# R/s Question

Discussion in 'Engine Building' started by K2e2vin, Feb 1, 2003.

1. ### K2e2vinSenior Member

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ok i know the ideal r/s is 1.75, but what if the r/s is higher than 1.75? would it still work or be better or something?

2. ### asmallsolSuper Moderator

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it would work just the engine can not rev as high

3. ### lsvtecGNU/Linux Evangelist

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Think about what R/S means for a second. Rod length / Stroke length. All it says is that ideally your rods are 1.75 times the length of you stroke. Now if you stop and consider why that ratio is so "good". The longer your rods are compared to you stroke the smaller the angles are between vertical and rod position as the pistons travels up and down. Envision a cross section of an inline reciprocating piston engine. As the piston starts to climb from BDC the rod forms an angle with vertical at the wrist pin. If you have taken any physics this is pretty basic. Think of a force diagram illustrating the forces acting on the wrist pin ( for simplicity sake assume that the piston and wristpin are one unit). The larger the angle between vertical and the rod, the larger the sine of that angle, and the larger the horizontal component of the force applied by the rod (meaning more side loading on the piston and cylinder wall causing ovaling of the cylinders). Now, at 1.75:1 the angle between horizontal and the rod is minimal.

I would assume that if we were dealing with free, mass less engine components, that greater than 1.75:1 would better "better", I believe that engine designers do not attempt to achieve greater than 1.75:1 for several reasons:
1. The longer the rod, the greater its mass. Adding mass is bad because it increases the amount of power generated by the engine that is consumed in keeping itself spinning.
2. Cost, larger rods cost more to manufacture, and with longer rods you need to have longer blocks which also costs more. You get the idea.
3. Diminishing returns. The relationship between side load and R/S ratio is not linear. Meaning that a .4 increase in the ration from 1.75 to 1.79 won't make as big a difference as from 1.71 to 1.75 did.

Cliff Notes: A R/S ratio larger than 1.75:1 is unnecessary and the costs out weigh the benefits.

4. ### BigJI'm just about that action Boss.VIP

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my god I learn something terribly valuable and new everyday I read the posts here.

5. ### lsvtecGNU/Linux Evangelist

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It's all in that class that everyone slept through in HS.

6. ### K2e2vinSenior Member

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Phew, thanks for the info. I asked that cuz i wanted to see if it is possible to run b16a cranks in b18 block and have a good r/s ratio. turns out its around 1.85. oh well.

7. ### lsvtecGNU/Linux Evangelist

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It would work and you could spin the hell out of it, but you would lose the extra displacement.

8. ### K2e2vinSenior Member

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unless you rebore/resleeve it?

9. ### lsvtecGNU/Linux Evangelist

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If you bore out to 84mm (the B20 bore) with a B16 crank that yeilds 1715.7cc's. (1.7 liters). Still 100 cc's less than the B18A/B.

10. ### E_SolSiMember of the 20 nut clubModeratorVIP

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what kind of effect would this have on the compression ratio??

11. ### lsvtecGNU/Linux Evangelist

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That is a good question, I haven't even stopped to think about it. It would lower the compression. Using the LS rods on a B16 crank with the pistons at TDC it will be sitting a little lower than it was using the LS crank.

12. ### timEspeedSenior Member

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<-------- see my avatar

Which isn't all that bad in certain applications where piston to head clearance is an issue.

13. ### pissedoffsolRETIRED

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so check it out its like this:

1.75:1 is the ideal ratio. why? because it is said to be by some mathematical formula that at this ratio, a motor will make a good amount of revs, while retaining some torque.

the lower you go, the more torque you get, the less your engine can rev out safely.
the higher you go, the less torque you get, the more your engine can rev out safely.

so a motor with a 2.0:1 r/s ratio will scream easily to 15k rpms (ideal motor with head work to hold it) but it won't make dick for power until 12 grand. in other words, a b16a
now, a motor with a lower r/s ratio lets say 1.5:1 will be a torque monster, but will not fair well to higher revs (read, V8)

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