3 motors all the same bore, stroke, cubic inch, just different rod lengths (which requires a different pin hieght)

For conversation sake say 408w with a 6.125, 6.2, 6.25?
What the difference in torque characteristics / HP? Discuss

 

Arguments can be made for the longer rod to increase dwell time and leverage. I think piston design/weight is more relevant. My vote is for the lightest rotating assembly of the 3.

 

I was looking at the wieghts.... was within grams of each other as far as the pistons, rods and crank was heavier on the longer rod

 

edited

NVM... thought you were talking about aftermarket 302 blocks.


*depends on the rest of the valvetrain and how many rpm's you want to spin.

 

Block or CI doesnt matter, 408 was just one of a few combinations that had the different rod lengths. Tell what you got on your brain Chris. Doesone rod length spin up faster or slower than the other? Its no secret...I'm looking for the best combo for a turbo motor....So rpm's will be less than 7200

 

A longer rod would create less wear on the piston/cylinder wall since the rods angle during strokes would be lesser than a rod of shorter length. wouldn't that combo make sense to be more stable through out the RPM's?...

 

Everything depends on which block you use since they are all different... but take for example the FRPP Boss block. The cylinder walls allow the piston skirt to extend below them at BDC with a shorter rod. I saw somewhere that the wrist pin is more than halfway below the cylinder wall with some short rod combos. When there is that much piston below, I'd be afraid to spin it high, especially under boost but 7,200 isn't horrible.

Since you won't be spinning over 8 grand, I wouldn't think there would be too much differences between rod lengths. If piston weights are similar, the rotating weight would be higher with a longer rod.. which would lead me to believe you'd have a bigger gain on the torque side. Keep everything as light as possible and the boost would come in quicker, giving the combo a slight horsepower edge.

Just something I'd think of, but I am not an engine builder... and don't want to give the impression I know for fact what I am saying on this. Just thinking out loud.

 

I would use as long a rod as possible as long as the piston pin height doesn't come out less then 1.25"..... Any shorter the that on the pin height and the rings are jammed too close together and the piston skirt gets too damn short causing the piston to have excessive rock in the bore.....

 

I have been kicking around building a long rod 306.

I know everyone says go 331 or 347. but I have a fresh 302 crank and block. I would like to get better rods and new pistons for anyway.

I found a set of Forged Keith Black pistons p/n PB747 with a weight of 428g. They have a compression height of 1.298.

These with a set of Eagle 5.4's should make for a interesting combo.

I ran it thru several engine combe calculators. It has a better rod to stroke ratio and the side loads on everything are seriously decreased.

 

In a max effort situation, a shorter rod has the potential to make more power. The increase in peak piston speed allows for a larger than normal intake port. This is called "Billy Glidden" style. You can find information regarding installing pistons with offest pin backwards, in an effort to achieve just this.

Brian
EFI-Unlimited