SBC Rod Length question
 

I'm planning a 383. A friend suggests going with a longer rod than the stock 5.7" for some 'free' horse power. My engine will be built for low end torque. What do you think?

What is the engine being used for? Street use, Drag racing etc. ?

TS1955

There is no "free horsepower" in rod length. The rod connects the piston to the crank, simple as that.

Choose a good piston and the crank you want to run, get a rod that connects those 2 togehter and be done.

The only way a longer rod would help is if the piston could be lighter because of the longer rod.

actually if he goes with a longer rod the compression hieght
for the pin on the piston would be higher in the bore allowing the
piston to travel deeper into the cylinder allowing for more swept volume
of fuel and air into the cylinder for a bigger detonation of power.

Right ????..... :lol: :lol: rod ratio to stroke..

The piston travel would be the same, but there are some minor gains, due the better sealing of the piston rings. The longer rod also puts less side pressure on the piston, which also helps and maybe makes a little more HP, The higher up in the piston creates less piston rock= better sealing of the rings. I don't know what the gains would be but NASCAR typically runs 6.2 rods on a 358" Chevy motor, so there has too be good reasoning for that.

JMO

Zip.

The shorter the rod the lower the torque band, If you truely want low end go with a stock 400 rod 5.565

Hey Mr. Russell

Where you been?, i've been meaning to ask you what do you recomment for piston clearance, on a street driven 406" forged 2618 mat'l piston that will probablly not see more than 7200 RPM. I've got .006 but i think .0045/.005 would have been better. Hopefully i can keep it cool no blk. filler.

Right on, i'm building one right now, 406" flattops, eagle 4340 forged crank, 2618 alum. forged flat tops, 5.565 (h) beams and track 1 heads, 850 DP'er, 1.76 PG with pro brake, for my prostreet 1998 stepside Sonoma, i think about 2800 lbs. I believe i can drive it to sonic and the track and run 10.50's with street slicks.

Zip.

If you don't run more than 7000 it will be better with a shorter rod. And for a street motor the factory 5.565 rods are fine with a good set of rod bolts.

Some technical data for all-

It is essential to use the longest rod you can afford to fit in the engine.
A longer rod causes the piston to "dwell" longer in the vicinity of top dead center. This allows pressure to build higher and translates into more torque and horsepower. Less side load (friction) is a free benefit.
The term usually used is rod ratio.

I can't take credit for this - some Florida guy named Smokey Yunick said this.

If the stroke is the same in two engines the piston is moving slower from TDC to 90 degrees with the longer rod.The piston is moving faster from 90 after to BDC with the longer rod.With most racing cams max. intake lift is 100 deg.-108 ATC.The piston moving faster when the valve is at max.lift is good.Bill.