John B

iTrader: (10)

Having experienced my forged LS6 shortblock making less HP than a stock shortblock makes me wonder.....are the stock tolerances for piston to wall clearance able to make 10-15 HP more than a forged piston and an aftermarket ring?

My forged shortblock only has 2.9% leakdown yet seems to make less power than when it was stock at about the same compression ratio. When I consulted a couple of well respected tuners about this they said that you can't have a stock cube forged motor equal the power of a stock cubed factory motor. Is it just that simple?

I noticed that both Brent's (Cartek's 3X H/C) and Tommy3gun's (LG Motorsports G5X3/Absolute H/C) motors are stock LS6 shortblocks and appear to be the only mild motors at 500RWHP. I have yet to see a forged motor equal these numbers (with similar compression and camshaft size). Opinions?

kp

iTrader: (34)

Reciprocating weight - I'm sure the forged stuff is heavier then stock.

Stock rings dont have much tension and just because they leak down a bit more then the better rings doesnt mean they make less power. Friction is a killer, look at the cylinder walls on a stock motor after 75K, the crosshatch still looks like a motor with 10k on it. I bet most aftermarket rings are higher tension then stock to control oil use/blowby.

That would be my opinion anyhow..

hellbents10

iTrader: (10)

wieght and friction.

CHRISPY

Forged alum pistons are LIGHTER than stock not heavier

I think alot has to do with how the pistons are positioned in the hole (Which is based both a combination of piston selection and decking of the block)

Ring gap is a factor as well.

If the piston is sitting .005-.008 out of the hole and a stock gasket is used it should make the same/more power.

If the rings are setup a little looser for spray then NA power will be very slightly down. Also low tension rings tend to make a little more power (at the expense of oil control) in comparision to regular/high tension rings.

kp

iTrader: (34)

I have never weighed them personally but the stock pistons are pretty light in these, they dont have the steel expansion plates like the old cast pistons so they are pretty light, same with the rods, they sure seem pretty light but I never weighed them compared to the aftermarkets. I'd be curious to see what the actual weights are.

Ring tension makes a pretty big difference in power, ring gap probably doesnt have much to do with it unless its extreme, huge gap = blowby, not enough = rings butting and wiping them out.

Pro Stock John

iTrader: (34)

Ring gap can make a difference, but usually tighter rings make more power.

kp

iTrader: (34)

describe 'tighter' rings

I have made the most power on an engine dyno with over 15% leakdown with back cut super light tension rings as opposed to medium tension gapless with 3% leak on the same combo. Ring end gap is not a power thing directly, its a clearance thing as far as I know. When the rings heat up they expand, idealy getting them just to the point of butting at peak temp. Big mistake is usually making the gap too small on a nitrous/supercharged engine and the rings will expand enough the butt - when they do that they will be trashed pretty quickly. Naturally way too big a gap will hurt power but its hard to say by how much, any decent motor will have the rings hand gapped anyhow and they will most likely be tighter then stock rings.

kp

iTrader: (34)

getting back to the original topic, there are a lot of factors involved and I wouldnt put to much stock in dynojet numbers from around the country but here are my thoughts as why a builder would tell you a forged motor wont make as much power as stock.

Piston ring tension/cylinder bore finish = friction. Add 8 pistons going up and down and that makes a pretty big difference at 6000rpm.

Weight, someone get some piston/rod/pin/ring weights between stock and commonly used 'street' forged stuff - I bet its heavier then stock (could be wrong but I doubt it).

Double roller chain = more friction.

Oil pump, most people shim the oil pump to get more pressure. More pressure = less power.

Neither one of the three by itself will make a huge difference, but start adding things up. Most people arent building all out drag race motors, lightened pistons, aluminum rods, big oil clearence, light tension rings etc and most build them for reliability while beating the crap out of them so you are going to pay a little price in power IMO. Sort of like putting a TH400 and a 9" ford in your car - 4L60 and a 10 bolt will be quicker, right? How much power who knows, you would think with hand gapped rings and better piston to bore clearance you would make up up for some of the loss of other things but until someone does it on an engine dyno and A/Bs stuff cant say for sure..

John B

iTrader: (10)

Does the piston really know where it is? If the quench distance is say....0.040" at 0.005" out of the hole or 0.040" with the piston at -0.005" (due to a thinner head gasket) how can the power change? The piston is in the head gasket material at 0.005" out not in the combustion chamber.

CHRISPY

I thought this too but as far as I remember the stock pistons are heavier than diamond forged.

Rods I am not sure about...

I am SURE Sled or Chris would be able to tell us for sure.

ROCNDAV

iTrader: (100)

I think he meant that thicknesses in head gaskets would affect compression ration, which would affect power.

D

CHRISPY

affects compression ratio, quench and squish