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-   -   do i really need forged? (http://www.hotrodders.com/forum/do-i-really-need-forged-181078.html)

muddbogg 07-14-2010 10:33 PM

do i really need forged?
 
Hi i am wondering how much rpms a cast crank,rods,and pistons will go? I have always wondered if it is worth spending the gazillion dollars more on forged. I understand preventative maintnence, but we will see where some of these answers lead to. Another question is ultimately if i have to go forged, what one peice(crank,rods,pistons) would you guys for sure get forged? this is for no specific rig, i work with small/big block chevies if that helps. so if cast limit is 6000rpm just say 6000 and below cast,6000 up forged or something along the lines of that. Thank you for helping me with my upcoming builds.

BOBCRMAN@aol.com 07-14-2010 11:19 PM

Properly balanced and oiled cast crank will take all you can give it on a street strip type motor. Six-seven thousand rpm in short bursts is common. I have more than several "Saturday Night Special" engines out that are cast crank and winners..

Hyper eutectic pistons also. I have left turners that get two-three seasons on them.. Secret is proper ring gap and wall clearance..

Only cast rods I deal with are Buick-Pontiac. Low rev motors..

SBC or BBC rods work well with new rod bolts and mag/resize. Unless you get int real big hp numbers. Then I go for aftermkt stuff..

muddbogg 07-14-2010 11:31 PM

thanks, so what ive gotten from the search engine and your post is this...
hypereutectic pistons for about 6500 rpm and below
cast crank will do for about 6500 rpm and below
And stock rods with ARP bolts are good for about the same rpm as the others.
So now i have a question and its how reliable are all these with 500 ft lbs of torque behind them?? but they will not be revved that high all the time. So would they last?

65smallblock 07-15-2010 12:42 PM

Stock rods with ARP bolts are typicaly good up to about 400 HP. If you are running @ 500 ft lbs then I assume HP will be similar and I would suggest aftermarket rods. Go that route and you "might" be fine with a cast crank and hyperutectic pistons, its kind of a crap shoot, but those parts can certianly withstand such work "from time to time".

RPMs are what kills a motor. If your gonna rev it, build it to take it. Hyperutectic pistons are proving to be very adequate for lots of work, a couple of huge assets in building for RPM's are lightweight parts (pistons and rods) and a good ballance job. A cast crank can withstand a LOT more work with those two assets because it reduces stress on the crank.

PatM 07-15-2010 01:46 PM

I agree totally with just about all of that. Cast cranks (SCAT, for instance) claim to be able to handle 500 HP. That of course will depend on the weight of the pistons and rods, as well as the power being produced. Lunati Street Race rods used to be rated (don't think they make them anymore) for 450 HP at 7000 RPM. Since RPM increases drive increases in acceleration forces, the rods should be more capable at lower RPMs. Like maybe 475HP at 6200-6500 RPM. With Rods, it's not the force from combustion, it's the inertial acceleration and deceleration created by higher RPM that makes the difference. Naturally, piston weight comes into play here as well as RPM. So, very light hypereutectic pistons, light weight forged high-strength (say, 4340) rods with good (like ARP) bolts, and a quality cast crank contribute to a bottom end that is likely to survive into the 475-500 HP arena.

muddbogg 07-15-2010 01:55 PM

alright guys,thank you.
one last question, and that is what if i go supercharged??
the compression would be lower(8.0 to 1 maybe). so is there anything you would definently change??thanks

PatM 07-15-2010 01:59 PM

I have no personal experience with non-NA engines. But I'd bet it'd be a gamechanger for the pistons. And be carefull with the cast crank as well. It's easy to go over 500 HP with a serious blower.

65smallblock 07-15-2010 02:03 PM

Compression is lowered for use with forced induction because otherwise the cylinder pressure would go through the roof and things would come apart. In other words, supercharging will bring the cylinder pressure back up, so any reduction in compression is a moot point. The bottom line is that you will be making considerably more power and so the strength of the bottom end would also have to be upgraded accordingly. I would not reccomend supercharging with a cast crank.

muddbogg 07-15-2010 02:06 PM

i have no personal experience with them easier. i had a little idea about adding a blower to a 400 sbc of mine so im just sort of breaking the ice to see if its for me possibly. i know with running NOS(which i dont) they reccomend forged but i heard people turn their fuel mixture rich and they go play and nothing ever goes wrong in the rotating assembly FOR THEM. Im not sure about to me though,just my luck.


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