crank and rod question

[wrench87]

what would be advantages and disadvantages of running a 398 crank with the stock style 6.4 rods, and what would the difference be in running the 3.98 crank with the 6.7 rods would one spin up quicker? which one would make more rpm and would they require different pistons because the length of the rod?

[blykins]

IMO, the short stroke crank and rods would be your best bet for turning high revs. But the difference would probably only be noticeable in REALLY high revs.

How high are you looking to spin it?

The 390 with stock rods wouldn't require any special pistons....neither would a 3.98" stroke crank with stock 6.490" rods.

They use 6.700" rods with 4.125" and 4.25" stroke cranks....so pistons are readily available for those combos.

To use a 3.98" stroke crank with 6.700" rods would require a compression height of 1.480" on a stock deck block and I think that would be a custom piston.

There are lots of theories on rod length/stroke ratios...so you'll get a bunch of difference answers on what the optimum length rod would be.

[wrench87]

i have seen the stock 3.98 crank advertised for use with the stock rod, then i have seen the stroker kits with the 3.98 crank with the 6.7 rod, i have also seen the 4.125 and the 4.250 with the 6.7 rod. i know any of the 4 combinations would be great in a 2100 lb cobra. i just have not decided what would be the best for me yet?.

[CHANMADD]

If you divide the rod length by the stroke you will obtain the rod/stroke ratio

6.4 divided by 3.98 =1.608
6.7 divided by 3.98 =1.68

The higher the ratio number the better it will be at revving higher. The shorter rods tend to give a little more torque.I have a 406 chevy with 6" rods and 3.750 stroke = 1.6 and I happily rev it to 7500rpm and sometimes 8000rpm.
So it depends on what you want. The stock 400 small block with 5.575 rods has a ratio of 1.48 very bad for high revs ,but it even took 7000rpm regularly

Longer rods require special pistons, mine the piston pin is behind the oil ring and works fine ...so far
Good luck