<VV> crank material, again

[djtcz at comcast.net]

Snipped and bottom posted
-------------- Original message -------------- 

> From: BobHelt at aol.com 
> Subject: Re: Crankshaft Question? 
> In a message dated 2/3/2008 1:43:33 PM US Mountain Standard Time, 
> westerncanadacorsa at shaw.ca writes: 
> 
> Has anybody ever heard that there is a difference between the standard and 
> nitrided cranks in terms of the metal they were constructed with? 
> 
> Regards, 
> Joel 
> 
> NO. Where did you hear this? 
> Regards, 
> Bob Helt 


----------------

I just found the Corvair SAE paper compilation published by Motorbooks ~ 1979.

It is only about early models.  


R. Thoreson and J. Brafford of the Chevrolet motor division are credited  for the paper entitled "The Corvair turbosupercharged engine."
In "The Corvair turbosupercharged engine" paper (no number listed), in the "engine modification" section, it (Chevy) says "Quite early in the development of the turbocharged Spyder engine it became evident that the  carbon steel used in the basic Corvair engine (SAE 1045) was not adequate for the loads to which the crankshaft would be subject in the turbocharged engine. Cracks appeared in the crankshaft fillets and radii during the rotary fatigue and stroking durability tests....... A chromium steel (SAE 5140) was substituted.......Use of the 5140 steel and the special treatment (low temp liquid nitriding) gives increased hardness and fatigue strength to the fillets, largely through more uniform and deeper penetration of the heat-treat than would be possible with plain carbon steel........"

They mention the stock 1040 cranks would develop cracks in about 100,000 test cycles, whereas the Spyder crank withstood over a half million cycles without cracking.

The last paragraph says the torsional vibration at full throttle and full boost was similar to normally aspirated (non-turbo) high performance Corvair  engines "consequently no harmonic vibration balancer is needed."

My conclusions - 
- The spyder crank is really recommended for 145 CID Hot Rod engines
- But dampers are not needed on 145s with stock rev range
-  If all the 164 cranks are the better material, that is good news
- Tufftrided cranks are preferred, if not required, for speed work. That means a factory Tufftrided crank , despite the 5140 material, really is less manly if ground undersize and not re-Tufftrided
- Crank "breakage" is fatigue failure after 100,000s of cycles (100,000 =  25 minutes at 4000 rpm) , starting from cracks that develop in the rod journal radii.    I'll continue to  keep my eyes out for factory Turbo/140 HP cranks with the "best" radii, in case there are some runts in the GM litters. 

Dan Timberlake