What benefit can I expect from using 6.0" rods over the stock 5.7" rods I just bought a 383 and it has the 6.0" Eagle H-beam rods.

 

your piston wont travel as fast at a given rpm.


adam

 

Well, that can be a bit misleading. If you study the geometry, the piston will not travel as fast (linearly) at PART of the stroke, but peak velocity at a 90° crank angle will be exactly the same. The piston will move a bit more slowly at the ends of the stroke, where it really doesn't matter a lot.

What is more important is that there will be slightly less torsional force on the piston as it moves through the bore, and that the angle of the rod to the crankshaft throw will be slightly lower, making the transmission of power to the crankshaft more efficient.

Of course, a 6" rod tends to have more mass, so there can be a trade-off. If you are buying an entire new rotating assembly, the longer rods are definitely worth looking into. Just watch the clearances.

 

6" rods are an old NASCAR trick. I have an old small block Chevy engine book by Smokey Yunick, and here's what he said about the subject:

"All the Chevy rods with a length of 5.7" are too short for building a real racing engine. When you start getting serious about making some horsepower, you'll have to go to a longer rod - something on the order of 6". The longer rod parks the piston at the top of the bore for a longer period of time. Up to a point, the longer you can park the piston at the top of the bore the better off you are, because that gives more time for the pressure to build up and shove the piston down. It must be remembered it's the pressure that pushes the piston down - not gasoline or air burning. .... Now if a guy doesn't intend to twist an engine pretty tight, he's wasting his time by playing with longer rods. The engine that turns in the area of 5000 to 8000 rpm will really appreciate a longer rod, but the engine built for the street and operating in the area of 3000 rpm or less won't benefit because the piston is moving so slow the pressure is already dropping off before the piston gets ready to come back down."

He had some other interesting things to say about rod length vs stroke, ignition timing, etc. He had an Indy car small block that used a 6.25" rod. I guess my take on it would be that nowadays with the easy, cost-effective availability of 6" rods I'd say "go for it". Back in the day when 6" rods were custom made (i.e., expensive), most guys on the street would run the standard 5.7" rod.

 

what points are the Rods measured form?

 

Con rod length measurement is a center-to-center distance from the wrist pin to the crank throw. You'll have to calculate the diameter of the wrist pin and journal, divide by two, and arrive at the center distance.

 

Thanks Vader.

 

but, pistons for a 6" rod are shorter than for a 5.7" rod, reducing their mass.
The end result is usually decreased mass. And, since it's reduced at the piston, you have reduced strain on the wristpin as well as on the rodbolts.
also, moving more slowly at the ends of the stroke reduces stress from having to reverse direction.

All in all, though, I agree - definatelty only something to look into if you're redoing the rotating assembley for other reasons. Tear down the engine just to swap 5.7" to 6"? Probably not worth it...

 

Well actually I just bought an unassembled 383 it came witha new crank Eagle H-beam rods (6") Keith Black hyper-whatever domed pistons moly-plasma rings clevite bearings and a block that has been fully machined. The motor is supposed to make 11:1 compression with 64cc heads. I just never really understood the advantage of using 6" rods. Thanks for the advise!!!!