hey guys,
I recently got done completing an engine biuld. I left the displacement at
350 ci. but added forged rods and pistons to the motor given I am running a supercharger with a LE2 pkg. I was just wondering why it was suggested that I go with 6" rods instead of the stock 5.7" legnth. Bret and Lloyd both told me to use the 6" rods, so I just listened to them. I was just wondering why they might have suggested that I do this, just curious, is it becuase 6" rods are better for high rpm's or what?? thanx for any answers....

 

Longer rods keep side angles lower so there is less tendency to push the piston sideways and thus produce less skirt scuffing.

They also give the piston more time at TDC so efficiency of the burn is increased.

 

I believe there was a recent good thread on this in the advanced section.
The talk of angles and sideloads are overblown, a lighter rotating assembly is the real advantage, just need to keep the pin clear of the oilring which is easy enough with a 3.48 stroke and 6" rod.

 

Mainly because it lowers the mass of the piston, even on a blower there is more than enough piston there with a 6" rod.

Bret

 

Gary,

I've seen 1.125" compression height pistons that don't need a bustons or oil support rails and have VERY thick decks. Considering that a forged piston is much tougher than a factory stock one and the underside of the piston has a constant thickness due to coutours in the forging to match the valve pockets I don't think that's much of a issue either.... what the issue IS deals with the basic physics of the setup. When you are building a 355 on a crankshaft that is designed to run 6250rpm max, with heavy stock parts on it lowering the mass of the parts that you hang on that crankshaft is a very good idea when you increase the max engine speed to 7000rpm. That 750rpm gain increases piston speed 435 ft/min(12%), but increases the piston g's 25.4%. Now if you can find me a affordable 1.550" compresion height piston to recomend with a inboard pin boss and a light strong piston pin then we might be able to get some nice mass savings for the cutomers. Thing is a .300" reduction in compression height along with a inexpensive 6" rod that is bushed and supplied with ARP rod bolts is a better alternative in the mass and strength department, plus the longer rod also lowers the max piston G's the motor sees. You can get up to a 2% improvement in piston G's.

I'm building a 355 in the shop now that has lowered the bobweight of the setup 16% over stock with affordable parts all in the name of longevity for the life of the motor. These recomendations do not just come off the hip it's the same thing that I would do if I was building the motor for a customer or myself.

BTW the way the physics and math work out that setup I mentioned above will now have less loads on the crank at 7000rpm than the stock setup has at 6200rpm.

Maybe all this isin't mumbo jumbo after all.

Bret