I am currently building a 1968 large journal 327 SBC. I have been told by a few people that I should ditch the stock length rods and upgrade to six inch ones. It is supposed to increase engine life beacause the piston travels up and down fewer times... is this true?

 

I don't think so...although I'm not positive.

The thing that would really increase piston life (among other things) is going with a shorter stroke to keep piston speed down. You have a 3.25" stroke which is good for higher revs than your standard 350 (3.48") so you are fine in that department.

 

rod lenth changes have mant effects on the engine, more then I want to type here, as far as the SBC, the six in rod is popular because it is #1 off the self parts # good HP gains. It does this by basicly parking the piston at TDC and BDC for MORE crankshaft degrees. It also releives some stress on the piston skirts. I have used in the past, 5.85, 6.0 and 6.125.

 

The LS1 uses a longer than normal rod as well. It is somewhere around 6" (6.125"?). I always thought longer rods were more succeptible to high rpm breakage because they have more mass to throw up and down, putting more stress on parts. Is this true?

 

I heard a long time ago that you shouldn't worry about it. When you get into rod length and rod ratio issues, you simply create more headaches than you need. I would be more concerned with oil control issues and piston quality. I say go for whatever will make the most power without sacrificing a ton of reliability. If you are comparing two rods, one gives you a 1.51 rod ratio, and the other a 1.55, and thats the only difference, and there is mroe benefit to the 1.51 rod ratio, then use it. You are not building a motor for 100,000 miles. (If you are, performance is not your main goal). The only time I would worry about rod ratio is if I were designing the engine from the ground up. Choosing deck height, etc.

 

I dont think so, from what I've seen the difference between a 5.7" and a 6.0" rod is no more than +/- 10grams. Plus with a 6.0" rod the piston is usually lighter which reduces the combined weight of the rod/piston even if the rod by itself is a shade heavier.

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Jason
93 TA, M6

 

uh, the piston still travels up and down the same number of times...

A longer rod will create less g-force on the piston at BDC and TDC, so there's less stress on the rod bolts, wristpin. Then, add in that a longer rod makes for a lighter piston/rod package, and you increase high-rpm longevity.

As for making power - you won't notice any power differences. You might notice that the motor would be a bit more 'snappy' and quicker to rev, but probably not too much.
Basically, you'll have peace of mind.
But if you're on a budget, well, 'back in the day' people revved the snot out of stock-rod 327's all the time Just get good rod bolts.

 

longer rods are better because they put less stress on the cylinder wall because of the angle a good way to decide what to buy is the rod length and stroke ratio.anywhere 1.6 and higher is probably more ideal.

 

a 327 has a 3.250 in sroke
with a 6 inch rod the ratio would be 1.85

 

There sure is a lot of disinformation on this thread! he he he !

"less revolutions" with a longer rod? Um, nope. Once around the crank is always once around. Your car will still move the same distance with long rods versus short rods.

Um, long rods yield a shorter stroke? um, nope again. If you have a 3.250 stroke crank, it'll still stroke 3.25 inches regardless of the length of the rod. The piston will move up and down 3.25 inches regardless of the rod length.

Blueyes is right: You'll have more dwell time at top dead center with the longer rod. It will also change the rate of acceleration of the piston. The piston spends a lower number of crank degrees accelerating with a longer rod: It's rate of acceleration is slightly higher in the middle of the stroke, but is slightly slower at the top and bottom of the stroke.

The rod length influences where the wrist pin is located on the piston. The longer the rod, the higher up in the piston the writ pin must locate. In the case of a stroker such as a 3.75 inch stroke motor, the wrist pin moves far enough up as to have to sit under a portion of the oil control ring. The compression rings also must be moved closer to the piston crown, exposing them to additional heat from the flame front.

However, most agree that today's current piston and ring designs can handle the added stress of being pushed closer to the piston crown. Thus, the disadvantages of a long rod (other than cost) are comparatively low.

Lastly, with a longer rod, there is generally little or no impact on mass. By relocating the wrist pin UP and closer to the crown, you need less material in the piston around the wrist pin. Thus, the additional material on the little end of the rod is offset by the reduction of material in the piston.

HTH,

Dan