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Tractor Talk Discussion Forum

Compression ratio and octane rating

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hogman_2002

03-26-2007 10:32:44




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Are there any charts on the internet that tell you what octane rating you need to run in an engine with 9.1:1 compression ratio?




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buickanddeere

03-27-2007 20:25:02




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 Re: Compression ratio and octane rating in reply to hogman_2002, 03-26-2007 10:32:44  
Depending on the cam grind and the design of the ports/manifolds etc. A 7.5 to 1 engine may knock on 87 octane and a 9.5 to 1 engine may run fine on the same 87 octane. Detonation depends on cylinder pressure, rate of compression(piston speed), air/fuel ratio, combustion chamber design/shape, cylinder size, air temp, engine temp & cam timing. Retarding the timing on a high comp engine just make them loose HP/fuel efficiency and tend to over heat. Odds are 87 octane will reduce engine power and efficiency. If your ears can hear detonation, it's already too late to retard the timing.

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KEB

03-26-2007 19:14:03




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 Re: Compression ratio and octane rating in reply to hogman_2002, 03-26-2007 10:32:44  
Also depends on whether its a carbureted or electronic fuel injected engine. Current electronic fuel injected engines have knock sensors, and will automatically retard the timing to whatever is necessary to avoid knock. As others have noted, for a carbureted engine there are a lot of variables. Does it have to meet emissions standards? EGR affects knock, as well as engine temperature and load, and the timing curve (there's probably other factors as well, but that's what comes to mind)

I can remember setting timing on older cars by advancing the timing until it would knock under load, then backing off just enough to avoid knocking.

Keith

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hogman_2002

03-26-2007 16:55:40




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 Re: Compression ratio and octane rating in reply to hogman_2002, 03-26-2007 10:32:44  
My brother bought a crate engine from Jegs to put in his 81 4x4 Suburban. The spec. sheet he got with it says to run 92 octane in it. He didn't find that out until the engine was shipped to him. Now he's looking for some way to run 87 in it. I was thinking he could retard the timing a few degress and run 87 and be ok. Like I said, the compression ratio is 9.1:1 and initial timing is set at 10 degrees BTDC.

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Bob

03-26-2007 12:33:30




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 Re: Compression ratio and octane rating in reply to hogman_2002, 03-26-2007 10:32:44  
The only way you will know for sure is by trial.

In most cases; regular 87 octane pump gas should be adequate; but you ears will let you know if it's not. If it "pings" under heavy acceleration; get off of the throttle!

What are you working with... is the timing adjustable? The advance curve?



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Tom43

03-26-2007 11:00:18




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 Re: Compression ratio and octane rating in reply to hogman_2002, 03-26-2007 10:32:44  
Dear Hogman,
There is more to octane requirements than just compression ratio. Combustion chamber design enters in as does total timing advance. I remember in 1985 Chrysler redesigned the heads on the 318 engine, raised the compression, reduced total timing 10 to 15 degrees and retained the same octane rating.



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Mike M

03-26-2007 10:57:55




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 Re: Compression ratio and octane rating in reply to hogman_2002, 03-26-2007 10:32:44  
There may be some guide lines ,but I know that it is not cut and dried as a different design inside the engine can sometimes let you run higher compression without knock troubles that other designs can have.



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lucasss

03-26-2007 17:55:44




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 Re: Compression ratio and octane rating in reply to Mike M, 03-26-2007 10:57:55  

we don build any gas engines over 8:1 anymore because of spark knock. lucas



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