Everybodys talk'n about low rod stroke ratio being bad for all motor application, because a motor with a higher r/s has a slower piston speed. i remember when i had my b16 with my built head, taken it to 9k rpms feelt natural.. that same head is now on my ls/vtec and when taking to to 9 k rpms, well is kind of scary.. i can hear the motor work extremly hard at 9k, feels like its spinning at 11k.. i can immeditaly tell the difference in piston speeds when swapped motors.. my question is, imagine both motors (b16, b18a/b) in the same car both spinning at 3k rpms, since the b16 has a high r/s isnt it really doing lower rpms??? sorry if it seems like a dumb questions, i never quite understand how these motors can be at the same rpm and sound so different =o(


Modified by 91integraLSVTEC at 12:31 PM 2/15/2004

 

Formulas for piston speed:

piston speed in fpm = stroke in inches x rpm / 6

rpm = piston speed in fpm x 6 / stroke in inches

or

.166 X Stroke (in.) X rpm X 3.28) / 60

 

piston speed and RPM are direct, as rpm increases piston speed increases, but because of a longer rod the pistons move up and down slower than on a shorter rod, long rod plus short stroke= higher rod stroke ratio, thats just what i think

 

Imagine a syringe submerged in water with your finger covering the tip.
Let water be "air" and your finger be the valve.

Assume that valve timing will be the same (same b16a head)
Keep your finger opening consistent.. same amount of gap, and open for the same amount of time. If you pull the syringe with a short stroke, it will suck in less amount of water than pulling it with a longer stroke (given the same valve event).

This is basically why both sound and feel different at 3k rpm (as you mentioned).
Even if both were operating at the same theoretical VE at that point, the longer stroke will still displace <u>more amount</u> of fluid.

At high rpm, the piston speed of the longer stroke will begin to out-run valve timing event. There is a limit to how fast air can move into the cylinder. After peak VE, torque begins to take dive as the cylinder gets filled less, and less... as rpm increases. That is why your b18a/b block sounds like its working harder than the b16a at 9k rpm.


Modified by Quick 200k Mile Motor at 12:24 AM 2/16/2004

 

doesnt the new s2000 have lower rpms but higher piston speed

 

It's all about stroke,the pistons in your ls/vtec move a larger distance then your B 16 did,so at the same rpm the ls/vtec is doing more work,and the pistons therefore have to move quicker to completle the cycle then the B16 will.

 

Here is how I think about it.

Think of the stroke as a circle. The crank has to make a 360 degree revolution so its not hard to grasp.

Now lets take two different sized circles. One that is 2ft in circumference the other that is 3ft in circumference. 1 RPM is 1 revolution in 1 minute. If you have to spin each circle around 1 time in 1 minute the larger circle will have to move faster to make the revolution in the same about of time because it has more distance to cover (3ft vs only 2ft). Think of a B16 and B18 stroke in this regard. The B16 stroke is the small circle the B18B the large circle.

I'm not an engineer but this is how I think about it.