Rpm limits on a cast crank?

[ZROSSA]

Howdy all,

Just pulled the sump of the sbc and found i have 2 bolt mains and a cast crank. Part # is 3932442 which is listed as a replacement 350 crank. Anyone know how hard I can spin this? I need it to hold at least to 6500 rpm. If not its forged crank time. Dont really have the $$$$ for that right now.

 

I really hope this will work as everthing is balanced and it just needs rings and bearings then we are ready to go.

 

Douglas

[Guest Magnum Rockwilder]

Here's everything you need to know about cast cranks:

 

Stock Car Racing magazine published an article entitled "The Cast Crank Blues" in Sep 1992 that described in detail the care needed in balancing a race engine using a cast crank.

 

Smokey Yunick mentions in this article that you should limit any cast crank to 6500 RPM max, no matter how good it is or how well it is balanced. So remember that statement when you plan your cam power band. An untouched stock crank shouldn't be revved beyond 6000rpm.

 

Bad gas (contaminated or too-low octane) and/or too much timing advance causing detonation is hard on the pistons and crankshaft, so run high octane gas and a conservative ignition timing advance.

 

Lugging the motor at low RPM is hard on the crankshaft, so keep the motor in its power band and never acclerate hard below the torque peak.

 

Never go full throttle until the engine is fully warmed up with oil temp over 180 degrees.

 

Replace or service your crank after 100k miles of mild street use or 50k miles or street/strip use.

[BrandonsZ]

OUCH!

[grumpyvette]

its NOT the rpms exactly its the STRESS LEVELS you need to look out for and that relates more to the feet per minute (fpm) in piston speed than just the RPMS

youll generally want to stay UNDER 4000 fpm MAXIMUM if you want the engine to live a long time

and I would seriously consider forged parts if NITROUS, is used or youll be looking to exceed 4000fpm in piston speed even ocassionally

 

BTW 48000/(stroke X 2) = piston speed

so a 327 with its 3.25" stroke reaches that at 7400rpm

but a 383 with its 3.75" stroke reaches that at 6400rpm

thats the PEAK LIMIT NOT THE OPERATING SPEED,or put another way, if you exceed that speed even on shifts,you WILL eventually hurt the engine DRASTICALLY... and in most cases its the valve train that causes you problems before the lower assembly, enter valve float rpm levels even for a second,have the valve train geometry slightly out, use hydrolic lifters over 6000rpm or not have the clearances correct and your engines eventually going to be expensive history(expensive junk!)

 

(here other usefull calc.)

http://users.erols.com/srweiss/index.html#jcalc

[blueovalz]

Grumpy? Is that you? Long time....

 

Oh yeah, while on the subject. Is the formula above valid regardless of the rod length to stroke ratio (or is this ratio simply a reflection of piston acceleration and not ultimate speed)?

[wheelman]

Hey Grumpy,

You need to revise your formula to read like this:

 

48000/(stroke X 2) = piston speed

 

Wheelman

[grumpyvette]

wheelman

thanks YEAH I WROTE IT WRONG!

 

blueovalz

your correct that the rod length effects the accelleration , but its just a well tested formula to keep the inertial loads within reasonable limits, its NOT meant to be exact, its meant to keep you from launching parts

 

valve train failures and rod bolts would tend to be the reason for engines failing before cranks and pistons if you stay under that limit! and the limit is there to keep from over stressing the rod bolts.

if your not having a lube failure its almost always valve train clearance or valve float that caused engine failures below that limit

once a piston contacts a valve for any reason its pretty comon for things to compound and get nasty real fast because pistons can,t compress bent valves without rods breaking or heads cracking ETC.

[ZROSSA]

Thanks guys. Thats good to here. I have decided to let the guy doing the heads do the bottom end as well. I am pretty good on theory but have very little practical knowledge on v8's. The way some people were talking I thought the crank would fly apart. The piston will probably be KB hyper.... at 10.5-1 and the cam is being custom ground on a bilet to match the flow of the heads. Looking for 450 hp on a 350 sbc. This is a road engine. No n02. The guy putting the engine together is pretty clued up. Has a whole stack of trophys from all different classes of racing.

 

Cheers

 

Douglas