My buddy brought up a point and i just want to shed some light on it does rod length change your compression ratio?

 

If you changed the rod length without changing pistons then yes, it would. But that is not really possible unless the rod lengths are very close to one another.

 

IOW, no. It will change the DCR, but only ever so slightly.

Rich

 

Doesn't it matter if the pin bore is moved up on the piston? I would think that the piston for a 6" rod is different than a piston for a 5.7" rod, otherwise they would not list a different part# piston for each rod length. It does change the piston dwell at the top, which is what produces more power. Incidentally, depending on the application, a 5.7 rod is better than a 6" rod for short track use. At least, that is the experience from the local builders.

 

There is NOT a nickle's worth of difference in either. Ya can't see it on the dyno or feel it at the track.
A rod is a good thing to connect the piston to the crank.

There is some advantage when using rod length to influence the intake port,that's a whole 'nuther book.

 

yea use longer rods if you have issues with the piston skirt hitting the crank counterweights, lol. the main thing is the piston still goes up and down your crankshaft's stroke and the rod doesnt affect that.

 

Thanks gusy so it really has little affect on compression

 

None at all.

If ya use 5.7" ya have to get the crank with smaller weights to clear the pistons.

 

Nobody is assuming you would use the same piston with a 6" that was designed for a 5.7" rod, or vice versa. It appears that the person in the original post is confused and thought that there would be a difference in CR between a 5.7" rod+appropriate piston and a 6" rod with the corresponding slug.

 

Just for purposes of illustration, ignoring mechanical interference.

CR with zero deck, flat tops = 12.5:1
CR with -0.3" deck = 6.77:1 (piston for 6" rod on a 5.7" rod)
CR with +0.3" deck (piston for 6" rod on a 5.7" rod = infinite?)

Rich

 

I rember that being brought up.....well anyways thanks alot guys i let him know whats up

 

This maybe so, but it does show in lap times. Apparently the shorter rod will accelerate faster than the longer rod. I have not verified this, but it is almost unanimous at the local tracks.