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Tractor Pulling Discussion Forum

Compression ratio and pump gas???

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Jim in OH

01-22-2007 10:01:10




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I know that the answer is more complex than just compression ratio (e.g, timing, air/fuel, etc..) but as a start, for a first time builder:

About how high can I go (compression ratio) and still expect to be able to use premium pump gas (92-93 octane)? About 10:1????? ?

Jim




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dboll

01-22-2007 16:22:13




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 Re: Compression ratio and pump gas??? in reply to Jim in OH, 01-22-2007 10:01:10  
premium is a joke if you have much compression, buy good gas it burns cleaner and has lead in it



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Jim in OH

01-22-2007 20:40:12




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 Re: Compression ratio and pump gas??? in reply to dboll, 01-22-2007 16:22:13  
this is to be a part time puller, and still be in the "field" sometimes... I guess I am not willing to make a trip to the airport for gas to do some brush-hogging.. :)

Thanks all for the answers... my current plans are to be at about 9:1 (just cc'd the heads, piston craters and did stroking/machining calculations)

Jim



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dboll

01-23-2007 12:39:27




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 Re: Compression ratio and pump gas??? in reply to Jim in OH, 01-22-2007 20:40:12  
what are you going to brush hog with a bored/ stroked motor? The only thing I can imagine would be mind altering plants!!!



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G/MAN

01-22-2007 13:47:48




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 Re: Compression ratio and pump gas??? in reply to Jim in OH, 01-22-2007 10:01:10  
As you already alluded to, there is no hard and fast rule for compression ratio vs. octane. Engine temp, load, camshaft specs and a ton more factors all affect that. 10:1 on 92-octane would be really pushing it in an old-fashioned tractor engine with an ineffecient chamber under high loads, in my opinion. Especially if you get into big-bore engines where the flame front has to travel very far across the cylinder.

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ChadS

01-24-2007 16:58:48




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 Re: Compression ratio and pump gas??? in reply to G/MAN, 01-22-2007 13:47:48  
Cyl pressure is relative to fuel octane. 200 psi wont run on 93 alone. The camshaft can let you run 87 octane in it if you put a big enough overlap in the grind in most cases. Spark plug location, and single fire ignition systems hinders firing big bore cyls with high cyl pressure.



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champspa

01-22-2007 12:53:36




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 Re: Compression ratio and pump gas??? in reply to Jim in OH, 01-22-2007 10:01:10  
yes



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