racerxadam

This is a topic that I don't know much about. I know that the theoretical perfect ratio is 1.75 but I don't really know what number does for an engine. In passing over other topics that have touched on this I have picked up that allows an engine to safely rev higher, but is that all? Is this even something that I should consider when I choose which block and crank I want to use for a build up? Is it something that should be considered more during a build up for a daily driver rather than a mostly track driven car? Some of the guys that build engines regularly please educate me.

92JDMSiR

a better rod/stroke ratio will put less stress on the sidewalls of the sleeves. if sidewall strength is to be taken under consideration while under boost the car with less sidewall stress is less likely to break a sleeve. and yes you are correct about rpm's, but that is not the only factor. there is about 10 other things I could think of off the top of my head that would contribute to a better revving engine and a good rod/stroke ratio is definitely one of them. a really good rod/stroke ratio will benefit you the most in full out race but for street where revving is not as important as race a slightly lower rod stroke ratio will not blow the motor. for example lets take a b20vtec and a b16. for the sake of arguement bother blocks have built heads and a block girdle. the b16 will rev more willingly because 1 there is less rotational mass and 2 there is a better rod stroke ratio. if you want a street motor a perfect rod/stroke ratio is good but not detramental to an engines longevity. if the car is going to be driven on the street just go with more size. you get more torque, more hp, with less revs. i feel like I have to be next to you in person to better descibe this to you.

racerxadam

my real question is what is the mathematical basis for deciding that 1.75 is the ideal ratio. some of the reasons that you listed were side loading of the pistons, and rotational mass. Now consider an engine that has a rod with a lenght of 10 units and a stroke with a length of 1 unit. the rod/stroke ratio is 10. Side loading would be very minimal. Now, consider an even more ridiculous engine, one with no stroke or almost no stroke. the bottom end of this motor could spin on and on with no worries besides balancing issues because it would almost be moving in a circle. the rod to stroke ratio of this motor would be very high. at the same time it would be very diffucult to make power from it because there isn't much of a "lever" for the rod to push on and create torque. is a 1.75 r/s ratio the best compomise between making torque and revability? I know there is more to safer reving than r/s ratio but I'm just considering bottom ends for the moment

bruthaboost

Well spoken. High revs are really a product of valve train capability and stroke length. Get far enough away from center even with a perfect r/s ratio and things will come flying apart.

The 1.7 r/s number comes from, among other things, flow characteristics. Given two engines with idenctical heads and displacements, the lesser r/s one develops more peak flow over the intake stroke, while the shorter stroke one will have the same amount of flow but spread more evenly. Rapid buildup in flow and high peak flow rates lower efficiency. Also at some point with a high enough flow rate the induction system becomes the bottle neck.

paraphased from desktop dynos. I don't plagerize (sp?)

JoShYo

The July issue of Sport Compact Car has some good info on rod/stroke ratio in the technobabble section...try checking that out

EE_Chris

Just a link for some further reading.

Another link.

And yet another link


[Modified by EE_Chris, 1:39 PM 7/30/2002]

DSF

Yep. Has a lot to do with piston speed/acceleration. A long stroke/short rod (low r/s) motor will accelerate the pistons faster than it's counterpart. Therefore, it's get more air volume moving at a faster rate at lower revs. It also spends considerably less time at TDC which at high revs hurts VE. This can be counteracted somewhat by going with absudly high CRs that can only be supported by race gas or alcohol. Conversely, a short stroke/long rod (high r/s) motor will take longer to accelerate the same mass of air. But because of it's slower piston speed and extended stay up at TDC, it improves VE at very high revs. Raising compression isn't as necessary on a high r/s revver. You could theoretically raise the r/s to some absurd number and you would have an engine that would have hardly any side loading, super high VE and super high rpm capability, but functionallity becomes an issue. You would need an 6, 7, 8+ speed gearbox, just to keep the engine in it's powerband. You could write books on the subject...oh wait, there are

My rant is primarily based on the physics to manipulate power, not mechanical reliability.

filetofit

I'm bored here at work so I made a computer model. Using a R/S ratio of 1.54 which is for a b18a and 1.74 which is for a b16a I calculated the inertial side load on the cylinder wall when the piston is halfway between TDC and BDC at 6000 RPM. For the b16a the load on the cylinder wall is 782 lbs, for the b18a the load on the cylinder wall is 924 lbs, 18 percent higher.

At 8000 RPM the loads on the cylinder wall are 1393 lbs and 1646 lbs for a b16a and b18a respectively.


[Modified by filetofit, 5:58 PM 7/30/2002]

Sonny

Here's another good article on the subject:

http://www.grapeaperacing.com/GrapeA...rodslength.cfm

Sonny