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  #31  
Old 11-10-2003, 03:30 PM
oab_au oab_au is offline
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Forged vises Cast.

Yes the pistons are a pressure casting and the crank and rods are forged steel.

The advantages with forged pistons is there strength at high rpm. The pressure cast pistons are fine for our motor, it is just when the RPMs are up around 7000 to 8000 that forging is best as the cast pistons can crack around the oil ring groove, and at the wrist pin bosses.

I think the WRX uses pressure cast pistons also, it is when the sustained RPMs are higher that forging is needed, as in the WRX 22B Sti. Cost is the factor, forged slugs take a lot of machine finishing as the section is thicker and the wrist pin cutaways have to be machined in, instead of cast in.

Pressure casting is a lot stronger than gravity cast piston, porosity is not a problem in pressure casting.

Harvey.
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  #32  
Old 11-10-2003, 03:31 PM
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ok so heres the deal(dilemma), i am purchasing a motor from a yard shortly here. I dont really want to lower the compression, especially if i never go FI, but theoreticlly i wonder much much i have to looser by slightly lowering compression and going with their forged pistons. I hate emiling them so much, is there a mathmatical calcuation or are their too manyother variables?
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  #33  
Old 11-10-2003, 03:34 PM
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Re: Forged vises Cast.

Quote:
Originally posted by oab_au
Yes the pistons are a pressure casting and the crank and rods are forged steel.

The advantages with forged pistons is there strength at high rpm. The pressure cast pistons are fine for our motor, it is just when the RPMs are up around 7000 to 8000 that forging is best as the cast pistons can crack around the oil ring groove, and at the wrist pin bosses.

I think the WRX uses pressure cast pistons also, it is when the sustained RPMs are higher that forging is needed, as in the WRX 22B Sti. Cost is the factor, forged slugs take a lot of machine finishing as the section is thicker and the wrist pin cutaways have to be machined in, instead of cast in.

Pressure casting is a lot stronger than gravity cast piston, porosity is not a problem in pressure casting.

Harvey.
ok harvey, now does shot peened= forging?
phil
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  #34  
Old 11-10-2003, 03:55 PM
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Re: Re: Forged vises Cast.

Quote:
Originally posted by Phast SVX


ok harvey, now does shot peened= forging?
phil
Yes we shot peen rods and other forged steel componets to remove the rough finnish that can cause the unit to crack. Don't do it to pistons though.

Harvey.
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  #35  
Old 11-10-2003, 04:02 PM
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Quote:
Originally posted by Phast SVX
ok so heres the deal(dilemma), i am purchasing a motor from a yard shortly here. I dont really want to lower the compression, especially if i never go FI, but theoreticlly i wonder much much i have to looser by slightly lowering compression and going with their forged pistons. I hate emiling them so much, is there a mathmatical calcuation or are their too manyother variables?
phil
Phil I would stay with the std pistons and the 10:1 unless you are going to do circuit racing, Where the high rpms are sustained for a longer time. Spend the money on other bits. I have run both in bike engines but would only spend the extra bucks for racing.

Harvey.
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  #36  
Old 11-10-2003, 04:11 PM
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Re: Re: Re: Forged vises Cast.

Quote:
Originally posted by oab_au


Yes we shot peen rods and other forged steel componets to remove the rough finnish that can cause the unit to crack. Don't do it to pistons though.

Harvey.
I thought Phil asked if forging was equivalent to shot peening. If that was the question, then the answer is no. Forging is where a powerful machine literally shapes a block of material under enormous amounts of pressure.

BTW, just for technical accuracy, shot peening helps to prevent cracking not only by removing stress risers due to small surface irregularities, but also by putting the surface layer of the material into elastic compression and by strengthing the surface layer.
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  #37  
Old 11-10-2003, 04:18 PM
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Quote:
Originally posted by Phast SVX
ok so heres the deal(dilemma), i am purchasing a motor from a yard shortly here. I dont really want to lower the compression, especially if i never go FI, but theoreticlly i wonder much much i have to looser by slightly lowering compression and going with their forged pistons. I hate emiling them so much, is there a mathmatical calcuation or are their too manyother variables?
phil
All the research I've done indicates that if you plan to do forced induction, you are best off with the forged pistons. Its not only for the strength, but the lower CR. I guess you just have to ask yourself at some point whether or not you are really going to go with forced induction.
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  #38  
Old 11-10-2003, 04:57 PM
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Quote:
Originally posted by mbtoloczko
... the high cylinder temps resulting from the very high pressures due to the 10:1 CR cause the edge of the piston to melt away...
I got to thinking about this statement and it didn't make sense because aluminum, cast or forged, melts at the same temp, so I called Cobb Tuning back up. The deal is that its not so much an issue of melting is that a piston designed for boost should have a much larger space between the top of the piston and the first ring for strength and heat transfer away from that corner, and the SVX piston does not have a large space, so the corner is not well supported, and when it gets hot from the cylinder temps, its easily chipped or deformed.
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  #39  
Old 11-10-2003, 04:57 PM
oab_au oab_au is offline
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Sorry Phil guess your shorthand was too short for me

