Your take on bore/stroke ratio, rod ratio, and strokers

[375InStroke]

How do you think these two motors would act?
1. 383 stroker
    496CID
    4.31 X 4.25 bore/stroke
    1.537 rod ratio
2. 400 stroker
    499CID
    4.375 X 4.15 bore/stroke
    1.628 rod ratio
You think it makes any difference?  Let's say both motors are exactly the same in every other way.  Would they have more than a 100RPM shift in torque curves from each other?  Do low rod ratios wear out motors faster?  What wears out?  From the motors I've torn apart, it looks like the cylinder walls at the top wear out, but do the piston skirts wear out, causing piston rock, or slap?
   

[andyf]

The 4.375 x 4.15 would be a slightly better combination.  The bigger the bore the better for breathing and the shorter the stroke the better in terms of friction.  But you might not notice much of a difference since those two configurations are very close.  Of course, the 383 block has to be 0.060 over to go to 4.31 while the 400 block is only 0.035 over to get to 4.375.

[Dcuda69]

 []{}Doesn't rod length come in to play as far as side(thrust) loading go? How does that affect durability?  I did a 470" LD w/3.900 crank,4.375 bore and 6.760 rod. So far so good,just curious?

[NYrr496]

Car Craft did a dyno test on two 496" big Chevies a year or so ago and found that there was no measurable difference between different bore/ stroke combinations.  I'll see if I can scare up the specs and the outcome.

Also, many years ago, I worked in a shop that built stroker BBC's for Cigarettes and such.  I can't tell you how many times during the summer we thrashed and put a 496 in place of a 482 or a 511.  We would install the heads, cam and intake from the wounded motor and drop the new one in next to the other and send the customer on their way to go do some more damage.   I was amazed how similar they all ran.

[speedymopars]

The difference is the dwell on the piston - ignition timing becomes more critical on a short rod engine, but it is less prone to detonation, but other than that, no real difference.

[Thin White Duke]

A shorter rod will side load the cylinder walls more.  You always want to put the longest rod possible in. 

[coors00]

Yep, that's why a 340 will beat a 350 chevy- the longer stroke of the 340 producing torque; the 350 is just higher rpm'ing, producing more hp. Torque moves you forward faster than HP.

[375InStroke]

More correctly, torque will beat HP if the car isn't geared low enough.  I smell a torque vs. HP war coming.

[coors00]

 

[chryco]

Torque gets there first .

[forphorty]

Yep, that's why a 340 will beat a 350 chevy- the longer stroke of the 340 producing torque; the 350 is just higher rpm'ing, producing more hp. Torque moves you forward faster than HP.

nope, sorry. stroke of 340 is 3.31. 350 chevy is 3.48. 360 mopar is longer at 3.58.

[coors00]

Here we go; maybe the larger valves in the 340 smoke the larger chevy engine.

[Steve DeTar]

depends... what are you looking for? 

[bigtimeauto]

Don't forget gear ratios, I'll take that high winding short rod engine anyday.

[FastmOp]

I'd want the 4.250 crank in the 400/4.375 bore 

[pwmax]

I would use the bigger bore for breathing, otherwise, there will be virtualy zero difference. Some gains come with longer rods, from frictional differences, and, the fact the piston is usualy a bunch lighter. There could be some benefits in a really high rpm piece from a longevity standpoint, from less piston speed, but for power, not a whole bunch. If you have a good 383 block, I wouldn;t hesitate to use it. What I have alwas done in the past, is look to what the "others" have for choices, and how well those work. For example, a VERY popular stroker combo for a BBC, is a 496. Its a 4.310 bore, and a 4.25 stroke. The Deck height is usualy 9.780 for 0, and you use typicaly a 6.385 rod, which is .250 longer than a stock BBC. Which, ends up with the same rod/stroke ratio, as the stock stroke is 4 inch. Thats also a .060 over bore. Those engines rock. Thats basicaly EXACTLY what your 383 stroked to 496 is. Same exact stroke, same bore size, same rod length typicaly, although, with the much taller deck, you can run a 440 length rod. The only advantage there, is the shorter compression height, which typicaly makes the piston lighter.
The old "rule of thumb" rod length is used to "tune" a certain cylinder head combo. Typicaly, a smaller, high velocity port favors a long rod, and a big slower port, tends to like a short rod, with more piston speed. The idea is that the longer rod doesn;t leave tdc as fast, and has less overall piston speed, so there isn;t as much yank if you will on the air fuel mix, so the higher velocity smaller port, works better. The opposite with a bigger slower port, the short rod, yanks the piston away from tdc faster, with overall faster piston speeds, so it artificialy increases the velocity in the runner. So, most serious race engines, favor shorter rod lengths, or rod/stroke ratios, and bigger high flow ports, the engines work better at high rpm, eben though the logic of the frictional issues would say its not worth it. Again, the differences are small, but in certain classes where very small differences in et or lap times matter, very small differences can be huge in the end results, such as pro-stock, where a few hundredths seperate number 1 qualifier from number 16, and, 1 thousandth of a second can get you in the show, or put you on the trailer, so, in those cases, it matters. But for a bracket guy, or street car, there is nothing there, your just loosing sleep splitting hairs


Frank

[bOb shingler]

exactly

[Thin White Duke]

For the street, the most torque at a reasonable rpm will not only spare parts, it will also get the car moving from a stop much better than a high hp screamer.

[Womanator]

Always waiting for your responce Frank.
             
                      Respectfully  Marv