The Dirt Forum for More Information
Topic: 6" RODS ?
Total posts: 14
posted July 31, 2001 06:42 PM
Can I get a few opinions on pro's and con's of running and building a 6" rod motor.
What are the advantages of doing this in a350 SBC.
Will I need to rethink my cam or be able to use the same type?
How will it affect gear ratio choices?
Any useful info is welcome!
Total posts: 219
posted August 03, 2001 12:41 AM
The length of the rod controls the angle of the rod, the angle controls the dwell time of the piston.............by this what I'm saying is this........put a degree wheel on a motor with 5.7 rods and use a dial indicator and check how many degrees of crank shaft rotation it takes to move the piston from .050 BTDC to .050 ATDC
Now try the same thing with a 6.00 rod.....
you will notice the piston will stay at TDC "longer" and with alchol for fuel this is a good thing. The other plus to doing this is to catch back up at BDC the piston speed is much faster with a 6 inch rod....we are talking about in the middle of the stroke here, this helps intake charge. As you can see without a doubt you will need to re-think you ideas of cam selection. intake, and headers too. This is a package deal, to achieve the most from the 6.00 rods you will want to change many other things
Rod angle is like a crow bar deal, when pulling nails out of the wall at home would you want a short crow bar or a long crow bar?
rod angle is the total thing here, you know about steering geometry, well the rod angle is the same thing.......wrist pin and big end rotating around the crank shaft is like the tie rods and drag link pivoting around the idler arms.......as they go over center the steer faster then as they move away they slow back down.........in a motor the piston will dwell longer at top dead center and bottom dead center longer, but will be much fast in degrees of crank shaft rotation while moving at the center of the stroke,with a longer rod....
Most people now are going to a 6.125 or some even go to a 6.250 rod. I don't like to get too much angle........for a carb, with flat tappet cam, I like the 6 inch rods best.
If you have a late model with a roller cam a 6.125........sprint car roller cam with injection, 6.250
as you notice, every time the rod got longer I liked it better in a lighter car with a bigger tire. For a IMCA modified, if you run a 6.125 rod the torque curve moves too far up the rpm range of the motor, You give up too much bottom end, and don't have enough top end with the flat tappet cam.....
I hope this helps you understand the reasons for the different choices of rod lengths...