Lately theres been much discussion on h-t about rod to stroke ratios and their importance and relavance to tuning, what do you guys think about Bore to Stroke ratio's and their impact on powerbands, rev limits and the such... anyone have any good knowledge on this subject?

 

cmon, one of you guys has to have some info on this!

 

A larger bore and a shorter stroke will give you a motor more suited to high rpm, high horse power. The large bore unshrouds the valves, letting the motor draw in air at high rpms due to the velocity. The short stroke keeps piston speeds down, allowing you to turn higher rpms without breaking stuff.

Two good examples are sport bikes and F1 cars. F1 engines in particular use very undersquare bores, with a tiny stroke, sometimes with twice the bore as stroke. The obvious effect is a retardedly high redline and tons of horsepower, at the expense of low end power.

More discussion please.

 

then why wouldnt honda, the king of high end horsepower, develop their commuter cars with undersquare b/s dimensions for power and reliability?

 

You mean they don't have power and reliability already?

81mm bore and 77mm stroke is undersquare, but to get a larger motor, it's probably cheaper and easier to put a longer stroke in than to make a larger bore.

 

Good info but you have it backwards. When the bore is bigger than the stroke it's called oversquare geometry.

 

Whoops, duh, you're right.

 

How did they determine it was bigger bore that made it oversquare? Mathematically, either way makes sense, correct?

 

I don't know, you're right though. I was just thinking that if you have a square and reduce the stroke, it's less than before, so undersquare, and increasing the stroke would be more, so oversquare. The terminology has been around for a while, so beats me.

 

I believe its referring to the over/under assembly of the engine. The bore is above the crank so anytime you have a larger bore than stroke its "over-square" in relation to the location of the two displacement factors.

Not dumbing it down for you, I just suck at explanations.

My take on Honda's choice for under-square motors is that they were trying to eek out every last ounce of usable power before VTEC. Im not sure why they didn't just increase the bore of a B16 to increase displacement other than the fact that short stroke/high rod ratio engines produce torque higher in the RPM range. Its not like any of Honda's engines could be called "torquey" anyway.

Thats just one guys take on it.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by notstock93 &raquo;</TD></TR><TR><TD CLASS="quote">I believe its referring to the over/under assembly of the engine. The bore is above the crank so anytime you have a larger bore than stroke its "over-square" in relation to the location of the two displacement factors.

Not dumbing it down for you, I just suck at explanations.

My take on Honda's choice for under-square motors is that they were trying to eek out every last ounce of usable power before VTEC. Im not sure why they didn't just increase the bore of a B16 to increase displacement other than the fact that short stroke/high rod ratio engines produce torque higher in the RPM range. Its not like any of Honda's engines could be called "torquey" anyway.

Thats just one guys take on it.</TD></TR></TABLE>

good post!