FuryZ28

What benefits are there by going lightweight? I'm not talking Forged steel vs. Titanium here, I'm referring to rods and cranks that are both 4340 forged steel and of the same manufacturer, yet the "lightweight" crank is only like 3 lbs less than the normal one. Same with rods- 535g for lightweight, vs. 630 for standard forged steel rods. I'm looking at building a forged 355 to run fairly high boost (12-15 psi), and I was wondering if going lightweight would be worth the extra $$, or if it's something trivial I don't need to concern myself over. My goal is to have a fast street/strip weekend driver. I'm not trying to build a strip-only car here, nor am I trying to set any track records either...

rskrause

For a given level of quality, stay away from the "lightweight" parts for a blower app. Think of them as parts designed for high rpm NA use, or else parts that will be inspected/replaced frequently (or both).

Rich

FuryZ28

Thanks! That's all I needed to know!!

Pasky

ever 1 lb of actual weight = 5lbs of rotating mass, so essentially those 3 lbs of weight lost off the crank + 3 lbs off the rods = 30 lbs off of rotating mass which translates to faster revs.

But aren't light weight items harder to balance?

contactpatch

Where is the weight, removed from?

IMO
removed from the center of a rod journal, useful, I guess.
removed from the axis of the crank, I don't see the
benefit other than lower car total weight.

do vendors publish the
'rotating moment of inertia', for their cranks?,
if not, how do you know what you get?

rskrause

The counterweights on a high-end crank tend to have a lighter bobweight than a lesser crank on the assumption that a lighter piston and rod assembly will be used. This can make balancing expensive in a case of say a blower application where a relatively heavy rod and piston is used. Manufactureres do publish the bobweight the crank is balanced against so you do have an idea of this when purchasing.

Saving any rotating weight is good. But the closer the weight is to the center of rotation, the less benefit is derived. Reducing reciprocating weight, especially at the end of the rod (piston and wrist pin) has a much bigger impact. But in both cases, weight removed at the expense of strength beyond what is acceptable is a poor compromise. You can have strong parts, light parts, or cheap parts - but you only get to choose two out of three. And at some point, really light stuff will not be strong enough no matter how much you spend.

Look, for example, at JE's piston catalog, small block Chevy section. There are many different piston weights, and these closely correlate with the intended use. All are of comparable "quality" in terms of design and material and all are about the same price. So, the variation in weights you will observe is a function of the differing design requirements. You will see small block pistons with weights that vary by nearly 50%. The lightest are for "restricted" applications. In that case, class restritions on the intake and valvetrain limit the stress the piston is subjected to and they are very light. Then look at the blower pistons, which are ~200gms heavier. You would never use one of the light weight pistons with a blower.

Remember: you first have to finish to finish first!

Rich