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What is the ideal bore to stroke ratio for an America V8 (ex. Ford 351, Chevy 350).
I want to build a 351W-based Ford Stroker motor. I've seen engine kits all the way up to 470 cubic inches (assuming a Dart or World Products 4-bolt block). But I also see many 408, 427, and 454 kits as well. My question is, what is the optimal bore to stroke ratio before the piston side loading is perhaps no longer ideal?

Mmfp_0711_04_z_ford_small_block_stroker_engine_build_stroker_crank_thumb
To produce a longer stroke, the offset of the rod journal is increased.
Mmfp_0711_05_z_ford_small_block_stroker_engine_build_rod_angularity_and_piston_side_thrust_thumb
You can see that for any given stroke length, the rod angularity quickly increases as the rod length is shortened.
Mmfp_0711_06_z_ford_small_block_stroker_engine_build_rod_to_stroke_ratio_thumb
Shown here are the dimensions involved in determining the engine's rod/stroke ratio. The ratio = B divided by A.
Mmfp_0711_10_z_ford_small_block_stroker_engine_build_torsional_vibrations_thumb
A crank rotates and receives forces from firing pulses, it advances and retards from its true nondeflected position

Asked by: BigBlock

User type: Enthusiast

Asked on: November 03, 2008 00:30

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Answers

Amv12front
TheRipper
User type: Enthusiast
Posted: about 1 year ago
Lots of different bores and strokes make up these different engines. You can get most of the aftermarket blocks in 4.125" bore and long what seems like 4.25" stroke cranks these days. Rod length is also a limiting factor to some of the combinations as you start getting into higher RPM ranges. Also heads may not work really well for some big cube stroker kits so be aware of things like that and other components such as cams. The larger cube motors do have some nice trade offs since you have Cubes, you don't have to rely on a peaky high compression (and racing gas) and winding motor. Cams that may be too radical for a 351 might be mild for a 427. The big stroke motors are easy to run on the street and make a lot of HP and Torque too. Side loading gets bad with long stroke short rod combinations. As the stroke gets longer to maintain good rod ratio gets difficult with some combinations. The worse are ones that don't use custom parts and may be a bad compromise at best.
M-gear
Motor8rMatt
User type: Enthusiast
Posted: about 1 year ago
I think the better question would be the ideal rod to stroke ratio rather than bore to stroke. Its the rod ratio that will determine how much piston side loading you will have. Ripper is correct is that piston side loading increases when a long stroke is used with a short rod. In the past, a decent rod ratio would not exceed 1.55-1.58 (rod length divided by stroke). However, with good forged pistons, a strong block and good rods, you will probably be fine with 1.4-1.5 range. I spoke to noted race engine builder Bennett Racing and this is what Jon Bennett told me, "When building a long stroke 351 4.125-4.25 stroke, we base the rod length soley on piston design. Example: If we are building a turbo/blower engine with 4.125-4.250 crank the shortest piston we can use would be 1.300 due to dish requirements & rod BE thickness, so we would be forced to accept a 6.050-6.070 rod. Nitrous or no power adder engines the longest rod we would use would be 6.200-6.250."

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This image has been reduced by 20%
Close_button 
Mmfp_0711_04_z_ford_small_block_stroker_engine_build_stroker_crank
To produce a longer stroke, the offset of the rod journal is increased.
This image has been reduced by 20%
Close_button 
Mmfp_0711_05_z_ford_small_block_stroker_engine_build_rod_angularity_and_piston_side_thrust
You can see that for any given stroke length, the rod angularity quickly increases as the rod length is shortened.
This image has been reduced by 20%
Close_button 
Mmfp_0711_06_z_ford_small_block_stroker_engine_build_rod_to_stroke_ratio
Shown here are the dimensions involved in determining the engine's rod/stroke ratio. The ratio = B divided by A.
This image has been reduced by 20%
Close_button 
Mmfp_0711_10_z_ford_small_block_stroker_engine_build_torsional_vibrations
A crank rotates and receives forces from firing pulses, it advances and retards from its true nondeflected position

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