Long Rod 408?

Has anyone built a long rod 408? By my calculations, if you run a 4" stroke crankshaft, with 6.385" rods (SCAT offers them), and the 347 pistons (1.1" compression height), that puts you at a rod/stroke ratio of 1.60:1 as well as having the piston 0.015" in the hole, which with a decked block, comes out near the top of the bore.

As long as the rod is 2.00 inches or more longer than the stroke, NOTHING GOOD will happen. The rod to stroke ratio is pretty much all BS


JOE SHERMAN RACING ENGINES

Originally Posted by BIGJOE

As long as the rod is 2.00 inches or more longer than the stroke, NOTHING GOOD will happen. The rod to stroke ratio is pretty much all BS


JOE SHERMAN RACING ENGINES

I had to run the numbers just to double check, but here's what I found:

Regardless of the rod length, with the same force on top of the piston, the moment created about the crankshaft is the same. So the rod/stroke ratio has no affect on torque, from a purely geometric standpoint.

Where the rod to stroke ratio plays a factor is this: The piston spends a longer time at the top and bottom of the cylinder, leading to better chamber filling (charge momentum). Not only that, but the sidewall forces are substantially higher: 7% lower sidewall force for the 6.385" rods versus the 6" rods with the numbers I used. With a pressure on the top of the piston of 200 psi and the crank at 90 degrees, that translates to a sidewall force of 900 lbs for the 6" rods, and 840 lbs for the 6.385" rods.

The increased sidewall force generates extra heat that goes right out of the cooling system. The only source of energy comes from the combustion cycle, and the energy going out the cooling system is effectively "wasted" energy that's been robbed from your combustion cycle leading to less power and a heavier coolant system.

Besides, I'd prefer this thread to not fall into a rod/stroke discussion, but rather focused on answering my question, which was whether or not any has built this combination. Would there be any downsides to this combination?

If you allready know the answer , Why ask the question ??

JOE SHERMAN RACING

Originally Posted by BIGJOE

If you allready know the answer , Why ask the question ??

JOE SHERMAN RACING

My question was asking whether or not anyone has built this combination..

Originally Posted by Echo

I had to run the numbers just to double check, but here's what I found:

Regardless of the rod length, with the same force on top of the piston, the moment created about the crankshaft is the same. So the rod/stroke ratio has no affect on torque, from a purely geometric standpoint.

Where the rod to stroke ratio plays a factor is this: The piston spends a longer time at the top and bottom of the cylinder, leading to better chamber filling (charge momentum). Not only that, but the sidewall forces are substantially higher: 7% lower sidewall force for the 6.385" rods versus the 6" rods with the numbers I used. With a pressure on the top of the piston of 200 psi and the crank at 90 degrees, that translates to a sidewall force of 900 lbs for the 6" rods, and 840 lbs for the 6.385" rods.

The increased sidewall force generates extra heat that goes right out of the cooling system. The only source of energy comes from the combustion cycle, and the energy going out the cooling system is effectively "wasted" energy that's been robbed from your combustion cycle leading to less power and a heavier coolant system.

Besides, I'd prefer this thread to not fall into a rod/stroke discussion, but rather focused on answering my question, which was whether or not any has built this combination. Would there be any downsides to this combination?

when you build an engine ,put a degree wheel on and see what happens and when

For a quick reference, I looked at KB's catalog.

They show a 408 piston with a 6.200" rod, it has a compression height of 1.28"

The 347 Piston shows a CH of 1.09

The difference is 0.190", and you are suggesting a rod that is 0.185 longer. Looks like you'd only be 0.005" in the hole!

Never built one, though. Sounds like the ring pack will be squeezed together rather tightly. The shorter piston will be less stable in the bore, possibly negating the benefits of the reduced sideloading of the longer rod. Oh, the longer rod will probably weigh more as well (the piston MAY be lighter, but the net effect is most likely a gain in weight).

But hey, it's fun to try different stuff! Try it, see what happens, and learn :-)

Lee

Originally Posted by L n L

For a quick reference, I looked at KB's catalog.

They show a 408 piston with a 6.200" rod, it has a compression height of 1.28"

The 347 Piston shows a CH of 1.09

The difference is 0.190", and you are suggesting a rod that is 0.185 longer. Looks like you'd only be 0.005" in the hole!

Never built one, though. Sounds like the ring pack will be squeezed together rather tightly. The shorter piston will be less stable in the bore, possibly negating the benefits of the reduced sideloading of the longer rod. Oh, the longer rod will probably weigh more as well (the piston MAY be lighter, but the net effect is most likely a gain in weight).

But hey, it's fun to try different stuff! Try it, see what happens, and learn :-)

Lee

I was just curious. I don't have the funds to be trying things out, so I just wanted to see if anyone has done it.

Done one with 6.300 rods. It replaced one that had 6.250. No difference. Sounds a little different idling but other than that, not a single number difference in ET or MPH.

Only reason I did mine that way was due to the shorter deck height of the 1969 Windsor block. 6.300 rods + 331 pistons = piston nearly at zero deck before "squaring" the decks.

I like folks who think outside the box...well done.

There are advantages to shorter rods also. You can play the game either way. The rod has to be a lot longer to see any real meaningful difference. I wrote a program to see how the piston motion differs during my decision to use 289 length rods and a 383W piston in a 331.

I concluded that unless the rod is a lot longer, which will start to show some significant difference in piston motion, the advantage to a long rod is lighter piston (less stress on the rod thus more rpm capability) and reduced friction loss.

Difference in piston motion of a 331 with 5.155 rods and a 331 with 5.4 rods was .012 max at 90 degrees....less everywhere else. The 331 with 5.155 rods has the piston down .012 more at 90 degrees....thus someone could argue that the shorter rod may fill the cylinder better.