stroke to rod ratio:

[C-10 simplex]

NOTE: Let's keep the discussion specific to the small block chevy---for now.


What would be better and why: A "normal" stroke---say 3 to 3.48" and a longer rod--say 6" ish. OR a very long stroke---3.75 to 4" and a shorter rod---like 5"?

OR is it possible to have a long strong AND rod?

[Marv D]

Lets make it easy,,, long rod reduces side loading on the bore. Less side loading, less friction, more power / rpm potential.

i.e look at the sbc 400,, short rod,,, made power but wasn't very rpm tolerant. The 'norm' today is 6" for anything stroker so you get clearance to the counterweights. Side effect is the piston having a very SHORT C.H. and a ring pack squeezed into a tiny area, or rings support rails to help hold up the oil rings in the area of the piston pin. Other draw back (really more asociated with the stroke than the rod length) is the short piston skirt wants to rock at TDC / BDC.

From a pure power potential,,, long rods give less side loading and reduced friction,,, and offer a change in the 'rate of compression' as the piston nears TDC. More 'dwell' at TDC and BDC has it's trade off tho. Piston speed!!! If the piston has to move the entire stroke every revolution,, and 'dwells' longer at TDC and BDC,, it hs to make up for the difference somewhere in piston speed and piston accceleration.

My concern with the long rod in a SBC is just that,, piston acceleration. Even a realitively light ~400g piston puts a TREMENDOUS load on the rod as it tries to yank it away from TDC on the intake stroke. Look at 1000 sbc rod failures and they don't fail because there was too much pressure put on them from the top,, they can tolerate MONSTEROUS compression forces. They fail by being pulled in half, or ripping the cap off the big end of the rod because the fastners cant tolerate the stretch loading.

If the piston weight has to accelerate daster and harder to make up for the 'dwell' at TDC/BDC,,, it only goes to reason that the long rod is more prone to failure than it's shorter counterpart. True?????

Ask a hundred engine designers and you'll get plenty of reasons on both sides of the issue.

[C-10 simplex]

Hmmm.....i never thought about the acceleration thing. So, to confuse the issue further, let's say we keep the rpm to a reasonable level---say 5500 to no more than 6000.

Actually, is it just an acceleration thing? After all the piston/rod combo has to change directions no matter how long the rod is; Again, i'm thinking rpm is really important here.

[Marv D]

Absolutely, rpm as well as piston weight! Engine analyer reports piston speed, and g forces on the piston (which we have to assume is nearly the same for the rod small end and piston pin) I don't have a version that works in Win7 to get some numbers for a real world # but you would be AMAZED at how closely the rod length effecs the G's on the piston. Just changing directions..., traveling about 3-1/2"..., changing directions again.. and doing this 200 times a second at 6000rpm ought to be enough to self destruct anything. Best recollection is a 350 at 6500rpm makes like 3000 g's on the piston at TDC
Add the piston is traveling something like 1 mile every minute.

Again the compression loads on the rod isn't really a concern unless your making like 1000HP, it's the no load,, actually the vacuum trying to pull the piston up and the weight not wanting to change directions that plays havoc on things. Rods can take the compressing, they can not take the stretching. That's why used aluminum rods are all over the internet for a fraction of new. In a serious motor they are good for only a couple of races,, then they are stretched to a point of 'not worth the risk' .

I got all wrapped up in the long rod effects building a 6" rod 383 for my truck some years ago. Started 'over thinking' things and trying to figure out the effects on the carb signal , that 'rate of compression' thing and trying to build a 12:1 motor that coud tolerate pump gas on the street with a soft tune, and would perform at the track like a 600HP motor with good fuel and leaning on it a bit. With the help of some pretty bright minds I buit this thing, put it in the truck and made 2 passes. Promptly put the motor in something that only saw the track, not the street.

So my experiment with the long rod motor was a bust so to speak, so take my rambelings with a grain of salt. But the motor did run great,, 10.2's @ 128 in a 2980 pound chassis. But my guess is it would have ran 10.2's no matter if it was 5.7" rods or 6.125" rods. Any more I pick a rod that mainstream, reputable, has easily replacable fastners (cap screws with a locating collar, not bolts) and is long enough to get the pistom up off the counterweights, and keeps the best ring pack for cylinder sealing.

[Torrey72]

Originally Posted by C-10 simplex

OR is it possible to have a long strong AND rod?

Yes, yes you can have a 6" rod in a 400. Or with a 3.75” stroke.