As for which to go with? You just have to decide what you are gong to do with the engine. Unblown stay with the std pistons and 10:1. Road work blown, you can still use the same pistons, depending on the amount of boost. I have run 14lbs boost on cast pistons at 9.5:1 on the road to 7000 rpm, no problem. I have also blown a hole in a forged piston on the road without a blower. Turbo charging produces little boost at low RPMs so there is not a lot of problems with it. The boost can go up as the rpm goes up. Water injection will hold the top end boost ok to say at 7 lbs.

If it is going to be raced, like 10 laps of a circuit using full boost, then use forged pistons and a higher boost say 14lbs plus water and every other device that will keep the combustion temp to a reasonable level.

It is the amount of time that it is going to sustain the boost and rpm that needs the lower compression ratio and forged pistons.

Harvey.
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  #40  
Old 11-10-2003, 07:46 PM
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for what's worth

Quote:
The rods and crankshaft are forged steel and the pistons are forged aluminum.
This is from http://my.voyager.net/~tmclane/index.html
wolf33h
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  #41  
Old 11-10-2003, 09:06 PM
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Re: for what's worth

Quote:
Originally posted by wolf33h

This is from http://my.voyager.net/~tmclane/index.html
wolf33h
Yeah, that is apparently incorrect.
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:: 2006 Silver Mitsubishi Evolution 9, E85, 34 psi peak, 425wtq/505whp DJ ::
1995 Laguna Blue SVX L AWD 5MT (sold)

Visit my locker

SVX Mods: ND iridium spark plugs, Impreza RS fpr, afr tuned to 13.2:1 using a custom MAF bypass, custom exhaust, WRX 5MT w/ STi RA 1st-4th gear & stock WRX 5th gear, Exedy 13 lb flywheel & Sport Clutch, STi Group N tranny & engine mounts, urethane spacers in rear subframe, rear diff mounts, and pitch stopper, SVX Sport Strut Springs (185f/150r), custom 19 mm rear swaybar, urethane swaybar mounts, Rota Torque 17x8", 225/45-17 Proxes 4 tires, Axxis Deluxe Plus organic brake pads.
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  #42  
Old 11-11-2003, 04:34 AM
wolf33h
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Exclamation Another for what's worth

Quote:
cdigerlando
Quote:
I am thinking of switching to turbo legacy pistons in my subaru SVX. Anyone know if they are forged aluminum like the SVX or are they steel?
Apparently cdigerlando was considering turboing the SVX engine when he asked the question in:
http://forums.nasioc.com/forums/showthread/t-40339.html
wolf33h
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  #43  
Old 11-11-2003, 08:35 AM
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Re: Another for what's worth

Quote:
Originally posted by wolf33h

Apparently cdigerlando was considering turboing the SVX engine when he asked the question in:
http://forums.nasioc.com/forums/showthread/t-40339.html
wolf33h
:-) He was also apparently incorrect. Its quite amazing that this rumor has been floating around for so long. I'm very confident in Cobb Tuning's statement that the pistons are cast because they've torn down SVX motors many times and have had the opportunity to carefully inspect the pistons.
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:: 2006 Silver Mitsubishi Evolution 9, E85, 34 psi peak, 425wtq/505whp DJ ::
1995 Laguna Blue SVX L AWD 5MT (sold)

Visit my locker

SVX Mods: ND iridium spark plugs, Impreza RS fpr, afr tuned to 13.2:1 using a custom MAF bypass, custom exhaust, WRX 5MT w/ STi RA 1st-4th gear & stock WRX 5th gear, Exedy 13 lb flywheel & Sport Clutch, STi Group N tranny & engine mounts, urethane spacers in rear subframe, rear diff mounts, and pitch stopper, SVX Sport Strut Springs (185f/150r), custom 19 mm rear swaybar, urethane swaybar mounts, Rota Torque 17x8", 225/45-17 Proxes 4 tires, Axxis Deluxe Plus organic brake pads.
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