Tuner wrote: ↑Sun Oct 28, 2018 8:30 pm
pamotorman wrote: ↑Sun Oct 28, 2018 1:13 pm
there was a dirt track modified that raced near here using the 6 cylinder ford engine that whipped up on the chevy V-8s pretty regularly.
I remember an article 35-40 years ago in "Stock Car Racing" or "Circle Track" about a guy (I think ASA in Wisconsin, but ?) who was successful in making power with a 300 Ford Six but the bottom end wouldn't hold together, the forged steel truck cranks he was using would crack. When he finally ran out of forged steel cranks he used a cast iron crank and had no more failures (at the time of the article). The opinion was the cast crank was easier to make with the counterweights in more ideal locations, the castings could have more strategically located counterweights.
In the early '70s I tuned some 500 2bbls on a 300 Ford for a drag racing deal that the flywheel came loose in just a few passes, time after time, better bolts, lock tabs, nothing would keep it tight. The guy finally tack welded the flywheel bolts and it still came loose. Another was '70 240Z Datsun that after it got the cam and headers treatment when the RPM went north of 8000 a few times the flywheel would get loose and start clunking. The '70 240Z crank has no counterweights at all, same as a 4bbl. Pontiac OHC 6, just bearing journals. The 1bb. OHC Pontiac crank has counterweights. As I recall you can tell a 6 cyl. OHC Pontiac crank from other GM 230 and 250 sixes because it has scallops in the counterweights to clear the block where the accessory drive housing mounts for the distributor, fuel pump and oil pump.
There are no non-counterweighted 240Z cranks. They all have counterweights. I have 2 70's, a 69, and MANY 71-84 cranks for these engines.
The vibration problems are not due to the counterweighting of the crank, they are due to the flywheel and damper masses being significantly different. You wanna wind up a sixer, that's your fix-get the flywheel as light and as small a diameter as possible, and get the front damper as close to the same size and weight as the flywheel. Use a good quality elastomer damper and it will need to be custom made for your application.
Source? I run the old datsun inline sixes to 8500+RPM for drag and land speed record runs. You will run into the problems you mention-and some more. Like the damper bolt that holds the damper in place-you need a bigger, stronger, longer one. Make sure you don't torque up on the nose of the crank, that the damper bolt clamps the damper in place-it's not the crankshaft keys that are holding the damper in place, it's the bolt torque and face friction ONLY. Those keys (on any engine I've ever worked on, actually-not just 6's...) have zero ability to handle the torque of that damper spinning.
Next, the damper needs to be a press fit onto the crank. Not much of a press fit in most applications, but if it just slips on there it's not gonna do the job. It'll get loose and cause just as much havoc as the loose flywheel.
The flywheel bolts need the same work that the damper bolt needs. Bigger, stronger, longer. Loctite will only get you so far if you can't get the torsional vibration down by reducing the flywheel diameter and getting the damper diameter up.
Lots of "It shook the flywheel bolts out, gotta be a balancing problem" myths out there. It's a torsional vibration problem caused by the long crankshaft twisting along it's length, and the front damper and rear flywheel trying to play catch-up and overshooting each other. The cure is lightness and symmetry.
As to the 300-6, that's my other race engine...if he was breaking steel truck cranks and not cast iron cranks, well...he's got exactly the opposite problem of every other 300-6 racer out there. The cast cranks will crack in the main radius area at about 600HP on the 5 and 6 mains, you guessed it...torsional vibration problems. I've only broken one through the throw area, that's when I switched to the old steel cranks and the problems stopped. I also have my steel cranks welded up to BBC rod journals and run BBC rods, but that's because I'm a cheap whippersnapper who doesn't want to buy 6 custom rods when I could run off-the-shelf rods and 1 custom crank instead.
