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Tractor Pulling Discussion Forum

Fuel octane

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RWWisehart

05-11-2007 10:28:03




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I just finished an Unstyled A with 9 inch stroke and 5 3/4 bore. What octane should I use, 87 or racing gas and why? Thanks




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compression

05-11-2007 10:36:28




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 Re: Fuel octane in reply to RWWisehart, 05-11-2007 10:28:03  
You forgot the most important stuff...
1. Compression ratio
2. Timing
3. RPM



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B-maniac

05-12-2007 05:25:39




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 Re: Fuel octane in reply to compression, 05-11-2007 10:36:28  
He's right. First of all , racing gas comes in quite a few octane ratings. Compression ratio will get you in the ball park as to what octane to start with. RPM and timing will tell you where to fine tune (blend)your octane level to get the most out of every intake charge. Assuming there are no availability issues , my opinion for base line is 87-93 pump gas for anything under 9:1 comp. , blend for 94-98 from 9:1-10:1. From 10:1- on up use leaded racing gas from 100 octane up to 115 depending on how high. Cam timing plays a big part also especially when your rpm drops below the level that you are tuned for. My guess is that you are in the 98 octane range. Trial and error and testing will get you on. Don't use aircraft fuel for racing gas. It is blended for high altitude use where there is less oxygen density.(planes don't spend much time running on the ground). As a rule , octane allows you to run high compression , lot of timing and lots of rpm , all of which is condusive to more hp , but depending on your setup , you may not even need it. Don't use it just for the smell , you may even cost yourself hp.

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buickanddeere

05-13-2007 11:21:58




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 Re: Fuel octane in reply to B-maniac, 05-12-2007 05:25:39  
Other people claim that airplane gasoline is blended for long stroke, large bore slow speed radial engines with open combustion chambers. Some of those tractor engines certainly are long stroke, large bore & slow speed with open combustion chambers. The Cam series of high octane gasoline works better in small combustion chambers with lots of quench, short stroke & small bores.Eg automotive & bike engines.

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B-maniac

05-13-2007 19:17:19




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 Re: Fuel octane in reply to buickanddeere, 05-13-2007 11:21:58  
I'm not so sure ALL aircraft engines are "radial" , or slow speed , or large bore/long stroke etc,etc, BUT , I am sure most of them run in very thin air at high altitude without the aid of forced induction. I know this creates an air/fuel ratio issue and it has to be variable to cope with elevation changes from 0-10,000 ft. , but I also think the blend of fuel and it's properties plays a key role in how well it adapts to all the conditions present in a flight. Any refinery lab techs out there prowling this board??

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buickanddeere

05-13-2007 19:51:38




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 Re: Fuel octane in reply to B-maniac, 05-13-2007 19:17:19  
The airpressure only affects the gasoline as to it's tendency to vapour lock. If flying at elevations where the airpressure is say 12 psi instead of 14.7 psi. The automotive gasoline will flash into vapour before the Av gas wiil. As for mixture ratio? Many aircraft engines run blowers or turbos so the intake manifold sees just about the same pressure at either 500 or 10,000ft. With nat aspirated. If the air is only 80% as dense at at altitude as it is at sea level. Then only 80% of the fuel delievery is required. The ratio doesn't change Air/fuel mixture concerns with AV gas v.s auto are a non issue. Switching from any brand or type of gasoline may require a slight jetting adjustment to obtain peak torque and/or HP. No news there.

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pukeanbeer

05-13-2007 17:21:08




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 Re: Fuel octane in reply to buickanddeere, 05-13-2007 11:21:58  
Got any dyno sheets for this?



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John Hurron

05-17-2007 12:54:47




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 Re: Fuel octane in reply to pukeanbeer, 05-13-2007 17:21:08  
With all his knowledge he doesn't need dyno sheets.



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Bob Barker

05-12-2007 07:37:15




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 Re: Fuel octane in reply to B-maniac, 05-12-2007 05:25:39  
Like B-maniac said, if you don't need it you'll cost yourself horsepower. Use the lowest octane you can without knocking or pinging occurring and you'll be set. Higher octane gas is just harder to ignite and if you don't need it the power loss comes from the gas not igniting as well and not burning as well.



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