Rod length doesn't matter??

[tiger9297]

I'm trying to read and learn as much as possible as I'm thinking on a 383 for my '90 K5. I ran across this article from "Hot Rod" magazine. So is this guy basically saying that the only advantage to "stroking" an engine is the increase in displacement? http://www.hotrod.com/tipstricks/34219/index30.html


Why Rod Length Doesn’t Matter



I’ve read as many articles and books as the next guy when it comes to connecting-rod length. Essentially, there are a couple of accepted theories Some state that that short rods are better, others that long rods are. Neither might be correct. Not only has Reher-Morrison plotted points on a graph using three big-block Chevy rod combinations (stock--6.135-inch; +0.250--6.385-inch and +0.400--6.535-inch) to show piston movement through crankshaft movement, and also built a test engine to prove the theory. To be honest, when the plotted graph points are overlaid on the same page, the traces are so close, it’s virtually impossible to differentiate between them. At 10-degrees ATDC (a point where the most pressure is present in the cylinder on the power stroke), the difference between the shortest rod combination and the longest rod is a mere 0.0004-inch (that’s not a misprint; it’s four ten-thousandths of an inch). A small block test engine showed basically the same results, only this time, it was on a running dyno mule. Contrary to what some experts have touted, the tests (which were very comprehensive) showed zero gain with long rods, particularly past 1500 rpm. In simple terms, the connecting rod links the piston pin to the crank. There’s nothing more to it, so don’t waste your time working with weird rod length combinations.

 

[ryan22re]

You got it.

 

[DEMON44]

Amen brother. Stroke is where it's at.

 

[4xcrazy]

Rod Length Doesn’t Matter

not according to my ex,,,

 

[tiger9297]

I can't find it but i've read an article on "Hot Rod" magazine's website where they discussed how in a 383 the 5.7" rods produced better TQ and the 6" rods better HP. Wish i could find it.

 

[Hogback Fabrication]

Yeah, rod length doesn't matter very much (in 383's anyway ). Go with 5.7" rods out of a 350 and call it good.

 

[resurrected_jimmy]

Yeah, rod length doesn't matter very much (in 383's anyway ). Go with 5.7" rods out of a 350 and call it good.

The difference between 5.7" rods and 6.0" rods in a 383 isn't worth the trouble of clearancing the block for the longer rods. Building trends change and people are getting away from the longer rods because they are a PITA to fit in a lot of cases. Don't get to hung up on the bore to stroke ratio.

 

[4X4HIGH]

The longer a rod is the less side loading there is on the cylinder wall. This would mean that a longer rod is the better choice. When building a 383 that is for a street driven vehicle or even for that matter an off road vehicle that is intending to stay together for a long time you don't want to run a 6" rod as that rod length on a 383 puts the wrist pin into the oil ring and requires some special attention.

 

[b454rat]

Ever tear apart a million mile 400?? The cylinders are egg shaped from a short rod, putting extra force in side to side motion. I would run 350 (5.7") rods for a 383 or 400. I was told that in a 383 or 400, instead of a torque peak, it was flat. I had a TBI406 in my old truck, and can verify that theory. It will pull off idle to you got scared and let off the pedal. Thing was a monster, and with only 3.08s in it, too.

 

[DEMON44]

A million mile SBC...............



....but aside from that, for sure its the side loading of the piston with a short rod and increased stroke that you want to avoid. But really if you're just building a truck engine or a weekend warrior street/strip application IMO its not worth the bother. A high end/ high rpm race engine is another story though.

 

[beater_k20]

a longer rod also allows the piston to dwell at TDC longer, creating a more efficient burn in the chamber...

6 of one, half dozen of the other.

 

[ntsqd]

I would consider an article in the old Circle Track Mag to be a far better piece of info than an article in Hot Rod.

They did the long rod vs. short rod test. Had a pair of matched rods & pistons made up. Total rod & piston weights were the same, big end weights were the same. One was a 5.0" rod length (AIR) and the other was a 6.0" rod length. Both sets were put in a 350 dyno mule engine. Tests were run to optimize the tune on each combo in the engine, but nothing other than the rods & pistons was changed.

What they found was that the rod length did not change the peak torque or HP points. Both happened at the same RPM and at the same amount. What the long rod did was flatten out the torque curve. Based on that a long rod engine would be easier to drive as it would be less like a MX bike's two-stroke engine. Translation: much harder to be in the 'wrong' gear.

The caveat to a long rod engine is that when it's a working engine (i.e. tow rig engine, etc.) that the long rod causes a longer piston TDC (& BDC) dwell. That increases the tendency to detonate in a working application.

Another thing to consider with a long rod engine is piston speed. If the long rod causes longer BDC & TDC dwell times then it has less time to get from one to the other. In a high reving engine you could possibly build a combo that wants to tear up pistons simply b/c they're being accelerated & deccelerated too fast.

Like a lot of things, it's a balancing act. How much of this are you willing to trade off to get that?