I'm in the process of gathering information for the buildup of either a 383 or 396 stroker. I've noticed that there are different rod lengths that can be used in these packages. Can someone please explain the pros and cons of the different length rods?

 

Dave: I think a lot of us are talked out on this subject. It is discussed here a lot. This thread, which is current, turned into a fairly technical discussion of rod length: http://web.camaross.com/forums/showt...hreadid=114767 and if you do a search you will find a lot more. Without getting into the theory too much, here's synopsis in the context of an LT1 (you didn't state what motor you were building). Keep in mind, the differences aren't large and that rod length is one of the more trivial choices you will need to make unless you are building an all-out race motor.

For an NA stroker use a 6" rod. Primary advantages are a lighter piston/rod assembly as well as less wear on the cylinder over the very long haul. Also may have some subtle effects on air flow as discussed in the thread I referenced.

For a supercharged setup, use a 5.7" rod. The shorter rod necessitates a piston with a taller compression height. This is heavier but allows the design to provide better ring support and a deeper dish without compromising the strength of the dome. It also may decrease the chance of engine damage by lowering the peak cylinder pressure since it will accelerate the piston away from TDC faster than a longer rod.

Or, you could use a 5.85" rod if you can't decide!

Rich Krause

 

Well said, Rich. I agree, it's all about the ring package/piston weight, and what your combination is. I use a 6.125" rod in my 377 (3.48" stroke). The piston has a 1.13" compression height. This necessitated oil ring spacers since the wrist pin was so high, even with a tight, thin ring package. This combo would not be adviseable for street-duty. For a street engine, I would keep the compression height of the piston above 1.25". That, coupled with your stroke and deck height should give you your rod length needed. Other than that, it's kind of like Dave Reher says, "The main function of the connecting rod is to join the piston to the crank." Hope this helps.

Shane

 

Yes, sounds right to me. Your 6.125 with a 3.48" stroke counts as a "very long" rod setup. Ideal for high rpm racing but too extreme for a street car.

I like that quote from Dave Reher, thanks!

Rich Krause

 

I also agree with Rich and Shane but Dave don't you also have a Procharger that you might install? If so then I would use a 5.85" rod with the 396 and a 6" rod with the 383, because if you're blown then you want a thicker piston crown and you want to move the top ring down to lessen the heat it will see from combustion.
Take your block deck height - 1/2 stroke length - rod length = piston compression height: for example my 421 breaks out like this:
deck height = 9.020
rod length = 5.85
stroke = 3.875 1/2 of which = 1.93
9.020-5.85-1.93=1.24 which is my piston compression height.
I choose this rod for the reasons above, this allowed me to have the ring package moved down .300 on the piston.

 

Thanks for the replies guys. Considering this if going to be a "street" motor, I really would like to put together something that will last for quite a while. I really don't want to build a dedicated blower motor which is why I want to keep the compression as close to stock as possible. Hopefully my current cam will have enough overlap to relieve some of the cylinder pressure under boost. My procharger is a 9 lb kit but can be easily dropped back to 7 lbs. I really don't expect to gain much, if any, power from the stroker, over my stock shortblock, but hopefully the strength of the forged bottom end will be able to handle my mildly boosted motor. I know there will be a significant increase in torque N/A. It would be nice to build a killer N/A stroker motor but emmissions compliance is one of my primary concerns which is why I'm considering reinstalling my blower for added power. From what I'm gathering from you guys, the shorter the rod, the thicker the piston crown will be, which will be better in a blown or nitrous application but also there will be an increase cylinder wall wear because of the rod to stroke ratio. In essence the 5.85 rod would be the best choice for what I'm looking for. I just hope I have enough "heads and cam" for this stroker motor.

 

Rod stroke ratio also effects the piston speeds. The shorter rod makes the piston speeds faster at the end of the strokes. While the longer rod will make the piston slower at the end of the strokes. However the longer rod will have a faster average piston speed wich gives more suction later in stroke when the intake valve is open more. The faster speeds from the shorter rod will have more force pushing out the exhaust and in the exhaust at the end of the exhaust stroke. This increases the exhaust velocity at the exhaust valve when the intake opens and will have more suction to scavenge the fresh air into the cylnder. The oposite effects would come from a longer rod.

The ange of the longer rod will always be less than a shorter this reduces side load from the piston and reduces piston and cylnder wear as rskrause mentioned.

In turbo motors the shorter rod takes advantage of having a near 0* overlap cam.