Lets talk about forged internals. what exactly is the difference? how do they make them? why is it needed? after how much hp should you have forged internals? is it just using juice?(NOS) or other power adders such as supercharger or turbos? please help me understand this subject a little better

 

A cast piece is something that is poured into a mold, a forged piece is something that starts out as a block of metal and is squeezed/pressed into shape after its heated up.

Forged is the best, though not necessary for most engines. It depends on what rpm you want to run to, and whether youre going to be running a high compression and/or forced induction.

Strictly speaking, having forged rods and pistons is better than a forged crank. Youve got to actually work at breaking a cast crank thesedays.

 

Then, in between, is hypereutectic. These are aluminum injected with a silicon to reduce expansion from heat.

Cast<hypereutectic<forged

Forged is basically good up to about 7k and with any kind of forced induction nitrous kits. Hypereutectic can rev just as high if not higher (lighter piston) but isn't recommended for forced induction (not as strong). Cast pistons are basically only what the factory offers, and they should stay below 6k with a stong/naturally aspirated motor.

 

They way I look at it is that the cost of forged pistons vs. hypereutic is very little. So, if you are going to build a new motor, I would recommend forged rods and pistons. A forged crank is not necessary unless you are going to sustain high rpms(6500+) or are pushing past the 500hp(flywheel) level.

I know plenty of people's hypo pistons work fine for them but mine failed, maybe even due to a casting flaw. The first piston broke where the pin meets the piston, the little round portion broke in two which led to full engine failure. If you are going to build a $3000 motor, why not spend the extra couple hundred for the piece of mind of forged.

 

:skep: So what do you go with for engines spinning over 7K? We twist some of our motors at work to 9500 with forged internals.

Hyper pistons are ok, but they can be brittle and therefore are vulnerable to detonation.

Cast is fine for stock street motors, but it really is junk as far as pistons are concerned. Cast pistons are usually used because they "grow" less when the engine comes up to temperature, they are easily reproduced, and they are cheap. Forged and to a lesser degree, hyper pistons will "knock" until they come up to temperature and expand in the cylinder bore.

 

Cast - soft and deformable - best for hi-mileage vehicles with lots of oilchanges

Hyper - Hardest and most heat reflective (hyer=high; sillicon-heat)- No power adders...shatters like glass best for stock+ motor for mileage-performance (LT4 corvette, Late model I-6 jeep engines)

forged - stiff and strongest (maintains shape) will take more of a tensile load and shock (rapid change in load=knock) best choice for power adders...I also include high compression [over 10.5] as a power adder (S/C'd cobras)

For materials it is really a questions of Hp/displacement and/or Torque/displacement ratios

Torque/displacement ratios usaully don't come into play unless you're looking at a cool diesels

HP/displacement really talks to heat generation/dissipation vs physical properties.
Here's my take
Cast .66 - 1 hp/displacement-ci
Hyper 1-1.25 hp/displacement-ci
Forged 1.2-1.5 hp/displacement-ci

now the question is "how do make more power than 1.5 HP/CI?" It's a trade off....here's what really drives it
detonation
Peak heat generated (power cycle) management
Average heat generate (compression/power/exhuast) management

What people do
high rod ratio
top ring to piston top distance
combustion chamber design
combustion chamber deburr
spark plug
"crash" distance (piston top at TDC to cylinder head)
intercoolers (not just for turbo guys and includes water injection)
time - drag racers give their motors a cooling period
compression ratio (road racers will have lower compression ratio than what is maximumly possible due to longer races-very fuel dependent)
etc...


[Modified by ZD1, 12:42 AM 3/11/2004]

 

Yeah, hypers suck, here's one I had(they were relatively new):

See anything wrong? :D Also note the coating and logo on the skirt.
There are 2 others with hairline cracks in the same locations also.
Only forged for me now.
I prefer USA made forged recip assemblies but a fellow forum member in the know said the chinese forgings are actually pretty good also.





[Modified by 89 Paul in Cal, 11:47 PM 3/10/2004]

 

Wow. sounds like I am going to definatly want to go with forged internals including a forged crank down the road when i do my rebuild.