Navy_CRX

Ive been reading and read that a 1.75/1 rod stroke is perfect. My question is why. And what will i get out of having a perfect R/S ratio. Thanks in advance

D-Rob

I'm going to save you a lot of headaches here.

Do NOT worry about rod-to-stroke ratio on an engine designed for street/weekend warrior use.

Trust me.

- Derek

jdm602

you see more torque with lower r/s ratios but higher ratios you dont have the larger rod angles and sidewall loading that the lower ratios have. those angles put a lot of stress on the rod itself and the sleeves too. why do you think the b16 is so rev happy? its got (i think the highest) r/s ratio...correct me if im wrong.

but in the end there's no such thing as a perfect ratio. youll be losing an advantage somewhere no matter which ratio you have.

RC000E

Well, to actually answer your question versus telling you don't worry about it, here's the most brief answer that can be given.

First and foremost though, to better place DRob's comment, you certainly should not overthink or overanalyze the rod ratio issue. This isn't a factor which you should heavily concern yourself with, especially in the world of street/strip cars. Modifying your motor to optimize rod ratios are a cost that is beyond necessary for a typical setup. That being said, here's the simplest way to answer your question.

Rod ratio directly effects piston acceleration and deceleration as well as how long you get linger at TDC which has an effect on cylinder fill. It also influences sideloading of the piston, as well as the angular leverage the rod has over the crank.

Long rod motors (i.e. higher rod/stroke ratio) tend to be more comfortable at higher rpms. They linger at top dead center longer, which allows them to have better cylinder fill at higher rpms. Higher rod ratio's also reduce ring wear and reduce bearing load. Longer rods in relation to stroke also keep pistons accel and decel more reasonable as you rev the motor higher. Geometrically speaking they heavily influence how the motor breathes and what kind of power curve the motor produces.

The issue with rod/stroke in Honda motors really comes down to packaging. Look at the B16...it's a motor built to rev, built to breath on the top end, but in order to fit that in a block that isn't too ridiculously tall, Honda had to sacrifice stroke. Increase stroke, as in the GSR motor and you increase torque, but you must sacrifice rod length to maintain the package height...i.e. rod to stroke goes down.

It's all a package of compromises. Rod/stroke is also only one piece of the geometric puzzle which makes a motor ideal. Ideal engine geometry really comes down the application and it's use. A short rod, long stroke motor will make big torque, but the piston will quickly go so fast it'll out accelerate the flame front at higher rpms and the usable powerband goes away. Add length to the short rod and you increase the usable powerband but sacrifice some initial torque...it's all a balance.

If you look at rod/stroke, you also have to consider undersquare/oversquare (bore versus stroke ratio). Bore influences alot of how an engine breathes, how an engine is timed, etc. In the end though, you take a package and you fit what you can inside that is best for the need.

Honda gives you alot of options though, and upgraded internals allow you to increase piston speeds and overcome some losses associated with poor rod ratio. Forced induction also makes things a little easier without having to think about all this much. The N/A guys are the ones who must ponder heavily on engine geometry when you look at big power builds.

Now you've probably got more questions than you started with....that's the nature of studying engines though. Cam science, geometry, exhaust and intake design...it consumes lives and is something you can never truly master.

D-Rob

RC000E, you're a lot more patient man than I.

- Derek

RC000E

LOL.....I just type fast enough that it doesn't bother me much...hahaha....90+wpm no mistakes. Forums alot of years and part time web design alot of years.