i still consider myself much a noob, you get a noob question. i keep hearing about how the R/S Ratio is what causes the LS/VTEC to fall apart/blow up/whatever. my problem is that i don't fully understand what rod to stroke ratio is, or even how it works...hence i'm in a state of confusion about the issue. i've read up on some sites and that ended me leaving in more confusion than i began with, so i'm asking you guys. i just did a search, and couldn't find anything, so help is greatly appreciated if somebody could give me some descriptions or whatnot...or even a good site to check out about this. thanks

 

R/S ratio is the side loading of the pistons on the cylinder walls, and the effects they have on the rods themselves.

1.4 is poor/ok
1.6 is good
1.8 is VERY GOOD
2.0 is EXCELLENT!!!

How to calculate R/S.

Length of Rod / Stroke of Crank in mm.

For instance. A GSR has a 87.2mm crank, and 138mm rods.

Divide 138 by 87.2 and you get 1.58

A LS block has a 89mm crank, and 137mm rods.

137 / 89 = 1.54

Following so far?

Here, lets do a CTR engine.

77.4mm crank / 144mm rods = 1.86

NOW, back to your LS/VTEC question. The reason MOST people blow their LS/VTEC engines are a couple of reasons.

1- They don't balance and weight match their bottom end. The LS block was never designed by Honda to rev past about 8000 rpm. Most VTEC heads in stock form can go to 8400 without a problem.

2- The LS rods are weak. This is often overlooked when building a LS/VTEC engine, and thus the weak link. The rod bolts can snap and have done so at about 8200 rpm give or take a few hundred. Thus, the rod makes a illegal exit through the block and no more LS block.

Hope this helps you out understanding LSVTEC's and R/S ratios.




[Modified by B18C-EJ1, 1:43 AM 12/28/2002]

 

The side loading of a more angled rod isn't the most important thing about rod/stroke ratio. In an engine with a longer rod length to stroke length, the piston comes down slower away from top dead center, allowing more complete cylinder filling at high rpm. As an engine revs higher, there is less real time for an intake stroke to occur and fill the cylinder well, and a better rod/stroke ratio makes the piston "dwell" around top dead center longer, because the piston accelerates away from TDC slower.

If you had two Honda B16As, and you had one standard and then the other with a piston with the pin moved up higher to use a longer rod, the longer rod engine's torque curve would fall off more gradually than the standard one. When the normal B16A's crank has rotated 30 degrees from TDC, the longer rod engine's pistons, at that same 30 crank angle from vertical, would be higher up in the bore.

Here's something most people don't realize. When the crank has turned 90 degrees from TDC, the piston isn't halfway down the bore. It is deeper than halfway. That is the nature of a crank/rod system. When piston is exactly halfway down the bore, the crank angle is more like 70 degrees, but that depends on the rod/stroke ratio. A longer rod engine's pistons would take more time (in crankshaft degrees) to reach that half-stroke point.

And since the piston has gone more than halfway down the bore in only 90 degrees, half of the crank's stroke, the last 90 degrees has less distance to move the piston than the first 90 degrees. More distance in the same amount of time means it moves faster, it accelerates faster, and that puts higher loads on the piston/rod that try to tear them apart. Longer rod engines have lower peak piston speeds. They still have the same average piston speed, because the piston ends up moving the same total distance in the same total time.