06:45 AM
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Should be fine on 91 octane, especially if you're up in the mountains. Those heads have really efficient combustion chambers, and good swirl, both of which helps resist detonation.
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Old 02-11-2012, 06:58 AM
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yes, aluminum heads and the big cam are lowering the octane requirement.

Other things that effect octane requirement is load on the engine (weight, gear), elevation, engine and air temperature.

I run a 11:1 362 cid sbc with Dart iron heads with 93 octane with a 244/244 cam at 0.050 in a 2800 lb car with 3.90 gears and have no issues.
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Old 02-11-2012, 12:41 PM
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On the KB calculator, your Dynamic Compression Ratio is 8.55:1, so that's why you can run pump gas without detonation. I'm guessin' that you built a good, tight squish into the motor also (0.889-1.143 mm).
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Old 02-11-2012, 12:42 PM
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10.4:1 is a workable compression ratio for good modern unleaded premium pump gas. Its up there, but workable. If you get any hint of detonation use the better fuel available. If you plan to run it hard ( racing, lots of WOT hard running) use the best fuel available.

You should not use the lowest octane pump fuel thinking you will make more power. This is false and a myth born by comparing modern unleaded gas that has a relativly fast burn speed, reguardless of its octane, to old school race gas for extremely high compression ratios that does have a slower burn speed. Compareing them in a low cr motor that does not benefit form the fuel octane.

Most all modern unleaded fuels (even the 114 octane unleaded) have a fast burn speed and tend to make a bit more power than the old school stuff.

In a 14:1 ++ cr race motor both type race fuels burn fast and make good power.

Its all what you compare it in.

Use the best fuel you can buy.

This is what we use here when called for. www.profuels.ca Blends great with pump gas.

when you get to 11:1+ you need a bit.
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Old 02-21-2012, 03:02 AM
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Well that was a learning experiance guys. As it turns out running 91 in my engine wasn't the right thing to do after all. It fouled some plugs recently, I had a feeling it may have been getting some crankcase oil in past the intake manifold gasket, so I removed the intake manifold. Most ports were stained a dark colour but not all the same. I showed my local engine builder and he said it was from pre-ignition. Fuel has been igniting early before the intake valve has closed! Yuk!
Something else I learned is that the advice I recieved from you lot is probably bang on, what you people dont realise is that our quality of fuel over here in little old NZ is absolute crap. So what works for you doesn't always work for us. Buga Buga Buga.
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