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Tractor Pulling Discussion Forum

More bore or stroke or both?

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Curious Mind

11-14-2006 22:33:23




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If you had a oliver 88 with a 4.25 bore and a 4.25 stroke OR a 4 3/8 bore by 4 inch stroke same amount of cubes which one would work better in diffrent situations (mph or open rpms or whatever)

No color war please! it is not about the color I want to learn more on engine building.




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toolz

11-15-2006 22:32:51




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 Re: More bore or stroke or both? in reply to Curious Mind, 11-14-2006 22:33:23  
I say bore it AND stroke it! No need to settle for one or the other!



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Mopower

11-15-2006 04:29:40




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 Re: More bore or stroke or both? in reply to Curious Mind, 11-14-2006 22:33:23  
Consider your crankshaft like a lever. When you break loose a rusty bolt, obviously you can loosen it easier with a longer wrench even though your arm is not pulling any harder. Say you have two engines with 200 cubic inches, identical heads, cam, rpm, etc. One with a 4x4 bore and stroke, one with 3.563x5 bore and stroke. The cylinders draw in similar abounts of air and fuel, but when the engine fires, the explosion pushing on a longer crankshaft with the 5" stroke generates much more torque even if h.p. numbers are similar. Usually the longer stroke works better at lower rpm (good torque) and shorter stroke good at higher speeds (good horsepower). A large bore also creates more vacuum sooner as the piston begins to move down. In tractor pulling, I would always opt for stroke size over bore size, but you do lose efficiency when your rod angle is extreme. The rod ratio is a factor better explained by an expert.

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buickanddeere

11-17-2006 18:42:30




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 Re: More bore or stroke or both? in reply to Mopower, 11-15-2006 04:29:40  
The longer stroke may gain on leverage. However the surface is of the piston is much smaller. So there is less thrust applied to the longer lever. Net result is no difference on crank torque. The long stroke also looses thrust on the crank as the rod is cocked on an increased angle = less leverage. The piston is also dragging heavier on the cylinder wall with the high angle. The long stroke engine have to run slower to prevent exceeding a workable piston speed. A large bore engine usually breathes better as there is room for larger valves.

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shrike

11-15-2006 02:47:28




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 Re: More bore or stroke or both? in reply to Curious Mind, 11-14-2006 22:33:23  
more stroke less bore = horsepower / higer RPM"s and needs lower gears to maintain draw bar hp

more bore less stroke = torque + higher gears / more power at lower RPM"s needs higher gears to keep RPM"s low



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RCP

11-15-2006 02:55:13




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 Re: More bore or stroke or both? in reply to shrike, 11-15-2006 02:47:28  
Hot Rod did a test by building identical displacment engines, one with long stroke small bore and one with short stroke big bore, the HP and Torque curves were virtually identical.



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B-maniac

11-15-2006 14:19:49




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 Re: More bore or stroke or both? in reply to RCP, 11-15-2006 02:55:13  
The curves were identicle but the rpm s applied to the curves weren't. A short stroke engine will NOT have the same torque OR hp at 2000 rpm as a long stroke will. The "curves" can still be the same shape and length. Just from a diff. rpm to a diff. rpm. Example; a 300 ci straight six Ford will have more torque at 2000 rpm than a 302 Ford v/8 (4"x3")everything else being equal. Same cfm intake and single exhaust. From there on up it's a different story. The curves may be the same but at diff. rpm range. A 190 cu. in. 6 cyl tractor will not out pull a JD "B" (190 ci) with a 1000 rpm limit on engine speed. More smaller cyllinders need to fire more times and faster to put the same forces on a shaft.

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SM-53

11-15-2006 04:35:49




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 Re: More bore or stroke or both? in reply to RCP, 11-15-2006 02:55:13  
Read the same article, could not believe the results. I think in that article the big stroke motor had alittle more torque, about 5%.

Bore and stroke are critical, pay attention to both in unison !



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