A longer rod will slow the piston speed down so the piston will dwell at (TDC) longer. This will help with combustion, help cool the piston with more time spent at quench and higher cylinder pressure due to more fuel burned before the piston travels down as a result. Cylinder filling (volumetric efficiency) will suffer slightly due to slower piston speed off of (TDC) though. This is made up for from the mechanical advantage you get from a longer Rod. Picture this; imagine that the crank is a hand wrench attached to a stubborn bolt. The rod is your arm. The cylinder combustion is your strength moving it all. You are your strongest or rather you can exert your maximum force to the bolt at the end of the wrench when your arm is at a 90 degree angle to the wrench. When your arm is at a sharp angle. Let’s say 45 Deg. or closer to parallel ether direction. Your force you transfer to the bolt at the end of the wrench is less. Despite the same amount of power exerted. A longer rod will dwell at an angle closer to the perfect 90 Deg. angle for more degrees of crank rotation. In turn, transferring more utilized combustion power to the lever (crank). A short rod will transfer the same power but for shorter time when the rod and crank are at crazy angles. Or off of the perfect 90 deg. sooner due to the piston speeding up at both ends of the stroke. The majority of this power is wasted and turned into side loading or loss due to lack of mechanical advantage. The force goes some place just not out to the transmission.

[C-10 simplex]

1) i meant long stoke, not long stong (whatever that means).

a) incidentally, what is the longest stroke and rod combo possible in a small block chev?

2) The more i think about it, the more maybe it's best for me not to worry too much about it-----a)since my next planned engine will be blown or nitroused---therefore rod ratio is probably not crucial.

b) And, at any rate, the engine will be for bracket work---therefore wringing every possible ounce of power out of the engine is not crucial.

c) This is all above my head.

[Torrey72]

Yes, a large amount of rod failures are due to the rod bolts pulling apart. This is not from a longer rod. Longer rods use a shorter piston (Lighter piston) that in turn lowers total mass. Engine loading due to decelerating typically is the root cause of the lateral force on the wrist pin, cap, and rod bolts. Rods are primarily designed to have a load or force pushing the wrist pin and crank journal parts of the rod together. This force happens during acceleration. The bad force (stretching force) happens in situations like when the engine is at a high rpm, you let off the gas, and you shift to a lower gear. Instead of the engine turning the transmission. The transmission is turning the engine causing the rods to experience a pulling, stretching force instead of a loaded pushing force. Other factures cause rod failure as well. But all things being equal. Long Rods experience less transitional and lateral forces at the wrist pin due to the reduced angles experienced. Imagine this. Pretend that the rod is four feet long and the crank has a four inch stroke. You can visualize how the long 4 foot rod stays in a straight up and down position under the piston as the crank turns. The up and down force of the piston is transferred in the same direction as the length of the rod. Even though the base of the rod at the crank journal swings from side to side 8 inches due to the 4 inch crank. Now pretend the rod is 6 inches long. As the crank rotates in its 8 inch circle and the piston goes up and down. You can visualize how the rod will swing extremely at the crank and cause sharp rod angles at the wrist pin due to the piston being so close to the crank rotation. This is a ridicules example but illustrates the point that longer rods are better. Also, long rods have a slower not faster piston speeds at (TDC) and (BDC) than short rods. This is more favorable to wrist pin and bolt survival when the piston changes direction.
To answer your question: You don’t want the longest rod length possible IMHO. I have seen rods longer than 6” in 400s with pistons so short that the oil control ring land runs right across the center of the wrist pin. Personally I would stick to a 6” rod in a tall deck block myself or a 5.7” rod in a 400 or 383 with standard deck height. You might want to talk to your Machinist about costs. Long rods = more money and clearance issues.

[C-10 simplex]

Yeah, i'm probaly not going to obsess over it too much.

i should add that i've never actually built an engine---i've swapped cams,carbs,heads, intakes etc., but never actually assembled an engine.


Yet, i'm asking about rod ratios vs. stroke length. How's that for chutzpah?

[ItsRandy]

I really enjoy reading posts like this, I get to learn stuff. I can't wait until they come out with the mechanical engineering version of trivial pursuit.

[Torrey72]

Chutzpah is encouraged. Be forewarned, everyone’s a Balmalocha here

[BigDan3131]

Everything I've read is that a slightly longer rod is fine in anything that is NA but...anything that is blown etc should stay away from the long rods to allow for more compression height. The turbo forums guys have a engineering section that really explains it in detail.
I was going to build a SBC with 6.0" or longer rods but after reading that there I changed my mind. In fact, I was thinking about a lightweight crank as well and that's a big no no too.

[Marv D]

Theres a point where you have no choice tho. take a 4" stroke and stuff into a sbc package and you can't use a 5.7 rod. It just doesn't clear the counterweights. Well,,, I've seen guys chuck up a crank and turn the counterweights down to nothing and then add back bucketloads of mallory to get it in balance. But WHY??? Common sense says in a stroker application your stuck with the long rod,, ring pack be damn'd. Other than the piston stability at TDC the ring pack thing doesn't scare me a bit. With Childs Plasma Moly rings, a torque plate honed bore, setting ring gap for N/A I typically see a 1.5-2% leak on a fresh motor. after 2-3 seasons of hard abuse still above 4% so ring seal and oil control is not a issue with the proper oil rail support and ring and bore prep. We just get back to the issue of side loading the bore with a short rod,,,